ELEC4611-14-Lec 4 - Overvoltage transients
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ELEC4611: Overvoltage Transients
p.4
The voltage waveform has a double exponential form: V(t) = Vo [exp (-1t) – exp (-2t)] In most respects the impulse voltage withstand requirement is the most important insulation test and specification for any power system. The power frequency overvoltage test is only of secondary importance in most cases. For this reason the impulse voltage withstand requirement is normally called the BIL or Basic Insulation Level of a power system.
ELEC4611: Overvoltage Transients
p.2
1
Sources of Transient Impulse Voltage in Power Systems
There are two major classifications of source: • • External – Impulse voltage generated externally and coupled into the system Internal – Impulse voltage generated within the system
1.1 External generation The prime cause of external impulse generation is lightning activity. This can produce impulse voltages in power systems by three possible mechanisms: (i) A direct lightning strike to a power line (ii) An induced impulse voltage in the line by inductive or capacitive coupling from the electric or magnetic field generated by a nearby lightning strike. (iii) Impulse voltage generation due to coupling by means of a rise in the local earth potential due to a lightning strike to the earth near the power line or electrical installation. The electric charge on clouds which generates the lightning strike may be tens of coulombs and the voltages may be tens of mega-volts. Typically, the lightning discharge currents are about 15kA to 20kA. When such a lightning
ELEC4611: Overvoltage Transients p.1
increase in the power frequency voltage level, perhaps sustained for times ranging from seconds to minutes or hours. The magnitudes of such “steady state” overvoltages are typically less than 1-2 per unit of rated voltage. These are generally not a matter of much concern to the power system equipment, although they are an increasing concern to items such as electronic equipment in the low voltage distribution system. All major items of electrical equipment must be tested for their ability to withstand the design levels of overvoltages. The power frequency test usually requires that about 2 per unit of voltage be applied for some time without insulation breakdown. However the test for the transient overvoltage is more onerous to the equipment and is also difficult to do. It must be done by application of an appropriate transient waveshape and possibly under a variety of test conditions. The lightning and switching impulse (Marx) generator is the most usual test source for such applications. The overvoltages that are thus of most concern are the short duration transient overvoltages. Such transient impulses can have voltage amplitudes many times the peak of the rated power frequency voltage. They also have very high frequencies and, in general, electrical insulation is more susceptible, in terms of potential breakdown, to high frequency transient overvoltages. We shall call them impulse or surge voltages.
ELEC4611: Overvoltage Transients p.3
stroke hits a power transmission line, the current will divide into two and each part will propagate away down the line in each direction. The voltage associated with the lightning strike on the line will be determined by the surge impedance of the line. Thus if the surge impedance is 300 ohms, then the voltage developed in each mode of propagation will be 20kA/2 x 300ohms = 3MV. This voltage will attenuate rapidly because of line resistance, but for short distances may cause flashover if the insulation is not adequately designed with enough spacing. All equipment must be tested for withstand ability against lightning effects. The standard waveform used for testing power system equipment is the 1.2/50 sec impulse voltage waveform, shown below. The risetime to peak is 1.2s and the decay time to half peak voltage is 50s.
ELEC4611 Power System Equipment
OVERVOLTAGE TRANSIENTS
Overvoltages in power systems are a relatively common occurrence and their presence must be accepted and protected against. The problem that they represent to the power system is that they may exceed the insulation strength of the power system and if this happens then dielectric (insulation) failure can occur with quite drastic consequences of such breakdowns for the power system and its component equipment. Two approaches are used in combination to provide protection of equipment against potential damage due to overvoltages. On the one hand the equipment is normally designed and constructed to tolerate a certain level of abnormal overvoltage without needing any additional protection. But some equipment that may be more susceptible to such events and which may be an important component (such as a transformer) will need some additional protective measures. In such cases the equipment is protected by surge or lightning arresters which act as a short circuit bypass to earth for the more dangerous overvoltages, which are usually the switching surge or lightning surge events. Within the general term “overvoltages” are included a range of different voltage events. They include simple
ELEC4611: Overvoltage Transients
p.4
The voltage waveform has a double exponential form: V(t) = Vo [exp (-1t) – exp (-2t)] In most respects the impulse voltage withstand requirement is the most important insulation test and specification for any power system. The power frequency overvoltage test is only of secondary importance in most cases. For this reason the impulse voltage withstand requirement is normally called the BIL or Basic Insulation Level of a power system.
