Construction of substation 220 kV Lafarge. Volume 2. Relay protection and automation, страница 8

Deceleration can use, for example, the input currents of all current transformer groups, which are connected with the protection.

Regulation of the minimum operating current of zero-sequence differential protection (I_{s.z min}) should be in range (0,05 – 2) I_nom.

Deceleration rate (К_torm) (characteristic response angle) should be in range (0,05-2,0).

Protection time delay can be может регулироваться в пределах (0-10,0) с.

Zero-sequence differential protection of the transformer must function properly during a short circuit within its zone even if the current transformers error have the values up to 50% (caused by saturation) at steady state when working on a resistive load and the values ​​of phase-to-ground fault primary currents are up to 20 • I_nom. It’s also must function properly during an external short circuit when the current transformers error have the values up to 5% (caused by saturation) at steady state when working on a resistive load and the values of phase-to-ground fault primary currents are up to 20 • I_nom.

For proper operation of differential protection the nominal parameters of the current transformers should be selected according to the requirements of manufacturers.

4.1.1.3. Gas protection of the transformer, on-load tap-changer (OLTC) and OLTC contactor.

Gas protection of the transformer and OLTC has two stages. First stage turns on alarm system, second switches off the transformer.

OLTC contactor protection (jet relays and pressure monitoring device) has one stage that switches off the transformer.

Second stages of gas protection of the transformer and OLTC acts on:

- switching off the circuit breakers on all sides of the transformer;

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

The operation of switching off circuit breakers should have memorization up to 1 second from the moment gas protection trips.

All the operations (alarm and switching off) of gas protection of the transformer, OLTC and OLTC contactor should be made via MP terminals of the main and back up protections (transformers must be equipped with a gas relay which has at least two signal contacts and two tripping contacts).

The cabinets of main and back up protections of T1, T2 should have a switch to transfer switching off levels to alarm system. There is also should be an ability to use signal level as switching off level during the first start of the transformer. This solution is possible if the terminals have programmable logic, available logic elements and a sufficient number of digital inputs (not less than 6). It makes possible to arrange the transfer of gas protection signals to the PCS (in the digital code).

There shall be a local alarm (terminal LEDs) of each gas protection stage, transferring switching stages of gas protection of the transformer and OLTC to «alarm» mode and alarm stages to «switching» mode.      

The power supply of gas protection from the operating direct current should be carried out through a separate circuit breaker in each kit of protection.

There must be a device which controls isolation reducing of circuits, used by gas/jet relay, and makes a signal with a time delay when these circuits are faulty.

 4.1.1.4. Overload protection on the MV and LV sides of the transformer

Overload protection is provided at the input 35 kV (6.6 kV) of the transformer.

Overload protection operates with a time delay on the signal and can have one phase current control.

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

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

Overload protection must have time delay setting adjustment range (0,0 – 10,0) seconds.

4.1.1.5. Overcurrent protection on the MVand LV sides of thetransformer

Overcurrent protection against short-circuits between phases on the side of the MV (LV) of the transformer has two stages.

Overcurrent protection is presented on the 35 (6,6) kV of the transformer.