Mitigation of Voltage Drop Using Pre-Insertion Resistor during Large Transformer Energization in a Weak System: PSCAD simulation

part of simulation also shows harmonics resonance phenomena.

Significant voltage drops can be expected during the energization of a large transformer in a weak system due to high inrush currents. Although controlled switching techniques are now available, pre-insertion resistors continue to be applied control the inrush current and mitigate the large voltage-depression.

As shown in simulation Fig.1 , large power transformer rating 800MVA at 230KV voltage system with 80% remanence flux draw large Inrush current 10KA lead to voltage dip 0.6 pu.

Multiple Run component used to find worth case scenario. By switching 100 times over one cycle (Point on wave switching).

Effect of pre-insertion resistance:

Any standard value of closing resistor between 200 and500 Ohms offered by a breaker supplier would provide similar performance in this application.

Effect of Insertion Time:

This dependence on closing instant can be completely eliminated only by ensuring that total minimum resistor insertion time is not less than 11 to 12 ms. (about 0.7 electrical cycle) Pre-insertion times less than about 8 ms (0.5 electrical cycle) provide limited benefit in energization events where the combination of remanence and energization instant are not favorable.

Note: for more simulation accuracy, the modeling must be at least 2 buses away from interesting area.

Harmonics resonance:

Due to interaction between inrush current low order harmonics and system equivalent impedance. harmonics resonance come into picture with TOV issue.

By using Impedance/frequence scan component, we can investigate parallel resonance point (240HZ near to second order harmonics).

Mitigation:

just by switching line reactor in service can move resonance point to upper frequence.