Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
AP
APPLICATION NOTES
Date:
16th June 2006
Hardware Suffix:
-305
Software Version:
-610
Connection Diagrams:
-404 (P631, P632, P634) -406 (P633)
P63x/UK AP/A54
Application Notes MiCOM P631, P632, P633, P634
AP
Application Notes MiCOM P631, P632, P633, P634
P63x/UK AP/A54 (AP) 6-1
CONTENTS (AP) 61.
INTRODUCTION
5
1.1
Protection of transformers
5
1.1.1
Introduction
5
1.1.2
Overview of existing practices
5
1.2
P63x protection relay
7
1.2.1
Protection functions
8
1.2.2
Non protection features
9
2.
APPLICATION OF INDIVIDUAL PROTECTION FUNCTIONS
10
2.1
Overall differential protection (87)
10
2.1.1
Biased elements
10
2.1.2
Ratio correction
13
2.1.3
Vector group correction and zero sequence current filtering
14
2.1.4
Magnetizing inrush stabilization
18
2.1.5
High set operation
18
2.2
Restricted earth fault protection
19
2.2.1
Basic principles
19
2.2.2
REF operating modes
20
2.2.3
Stability requirements for high impedance REF
20
2.2.4
Use of METROSIL non-linear resistors
21
2.3
Overfluxing protection and blocking
24
2.3.1
Basic principles
24
2.3.2
Transformer overfluxing
24
2.3.3
Time delayed overfluxing protection
24
2.3.4
5th Harmonic blocking
25
2.3.5
Required settings
26
2.4
Auto-transformer protection
26
2.4.1
Unloaded delta tertiary
28
2.4.2
Neutral earthing with phase-segregated CTs
28
2.4.2.1
Amplitude matching
28
2.4.2.2
Vector group matching
28
2.4.2.3
Zero-sequence current filtering
28
AP
P63x/UK AP/A54 (AP) 6-2
AP
Application Notes MiCOM P631, P632, P633, P634
2.4.2.4
Inrush stabilization
28
2.4.3
CTs in series with delta tertiary winding
29
2.4.3.1
Amplitude matching
29
2.4.3.2
Vector group matching
29
2.4.3.3
Zero-sequence current filtering
29
2.4.3.4
Inrush stabilization
29
2.4.4
CTs outside delta tertiary winding
30
2.4.4.1
Tripping characteristic
31
2.5
Busbar/mesh corner differential protection
31
2.5.1
Busbar protection settings
32
3.
SETTING RECOMMENDATIONS
33
3.1
Introduction
33
3.2
Enabling the relay
33
3.3
Enabling a protection function
33
3.4
Configuring a trip command and output contact for CB tripping
33
3.5
Configuring a watchdog contact
34
3.6
HMI read key assignment
34
3.7
Operation panel configuration
34
4.
CURRENT TRANSFORMER REQUIREMENTS
35
4.1
Knee point voltage offered by IEC “P” class CTs
36
4.2
Use of ANSI/IEEE CTs
37
5.
AUXILIARY SUPPLY FUSE RATING
38
FIGURES Figure 1:
Typical transformer protection package
6
Figure 2:
Typical protection package for a generator transformer
7
Figure 3:
P63x bias characteristic of transformer differential protection
11
Figure 4:
Amplitude matching factor, kamp
14
Figure 5:
Yd5 transformer example
15
Figure 6:
Vector group selection
17
Figure 7:
Zero sequence current filtering
18
Figure 8:
Fault limitation on an impedance earthed system
19
Figure 9:
Fault limitation on a solidly earthed system
19
Application Notes MiCOM P631, P632, P633, P634
P63x/UK AP/A54 (AP) 6-3
Figure 10:
High Impedance REF principle
20
Figure 11:
Variable time overfluxing protection characteristic
25
Figure 12:
Auto-transformer with tertiary winding
26
Figure 13:
Auto-transformer with tertiary winding
27
Figure 14:
Auto-transformer - unloaded delta tertiary winding
28
Figure 15:
Auto-transformer - neutral earthing with per phase neutral CTs
29
Figure 16:
Auto-transformer - CTs in series with delta tertiary winding
30
Figure 17:
Auto-transformer - CTs outside delta tertiary winding
31
Figure 18:
P63x busbar differential protection scheme
31
Figure 19:
P63x bias characteristic of busbar differential protection
32
AP
P63x/UK AP/A54 (AP) 6-4
AP
Application Notes MiCOM P631, P632, P633, P634
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
1.
INTRODUCTION
1.1
Protection of transformers
1.1.1
Introduction
(AP) 6-5
The development of modern power systems has been reflected in the advances in transformer design. This has resulted in a wide range of transformers with sizes from a few kVA to several hundred MVA being available for use in a wide variety of applications. The considerations for transformer protection vary with the application and importance of the transformer. To reduce the effects of thermal stress and electrodynamic forces it is advisable for the overall protection to minimize the time that a fault is present within a transformer. On smaller distribution transformers effective and economically justifiable protection can be achieved by using either fuse protection or IDMT/instantaneous overcurrent relays. Due to the requirements of co-ordination with the downstream power system protection this results in time delayed fault clearance for some low level faults. Time delayed clearance of major faults is unacceptable on larger distribution, transmission and generator transformers, where the effects on system operation and stability must be considered. High speed protection is desirable for all faults. Transformer faults are generally classified into four categories: •
Winding and terminal faults
•
Core faults
•
Abnormal operating conditions such as overvoltage, overfluxing and overload
•
Sustained or uncleared external faults
All of the above conditions must be considered individually and the transformer protection designed accordingly. To provide effective protection for faults within a transformer and security for normal operation and external faults, the design and application of transformer protection must consider factors such as: •
Magnetizing inrush current
•
Winding arrangements
•
Winding connections
•
Connection of protection secondary circuits
The way that the protection of larger transformers is typically achieved is best illustrated by examining the protective devices associated with common applications. 1.1.2
Overview of existing practices Figure 1 shows typical protection functions for a sub-transmission or large distribution transformer.
AP
P63x/UK AP/A54
Application Notes
(AP) 6-6
MiCOM P631, P632, P633, P634
WT
B
OT
51
50N
51N
ICT 64
87
AP
WT =
Winding temp'
B
=
Buchholz
OT
=
Oil temp'
64
=
REF
87
=
Biased diff'
51N =
Standby E/F
50N =
Inst' earth fault
51
=
IDMT overcurrent
24
=
Overfluxing relay P1937ENa
Figure 1:
Typical transformer protection package
High speed protection is provided for faults on both the HV and LV windings by biased differential protection (87). The relay operates on the basic differential principle that HV and LV CT secondary currents entering and leaving the zone of protection can be balanced under load and through fault conditions, whereas under internal fault conditions balance will be lost and a differential current will cause the relay to trip. The zone of protection is clearly defined by the CT locations and, as the protection is stable for through faults, it can be set to operate without any intentional time delay. In Figure 1 the application of the P63x differential relay includes software vector group and amplitude matching to provide phase and ratio correction of CT signals in addition to filtering LV zero sequence current to prevent maloperation of the differential element for external LV earth faults. Interposing CTs (ICTs) are no longer required. More sensitive high speed earth fault protection for the LV winding is provided by restricted earth fault protection (64). Due to the limitation of phase fault current on the HV side for LV winding earth faults and the fact that any unrestricted earth fault protection in the transformer earth path requires a discriminative time delay, restricted earth fault protection is widely applied. The application of restricted earth fault protection is further discussed in section 2.2. Earth fault protection is provided on the HV winding by the inherently restricted earth fault element associated with the HV overcurrent protection (50N). The delta winding of the transformer draws no HV zero sequence current for LV earth faults and passes no zero sequence current to upstream HV earth faults, hence there is no requirement to grade this element with other earth fault protection and it can be set to operate without any intentional time delay. Sustained external LV faults are cleared by the IDMT overcurrent protection on the HV winding (51) or by the standby earth fault protection (51N) in the transformer earth connection. The extent of backup protection employed will vary according to the transformer installation and application.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-7
The protection scheme may be further enhanced by the use of other protective devices associated with the transformer, such as the Buchholz, pressure relief and winding temperature devices. These devices can act as another main protective system for large transformers and they may also provide clearance for some faults which might be difficult to detect by protection devices operating from line current transformers, e.g. winding inter turn faults or core lamination faults. These devices are connected to directly trip the breaker in addition to operating auxiliary relays for indication purposes. WT
B
OT
64 24
51N 64
87
ICT WT B OT 64 87 51N 51 24
= = = = = = = =
Winding temp' Buchholz Oil temp' REF Biased diff' Standby E/F IDMT overcurrent Overfluxing relay P1938ENa
Figure 2:
Typical protection package for a generator transformer
The protection of a generator transformer is similar to that for any other large transformer. High speed protection is provided for phase to phase faults by the provision of biased differential protection. In addition, for large generators, the transformer is commonly included within an overall second main differential arrangement, which incorporates the generator and transformer within the overall zone of protection. Earth fault protection is provided by a restricted earth fault element on the star winding. Overfluxing protection is commonly applied to generator circuits to prevent generator or transformer damage from prolonged overfluxing conditions. Other protection devices will again complement the main relay protection. Auto-transformers are commonly used to couple EHV and HV power networks if the ratio of their voltages is moderate. The protection arrangements for an auto-transformer are similar in most respects to the protection of a two winding transformer. Protection of all windings can be offered by a biased differential relay such as the P63x, this is further discussed in section 2.4. 1.2
P63x protection relay The P63x relay has been designed to bring the latest numerical technology to the protection of power transformers. The increased functionality of numerical relays allows enhanced protection functions to be offered for a wide variety of applications, which, when combined with a host of non-protective features, can provide power system control and monitoring requirements.
