Guodian Nanzi PST 641UX Transformer Differential Protection Device

1 Overview
1.1 Scope of Application
The PST 641UX Transformer Differential Protection Device provides percentage differential protection with second harmonic restraint and instantaneous differential protection, suitable for transformers in power systems with voltage levels of 66kV and below.
1.2 Protection Function Configuration
a) Instantaneous differential protection;
b) Percentage differential protection;
c) TA broken wire detection;
d) Non-electrical protection (signal recording).
1.3 Measurement and Control Function Configuration
a) 20 external switch input telemetry collection;
b) Fault telemetry;
c) GPS time synchronization input.

2 Protection Functions and Principles
2.1 Instantaneous Differential Protection
When the differential current of any phase exceeds the instantaneous differential protection setting value, it activates the main output relay. It is used to quickly disconnect the transformer in the event of severe faults within the transformer differential zone. The instantaneous differential setting should avoid the maximum unbalanced current during external faults and the inrush current when energizing the transformer, typically set at 6 to 12 times the rated current.
After the main output relay operates, it outputs 8 pairs of contacts, namely (X3:1, X3:2), (X3:3, X3:4), (X3:5, X3:6), (X3:11, X3:12), (X6:9, X6:10), (X6:11, X6:12), (X6:13, X6:14), (X6:15, X6:16).

2.2 TA Broken Wire
It features instantaneous TA broken wire blocking or alarm function and differential current over-limit alarm function.
The TA broken wire initiation condition is when the differential current exceeds the differential starting current. The device disables instantaneous TA broken wire detection under any of the following conditions:
1) Before initiation, the maximum phase current on one side is less than 20% of the rated current on that side, then that side is not judged;
2) After initiation, the maximum phase current exceeds 120% of the rated current on that side;
3) After initiation, the current on any side increases compared to before initiation.
If none of the above three conditions are met, and one side simultaneously satisfies the following conditions, it is judged as TA broken wire:
1) Only one phase current is zero;
2) The other two phase currents remain unchanged from before initiation.
If the differential current exceeds 15% of the differential starting current, after a 10s delay, the device reports a differential current over-limit alarm but does not block the differential protection. This function can also serve as a monitoring function for the AC sampling circuit of the protection device.
2.3 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 PST 641UX main protection device for event recording. The records can be uploaded to the background computer via MMI.
Corresponding to the non-electrical protection device, the PST 641UX device uses soft plates to select which non-electrical information needs to be recorded.
For "cooling failure," it can be selected via control word KG1.0 whether to trip after a delay. The delay time can also be set. When the delay time is reached, a trip contact "(X3:8, X3:9)" is output to the non-electrical protection device to initiate the main trip output.