Lecture Slides Chapter 4 Insulation Coordination

SEE 4463 HIGH VOLTAGE TECHNOLOGY Dr Nouruddeen Bashir Umar Email: nour@fke.utm.my Tel: 0177696962 Course materials (notes, lecture slides and tutorials) can be downloaded from: http://nour.fke.utm.my/see-4463 Chapter : Insulation Coordination • When any over voltage appears in the electrical system , then there may be a chance of failure of its insulation system • Probability of failure of insulation, is high at the weakest insulation point nearest to the source of over voltage • Insulators in some points are easily replaceable and repairable compared to others • However, at other points, the insulators are not so easily replaceable and repairable and the replacement and repairing may be highly expensive, and require long interruption of power • Therefore failure of insulator at these points may causes bigger part of electrical network to be out of service • So it is desirable that in situation of insulator failure, only the easily replaceable and repairable insulator fails To arrange the electrical insulation levels of different components in the electrical system in such a manner, that the failure of insulator, if occurs, confides to the place where it would result in the least damage of the system, easy to repair and replace, and results least disturbance to the power supply Insulation level - An insulation strength expressed in terms of a withstand voltage To ensure that the probability of insulation breakdown is limited to an acceptable value and that any breakdown is restricted to self-restoring insulation External insulation • Is the distances in open air or across the surfaces of solid insulation in contact with open air that are subjected to dielectric stress and to the effects of the atmosphere • Example transmission line insulators Internal Insulation • Internal insulation is the internal solid, liquid, or gaseous parts of the insulation of equipment that are protected by the equipment enclosures from the effects of the atmosphere • Example transformer and bushing insulation Self-restoring insulation • Insulation that completely recovers insulating properties after a disruptive discharge (flashover) caused by the application of a voltage is called self-restoring insulation • This type of insulation is generally external insulation Non Self-restoring insulation • This is the opposite of self-restoring insulators, insulation that loses insulating properties or does not recover completely after a disruptive discharge caused by the application of a voltage • This type of insulation is generally internal insulation • Voltage Stress: – Magnitude of Surge – Duration of surge – Distribution of stress • Current Stress: – Magnitude of surge - Duration of surge – Length of surge • Dielectric Strength of insulation • Surge protective devices – Device characteristics – Device placement • Cost Selection of the reliability criteria Determination of the electrical stress placed on the equipment or the air clearance Comparison of the insulation strength characteristic, from which a strength is selected If the insulation strength or the clearance is considered to be excessive, then the stress can be reduced by use of ameliorating (improved) measures such as surge arresters, protective gaps, shield wires and closing resistors in the circuit breakers • In a power system various equipment like transformers, circuit breakers, bus supports etc have different breakdown voltages and hence the volt-time characteristics • Thus for proper protection of such equipment, it is therefore required that their insulations be properly coordinated with the insulation of the various protective devices • Conventional Method of Insulation Coordination involves the correlation of the insulation of the various equipment in a power system to the insulation of the protective devices used for the protection of those equipment against overvoltages • The basic concept of insulation coordination is illustrated in Figure (i) showing the desired positions of the volt-time curves of the protecting device and the equipment to be protected • Curve A is the volt-time curve of the protective device and B the volt-time curve of the equipment to be protected • Thus, any insulation having a withstand voltage strength in excess of the insulation strength of curve B is protected by the protective device of curve A • The breakdown voltage for a particular insulation of flashover voltage for a gap is a function of both the magnitude of voltage and the time of application of the voltage • The volt-time curve is a graph showing the relation between the crest flashover voltages and the time to flashover for a series of impulse applications of a given wave shape Figure (i) • Overvoltages are a random phenomenon and it is uneconomical to design plant with such a high degree of safety that they sustain the infrequent ones • It is also known that insulation designed on conventional method basis does not give 100% protection and insulation failure may occur even in well designed plants and, therefore, • It is desired to limit the frequency of insulation failures to the most economical value taking into account equipment cost and service continuity • Insulation coordination, therefore, should be based on evaluation and limitation of the risk of failure than on the prior choice of a safety margin • Therefore the modern practice is to make use of probabilistic concepts and statistical procedures especially for very high voltage equipment • This method is based on knowledge of overvoltage occurrence (the statistical distribution of overvoltages) and flashover probability statistics and not the highest overvoltage possible • Relies on statistical approach which relates directly the electrical stress and the electrical strength • This method requires a knowledge of the distribution of both the anticipated stresses and the electrical strengths of the insulation • Designed based on acceptable risk of flashover • Risk of failure diminishes as the insulation is strengthened • This method is laborious however very useful • Main aim of this method: To quantify the risk of failure of insulation through numerical analysis of the statistical nature of the overvoltage magnitudes and of electrical withstand strength of insulation To coordinate the electrical stresses with electrical strengths the overvoltage distribution is represented in the form of probability density function (Gaussian distribution curve and the insulation breakdown probability by the cumulative distribution function The knowledge of these distributions enables us to determine the ‘risk of failure’ P 1.0 r(V) = P(V) * f(V) (Area under curve) r(V) = risk of failure P(V) = Probability of failure at level V f(V) = Frequency of surge at level V V • Finding a suitable insulation such that the withstand distribution does not overlap with the overvoltage distribution is not practical, • Thus in the statistical method of analysis, the insulation is selected such that the 2% overvoltage probability coincides with the 90% withstand probability CONVENTIONAL STATISTICAL Characteristics/type of the insulation • • The insulation strength of air is usually described by a normal cumulative distribution, so this strength distribution may be convolved with the stress distribution to determine the probability of flashover The insulation strength of transformer insulation is specified by a single value for BIL and BSL hence no statistical distribution of the strength is available and thus, the conventional method is applied System voltage level • Cost of insulation for system of the voltage more than 380 kV is proportional to square of the voltage Critical flashover voltage (CFO) • Recall the statistical BIL or BSL is defined statistically or probabilistically • For every application of an impulse having the standard waveshape and whose crest is equal to the BIL or BSL, the probability of a flashover or failure is 10% • In general, the insulation strength characteristic may be represented by a cumulative Gaussian distribution • The mean of this distribution or characteristic is defined as the critical flashover voltage or CFO • CFO is the voltage level at the condition of the insulation results in a 50% probability of flashover (half the impulse flashover) Critical flashover voltage (CFO) Critical flashover voltage (CFO) Critical flashover voltage (CFO) ... nour@fke.utm.my Tel: 0177696962 Course materials (notes, lecture slides and tutorials) can be downloaded from: http://nour.fke.utm.my/see -44 63 Chapter : Insulation Coordination • When any over voltage appears... called self-restoring insulation • This type of insulation is generally external insulation Non Self-restoring insulation • This is the opposite of self-restoring insulators, insulation that loses... conventional insulation coordination approach seeks the impulse voltage level at which the equipment insulation will not show any disruptive discharge • This approach to insulation coordination