AP
P63x/UK AP/A54 (AP) 6-8 1.2.1
Application Notes MiCOM P631, P632, P633, P634
Protection functions The main protection functions offered by the P63x are listed below: •
Biased differential protection (87)
•
Restricted earth fault protection for individual transformer windings (64)*
•
Instantaneous/time delayed phase overcurrent protection (50/51)
•
Instantaneous/time delayed earth fault protection (50N/51N)
•
Instantaneous/time delayed negative sequence overcurrent protection (46)
•
Thermal overload protection (49)
•
Under/over voltage protection (27/59)*
•
Under/over frequency protection (81)*
•
Overfluxing protection (24)*
•
Opto-isolated inputs and programmable logic for alarm/trip indication of external devices
* These protection functions are not available in the P631. The biased differential element has a triple slope bias characteristic to ensure sensitivity, with load current, to internal faults and stability under heavy through fault conditions.
AP
The differential element can be blocked for magnetizing inrush conditions based on the ratio of second harmonic to fundamental current. In addition, the differential element can be blocked during transient overfluxing conditions based on the ratio of fifth harmonic to fundamental current. Fast operating times for heavy internal faults can be achieved by use of the unrestrained instantaneous differential high set elements. Restricted earth fault protection is available for up to three transformer windings to offer increased sensitivity to low-level winding earth faults. The principle of operation is selectable and allows either the high impedance or biased (low impedance) restricted earth fault method to be implemented. Both the definite-time and the inverse-time overcurrent protection operate with separate measuring systems for the evaluation of the three phase currents, the negative-sequence current and the residual current. Three stages each are provided for the three measuring systems of the definite-time overcurrent protection. The inverse-time overcurrent protection offers a multitude of tripping characteristics for the individual measuring systems. Thermal overload protection can be used to prevent electrical plant from operating at temperatures in excess of the designed maximum withstand. Prolonged overloading causes excessive heating, which may result in premature ageing of the insulation, or in extreme cases, insulation failure. The relay incorporates a current based thermal replica, using rms load current to model heating and cooling of the protected plant. The element can be set with both alarm and trip stages. The V/f overfluxing element provides protection against transformer damage that may result from prolonged operation at increased voltages and/or decreased frequency. Independent alarm and trip characteristics are provided to enable corrective action to be undertaken prior to tripping being initiated. Use of the opto-inputs as trip repeat and alarm paths for other transformer protection devices, (Buchholz, Oil pressure, winding temperature etc.,) allows operation of these devices to be event-logged. Interrogation of the relay fault, event and disturbance records offers an overall picture of an event or fault, of the transformer protection performance and sequences of operation. All models of the P63x are three phase units with internal phase compensation, CT ratio correction and zero sequence filtering, thus eliminating the need for external interposing transformers. Up to four biased inputs can be provided to cater for power transformers with more than two windings and/or more than one set of CT’s associated with each winding, e.g. in mesh or one-and-a-half circuit breaker substation arrangements.
Application Notes MiCOM P631, P632, P633, P634
P63x/UK AP/A54 (AP) 6-9
The variety of protective functions offered by the P63x makes it ideal not only for the protection of power transformers but also for a variety of applications where biased differential or high impedance protection is commonly applied, these include:
1.2.2
•
Busbars/mesh corners
•
Overall generator/transformer protection
•
Generators
•
Reactors
Non protection features In addition to providing all of the common relaying requirements for a transformer protection package, the P63x relay shares many common features with the other relays in the MiCOM range. The P63x offers this variety of additional features by virtue of its digital design and standardization of hardware. These features are listed below: •
Electrical Instrumentation with local/remote display
•
Fault records (summary of reasons for tripping etc.)
•
Event records (summary of alarms and relay events)
•
Disturbance records (record of analogue wave forms and operation of opto-inputs and output relays)
•
Date and time tagging of all records
•
Commissioning aids
•
Optional remote communications
•
High level of continuous self monitoring and diagnostic information
•
Relay menu displayed as standard English or Regional English language variant -800
AP
P63x/UK AP/A54
Application Notes
(AP) 6-10
2.
MiCOM P631, P632, P633, P634
APPLICATION OF INDIVIDUAL PROTECTION FUNCTIONS The following sections detail individual protection functions in addition to where and how they may be applied. Each section provides some worked examples on how the settings are applied to the relay.
2.1
Overall differential protection (87) In applying the well established principles of differential protection to transformers, a variety of considerations have to be taken into account. These include compensation for any phase shift across the transformer, possible unbalance of signals from current transformers either side of windings and the effects of the variety of earthing and winding arrangements. In addition to these factors, which can be compensated for by correct application of the relay, the effects of normal system conditions on relay operation must also be considered. The differential element must be blocked for system conditions which could result in maloperation of the relay, such as high levels of magnetizing current during inrush conditions or during transient overfluxing. In traditional transformer differential schemes, the requirements for phase and ratio correction were met by the application of external interposing current transformers, as a secondary replica of the main transformer winding arrangements, or by a delta connection of main CTs (phase correction only). Within the P63x, settings are provided to allow flexible application of the protection to a wide variety of transformer configurations, or to other devices where differential protection is required, without the need for external interposing CTs or delta connection of secondary circuits.
