Overcurrent protection of the HV side:
- should include combined start with voltage level check.
- carries out in a separate MP terminal.
- should be connected to the current transformers on the HV side of the power transformer.
Protection body must be three-phase and respond to the current rising of any one phase.
Overcurrent protection must have an independent characteristic of time delay.
The acceleration of protection must be provided during manual (remote or local) turning on the circuit breaker of the 220 kV transformer. Time regulation of this acceleration should be available in the range (0-1,5) seconds;
Voltage protection bodies should have next settings:
- voltage direct sequence - (20,0 – 80,0) V;
- negative sequence voltage – (3,0-20,0) V.
Overcurrent protection which has time delay (0-10 sec.) should act to:
- switching off the circuit breakers on all sides of the power transformer;
- starting breaker failure protection on the HV (220 kV) side of the transformer and blocking auto-reclosing.
4.1.3. Breaker failure protection of the HV (220 kV)
There should be breaker failure protection which is connected to the current transformers of the 220 kV circuit breaker.
Operate current of breaker failure protection should be regulated in the range (0,05-0,5) A.
Reset ratio of the current body of breaker failure protection should be at least 0,9.
The operating time of the current body of breaker failure protection must not exceed 0.03 in case when the input current is twice higher than operating current.
The return time of the current body with input current reset from 30 ∙ Is.r to zero should not exceed 0.03 s.
Breaker failure protection starts if there is current in the circuit breaker and protections of the transformer is carrying out.
Breaker failure protection should have two-step operation:
- switching off reserved circuit breaker without any time delay;
- switching off the adjacent circuit breaker with blocking of auto-reclosing
Breaker failure protection direct time should be regulated in the range (0,1–0,6) sec.
In this case the signal "switching off with blocking of auto-reclosing" is being transmitted on RP 220 kW Stany using the communication channel between the two substations.
In the repair mode, when one of the HV lines is out of work, and two transformers is powered by the rest line, there must be a transferring of the circuits of breaking failure protection to the circuit breaker of this line.
In the repair mode of 220 kV line Stany-Lafarge 1(2) (when the bridge repair is close) the signal "switching off with blocking of auto-reclosing" is introduced at 220 kV line Stany-Lafarge 2(1).
4.1.4. On-load tap-changer (OLTC)
The substation Lafarge involves the installation of the transformers T1 and T2 220/35/6,6 kV with OLTC.
Regulation under the load is carried on the excited and loaded transformer without any interruptions of the power supply and without switching off (local or remote) the transformer.
Automatic regulation under the load support specified voltage level on one of the bus bars of the substation, which is called «regulated». If necessary, the regulation may have compensation of the voltage loss to current.
Automatic regulation of transformation ratio of the transformers 220/35/6,6 kV supports the voltage level on bus bars 35 kV.
The blocking of automatic regulation introduces when:
- inappropriate drop of oil temperature in the tank of OLTC contactor;
- harmful overcurrent of OLTC;
- withdrawal of OLTC gas protection when refilling oil or operations with circuits of the gas protection;
- one step mismatch of OLTC phases - with the help of the regulator.
OLTC fault signal is transmitted to the workstation of the operator.
4.2. Relay protection of switchgear35 kV
According to single-line diagram of the substation Lafarge there is switchgear 35 kV with two busbars. Switchgear 35 kV is powered by T1 and T2 transformers. Sections of switchgear are connected by busbar sectionalising switch.
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