Alstom CT calculation guide

MiCOM P544, P546

8 Application Examples

7

CURRENT TRANSFORMER (CT) REQUIREMENTS

7.1

Recommended CT Classes (British and IEC) 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:

7.2

VA =

Voltampere burden rating

 ALF =

Accuracy limit factor factor

In

CT nominal secondary current

=

Current Differential Requirements For accuracy, class X or class 5P Current Transformers (CTs) are strongly recommended. The knee point voltage of the CTs should comply with the minimum requirements of the formulae shown below. Vk 

K*In*(Rct + 2*RL)

Where: Vk =

Required IEC knee point voltage

K =

Dimensioning factor

In =

CT nominal secondary current

Rct=

CT resistance

RL =

One-way lead impedance from CT to relay

K is a constant depending on: If =

Maximum value of through fault current for stability (multiple of In)

X/R=

Primary system X/R ratio

K is determined as follows:

1. Transient Bias = Disabled

OBSOLETE

For IEDs set at Is1 = 20%, Is2 = 2 In, k1 = 30%, k2 = 150%: K must be the highest of: K =

40 + (0.07 x (If x X/R))

Or K =

65

This is valid for (If x X/R)  1000 For higher (If x X/R) up to 1600: K =

107

For IEDs set at Is1 = 20%, Is2 = 2 In, k1 = 30%, k2 = 100%:

P544&6/EN/TM/E

8-87

MiCOM P544, P546

8 Application Examples

K must be the highest of: K =

40 + (0.35 x (If x X/R))

Or K =

65

This is valid for (If x X/R)  600 For higher (If x X/R) up to 1600: K =

256

2. Transient Bias = Enabled For IEDs set at Is1 = 20%, Is2 = 2 In, k1 = 30%, k2 = 150%: K =

-3

(1.42*If + 53.7) * (6.06*10 *X/R + 0.515)

If Two Relay are Connected

For IEDs set at Is1 = 20%, Is2 = 2 In, k1 = 30%, k2 = 100%: K =

-3

(7.47*If + 77.8) * (8.19*10 *X/R + 0.345)

If Three Relays are Connected

This is valid for If  50 pu and X/R  80

7.3

Distance Protection CT Requirements:

7.3.1

Zone 1 Reach Point Accuracy (RPA) Vk 

KRPA x IF Z1 x (1+ X/R). (RCT + RL) Where:

7.4

Vk =

Required CT knee point voltage (volts)

KRPA =

Fixed dimensioning factor = always 0.6

IF Z1

=

X/R =

Primary system reactance/resistance ratio

RCT =

CT secondary winding resistance ()

RL =

Single lead resistance from CT to relay ( )

Max. secondary phase fault current at Zone 1 reach point (A)

Zone 1 Close-up Fault Operation  An additional calculation must be performed for all cables, and any lines where the source impedance ratio might be less than SIR = 2. Vk 

Kmax x IF max x (RCT + RL) Where: Kmax

=

Fixed dimensioning factor = always 1.4

IF max

=

Max. secondary phase fault current (A).

Then, the highest of the two calculated knee points must be used. Note:

8-88

It is not necessary to repeat the calculation for earth faults, as the phase reach calculation (3  ) is the worst-case for CT dimensioning.

P544&6/EN/TM/E