2.1.1
AP
Biased elements The number of biased differential inputs required for an application depends upon the transformer and its primary connections. It is recommended that, where ever possible, a set of biased CT inputs is used per set of current transformers. There are four basic models of the P63x relay; •
P631 Two biased differential inputs (without REF or voltage based protection)
•
P632 Two biased differential inputs
•
P633 Two or three biased differential inputs
•
P634 Two, three or four biased differential inputs
Where a P634 or P633 is chosen they can be programmed to provide 2, 3, 4 and 2 or 3 biased windings respectively. Table 1 shows the variety of connections which can be catered for by the range of P63x relays. Configuration
No. of CT sets
Recommended Relay
2
P631, P632, P633 or P634
3
P633 or P634
3
P633 or P634
3
P633 or P634
HV
LV
HV LV1
LV2
HV LV HV LV
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-11
Configuration
No. of CT sets
Recommended Relay
HV
4
P634 only
4
P634 only
4
P634 only
LV2
LV1
HV LV1
LV2
HV
LV
Table 1:
Applications of the P63x transformer differential protection relay
The P63x relay achieves stability for through faults in two ways, both of which are essential for correct relay operation. The first consideration is the correct sizing of the current transformers as described in section 4, the second is by providing a relay bias characteristic as shown below: 8.00
I d / I ref
Tripping area 6.00
Fa u for lt cu rr sin gle ent ch -s i ar de a fee cte ris d t
ic
AP
2.00
m
m
1
=0
.7
Blocking area
= 0.3
I R ,m2 / I ref I d > / I ref
2
12200e.DS4
4.00
= 4.0
= 0.2 0.00
2.00
4.00
6.00
8.00 I R / I ref
I
P1764ENa
Figure 3:
II
III
P63x bias characteristic of transformer differential protection
The differential and restraining current variables for each measurement system are calculated from the current variables after amplitude and vector group matching (Refer to the following sections). The formation of the restraining variables differs between two and three-winding protection. The following equations are valid for uniformly defined current arrows relative to the protected equipment, i.e. the current arrows of all windings point either towards the protected object or away from it. Calculation of differential and restraining currents for two-winding protection: Id,y = Is,y,a + Is,y,b IR,y = 0.5 ⋅ Is,y,a − Is,y,b
P63x/UK AP/A54
Application Notes
(AP) 6-12
MiCOM P631, P632, P633, P634 When the infeed to an internal fault from both ends is exactly equal as regards amplitude and angle, then both currents cancel one another out, i.e. the restraining current becomes zero and the restraining effect disappears. Disappearance of the restraining effect when there is an internal fault is a desirable result since in this case transformer differential protection attains maximum sensitivity. Calculation of differential and restraining currents for three or four-winding protection: Id,y = Is,y,a + Is,y,b + Is,y,c + Is,y,d
[
IR,y = 0.5 ⋅ Is,y,a + Is,y,b + Is,y,c + Is,y,d
]
In this case the restraining effect never disappears when there is an internal fault; the restraining effect is even reinforced in the case of multi-end infeed. However, the restraining current factor ½ means that the differential current Id has twice the value of the restraining current IR so that safe and reliable tripping is also guaranteed in the case of multi-end infeed. The tripping characteristic of the differential protection device P63x has two knees. The first knee is dependent on the setting of the basic threshold value DIFF: Idiff> SGx and is on the load line for single-side feed. The second knee of the tripping characteristic is defined by the setting DIFF: IR,m2 SGx. The characteristic equations for the three different ranges are given below. Figure3: shows the tripping characteristic.
AP
The first section (Area I) represents the most sensitive region of the tripping characteristic in the form of the settable basic threshold value Id>. The default setting of 0.2 takes into account the magnetizing current of the transformer, which flows even in a no-load condition and is generally less than 5% of the nominal transformer current. When protecting generators and other items of plant, where shunt magnetizing current is not present, a lower differential setting can be used and 0.1 would be more typical. Characteristic equation for the range
0 ≤ I R ≤ 0 .5 ⋅ I diff > : (Area I)
Id I > = diff Iref Iref
The second section (Area II) of the tripping curve covers the load current range, so that in this section we must account for not only the transformer magnetizing current, which appears as differential current, but also with differential currents that can be attributed to the transformation errors of the current transformer sets. If we calculate the worst case with IEC class 10P current transformers, then the maximum allowable amplitude error according to IEC 60044-1 is 3 % for nominal current. The phase-angle error can be assumed to be 2° for nominal current. The maximum allowable total error for nominal current is then obtained, in approximation, as (0.0-3 + sin 2°) ≈ 6.5 %. If the current is increased to the nominal accuracy limit current, then the total error for Class 10P current transformers can be 10 % maximum. Beyond the nominal accuracy limit current, the transformation error can be of any magnitude. The dependence of the total error of a current transformer on current is therefore non-linear. In the operating current range, i.e., in the current range below the nominal accuracy limit current, we can expect a worst case total error of approximately 10 % per current transformer set. The second section of the tripping characteristic forms a straight line, the slope of which should correspond to the cumulative total error of the participating current transformer sets. The curve slope m1 can be set. The default setting for m1 is defined as 0.3 with respect to protection of three-winding transformers, i.e. 3 x 10%. Characteristic equation for the range 0,5 ⋅ I an < I H ≤ 4 ⋅ I B : (Area II) Id I > I = m1 ⋅ R + diff ⋅ (1 − 0.5 ⋅ m1 ) Iref Iref Iref
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-13
The second knee point of the tripping characteristic determines the end of the overcurrent zone in the direction of increasing restraining current in fault-free operation. It can be as high as four times the nominal current in certain operating cases - such as when a parallel transformer has failed. Therefore, the second knee point can be set (IR,m2) for a default setting of 4·Iref. IR,m2 must be set in accordance with the maximum possible operating current. Restraining currents that go beyond the set knee point are then evaluated as continuous fault currents. For truly continuous fault currents, the third section of the tripping characteristic could therefore be given an infinitely large slope. Since, however, we also need to take into account the possibility that a fault can occur in the transformer differential protection zone as the result of the system fault, a finite slope m2 is provided for the third section of the tripping curve. The default setting for m2 is 0.7. Characteristic equation for the range 4⋅ I < I B H
: (Area III)
IR,m2 Id I I = m 2 ⋅ R + diff > ⋅ (1 − 0.5 ⋅ m1 ) + ⋅ (m1 − m 2 ) Iref Iref Iref Iref
Iref: reference current
2.1.2
m1: gradient of characteristic in range
0,5 ⋅ I an < I H ≤ 4 ⋅ I B
m2: gradient of characteristic in range
4⋅ IB < I H
Ratio correction To ensure correct operation of the differential element it is important that under load and through fault conditions the currents into the differential element of the relay balance. In many cases, the HV and LV current transformer primary ratings will not exactly match the transformer winding rated currents. Ratio correction factors are therefore provided. The CT ratio correction factors are applied to ensure that the signals to the differential algorithm are correct. In order to set the amplitude matching for the protected object, a reference power, identical for all windings, needs to be defined at “Settings/Function Settings/Common Settings/DIFF: Rated Ref Power Sref” [019.016]. For two-winding arrangements, the nominal power will usually be the reference power. For three or four-winding transformers, the nominal power of the highest-power winding should be set as the reference power. The individual reference currents for each winding of the protected object are then calculated by the P63x on the basis of the set reference power and the set primary nominal voltages of the transformer. Iref ,a =
Iref ,c =
S ref 3 ⋅ Vnom,a
S ref 3 ⋅ Vnom,c
Iref ,b =
Iref ,d =
S ref 3 ⋅ Vnom,b
S ref 3 ⋅ Vnom,d
Sref:
reference power
Iref,a, b, c or d:
reference current of winding a, b, c or d
Vn,a, b, c or d:
nominal voltage of winding a, b, c or d
The P63x calculates the matching factors on the basis of the reference currents and the set primary nominal currents of the system transformers. Note:
Where on-load tap changing is used, the nominal voltage chosen should be that for the middle tap position.
