Guodian Nanzi PST 645U Transformer Protection and Control Device

1 Overview
The PST 645U Transformer Protection and Control Device is suitable for station service transformers or grounding transformers in non-directly grounded systems or small resistance grounding systems with voltage levels below 110KV. It can be installed in panels or locally.
1.1 Protection Function Configuration
1) Three-stage composite voltage blocking overcurrent protection (Stage III can be set as inverse time)
2) Three-stage zero-sequence overcurrent protection on the high-voltage side (Stage III alarm/trip optional)
3) Three-stage zero-sequence overcurrent protection on the low-voltage side (Stage III can be set as inverse time)
4) Overload (alarm/trip optional)
5) Zero-sequence overvoltage protection (alarm/trip optional)
6) Undervoltage protection
7) Manual closing acceleration protection
8) 4 non-electrical quantity protection circuits
1.2 Measurement and Control Function Configuration
1) 22 external switch input telemetry collection;
2) Circuit breaker position, manual tripping, and accident telemetry;
3) Normal circuit breaker remote control opening and closing;
4) 2 pulse inputs, electric energy accumulation;
5) GPS time synchronization input (adaptive to minutes and seconds).

2 Technical Parameters
2.1 Accurate Operating Range of Protection Components

1 Voltage 1.0V～150.0V
2 Phase current 0.04IN～20IN
3 Zero-sequence current 0.04A～12A

2.2 Setting Error of Protection Components

1 Voltage component ≤±2.5% setting value or ±0.005UN
2 Current component ≤±2.5% setting value or ±0.01IN
3 Time component: Definite time ≤±1% setting value + 40ms

3 Protection Functions and Principles
3.1 Phase Overcurrent Protection
When performing three-stage overcurrent discrimination, the device follows the same logic for each stage. The action conditions are as follows:
1) I? > Idn; Idn is the n-stage current setting, I? is the phase current;
2) T > Tdn; Tdn is the n-stage time delay setting.
Its undervoltage blocking element and negative-sequence voltage blocking element are controlled by corresponding control words.
3.1.1 Undervoltage Blocking Element
The undervoltage element operates when any of the three line voltages is below the undervoltage setting value, unlocking the blocked protection element. This element ensures the device does not malfunction under non-fault conditions such as motor feedback.
3.1.2 Negative-Sequence Overvoltage Blocking Element
When the negative-sequence voltage exceeds the negative-sequence overvoltage setting value, it unlocks the blocked protection element. It can form a composite voltage blocking element together with the undervoltage blocking element, working with the overcurrent element to create composite voltage overcurrent protection.