Chapter 4-Example (New).docx - Example Chapter 4 Question 1...

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Example Chapter 4 Question 1 a) Describe in details with the appropriate diagrams, the methods of insulation coordination for over voltages. Answer: a) There are two methods of insulation coordination for overvoltages: i) Conventional Method. This is the method used when the probability of failure for a given overvoltage is unknown, ie. with non self restoring insulation. The known probability distribution of overvoltage amplitudes is used only to determine the maximum surge which is liable to occur. This is added a safety margin to allow for the unknown flashover probability distribution. The resultant voltage level is specified as a ‘withstand’ voltage, V w which the insulation must be able to hold off. ii) Statistical Method This is based on knowledge of the statistics of both overvoltage occurance and of flashover probability. The design is then based on an acceptable risk of flashover. If at a voltage level V, the probability of failure is P(V) and the frequencies of occurrence or surges of that level is f(V) then the risk function is defined as, r(V) = P(V).f(V). The risk function is shown below,
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For a given insulation design the total risk of failure will be: 0 P ( V ) . F ( V ) dV The risk is therefore determined by the area under the r(V) curve. As the insulation is strengthen by using larger gap or longer insulation strength, the risk of failure diminishes as in the diagram below: For a chosen risk of failure it is therefore possible to position the curve. We can then choose a reference probability and quote the corresponding voltage as a statistical withstands voltage V w . In practice the reference probability is normally taken as P(V) = 0.1, ie. V w is the voltage at which the insulation has a 90% probability of withstand. The system design engineer will therefore specify that the insulation must
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have a withstand of V w based on the acceptable risk criterion, and the insulation design engineer will develop insulation suitable for that withstand level. b) Describe the differences between Basic Lightning Impulse Insulation Level (BIL) and Basic Switching Impulse Insulation Level (BSL) and explain how they affect in the design of power system network. Answer: BIL: This is the reference insulation level which is expressed as peak impulse voltage having a standard lightning impulse waveform of 1.2/50μs. It is determined by tests made using impulses of the standard lightning impulse wave shape. BSL: Similar to BIL but is defined for a standard switching impulse voltage of 250/2500 μsec waveform. It is generally specified for equipment with rated voltage at >= 300 kV rms . The level of voltage supply from customer side to the generation side can be categorized as i) Low Voltage (LV) ii) Medium Voltage (MV) iii) High Voltage (HV) iv) Extra High Voltage (EHV) v) Ultra High Voltage (UHV) These systems are affected by transient overvoltage generally due to external source such as lightning phenomenon and internally generated is the switching activities. In
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