AP
P63x/UK AP/A54
Application Notes
(AP) 6-14
MiCOM P631, P632, P633, P634
Winding a
Winding b
C C B B A A
k amp,a
Figure 4: k am,a =
kamp,b
Amplitude matching factor, kamp
Inom,a Iref ,a
k am,b =
Inom,b Iref ,b
k am,c =
Inom,c Iref ,c
k am,d =
Inom,d Iref ,d
With:
AP
kam,a, b, c or d: amplitude-matching factor of winding a, b, c or d In,a, b, c or d:
primary nominal currents of the main current transformers
Reference currents and matching factors are displayed by the P63x. The values are also displayed in the setting file, however the setting file will not update any changes to the matching factors as processing by the relay operating system is required to determine new matching factors. The P63x checks that the reference currents and matching factors are within their permissible ranges. The matching factors must satisfy the following conditions: •
The matching factors must always be ≤ 16
•
The value of the lower matching factors must be ≥ 0.5
In three or four-winding protection, the “weakest” end, that is the end with the smallest primary nominal transformer current, is thus not associated with any restriction of the settings for the amplitude matching. Should the P63x calculate reference currents or matching factors not satisfying the above conditions then a warning will be issued and the P63x will be automatically blocked. The measured values of the phase currents of the windings of the protected object are multiplied by the relevant matching factors and are then available for further processing. Consequently, all threshold values and measured values always refer to the relevant reference currents rather than to the transformer nominal currents or the nominal currents of the device. 2.1.3
Vector group correction and zero sequence current filtering To compensate for any phase shift between two windings of a transformer it is necessary to provide vector group correction. This was traditionally provided by the appropriate connection of physical interposing current transformers, as a replica of the main transformer winding arrangements, or by a delta connection of main CTs.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-15
Basically, this matching operation can be carried out regardless of the phase winding connections, since the phase relation is described unambiguously by the characteristic vector group number. Vector group matching is therefore performed solely by mathematical phasor operations on the amplitude-matched phase currents of the low-voltage side in accordance with the characteristic vector group number. The vector group is the clock-face hour position of the LV A-phase voltage, with respect to the A-phase HV voltage at 12-o’clock (zero) reference. Phase correction is provided in the P63x via “Settings/Function Settings/Common Settings/DIFF: Vector Grp. ends b-a” [019.010] for phase shift between windings a and b. Similar settings are provided for phase shift between further windings in the P633 and P634 relays. This is shown in the following figure for vector group characteristic number 5, where vector group Yd5 is used as the example:
AP
Figure 5:
Yd5 transformer example
No operation is carried out on the high-voltage side in connection with vector group matching. In addition to mimicking the phase shift of the protected transformer, it is also necessary to mimic the distribution of primary zero sequence current in the protection scheme. The necessary filtering of zero sequence current has also been traditionally provided by appropriate connection of interposing CTs or by delta connection of main CT secondary windings. In the P63x, zero sequence current filtering is implemented in software via “Settings/Function Settings/Setting Group SG1/DIFF: I0 filt a Enab SG1” [072.155] for setting group 1, winding a. Similar settings are provided for the remaining ends in each setting group. Where a transformer winding can pass zero sequence current to an external earth fault it is essential that some form of zero sequence current filtering is employed. This ensures out of zone earth faults will not cause the relay to maloperate.
P63x/UK AP/A54
Application Notes
(AP) 6-16
MiCOM P631, P632, P633, P634 An external earth fault on the star side of a Dyn11 transformer will result in zero sequence current flowing in the current transformers associated with the star winding but, due to the effect of the delta winding, there will be no corresponding zero sequence current in the current transformers associated with the delta winding. In order to ensure stability of the protection, the LV zero sequence current must be eliminated from the differential current. Traditionally this has been achieved by either delta connected line CT’s or by the inclusion of a delta winding in the connection of an interposing current transformer. In accordance with its definition, the zero-sequence current is determined as follows from the amplitude-matched phase currents:
Iamp,zero,z =
(
1 ⋅I +I +I 3 amp,A,z amp,B,z amp,C,z
)
The following tables show that for all odd-numbered vector group characteristics the zero-sequence current on the low-voltage side is basically always filtered out, whereas for even-numbered vector group characteristics the zero-sequence current on the low-voltage side is basically never filtered out automatically. The latter is also true for the high-voltage side since in that case, as explained above, no mathematical phasor operations are performed. Vector group matching and zero-sequence current filtering must therefore always be viewed in combination. The following tables list all the mathematical phasor operations. Mathematical operations on the high-voltage side:
AP
With Izero Filtering
Without Izero Filtering
Ivec,y,z = Iamp,x,z − Iamp,zero,z
Ivec, y, z = Iamp, x,z
Mathematical operations on the low-voltage side for an even-numbered vector group characteristic: VG
With Izero Filtering
Without Izero Filtering
0
Ivec,y,z = Iamp,x,z − Iamp,zero,z
Ivec, y, z = Iamp, x,z
2
Ivec,y,z = − Iamp,x +1,z − Iamp,zero,z
4
Ivec,y,z = Iamp,x −1,z − Iamp,zero,z
6
Ivec,y,z = − Iamp,x,z − Iamp,zero,z
8
Ivec,y,z = Iamp,x +1,z − Iamp,zero,z
10
Ivec,y,z = − Iamp,x −1,z − Iamp,zero,z
(
(
(
)
Ivec,y,z = −Iamp,x +1,z Ivec,y,z = Iamp,x −1,z
)
Ivec,y,z = −Iamp,x,z Ivec,y,z = Iamp,x +1,z
)
Ivec,y,z = −Iamp,x −1,z
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-17
Mathematical operations on the low-voltage side for an odd-numbered vector group characteristic: VG 1
3
5
7
9
11
With Izero filtering
Ivec ,y,z =
1
Ivec ,y,z =
1
Ivec ,y,z =
1
Ivec ,y,z =
1
Ivec ,y,z =
1
Ivec ,y,z =
1
3
3
3
3
3
3
(
⋅ Iamp,x,z − Iamp,x +1,z
Without Izero filtering
)
(
⋅ Iamp,x −1,z − Iamp,x +1,z
)
Ivec,y,z =
1
Ivec, y, z =
1
3
3
(
)
Ivec, y, z =
1
(
)
Ivec, y, z =
1
Ivec, y, z =
1
Ivec,y,z =
1
⋅ Iamp,x −1,z − Iamp,x,z
⋅ Iamp,x +1,z − Iamp,x,z
(
⋅ Iamp,x +1,z − Iamp,x −1,z
(
⋅ Iamp,x,z − Iamp,x −1,z
)
)
3
3
3
3
(
)
⋅ Iamp,x,z − Iamp,x +1,z + Iamp,zero,z
(
)
⋅ Iamp, x −1, z − Iamp, x +1, z + Iamp,zero, z
(
)
(
)
⋅ Iamp, x −1, z − Iamp, x, z + Iamp, zero, z
⋅ Iamp, x +1, z − Iamp, x, z + Iamp, zero, z
(
)
⋅ Iamp, x +1, z − Iamp, x −1, z + Iamp,zero, z
(
)
⋅ Iamp,x,z − Iamp,x −1,z + Iamp,zero,z
Setting the vector group matching function is very simple and does not require any calculations. Only the characteristic vector group number needs to be set:
Figure 6:
Vector group selection
Other nameplate designations may be used instead of the clock notation - common examples are: Alternatives
Equivalent Standard
LV Group Setting
DAB/Y
DAB - Y
Dy1
1
DAC/Y
DAC - Y
Dy11
11
Y/Y
Y0 - Y0
Yy0
0
Y/Y
Y0 - Y6
Yy6
6
Setting the zero-sequence current filtering function is very simple and does not require any calculations. Zero-sequence current filtering should only be activated for those ends where there is operational earthing of a neutral point:
AP
P63x/UK AP/A54
Application Notes
(AP) 6-18
MiCOM P631, P632, P633, P634
Figure 7: 2.1.4
Zero sequence current filtering
Magnetizing inrush stabilization The magnetizing inrush phenomenon is associated with a transformer winding which is being energized where no balancing current is present in the other winding(s). This current appears as a large operating signal for the differential protection. Special measures are taken with the relay design to ensure that no maloperation occurs during inrush. The fact that the inrush current has a high proportion of harmonics having twice the system frequency offers a possibility of stabilization against tripping by the inrush current. The P63x filters the differential current. The fundamental I(fn) and second harmonic components I(2*fn) of the differential current are determined. If the ratio I(2*fn)/I(fn) exceeds a specific adjustable value (typical setting 20%) in at least one measuring system, tripping is blocked optionally in one of the following modes:
AP
•
Across all three measuring systems
•
Selectively for one measuring system
There is no blocking if the differential current exceeds the high set threshold DIFF: Idiff>> SGx. 2.1.5
High set operation The P63x relay incorporates an independent differential high set element, DIFF: Idiff>> SGx, to complement the protection provided by the biased differential low set element. The instantaneous high set offers faster clearance for heavy internal faults and it is not blocked for magnetizing inrush or transient overfluxing conditions. Stability is provided for heavy external faults, but the operating threshold of the high set differential element must be set to avoid operation with inrush current. As described in section 2.1.4 when a transformer is energized, a high magnetizing inrush current is drawn. The magnitude and duration of this inrush current is dependant upon several factors which include; •
Size and impedance of the transformer
•
Point on wave of switching
•
Remnant flux in the transformer
•
Number of transformers connected in parallel
It is difficult to accurately predict the maximum anticipated level of inrush current. Typical waveform peak values are of the order of 8 - 10x rated current. A worst-case estimation of inrush could be made by dividing the transformer full load current by the per-unit leakage reactance quoted by the transformer manufacturer.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-19
A setting range of 2.5 - 30Iref (RMS values) is provided on the P63x relay. The high set setting should be set in excess of the anticipated or estimated peak value of inrush current after ratio correction. Note:
If the differential current exceeds the adjustable threshold DIFF: Idiff>>> PSx>, the restraining current and the saturation discriminator are no longer taken into account either, that is the P63x will trip regardless of the restraining variable and the saturation discriminator.
