In today’s article, we are going to know about the overvoltages and its causes. There are so many sources concerned with overvoltages like generators, transformers, and transmission lines. These over voltages can be caused by lightning, an opening of circuit breakers, the grounding of a conductor etc. These overvoltages may not be huge in magnitude but its effect can interrupt the protection system or devices connected in the system. Engineers always trying to limit and control the effect of over voltages on operating types of equipment. Now we are trying to find out the main causes of over voltages. How they can be generated and can affect the operating system.
“A sudden rise in voltage for a very short duration on the power system is known as over voltages or transients”
these transients or surges are of temporary nature and short duration (a few hundred µs) but they cause over voltages on the power system. They originate from switching and from other causes but the most important transients are those which are generated from the natural lightning strokes on transmission lines. When lightning strikes a line, the surge rushes along the line and a flood of huge voltages rushes into the system which is capable of damaging every single protective and electrical appliances.
The above figure shows the lightning surge waveform. The voltage build-up along the x-axis and the time along the y-axis. It seems quite clear that the surge generates a very steep forward wave in a very short time which is called wavefront. This buildup is very rapid, being of the order of 1-5µs. While the back side called wave tail passes slowly compare to the wavefront. It takes a little more time than a steep wave.
Causes of over voltages
The over voltages on the power system can be broadly divided into two main categories.
A) Internal causes B) External causes
Internal causes do not produce surges of large magnitude. Experience shows that internal surge hardly increases the system voltage to twice of the normal voltage.
- Switching surges: The first type of internal over voltage is switching surges. The over voltages produced on the power system due to switching operations are known as switching surges.1) It can be caused when an unloaded line is connected to the voltage source. 2) Also, it can be caused when a loaded line is connected. Suppose a loaded is suddenly interrupted. This will set up a voltage of transients.3) the most important cause of over voltages in switching surges is current chopping. Current chopping results in the production of high voltage transients across the contacts of the air blast circuit breaker.
- Insulation failure: the most important case of insulation failure in a power system is the grounding of the conductor. Suppose a conductor grounds at the center. It produces negative voltages which travel along both sides.
- Arcing ground: The phenomenon of intermittent arc taking place in the line to the ground fault of 3 phase system with consequent production of transients is known as arching ground. These transients are cumulative; they can be very dangerous and can cause some serious damage to the power system by causing insulation failure. Arcing ground can be prevented by earthing the neutral.
- Resonance: Resonance is that situation in the power system when the inductive reactance of the circuit becomes equal to the capacitive reactance. Under resonance the impedance and resistance become equal and the power factor is unity. Resonance causes high voltages in the transmission lines.
- External cause: External cause includes lightning only. “An electric discharge between cloud and earth, between clouds or between the charges center of the same cloud is known as lightning”. We will discuss this cause in the next article with a great detail.