CAUSES OF OVERVOLTAGES
The over voltages on a power system may be broadly divided into two main categories:
1. Internal causes
(i) Switching surges (ii) Insulation failure
(iii) Arcing ground (iv) Resonance
2. External causes i.e. lightning
Internal causes do not produce surges of large magnitude. Experience shows that surges due to internal causes hardly increase the system voltage twice the normal value. Generally, surges due to internal causes are taken care of by providing proper insulation to the equipment in the power system. But surges due to lightning are very severe and may increase the system voltage to several the normal value. If the equipment in the power system is not protected against lightning surges, these surges may cause considerable damage.
INTERNAL CAUSES OF OVERVOLTAGES
Internal causes of over voltages on the power system are primarily due to oscillations set up by the sudden changes in the circuit conditions. This circuit change may be anormal switching operation such as opening of a circuit breaker, or it may be the fault condition such as grounding of a line conductor.
1. Switching Surges – The over voltage, produced on the power system due to switching operations are known as switching surges. A few cases will be discussed by way of illustration:
(i) Case of open line – During switching operations of an unloaded line, traveling waves are set up to produce over voltages on the line. As an illustration consider an unloaded line being connected to a voltage source as shown in Fig 2.
When the unloaded line is connected to the voltage source, a voltage wave is set up which travels along the line. On reaching the terminal point A, it is reflected back to the supply end without change of sign. This causes voltage doubling i.e. voltage on the line becomes twice the normal value. If E rms is the supply voltage, then the instantaneous voltage that the lines have to withstand will be 2√2 E. This over voltage is of temporary nature. It is because the line losses attenuate the wave and in a very short time, the line settles down to its normal supply voltage E. Similarly if an unloaded line is switched off, the line will attain a voltage 2√2 E for a moment before settling down to the normal value.
(ii) Case of a loaded line - Over voltages will also be produced during switching operations of a loaded line. Suppose a loaded line is suddenly interrupted. This will set up a voltage of 2 Zn I across the break. (i.e. switch) where I is the instantaneous value of current at the time of opening of line and Z is the natural impedance of the line.
(iii) Current chopping – Current chopping results in the production of high voltage transients across the contacts of the air blast circuit breaker. Unlike oil circuit breakers, which are independent for the effectiveness on the magnitude of the current being interrupted, air-blast circuit breakers retain the same extinguishing power irrespective of the magnitude of this current. When breaking low currents (e.g. transformer magnetizing current) with air-blast breaker, the powerful de-ionising effect of air-blast causes the current to fall abruptly to zero well before the natural current zero is reached. This phenomenon is called current chopping and produces high transient voltage across breaker contacts. Over voltages due to current chopping are prevented by resistance switching.
2. Insulation failure - The most common case of insulation failure in a power system is grounding of conductor (i.e. insulation failure between line and earth) which may cause over voltages in the system. This is illustrated in Fig 3.
Suppose a line at potential E is earthed at point X. The earthing of the line causes two equal voltages of –E to travel along XQ and XP containing currents –E/Zn and +E/Zn respectively. Both these current pass through X to earth, so that current to earth is 2 E/Zn.
3. Arcing ground – The phenomenon of intermittent arc taking place in line-to-ground fault of a 3 phase system with consequent production transients is known as arcing ground. The transients produced due to arcing ground are cumulative and may cause serious damage to the equipment in the power system by causing breakdown of insulation, Arcing-ground be prevented by earthing the neutral.
4. Resonance – Resonance in an electrical system occurs when inductive reactance of the circuit becomes equal to the capacitive reactance. Under resonance, the impedance of the circuit is equal to inductance of the circuit and the p.f. is unity. Resonance causes high voltages in the electrical system.