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PROTECTION
SKD and SKE Check Synchronising Relays
Features • Low burden, static design. • Accurate and reliable. • Immune to transient and surges; withstands 5 kV impulse voltage test.
a pre-determined value. Referring to the block diagram the output from the phase measurement circuit drives an accura accurately tely calibrated timer. The timer operates the relay on completion of its timing cycle, if the output from the phase measurement circuit persists.
General Description Phase difference measurement Phase measurement is achieved by algebraically subtracting the two supply voltage waveforms and comparing the resultant modulated beat waveform envelope with a dc reference voltage (See Figure 1).
Check synchronising relays are used to prevent interconnection of badly synchronised synchronis ed supplies. Type SKD relays are generally used in auto reclosing sequence of interconnectors along with the auto reclosing relay. Type SKE relays are used to safeguard manual synchronising of generators.The check synchronising relay contacts are normally connected in series with the circuit breaker closing circuit and ensures that the differences in phase, voltage and frequency are within pre-selected limits before switching in an incoming alternator or interconnecting two sections of a power system.
The dc reference is proportioned to the sum of the peaks of the two supply voltages to provide phase measurement independent of supply voltage variation. Referring to Figure 2, the resulting voltage produced after subtraction is passed via the gate (which is opened when the VTI supply is present) to the phase angle comparator and integrating amplier. The signal is compared with the dc reference and produces a smoothed output to the relay drive unit if the signal is less than dc reference. The relay drive circuit prevents chatter chatter of the relay at the just-operate just -operate condition. condition.
Frequency diffe d ifference rence measurement
The relationship between timer setting and frequency difference is given in Table 1.
Voltage difference measurement Two types of voltage lockout are available. Type SKD compares the amplitude of the two supply voltages, with a stabilised dc reference which is proportioned to the nominal supply voltage. The Voltage lockout circuit inhibits the gate if either voltage is below the preset limit. Type SKE compares the two supply voltages and inhibits the gate if the difference between the supply voltages exceeds the preset limit.
Customer Benefts
• Immune to transients. • Static design and low burden. • Prevents interconnection of badly synchronized supplies.
Frequency difference measurement is achieved by checking that the phase angle traversed in a dened period is less than
GRID 1
SKD and SKE Technical data Voltage rating 110 volts/63.5 volts ac, 50 Hz.
Auxiliary supply 50V, 110V and 220V dc. For 110V dc and above, external dropping resistors are used.
Minimum operating voltages Type SKD Either supply below setting.
Type SKE Both supplies at 37%.
Settings Phase angle, frequency difference and voltage difference settings as shown in Table 1.
5
1
Operating time
3
200 m.sec. with timer disconnected. 2
Burden 2.5VA at rated voltage on each supply.
Accuracy
6
Rectified Envelope (by subtraction of
4. In-phase condition
running and incoming voltages)
5. 180 out of phase
��
D.C. reference level
6. Time to cover one slip cycle (360 )
��
Angle over which synchronising can occur
��
±5% on voltage lockout setting.
Thermal rating
4
o
o
AC supply voltage 115% of rating. Figure 1: Static check synchroniser rectified beat waveform showing dc reference level
DC supply 120% of rating.
Figure 2: Composite block diagram
2
SKD and SKE
Table 1: Check synchroniser types and settings Relay
Difference continuously variable. Voltage difference calibrated. At 2%,4%,6%,8% & 10% nominal voltage
Contacts
High frequency interference
Two normally open contacts, or one make and one break.
The relay complies with IS 8686-1977/IEC 25522-1 Appendix C to class III.
Contact rating Make and carry 7500VA for 3 seconds with maximum of 30A and 660 volts.
Case 1D drawout cases suitable for ush/projection mounting.
Insulation The relay meets requirements of IS 3231-1965/IEC 255-5 Series C-2 kV for 1 minute.
Impulse voltage The relay complies with the requirements of IS 8686-1977/ IEC 255-4 Appendix E to class III.
Information required with order • Relay type • Voltage rating • Auxiliary supply voltage • Type of mounting
3
SKD and SKE
Figure 3 : Case and panel cut-out dimensions for case 1D ( All dimensions in mm)
ALSTOM T&D INDIA LIMITED Pallavaram Works 19/1, G.S.T. Road, Pallavaram, Chennai - 600 043 Tel: 91-44-2264 8000 Fax: 91-44-2264 0040
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