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PROTECTION SWITCH GEARS

PROTECTION AGINST OVER VOLTAGES

CAUSES OF OVERVOLTAGES: The overvoltage's on a power system network may be broadly divided into two main categories. Internal causes: Switching surge. Insulation failure. Arcing ground. Resonance. External causes: Lighting.

INTERNAL CAUSES OF OVERVOLTAGES: Internal causes of overvoltage on the power system are primarily due to oscillation setup by the sudden changes in the circuit condition. This circuit changes may be a normal switching operations such as opening of circuit breaker, or it may be the fault condition such as grounding of a line conductor.

SWITCHING SURGE: The over voltage produced on the power system due to switching operations are known as switching surge. A few causes will be discussed by way of illustration. CASE OF AN OPEN LINE: During switching operations of an loaded line, travelling wave are set up which produce overvoltage’s on the line. As an illustration consider an unloaded line begin connected to a voltage source.

When unloaded line is connected to the voltage source a voltage wave is step up which travels along the line. On reaching the terminal point A, it is reflected back to the supply end with out change of Sign. This causes voltage doubling twice of the normal voltage. 2 2 E. If any unloaded line switched off, the line will attain a voltage of 2 2E for a moment before Settling down to the normal value.

CAUSES OF A LOAD LINE: For example; Overvoltage’s will also be produced during the switching operations of a load line suppose a loaded line is suddenly interrupted. This will setup a voltage of 2 Zn is across the break (switch) I is the instantaneous value of current at time of opening of line and *Zn is the natural impedance. For example; Zn=1000ohm cries current of 100A(rms) Voltage across the breaker=2 2 *100*1000/1000=282.8kV. If Vm is the peak value of voltage in kV, the maximum voltages of line may be(Vm+282.8)kV.

INSULATION FAILURE: The most common cause of insulation failure in a power system is the grounding of conductors which may causes overvoltage in the system Both these current pass through X to earth so that current to earth is 2E/Zn

ARCING GROUND: In the early days of transmission the neutral of three phase lines was earthed. Is a phenomenon which is observed in underground three phase system, operating in a healthy balanced condition. Capacitance are formed between the conductors and ground. The transient produce due to arcing ground are cumulative and may causes damage to the equipment in the power system by causing breakdown of insulation. Arcing ground can prevented by earthing the neutral.

RESONANCE: Resonance is an electrical system occurs when inductive reactance of the circuit become equal to capacitive reactance. Under resonance the impedance of the circuit is equal to the resistance of the circuit and the p.f is unity. Resonance causes high voltage in the electrical system. In usually transmission lines, he capacitance is very small so that the resonance rarely occurs at the fundamental supply frequency.

LIGHTNING: An electric discharge between cloud and earth clouds or between the charge centre of the same cloud is known as lightening.

OVER VOLTAGES DUE TO DIRECT LIGHTNING STRIKES: These can take two forms: When lightning strikes a lightning conductor or the roof of a building which is earthed, the lightning current is dissipated into the ground. The impedance of the ground and the current flowing through it create large difference of potential: this is the overvoltage. This overvoltage then propagates throughout the building via the cables, damaging equipment along the way. When lightning strikes an overhead low voltage line, the latter conducts high currents which penetrate into the building creating large overvoltage. The damage caused by this type of overvoltage is usually spectacular (e.g. fire in the electrical switchboard causing the destruction of buildings and industrial equipment) and results in explosions.

OVERVOLTAGES DUE TO THE INDIRECT EFFECTS OF LIGHTNING STRIKES The overvoltages previously mentioned are also found when lightning strikes in the vicinity of a building, due to the increase in potential of the ground at the point of impact. The electromagnetic fields created by the lightning current generate inductive and capacitive coupling, leading to other overvoltages. Within a radius up to several kilometres, the electromagnetic field caused by lightning in clouds can also create sudden increases in voltage. Although less spectacular than in the previous case, irreparable damage is also caused to so called sensitive equipment such as fax machines, computer power supplies and safety and communication systems

GENERAL POINTS ON LIGHTNING AND ITS RISKS: CAUSES OF TRANSIENT OVERVOLTAGES Overvoltages due to operating or switching actions: Equipment containing electronic switching components is also likely to generate electrical disturbances comparable to overvoltages. The consequences of which on sensitive equipment, albeit not visible, are no less detrimental premature ageing and unpredictable or fleetingbreakdowns. Operating overvoltages are produced when reactive or capacitive equipment is switched on and off. Furthermore, interrupting factory production, lighting or transformers can generate overvoltages which will themselves cause greater damage tonearby electrical equipment.

Representation of the various disturbances on electrical networks

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