2.2
Restricted earth fault protection
2.2.1
Basic principles The P63x uses biased differential protection to provide fast clearance for faults within the protected zone. The value of earth fault current, however, may be limited by any impedance in the earth path or by the percentage of the winding involved in the fault. The P63x offers a restricted earth fault element for up to 3 windings of the protected transformer to provide greater sensitivity for earth faults which will not change with load current. The levels of fault current available for relay measurement are illustrated in below. If an earth fault is considered on an impedance earthed star winding of a Dyn transformer (Figure 8), the value of current flowing in the fault (If) will be dependant upon two factors. These are the value of earthing impedance and the fault point voltage, which is governed by the fault location. The value of fault current (If) is directly proportional to the location of the fault. A restricted earth fault element (64) is connected to measure If directly, to provide more sensitive earth fault protection. The overall differential protection is less sensitive, since it only measures the HV current Is. The value of Is is limited by the number of faulted secondary turns in relation to the HV turns. 87
87 If
Source
If
Is
If
64
1.0
64
10
I Current (x full load)
If
Source
Is
Current (x full load)
F
8
I
F
6
4
0.2
IS
0.2
1.0 0.2 Fault position from neutral (Impedance earthing)
Figure 8:
Fault limitation on an impedance earthed system
IS
2
0.4
0.6
0.8
1.0
Fault position from neutral (Solid earthing)
Figure 9:
Fault limitation on a solidly earthed system
If a fault on a solidly earthed star winding (Fig 9) is considered, the fault current is limited by the leakage reactance of the winding, any impedance in the fault and by the fault point voltage. The value of fault current varies in a complex manner with fault location. As in the case of the impedance earthed transformer, the value of current available as an overall differential protection operating quantity is limited. More sensitive earth fault protection is provided by a restricted earth fault relay (64), which is arranged to measure If directly. Although more sensitive protection is provided by REF, the operating current for the overall differential protection is still significant for faults over most of the winding. For this reason,
AP
P63x/UK AP/A54
Application Notes
(AP) 6-20
MiCOM P631, P632, P633, P634 independent REF protection may not have previously been considered necessary for a solidly earthed winding; especially where an additional relay would have been required. With the P63x, the REF protection is available at no extra cost if a neutral CT is available. Restricted earth fault protection is also commonly applied to Delta windings of large power transformers, to improve the operating speed and sensitivity of the protection package to winding earth faults. When applied to a Delta winding this protection is commonly referred to as “balanced earth fault protection”. It is inherently restricted in its zone of operation when it is stabilized for CT spill current during inrush or during phase faults. The value of fault current flowing will again be dependant upon system earthing arrangements and the fault point voltage.
2.2.2
REF operating modes One of three operating modes for restricted earth fault protection can be selected via settings. •
Low impedance REF biased by residual current (Low-Z Iph Sum Bias)
•
Low impedance REF biased by maximum phase current (Low-Z Iph Max. Bias)
•
High impedance REF (High Impedance)
The biasing techniques operate by measuring the level of through current flowing and altering the relay sensitivity accordingly. The high impedance technique ensures that the relay circuit is of sufficiently high impedance such that the differential voltage that may occur under external fault conditions is less than that required to drive setting current through the relay. Historically the high impedance principle has been most widely implemented and an example illustrating the principle is included in the following section. Details on the application of the low impedance REF types are not included here, however the Operation OP section contains details on the characteristics and principles involved. Low impedance biased REF settings are similar to those of the biased differential protection function.
AP
Note:
2.2.3
The Low-Z Iph Sum Bias mode cannot be used for balanced earth fault protection of delta windings.
Stability requirements for high impedance REF
Rl
If(prim) Rct
Rl R stab'
64 Rl
Figure 10: High Impedance REF principle
Rl
If(prim)
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-21
The high impedance REF element shall maintain stability for through faults and operate in less than 40ms for internal faults provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor: Vk > 2 × Idiff > × Rs Rs = 1.1×
If × (Rct + 2Rl ) Idiff >
For faster operation of the REF element, a larger knee-point voltage will provide reduced operating times. Refer to the graph below showing the operating time of the REF element for differing ratios.
45 40
Operating time (ms)
35 30 25 20 15
AP
10 5 0 0
5
10
15
20
Vk / (Idiff> × Rs)
Note:
2.2.4
The diagram is the result of investigations which were carried out for impedance ratios in the range of 5 to 120 and for fault currents in the range of 0.5 to 40 In.
Use of METROSIL non-linear resistors Metrosils are used to limit the peak voltage developed by the current transformers under internal fault conditions, to a value below the insulation level of the current transformers, relay and interconnecting leads, which are normally able to withstand 3000V peak. The following formulae should be used to estimate the peak transient voltage that could be produced for an internal fault. The peak voltage produced during an internal fault will be a function of the current transformer kneepoint voltage and the prospective voltage that would be produced for an internal fault if current transformer saturation did not occur. Vp
= 2 2Vk ( Vf - Vk )
Vf
= Ι'f (Rct + 2RL + RST)
P63x/UK AP/A54
Application Notes
(AP) 6-22
MiCOM P631, P632, P633, P634 Where: Vp
= Peak voltage developed by the CT under internal fault conditions
Vk
= Current transformer kneepoint voltage
Vf
= Maximum voltage that would be produced if CT saturation did not occur
Ι'f
= Maximum internal secondary fault current
Rct
= Current transformer secondary winding resistance
RL
= Maximum lead burden from current transformer to relay
RST = Relay stabilizing resistor When the value given by the formulae is greater than 3000V peak, Metrosils should be applied. They are connected across the relay circuit and serve the purpose of shunting the secondary current output of the current transformer from the relay in order to prevent very high secondary voltages. Metrosils are externally mounted and take the form of annular discs. characteristics follow the expression: V
Their operating
CΙ0.25
=
Where:
AP
V
=
Instantaneous voltage applied to the non-linear resistor (Metrosil)
C
=
Constant of the non-linear resistor (Metrosil)
I
=
Instantaneous current through the non-linear resistor (Metrosil)
With a sinusoidal voltage applied across the Metrosil, the RMS current would be approximately 0.52 x the peak current. This current value can be calculated as follows:
⎛ Vs (rms) x 2 ⎞ 4 ⎟ ⎝ ⎠ C
Ι(rms) = 0.52 ⎜ Where:
Vs(rms) = rms value of the sinusoidal voltage applied across the Metrosil This is due to the fact that the current waveform through the Metrosil is not sinusoidal but appreciably distorted. For satisfactory application of a non-linear resistor (Metrosil), its characteristic should be such that it complies with the following requirements: 1.
At the relay voltage setting, the non-linear resistor (metrosil) current should be as low as possible, but no greater than approximately 30mA rms for 1A current transformers and approximately 100mA rms for 5A current transformers
2.
At the maximum secondary current, the non-linear resistor (metrosil) should limit the voltage to 1500V rms or 2120V peak for 0.25 second. At higher relay voltage settings, it is not always possible to limit the fault voltage to 1500V rms, so higher fault voltages may have to be tolerated
The following tables show the typical Metrosil types that will be required, depending on relay current rating, REF voltage setting etc. Metrosil Units for Relays with a 1 Amp CT The Metrosil units with 1 Amp CTs have been designed to comply with the following restrictions: 3.
