In rural California, a customer with a large ground-mount photovoltaic (PV) system had been generating electricity with no problem until, one day, fault codes occurred. The PV system was utilizing microinverters mounted on each panel. The fault codes indicated “utility frequency out of range.” This was an interesting fault code since the generators and electric utilities in the United States are required to hold a strict tolerance on frequency variations. What was causing the trouble?
To find out, a power quality monitor was set at the main electrical service to record, and within a short time, the problem was clear. It was a frequency problem, but not in the way one would assume. The fundamental frequency of 60 Hz was fine; however, the total harmonic distortion (THD) of the voltage was 6.3%. The THD was comprised of 5th, 7th, and 11th harmonics. With this customer being the only one fed from the transformer, one would assume the problem must be on the 12kV system feeding the area. Using IEEE 519-2014 Recommended Practice and Requirements for Harmonic Control in Electric Power Systems to identify the harmonic limits, it became clear that the utility 12kV system had a problem. Table 1-Voltage Distortion Limits provide the individual harmonics and THD limits categorized by voltage level, and12kV has a THD limit of 5%. Note that all harmonic measurements should be taken at the point of common coupling (PCC) when establishing limits.
Clearly, this was a utility problem — or was it? With conventional power generation and transformation, harmonics are rarely injected by utility generation into the power system. Some exceptions to this can be caused by renewable generation when the conversion from DC to AC takes place. In this case, the 12kV circuit had no renewable generation. The harmonic frequency components can help to identify what type of equipment is the cause. Since 5th, 7th, and 11th harmonics are typical of adjustable speed drives (ASDs), the search began. Further investigation and power quality monitoring found two large agricultural water pumps powered by ASDs on the 12kV circuit.
Current harmonics generate the voltage harmonics; therefore, locating the current harmonic source is critical to identifying the solution. To understand if these ASDs were the source of the problem, the power quality data was analyzed by using the short-circuit duty at the PCC and Table 2-Current Distortion Limits. The individual current harmonic limit is provided through calculation described in Section 5.2 of IEEE 519-2014. Both ASDs had high current harmonics; one ASD exceeded the 5th harmonic and the other the 5th, 7th and 11th harmonics. The harmonic frequency and current value from this calculation was crucial in determining the proper harmonic filter to install. Harmonic filters on each of the ASDs were the solution to reduce the 6.3% voltage THD on the 12kV utility system. Once the THD was reduced, the PV system generated with no more frequency fault codes.
