Results
After installation of two high-pass damped filters, failures about the capacitors of filters are reduced.
Room for Improvement
Low levels of non-characteristic harmonics should also be considered when designing filters. A latent crisis was hiding in the system. Any change in the filter capacitance, system impedance, and others could actuate the harmonic amplification problems.
Conclusion and Learning
The success of this case attributes to the joint effort of the plant and the research team. Both parties should positively face the problem, conduct testing and determine the best possible and cost-effective solution.
Reduce Impact Caused by Current Unbalance on Feeders Load Transfer
Background
A secondary substation which supplies power to industrial park located in central Taiwan. One of the feeders performs routine maintenance last year. In order to avoid power interrupt, the distribution office proceeds load transfer firstly. when proceed to distribution feeders load transfer, ground overcurrent relay （LCO ）energized and feeders tripped. It was identified that such tripping was caused by three phase current unbalance during transferring. Disconnecting switches（DS） were applied in this case. Load characteristic of the feeders will be changed during load transferring, and may produce severe current unbalance. One of the feeders supplied a steel plant which installed a large capacitor bank（about 1/3 of feeder rated MVA）, the distribution office also doubt about the effect of this capacitor bank on load transfer.
Investigation and Finding
The distribution office and the power research institute met and aimed at finding the best solution to reduce the impact. The first thought was measurement the effect of customer’s large capacitor bank. Those are about 40° current phase angle different between the feeders, and will be over 90° different when customer’s large capacitor bank switched on. It may produce large transient current during transferring, and has a big chance to energize the LCO.
The second thought was measurement the current change during load transfer. The same load transfer procedure will be performed, but the test time was choosing at noon rest time to minimize the possible interrupting effect. Load at that time was smaller than usual（about 13% of feeder rated current）. Test results shown that, the largest neutral currents was 65A （about 46% of total load current）during DS switching on (parallel operation), and was 83A（about 60% of total load current）during DS switching off （load transferring）, those neutral currents didn’t energize LCO （rated 120A）. When feeders load current large enough, the LCO will be energized. It was the cause of LCO tripping last year.
During or after parallel operation, feeder’s phase current will be redistributed according to the feeder voltage. In this test case, the feeder with customer’s capacitor bank（feeder voltage higher than the other one） increased to two times phase current.
