Construction of substation 220 kV Lafarge. Volume 2. Relay protection and automation, страница 9

Overcurrent protection is connected with the built-in current transformers on the MV (LV) sides of the transformer.

Overcurrent protection should include combined start with voltage level check.

This protection is also used by arc protection in order to check current level at input cubicle (35 or 6,6 kV).

Overcurrent protection must have regulation of operation current in the range (0.4 - 12) ∙ I_nom A.

           Voltage settings:

- voltage direct sequence - (20,0 – 80,0) V;

- negative sequence voltage - (3,0 - 20,0) V.

            The first stage of overcurrent project (current cutoff) should act to:

-  switching off the transformer (using output relays of every kit of main protection);

-  starting breaker failure protection on the HV (220 kV) side of the transformer and blocking  auto-reclosing.

The second stage should have two delays with independent regulation of each in the range (0,1 – 10) sec.

The second stage of overcurrent project should act to:

- switching off the circuit breakers on MV and LV sides with the first time delay.

- switching off the transformer (using output relays of every kit of main protection), starting breaker failure protection on the HV (220 kV) side of the transformer and blocking 220 kV circuit breaker auto-reclosing.

There shall be an automatic acceleration of the second stage while testing transformer. Time of the acceleration should be adjustable in the range (0-1.5) seconds.

4.1.1.6. Earth faults alarm on the 6,6 kV and 35 kV sides of the transformer

Earth faults alarm is connected to the voltage transformers at input cubicles of 35 kV (6,6 kV).

There should be an ability of measuring or calculation of zero and negative sequence voltages.

Zero sequence voltage setting should be regulated in the range (3-40) V.

Negative sequence voltage setting should have the ability of regulation in the range (3-20) V.

Earth faults alarm must have time delay regulation in the range (0,1 – 10) seconds.

There must be blocking of earth faults alarm when the circuits of MV (LV) transformers are faulty, for example, fuse links have burned out.

4.1.1.7. Condition control of the voltage transformers on the 6,6 kV and 35 kV sides of the transformer.

Condition control is connected to the voltage transformers at input cubicles of 35 kV (6,6 kV). There should be an ability of measuring or calculation of zero and negative sequence voltages.

Condition control signal must have time delay regulation in the range (0,1 – 10) seconds.

Condition control of the voltage transformers shouldidentify, if possible, faults both in primary and secondary circuits.

Condition control must distinguish short-circuit states in primary circuit, earth faults on the LV (6,6 kV) side from cases when secondary circuits of the voltage transformer are faulty.

4.1.1.8. Trip circuits of the technological protection

Switching off the MV and LV sides of the transformer (from oil overheating protection,  cooling fault protection and etc.) should be carried out through the kits of main and back up protections of the transformers (T1 and T2).

There must be reducing insulation control device for the circuits, which are coming to the sensors of technological protections and acts (signal) with a time delay when these circuits are faulty.

4.1.1.9. Requirements for the number of digital inputs and outputs of the terminals

The number of inputs and outputs specified for the terminal, which contains a group of relay outputs belonging to this kit of protections. Contacts of different relays, if possible, should not have common points.

In the terminal, which contains a group of relay outputs of this kit of protections, there should be no less than 30 digital inputs and 30 digital outputs.

Number of digital inputs and outputs should be specified in the working project documentation.

4.1.2. Back up protection on the HV (220 kV) side of the transformer

4.1.2.1. Overcurrent protection on the HV side of the transformer

It must be impossible for this protection to get zero-sequence currents, which appears with earth faults in 220 kV network.