Beijing Sifang CSC-241A Digital Transformer Differential Protection Device

1.1 Device Introduction

The CSC-241A is a differential protection device designed for two-winding transformers at voltage levels of 66kV and below, which can be installed locally in switchgear. It generally needs to be used in conjunction with the CSC-241B backup protection device (or CSC-241C plant transformer protection device) from the same series to form a complete protection system.

1.2 Main Functions

 Differential instantaneous protection responding to faults within the protection zone;

 Percentage differential protection with second harmonic restraint responding to faults within the protection zone;

 TA (current transformer) break function;

 Differential current over-limit alarm function;

 Fault recording;

 Collection of 9 remote signaling inputs (including acquisition of non-electrical protection output contacts and information upload function), device remote signaling change, and accident remote signaling;

 Various event SOE (Sequence of Events), etc. 

1.3 Protection Elements
1.3.1 Differential Instantaneous Element
When the differential current in any phase exceeds the differential instantaneous setting value, it operates to trip. It is used to quickly disconnect the transformer in case of severe faults within the transformer differential zone. The differential instantaneous setting should be able to avoid the maximum unbalanced current during external faults and the inrush current when energizing the transformer, generally set at 6 to 12 times the rated current.

1.3.2 Percentage Differential Element
To ensure sensitive operation of the differential protection during internal faults while preventing maloperation due to transient unbalanced currents during external faults or inconsistencies in CT characteristics on different sides of the differential, this device adopts a three-section percentage differential principle.

1.3.3 Second Harmonic Restraint Element
The second harmonic in the three-phase differential current is used as the criterion for blocking inrush current. The operating equation is as follows:
Id2 > Kxb·Id, where Id2 is the second harmonic in the three-phase differential current, Id is the corresponding three-phase differential current, and Kxb is the second harmonic restraint coefficient. If any phase in the three-phase differential current meets the above condition, the three-phase percentage differential protection is blocked. Kxb is generally set between 10% and 30%.
1.3.4 TA Break and Differential Current Over-Limit Alarm
1.3.4.1 Instantaneous TA Break Blocking or Alarm:
Under normal conditions, TA break is detected by checking if one or two phases of current in all phases have no current and there is differential current, which is judged as a TA break.
During a sudden current change, the criteria are as follows:
1) The current decreases (rather than increases) after the change.
2) One or two phases of the three-phase current on this side have no current, and the three-phase current on the other side remains unchanged.
If the above conditions are met, it is judged as a break in the TA secondary circuit.
After a TA secondary break, an alarm signal is issued, and the differential protection trip can be optionally blocked or not blocked.
12.3.4.2 Differential Current Over-Limit Alarm:
If the differential current exceeds 25% of the differential setting current and this condition persists for more than 10 seconds, an alarm signal is issued. It reports the differential current over-limit but does not block the differential protection. This function serves as a monitoring feature for the AC sampling circuit of the protection device.
12.3.5 Event Recording for Non-Electrical Protection
Contacts from the transformer body are relayed through the non-electrical protection device, and a set of dry contacts is output to the main differential protection device for event recording. The records can be uploaded to the background computer via network or field bus.