At the relay voltage setting, the Metrosil current should be less than 30mA rms
4.
At the maximum secondary internal fault current the Metrosil unit should limit the voltage to 1500V rms if possible
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-23
The Metrosil units normally recommended for use with 1Amp CT's are as shown in the following table: Relay Voltage Setting Up to 125V rms 125 to 300V rms Note:
Nominal Characteristic C
β
450 900
0.25 0.25
Recommended Metrosil Type Single Pole Relay 600A/S1/S256 600A/S1/S1088
Triple Pole Relay 600A/S3/1/S802 600A/S3/1/S1195
Single pole Metrosil units are normally supplied without mounting brackets unless otherwise specified by the customer.
Metrosil units for relays with a 5 amp CT These Metrosil units have been designed to comply with the following requirements: 1.
At the relay voltage setting, the Metrosil current should be less than 100mA rms (the actual maximum currents passed by the units shown below their type description
2.
At the maximum secondary internal fault current the Metrosil unit should limit the voltage to 1500V rms for 0.25secs. At the higher relay settings, it is not possible to limit the fault voltage to 1500V rms hence higher fault voltages have to be tolerated (indicated by *, **, ***)
3.
The Metrosil units normally recommended for use with 5 Amp CTs and single pole relays are as shown in the following table:
AP
Recommended Metrosil Type
Secondary Internal Fault Current
Relay Voltage Setting
Amps rms
Up to 200V rms
250V rms
275V rms
300V rms
50A
600A/S1/S1213 C = 540/640 35mA rms
600A/S1/S1214 C = 670/800 40mA rms
600A/S1/S1214 C =670/800 50mA rms
600A/S1/S1223 C = 740/870* 50mA rms
100A
600A/S2/P/S121 7 C = 470/540 70mA rms
600A/S2/P/S1215 C = 570/670 75mA rms
600A/S2/P/S1215 C =570/670 100mA rms
600A/S2/P/S1196 C =620/740* 100mA rms
150A
600A/S3/P/S121 9 C = 430/500 100mA rms
600A/S3/P/S1220 C = 520/620 100mA rms
600A/S3/P/S1221 C = 570/670** 100mA rms
600A/S3/P/S1222 C =620/740*** 100mA rms
Note:
*2400V peak
**2200V peak
***2600V peak
In some situations single disc assemblies may be acceptable, contact AREVA T&D for detailed applications. Note: 1.
The Metrosil units recommended for use with 5 Amp CTs can also be applied for use with triple pole relays and consist of three single pole units mounted on the same central stud but electrically insulated from each other. To order these units please specify "Triple pole Metrosil type", followed by the single pole type reference
2.
Metrosil units for higher relay voltage settings and fault currents can be supplied if required
P63x/UK AP/A54 (AP) 6-24
Application Notes MiCOM P631, P632, P633, P634
2.3
Overfluxing protection and blocking
2.3.1
Basic principles The P63x relay offers an overfluxing protection element which can be used to raise an alarm or initiate tripping in the event of prolonged periods of transformer overfluxing. In addition, a differential current 5th harmonic blocking feature is also provided within the P63x, which can be used to prevent possible maloperation of the differential element under transient overfluxing conditions. To make use of the time delayed overfluxing protection, the P63x relay must be supplied with a voltage signal which is representative of the primary system voltage on the source side of the transformer. The 5th harmonic blocking feature does not require a voltage signal. A 5th harmonic signal is derived from the differential current wave form on each phase and blocking is on a per phase basis.
2.3.2
Transformer overfluxing Transformer overfluxing might arise for the following reasons: •
High system voltage Generator full load rejection Ferranti effect with light loading transmission lines
•
Low system frequency Generator excitation at low speed with AVR in service
AP
•
Geomagnetic disturbance Low frequency earth current circulation through a transmission system
The initial effects of overfluxing will be to increase the magnetizing current for a transformer. This current will be seen as a differential current. If it reaches a high level without a waveshape which would cause operation of the inrush blocking system, there would be a risk of differential protection tripping. Persistent overfluxing may result in thermal damage or degradation of a transformer as a result of heating caused by eddy currents that may be induced in non-laminated metalwork of a transformer. The flux levels in such regions would normally be low, but excessive flux may be passed during overfluxed operation of a transformer. The following protection strategy is proposed to address potential overfluxing conditions: •
Maintain protection stability during transient overfluxing
•
Ensure tripping for persistent overfluxing
In most applications, the recommended minimum differential trip threshold for P63x, its filtering action and possible operation of the inrush detector will ensure stability of the differential element. If more difficult situations exist, the P63x relay is offered with a 5th harmonic differential current blocking facility. This facility could be applied with some study of the particular problem. To ensure tripping for persistent overfluxing, due to high system voltage or low system frequency, the P63x is provided with time delayed Volts per Hertz protection. Where there is any risk of persistent geomagnetic overfluxing, with normal system voltage and frequency, the 5th harmonic differential current facility could be used to initiate tripping after a long time delay. 2.3.3
Time delayed overfluxing protection Two independently adjustable V/f elements are available for overfluxing protection. A definite-time element, with a time setting range of 0 - 10,000 seconds, is provided for use as an alarm element. The settings of this element should be such that the alarm signal can be used to prompt automatic or manual corrective action.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-25
Protection against damage due to prolonged overfluxing is offered by a V/f protection element with a variable time tripping characteristic. The setting flexibility of this element, by adjustment of the time delay at various V/f values, makes it suitable for various applications. The manufacturer of the transformer or generator should be able to supply information about the short-time over-excitation capabilities, which can be used to determine appropriate settings for the V/f tripping element. The variable time overfluxing protection would be used to trip the transformer directly. If preferred, the V/f tripping element can be set with a definite time characteristic.
AP Figure 11: Variable time overfluxing protection characteristic 2.3.4
5th Harmonic blocking The 5th Harmonic blocking feature is available for possible use to prevent unwanted operation of the low set differential element under transient overfluxing conditions. When overfluxing occurs, the transformer core becomes partially saturated and the resultant magnetizing current waveforms increase in magnitude and become harmonically distorted. Such waveforms have a significant 5th harmonic content, which can be extracted and used as a means of identifying the abnormal operating condition. The 5th harmonic blocking threshold is adjustable between 10 - 80% differential current. The threshold should be adjusted so that blocking will be effective when the magnetizing current rises above the chosen threshold setting of the low-set differential protection. Where the magnetizing current is just in excess of the differential element setting, the magnetizing inrush detection will not be effective in all applications with all types of transformers. To offer some protection against damage due to persistent overfluxing that might be caused by a geomagnetic disturbance, the 5th harmonic blocking element can be routed to an output contact via an associated timer. Operation of this element could be used to give an alarm to the network control centre. If such alarms are received from a number of transformers, they could serve as a warning of geomagnetic disturbance so that operators could take some action to safeguard the power system. Alternatively this element can be used to initiate tripping in event of prolonged pick up of a 5th harmonic measuring element. It is not expected that this type of overfluxing condition would be detected by the AC overfluxing protection. This form of time delayed tripping should only be applied in regions where geomagnetic disturbances are a known problem and only after proper evaluation through simulation testing.
P63x/UK AP/A54 (AP) 6-26 2.3.5
Application Notes MiCOM P631, P632, P633, P634
Required settings The pick up for the overfluxing elements will be dependant upon the nominal core flux density levels. Generator transformers are generally run at higher flux densities than transmission and distribution transformers and hence require a pick up setting and shorter tripping times which reflect this. Transmission transformers can also be at risk from overfluxing conditions and withstand levels should be consulted when deciding upon the required settings.
2.4
Auto-transformer protection Auto-transformers are designed as three-phase units or consist of a group of three single-phase units. They are used for interconnection of solidly earthed EHV and HV networks if the rated voltages of both networks within a factor of 2 to 3 times. Material and weight as well as losses can be saved by autotransformers compared with separate-winding transformers. Auto-transformers with star connection of primary and secondary winding (serial and common winding) are usually equipped with a delta stabilizing winding (tertiary winding) rated about one third of the throughput rating.
