Solution and Alternatives
To avoid the unbalance current impact during load transfer, except set LCO tap to higher level or shunt with a reactor, other suggestion include: two feeders total current should less than one feeder rated current (such as operate at duty off period), switch off the customer’s large capacitor bank, keep two feeder’s voltage magnitude and phase angle closely, and replace the DS as load breaker switch (LBS). LBS can operate the three phase simultaneously.
Decision and Recommendation
The Distribution Office accepted the suggestion and installed the LBS instead the DS, the large unbalance current during transferring will be avoided.
Results
After applying the new load transfer procedure and the LBS, the failure caused by current unbalance didn’t happen.
Room for Improvement
To avoid load transferring failure, the following items worth mentioning:
- Before load transfer, the new load distribution of each feeder should calculate firstly. Some feeders’ current may become large enough after load redistribution, and may operate the phase overcurrent relay.
- Rearrangement the single phase load in three-phase four-wire distribution system to keep the three phase load balance.
Conclusion and Learning
The success of this case attributes to the joint effort of the distribution office and the power research institute. Both parties should positively face the problem, conduct testing and determine the best possible and feasible solution.