ELEC4611: Overvoltage Transients
p.2
1
Sources of Transient Impulse Voltage in Power Systems
There are two major classifications of source: • • External – Impulse voltage generated externally and coupled into the system Internal – Impulse voltage generated within the system
1.1 External generation The prime cause of external impulse generation is lightning activity. This can produce impulse voltages in power systems by three possible mechanisms: (i) A direct lightning strike to a power line (ii) An induced impulse voltage in the line by inductive or capacitive coupling from the electric or magnetic field generated by a nearby lightning strike. (iii) Impulse voltage generation due to coupling by means of a rise in the local earth potential due to a lightning strike to the earth near the power line or electrical installation. The electric charge on clouds which generates the lightning strike may be tens of coulombs and the voltages may be tens of mega-volts. Typically, the lightning discharge currents are about 15kA to 20kA. When such a lightning
ELEC4611: Overvoltage Transients p.1
increase in the power frequency voltage level, perhaps sustained for times ranging from seconds to minutes or hours. The magnitudes of such “steady state” overvoltages are typically less than 1-2 per unit of rated voltage. These are generally not a matter of much concern to the power system equipment, although they are an increasing concern to items such as electronic equipment in the low voltage distribution system. All major items of electrical equipment must be tested for their ability to withstand the design levels of overvoltages. The power frequency test usually requires that about 2 per unit of voltage be applied for some time without insulation breakdown. However the test for the transient overvoltage is more onerous to the equipment and is also difficult to do. It must be done by application of an appropriate transient waveshape and possibly under a variety of test conditions. The lightning and switching impulse (Marx) generator is the most usual test source for such applications. The overvoltages that are thus of most concern are the short duration transient overvoltages. Such transient impulses can have voltage amplitudes many times the peak of the rated power frequency voltage. They also have very high frequencies and, in general, electrical insulation is more susceptible, in terms of potential breakdown, to high frequency transient overvoltages. We shall call them impulse or surge voltages.
ELEC4611: Overvoltage Transients p.3
stroke hits a power transmission line, the current will divide into two and each part will propagate away down the line in each direction. The voltage associated with the lightning strike on the line will be determined by the surge impedance of the line. Thus if the surge impedance is 300 ohms, then the voltage developed in each mode of propagation will be 20kA/2 x 300ohms = 3MV. This voltage will attenuate rapidly because of line resistance, but for short distances may cause flashover if the insulation is not adequately designed with enough spacing. All equipment must be tested for withstand ability against lightning effects. The standard waveform used for testing power system equipment is the 1.2/50 sec impulse voltage waveform, shown below. The risetime to peak is 1.2s and the decay time to half peak voltage is 50s.
ELEC4611 Power System Equipment
OVERVOLTAGE TRANSIENTS
Overvoltages in power systems are a relatively common occurrence and their presence must be accepted and protected against. The problem that they represent to the power system is that they may exceed the insulation strength of the power system and if this happens then dielectric (insulation) failure can occur with quite drastic consequences of such breakdowns for the power system and its component equipment. Two approaches are used in combination to provide protection of equipment against potential damage due to overvoltages. On the one hand the equipment is normally designed and constructed to tolerate a certain level of abnormal overvoltage without needing any additional protection. But some equipment that may be more susceptible to such events and which may be an important component (such as a transformer) will need some additional protective measures. In such cases the equipment is protected by surge or lightning arresters which act as a short circuit bypass to earth for the more dangerous overvoltages, which are usually the switching surge or lightning surge events. Within the general term “overvoltages” are included a range of different voltage events. They include simple