AP
Figure 12: Auto-transformer with tertiary winding Shunt reactors or capacitors for power factor correction can be connected to such a tertiary winding. A booster transformer consisting of energizing and regulating winding for voltage adjustment by in-phase or phase-angle regulation can be accommodated in the same tank.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-27
C B A
C B A
Serial winding
Energizing winding Regulating winding
Common winding
-60°/0°/+60° phase-angle Delta connection
Tertiary winding
C B A
Figure 13: Auto-transformer with tertiary winding Depending on the application various possibilities with different features can be used for differential protection of autotransformers: Unloaded Delta Tertiary Winding
Neutral Earthing With PhaseSegregated CTs
CTs in Series CTs Outside Delta With Delta Tertiary Winding Tertiary Winding
Differential protection
Two-end
Three-end
Three-end
Three-end
Amplitude matching
Vnom,a ≠ Vnom,b
Vnom,a = Vnom,b = Vnom,c
√3·Vnom,c
Vnom,c
Vector group matching
VGa-b = 0
VGa-b = 0 VGa-c = 0
VGa-b = 0 VGa-c = 0
VGa-b = 0 VGa-c = odd
Zero sequence current filtering
Enabled
Disabled
Disabled
Enabled
Inrush stabilization Enabled
Disabled
Enabled
Enabled
Phase-segregation No
Yes
Yes
No
Affected by voltage Yes adjustment
No
No
Yes
Sensitivity for earth Low faults
High
High
Low
Protection against turn-to-turn faults
Yes
No
Yes
Yes
Protection of the delta tertiary winding
No
No
Yes
Yes
Individual applications are discussed below.
AP
P63x/UK AP/A54 (AP) 6-28 2.4.1
Application Notes MiCOM P631, P632, P633, P634
Unloaded delta tertiary A two-end differential protection may be applied in any case if the tertiary winding is used as delta stabilizing winding only i.e. if there is no additional feeding from the tertiary winding:
C B A
C B A
I d/I
AP
Figure 14: Auto-transformer - unloaded delta tertiary winding The setting of the differential protection corresponds to the setting of a separate-winding transformer with neutral earthing at both ends. 2.4.2
Neutral earthing with phase-segregated CTs In case of neutral earthing with per phase neutral CTs it’s the ideal solution to apply a three-end differential protection. The protected zone corresponds to a single winding; HV extremity, LV tap and neutral tail CT.
2.4.2.1
Amplitude matching Because the protection is effectively performing a Kirchoff current summation at the center tap, the primary nominal voltages of all three ends have to be set to the same value (primary nominal voltage of the serial or of the common winding).
2.4.2.2
Vector group matching Both vector group numbers have to be set to ‘0’.
2.4.2.3
Zero-sequence current filtering As zero sequence current flow will not upset the Kirchoff current summation, the zero-sequence current filtering may be disabled for all three ends.
2.4.2.4
Inrush stabilization Because of the galvanic connected electrical node there is no transformer coupling and therefore inrush stabilization may be disabled.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-29
C B A
C B A
I d/I
Figure 15: Auto-transformer - neutral earthing with per phase neutral CTs The differential protection described above operates strictly phase-segregated particularly due to the fact that inrush stabilization is not required. Sensitivity for earth fault detection is high. However turn-to-turn faults and faults on the tertiary winding cannot be detected in principle. 2.4.3
CTs in series with delta tertiary winding If the corresponding current through the tertiary winding is measured instead of the neutral-to-earth current per phase a transformer coupling will be given.
2.4.3.1
Amplitude matching Because of transformer coupling amplitude matching has to be based on the individual primary nominal voltages of the ends. Considering that the CTs of the third end are located in series with the delta tertiary winding √3 times of the corresponding primary nominal voltage has to be used for amplitude matching calculation.
2.4.3.2
Vector group matching Because of the limb related measuring systems both vector group numbers have to be set to ‘0’.
2.4.3.3
Zero-sequence current filtering Because of the inclusion of the neutral-to-earth current via measuring the current through the transformer coupled tertiary winding zero-sequence current filtering may be disabled for all three ends.
2.4.3.4
Inrush stabilization Because of the transformer coupling within the protected zone inrush stabilization has to be enabled.
AP
P63x/UK AP/A54 (AP) 6-30
Application Notes MiCOM P631, P632, P633, P634
C B A
C B A
I d/I
C B A
Figure 16: Auto-transformer - CTs in series with delta tertiary winding The differential protection described above provides the same degree of earth fault sensitivity in comparison to the differential protection according to chapter 2.4.2. Furthermore turn-to-turn faults can be detected on principle due to transformer coupling of the measured currents and the tertiary winding is included in the protected zone. Earth faults on the regulating winding will be detected too whereas the differential measuring systems are not affected by voltage adjustment. Only the requirement of inrush stabilization is unfavorable.
AP
2.4.4
CTs outside delta tertiary winding If the CTs of the tertiary winding are not located in series but outside the delta winding a three-end differential protection may be applied. This differential protection offers the largest protection zone in comparison to all the applications described above. However the requirement of zero-sequence current filtering leads to reduced earth fault sensitivity. The setting of the differential protection corresponds to the setting of a separate-winding transformer. The differential measuring systems are affected by in-phase or phase-angle regulation.
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
(AP) 6-31
Figure 17: Auto-transformer - CTs outside delta tertiary winding 2.4.4.1
Tripping characteristic This overall differential protection is affected by voltage adjustment. This has to be taken into consideration for the setting of the tripping characteristic according to section xx.
2.5
Busbar/mesh corner differential protection The P633 and P634 relays may be applied to simple busbar/mesh corner configurations to provide biased differential protection of the zone. The figure below shows the basic concept of the scheme when applied as busbar protection.
P634
BB1
P633
Figure 18: P63x busbar differential protection scheme The maximum size of any zone is limited to 4 CT sets when the P634 relay is used.
AP
P63x/UK AP/A54 (AP) 6-32 2.5.1
Application Notes MiCOM P631, P632, P633, P634
Busbar protection settings The reference power setting “Settings/Function Settings/Common Settings/DIFF: Rated Ref Power Sref (019.016)” is set based on the CT primary rating and the system voltage. In is then equal to P63x reference current, Iref. The minimum differential current pickup setting, “Idiff>”, is set above maximum load current to prevent maloperation of the scheme if CT secondary wiring becomes open circuited, presenting the relay with a differential current equivalent to load. In the settings shown on the characteristic below, “Idiff>” has been set to 1.2 times the nominal current.
AP
Figure 19: P63x bias characteristic of busbar differential protection If there is the possibility for resistive busbar faults to occur, e.g. in GIS chambers, then the “Idiff>” setting needs to be set less than load current. As bus zone protection is generally the most critical area where disconnection of a CT could cause problems, e.g. tripping a whole bus zone in error, the application of CT supervision is recommended to prevent maloperation of the biased differential protection when there is an open circuited CT secondary. The CTS feature can be used for de-sensitizing or blocking of the biased differential protection. De-sensitization is achieved by raising the differential current pickup setting to the value of “Idiff> (CTS)”.
Application Notes MiCOM P631, P632, P633, P634
3.
SETTING RECOMMENDATIONS
3.1
Introduction
P63x/UK AP/A54 (AP) 6-33
By default all protection, logic and I/O settings are disabled in the P63x when delivered from the manufacturer. The following sections are intended to provide recommendations on the setup of auxiliary functions available in all relay configurations. The settings may be configured in the MiCOM S1 S&R-103 setting software with out the need to access the relay. The configured file may be saved as a standard “default” and used as a starting point when applying the P63x to specific installations requiring calculated protection and logic settings. The relevant sections of the application notes should be cross referenced prior to applying the settings ensuring they are correct for the application. 3.2
Enabling the relay The relay must be enabled, switching it on-line: (003.030) “Settings/Function Settings/Global/MAIN: Protection Enabled” set to “Yes”.
3.3
Enabling a protection function As an example, the following steps are required to enable differential protection in Setting Group 1: 1.
Firstly, differential protection settings must be enabled globally within the relay: (056.027) “Settings/Configuration/DIFF: Differential PR (87)” set to “Enabled”
2.
