Construction of substation 220 kV Lafarge. Volume 2. Relay protection and automation, страница 7

Current transformers, which connects with differential protection, must have a star connection.

Differential protection should act without any delay time:

- switch off all sides of transformer;

- breaker failure protection start of the circuit breaker on the 220 kV side with auto-reclose prohibition.

Software alignment of input currents both in amplitude and phase must be provided according the group of the connection of the transformer windings.

Alignment error - no more than ± 2% of the nominal current of the transformer.

Protection consists of sensitive differential protection and differential cutoff.

Protection has the deceleration for time grading from the current imbalance occurring at external currents of short-circuit up to 20·I_nom in steady and transient conditions, as well as the inclusion of T under voltage. 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 sensitive differential protection (I_{s.z min}) should be in the range (0,2 – 0,8) I_nom.

When using minimum trip setting differential protection must have time grading from any magnetizing current inrushes.

If deceleration realized by the arithmetic sum (half-sum) of input currents (from all groups of CTs), the deceleration ratio (К_torm)  must be adjusted in the range from 0,2 to 0,8.

For the time grading from magnetizing currents it is possible to use blocking, which activates when ratio of the second (I_diff2 when f=2f_nom) and the fundamental (I_diff when f=f_nom) harmonics rises.

Trip setting for the blocking (I_diff2 / I_diff) can vary from 8 to 20%.

Adjustment of the differential cutoff minimum operating current (I_{s.z cutoff.}) should be in range (6,5-12) I_nom.

Tripping time of the differential protection of the transformer should be no more than 0,03 seconds in case when the current exceeds twice the value of the operation current.

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 short circuit 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 short circuit 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.

4.1.1.2. Differential zero-sequence current protection of the transformer

This protection is used to provide greater sensitivity in zone of protection (MV winding of the transformer) during earth faults.

This protectionis located at the same terminal as differential longitudinal overcurrent protection with deceleration and, accordingly, placed in a separate cabinet.

This protection needs the installation of neutral current transformers with accuracy class 5P and secondary current 5 A. The main protection terminal must have an analog input to connect with neutral current transformer.

Current transformers, which connects with differential zero-sequence current protection, should have a star connection on the MV (35 kV) side.

Differential zero-sequence current protection should act without any delay 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.

There must be software alignment of zero-sequence current inputs in amplitude.

Alignment error - no more than ± 2% of the nominal current of the transformer.

Protection can support deceleration for time grading from the current imbalance occurring at external currents of phase-to-ground faults up to 20·I_nom in steady and transient conditions.