Published In December 2001
Last month’s article focused on power factor (PF) capacitor switching transients. Although most power quality (PQ) disturbances originated from within a facility, we will again focus on an upstream event relative to the point-of-common-coupling (PCC), alias the service entrance. Sags are the most common type of power quality (PQ) phenomena, which is when the voltage goes typically below 90 percent of nominal. If it continues to drop to below 10 percent of nominal, then it is classified as an interruption. Based on how long the sag or interruption lasts, there are additional labels associated with the event, such as instantaneous (0.5 to 30 cycles long), momentary (30 cycles to three seconds), temporary (three seconds to one minute), and sustained (greater than one minute). Faults on the distribution system are caused by weather (lightning and wind), equipment failure, construction or traffic accidents, animals, and tree limbs. Most often, one phase conductor will come in contact with something at ground potential, causing an electric arc. This arc may be large enough to cause a flashover to another phase conductor. Although very rare, three-phase faults can occur when all phases are electrically shorted to each other and/or ground. Just like the breakers in a distribution panel within a facility, the breakers in a substation are designed to protect the wiring and equipment that provides the power. There are also fuses located on the utility poles to protect sections of the wires. The entire protection scheme is quite complex, but the gist of it is to try to clear the fault by interrupting the power flow to the smallest section of the grid that is possible, resulting in the disruption in service to the fewest customers. For this reason, the fuses react faster than some of the other system protection devices (such as breakers), typically within a cycle. However, once a fuse blows, it requires human intervention to restore service to those customers affected, once the fault is removed. The distribution substation breakers will typically react in four to 10 cycles. The breaker(s) will open to try to clear the fault. If the fault was caused by an arc from a critter, such as a squirrel, coming in contact with two conductors at once, the arc will be extinguished when the breaker is opened, and wouldn’t likely recur when the circuit is re-energized (as the squirrel is usually vaporized). If the fault is still there, such as a downed electrical wire on a vehicle that hit the utility pole, then the breaker will reopen and try to reclose again. This recloser sequence can last three to 10 times, after which the breaker will be locked out until human intervention can correct the problem and reset the breaker. The fault will draw very large current levels, which cause the fuse to blow or the overcurrent relays to operate the breaker. These large current flows cause voltage drops in the distribution feeders. The result is a sag. A sag can show up on parallel substation feeders that do not have the fault, if they are fed from the same voltage bus in the substation that has a faulted feeder. Once the breaker operates, all of the customers downstream of the fault will experience an interruption, until the breaker recloses. Those on parallel feeders will see the voltage return to normal. By noting the magnitude, duration, and recurrence of the sags and/or interruptions, the source of the disturbance can be determined as originating from the electric utility side. In addition, monitoring the current at the same time will usually show that the current changed only slightly during the sag. If the fault had been downstream from the monitoring point, there would most likely be a large increase in the monitored current level at the PCC. Consider a sag that was monitored at the incoming transformer to a bulk mail handling facility. Only two of the three phase currents had current transformers (CTs) to monitor the current. The flashover at the peak of the one phase voltage can be seen to start the sag, which is cleared approximately five cycles later. The current reduces during this time, due to the large percentage of linear loads in the facility. These waveforms indicate that it was an upstream sag on a parallel feeder that was cleared by the utility system protection breakers operating. In another case, the monitoring point was a facility that was on the feeder that had the fault. After five cycles of fault current, the breaker operated, resulting in an interruption. The slowly decaying voltage is the result of electrical motors that are backfeeding the circuit. An unpowered spinning motor becomes a generator. As the speed winds down, so do the amplitude and frequency of the voltage. This, as with the first example, was initially a single-line-to-ground (SLTG) fault, but this time, it became a three-phase interruption. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.