Then the differential protection must be enabled across the setting groups globally: (019.080) “Settings/Function Settings/Common Settings/DIFF: General Enable USER” set to “Yes”
3.
Differential protection must then be enabled in the required setting group, e.g. SG1: (072.152) “Settings/ Function Settings/Setting Group 1/DIFF: Enable SG1” set to “Yes”
The protection system settings for differential protection can then be set as required for the particular installation with reference to the appropriate sections of this Technical Guide. The above procedure is also required for configuring other types of protection such as overcurrent, restricted earth fault, etc. 3.4
Configuring a trip command and output contact for CB tripping It should be noted that the trip signals of the various protection functions are not suitable for trip command purposes, due to their undefined timing. For command purposes, P63x devices provide four trip commands. The functions to effect a trip are selected from those available via the following setting, e.g. for Trip Command 1: (021.001) “Settings/Function Settings/Global/MAIN: Fct.Assig Trip Cmd.1”
Having assigned the trip command, the corresponding signal may be assigned to the desired output relay. It is recommended to use output relay 01 on the power supply module which provides 2 N/O contacts, although any available contact in a P63x may be configured for circuit breaker tripping. For P632/P633/P634: (151.201) “Settings/Configuration/OUTP: Fct. Assignm. K 2001” set to “Gen. Trip Command 1”
For P631: (150.193) “Settings/Configuration/OUTP: Fct. Assignm. K 901” set to “Gen. Trip Command 1”
The closing pulse time of the output contact is then controlled by the setting: (021.003) “Settings/Function Settings/Global/MAIN: tDWELL Trip Cmd 1”
AP
P63x/UK AP/A54 (AP) 6-34
Application Notes MiCOM P631, P632, P633, P634
The logic equations should not be used for fast operating protection tripping functions, as they may introduce a small delay in operation. Any grouping of trip commands should be performed in the trip command described above. 3.5
Configuring a watchdog contact It is recommended to use output relay 08 on the power supply module as the watchdog contact: For P632/P633/P634: (151.222) “Settings/Configuration/OUTP: Fct. Assignm. K 2008” set to “MAIN Blocked/Faulty”
For P631: (150.214) “Settings/Configuration/OUTP: Fct. Assignm. K 908” set to “MAIN Blocked/Faulty”
This output relay provides a C/O contact which should be programmed to changeover when auxiliary power is applied and the relay has not detected any conditions preventing correct operation of protection. The operation mode for P632/P633/P634: (151.223) “Settings/Configuration/OUTP: Oper. Mode K 2008” set to “Inverted”
For P631: (150.215) “Settings/Configuration/OUTP: Oper. Mode K 908” set to “Inverted”
3.6
HMI read key assignment The Read ckey may be assigned to display the most recent fault recordings by configuring the following setting:
AP
(080.110) “Settings/Configuration/HMI: Read Key Assignment” set to “OSCIL Fault Recording 1”
3.7
Operation panel configuration The information displayed on the HMI LCD under normal operating conditions can be configured via the setting: (053.007) “Settings/Configuration/HMI: Operation Panel fct”
This enables display of e.g. load current, voltage, frequency measurements on the front panel display during normal operation. When more than two measurements are selected the HMI will cycle through the configured measurements at a rate set by: (031.075) “Settings/Configuration/HMI: Panel Hold-Time”
Application Notes
P63x/UK AP/A54
MiCOM P631, P632, P633, P634
4.
(AP) 6-35
CURRENT TRANSFORMER REQUIREMENTS CT specification IEC 60044-1 accuracy class 5P or equivalent. Minimum knee-point voltage (IEC knee) Differential protection The required knee-point voltage must be calculated for phase fault current and also for the earth fault current. The higher of the two calculated knee-point voltages is used. The CT requirements are based on the default settings. For transformer differential protection; Idiff> = 0.2 Iref, m1 = 0.3, m2 = 0.7, IR,m2 = 4 Iref, and for busbar differential protection; Idiff> = 1.2 Iref, m1 = 0.2, m2 = 0.8, IR,m2 = 1.8 Iref. Phase fault differential protection
Vk ≥ K × (Rct + Rl )
Earth fault differential protection
Vk ≥ K e × (Rct + 2Rl )
K is a constant depending on the maximum value of through fault current (as a multiple of In) and the primary system X/R ratio. For phase faults, K is determined as follows: When (If × X/R) ≤ 500 × In:
K = 0.14 × (If × X / R) When 500 × In < (If × X/R) < 1200 × In:
K = 70 For earth faults, Ke is determined as follows: When (Ife × X/R) ≤ 500 × In:
K e = 0.14 × (Ife × X / R) When 500 × In < (Ife × X/R) < 1200 × In:
K e = 70 Typical knee-point voltage requirement for transformer differential protection. The through fault stability required for most transformer applications is determined by the external through fault current and transformer X/R ratio. The through fault current in all but ring bus or mesh fed transformers is given by the inverse of the per unit reactance of the transformer. For most transformers, the reactance varies between 0.05 to 0.1pu, therefore typical through fault current is given by 10 to 20In. For conventional transformers (non-autotransformer), the X/R ratio is typically 7. This cancels out the 0.14 multiplier, leaving only the maximum secondary through fault current (If) to multiply with the loop resistance, giving:
Vk ≥ If × (Rct + 2Rl ) Alternatively, as a conservative estimate: Vk ≥
(Rct + 2Rl ) Xt
Low impedance REF protection The CT requirements for low impedance REF protection are generally lower than those for differential protection. As the line CTs for low impedance REF protection are the same as those used for differential protection the differential CT requirements cover both differential and low impedance REF applications.
AP
P63x/UK AP/A54
Application Notes
(AP) 6-36
MiCOM P631, P632, P633, P634 High impedance REF protection The high impedance REF element shall maintain stability for through faults and operate in less than 40ms for internal faults provided the following conditions are met in determining the CT requirements and value of associated stabilizing resistor: Vk > 2 × Idiff > × Rs Rs = 1.1×
If × (Rct + 2Rl ) Idiff >
For faster operation of the REF element, a larger knee-point voltage will provide reduced operating times. Refer to the graph below showing the operating time of the REF element for differing ratios.
45 40
Operating time (ms)
35
AP
30 25 20 15 10 5 0 0
5
10
15
20
Vk / (Idiff> × Rs)
Note:
4.1
The diagram is the result of investigations which were carried out for impedance ratios in the range of 5 to 120 and for fault currents in the range of 0.5 to 40 In.
Knee point voltage offered by IEC “P” class CTs Class X current transformers with a knee point voltage greater or equal than that calculated can be used. Class 5P protection CTs can be used, noting that the knee point voltage equivalent these offer can be approximated from: Vk
= (VA x ALF)/In + (RCT x ALF x In)
Where: VA
= Voltampere burden rating,
ALF = Accuracy Limit Factor, In
= CT nominal secondary current.
Application Notes MiCOM P631, P632, P633, P634 4.2
P63x/UK AP/A54 (AP) 6-37
Use of ANSI/IEEE CTs Where American/IEEE standards are used to specify CTs, the C class voltage rating can be checked to determine the equivalent Vk (knee point voltage according to IEC). The equivalence formula is: Vk
= [ (C rating in volts) x 1.05 ] + [ 100 x RCT ]
AP
P63x/UK AP/A54 (AP) 6-38
5.
Application Notes MiCOM P631, P632, P633, P634
AUXILIARY SUPPLY FUSE RATING It is recommended that the auxiliary supply wiring is protected by fuses of standard ratings between 6A and 16A. Low voltage fuse-links rated for 250V minimum and compliant with IEC60269-1 general application type gG with high rupturing capacity are acceptable. This gives equivalent characteristics to HRC "Red Spot" fuse types NIT/TIA often specified historically. Where only one or two relays are wired as a fused spur, it is acceptable to use a 6A rating. Generally, five relays could be connected on a spur protected at 10A, and ten relays for a 15/16A fuse. Alternatively, miniature circuit breakers (MCB’s) may be used to protect the auxiliary supply circuits.
AP
