Current protection systems for distribution networks are typically designed to handle fast-impulse overvoltages, such as from switching and lightning, but not power frequency overvoltages that are relatively long duration.
For example, should contact occur between a transmission and a distribution line or between distribution lines having different operating voltages, the higher voltage is transferred to the lower voltage line. Possible damage to the primary system can include breakdown of existing distribution class arresters and damage to transformers. The voltage spike being passed to the secondary system also risks damaging household electronic devices.
Methods to mitigate this problem are based mostly on preventing such contact. They can range from avoiding use of underbuilt structures in the first place to increasing transmission line tension or separation distance, to placement of a ground plane between nearby distribution and transmission lines.
A more cost-effective solution, however, involves applying station class arresters for lines that have underbuilt distribution. FortisBC in Canada and Dominion Virginia Power in the U.S. are two utilities that have successfully employed this concept to mitigate power frequency overvoltages. In the case of the Canadian utility, the main goal was dealing with transmission to distribution contact faults while Dominion was concerned mostly with contact faults between different distribution voltages. The experience for both is reported in this INMR article from 2013 contributed by FortisBC Engineering Manager, Aram Khalil-Pour and Dominion Principal Engineer, Dan Ward.
Types of Overvoltages
There are three main categories of overvoltage that can affect power systems: lightning, switching and power frequency – all with their own causes, consequences and ideal preventative measures.
Lightning overvoltages, the most common cause of transient faults, arise when a lightning strike occurs on or near a power line. Resulting surges generally travel along the line at close to the speed of light causing line voltage to increase rapidly for a short duration. While lightning can cause temporary faults on distribution circuits, usually less than 20% of strikes lead to permanent damage. In certain cases, lightning strike can trigger a relatively long outage if it causes a fault that overloads the fuse in a cutout or locks out a recloser. The strategy generally used to protect against this type of event involves applying distribution class arresters to protect equipment susceptible to permanent damage by lightning as well as automatic reclosing to re-energize the circuit once the fault has been cleared.
A switching overvoltage is caused by operation of line switches to either energize or de-energize HV equipment. The arc generated as the switch opens or closes could lead to a high frequency transient voltage being superimposed over the power frequency. Switching overvoltages are also generally successfully controlled by arresters since, as with lightning overvoltage, they involve transient voltage spikes that can be mitigated the same way.
A power frequency overvoltage can result from direct contact between a transmission and distribution line or through contact between distribution lines with different operating voltages. Such events could occur, for example, on underbuilt structures should ice buildup weigh down one of the lines. Similarly, high winds can damage structures or falling trees can cause lines to touch.
Unlike for lightning and switching, the resulting power frequency overvoltage is of comparatively long duration, (i.e. tens of milliseconds). This presents problems for protection systems not typically intended to handle such an extended overvoltage. As discussed, the result can be failure of the protection devices and risk of damage to distribution and customer equipment
Effect on Utility & Customer Equipment
Overvoltage surges caused by line contact can increase distribution voltage to several times normal values. For example, contact of a 69 kV with a 13 kV line will raise the distribution feeder voltage by 5 times normal, resulting in 300+V on the secondary side of the distribution transformers near the fault. Such faults are typically lower magnitude but longer duration than lightning surges.
When a power frequency overvoltage occurs, distribution arresters transfer the surge to ground but immediately overload and become short to earth until the 69 kV breaker clears the event. But once the 69 kV line is reenergized, the distribution arresters, already out of the circuit, cannot transfer the second application of high voltage to ground. Some portion of the surge will then likely also be transferred to the secondary side of transformers, resulting in risk of damage to customer equipment.
Basically, the sequence of events in this case is as follows:
1. Accidental contact made
2. Distribution arresters clamp
3. Distribution arresters fail and disconnect
4. 69 kV breaker opens
5. 13.2 kV system normal
6. 69 kV breaker recloses
7. 32.2 kV system rises to 69 kV (distribution arresters that had previously clamped have all automatically disconnected from the circuit with their ground lead disconnectors and therefore do not clamp when the 69 kV voltage is re-applied)
8. secondary voltage rises to 300+ volts