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1. Symmetrical Fault

2. Fault Analysis Types of Fault - Open Circuit (OC) Fault - Short Circuit (SC) Fault Occurrence of Fault Generator/Transformer/Bulk Motors/Reactor – 1) Winding Insulation failure – SC, 2) Winding may cut – OC Substation Equipment's – Failure, mal operation, Overload effect, T&D Line – SC on line, OC on Line, Climatic changes, Load Side – SC, Sudden shut down of bulk loads Lightening – Substation/equipment/T&D lines

3. Fault Analysis A fault in a circuit is any failure that interferes the normal system operation. Lighting strokes cause most faults on high voltage transmission lines producing a very high transient that greatly exceeds the rated voltage of the line. This high voltage usually causes flashover between the phases and/or the ground creating an arc. Since the impedance of this new path is usually low, an excessive current may flow. Faults involving ionized current paths are also called transient faults. They usually clear if power is removed from the line for a short time and then restored.

4. Differences in representation of power system for load flow and short circuit studies For load flow studies both the resistances and reactance are considered whereas for fault analysis the resistances are neglected. For load flow studies the bus admittance matrix is useful whereas for short circuit studies bus impedance matrix is used The load flow study is performed to determine the exact voltages and currents whereas in short circuit studies the voltages can be safely assumed as 1 p.u . and the pre-fault current can be neglected

5. Importance of fault studies Fault studies form an important part of power system analysis. The problem consists of determining bus voltages and line currents during various types of faults. The information gained from fault studies are used for proper relay setting and coordination. The three-phase balanced fault information is used to select and set phase relays, while the line-to-ground fault is used for ground relays. Fault studies are also used to obtain the rating of the protective switchgears.

6. BEEE306L: Power Systems Analysis 12 Short circuit MVA or Fault level During a fault very large currents used to flow. In order to minimize the damage caused by the short circuit, the faulty section need to be disconnected. Circuit breakers are used to disconnect the faulty section. The rating of the circuit breaker depends on the Short Circuit MVA which is also known as FAULT LEVEL or FAULT MVA . The circuit breaker breaking capacity must be equal to or greater than the short circuit MVA . The estimation of circuit breaker capacity is made on the basis that it must clear a three phase fault with zero fault impedance as that is generally the worst case. By simulating three phase fault at a point and using sub-transient reactance of the machines short circuit level at that point can be computed as Short Circuit MVA = prefault voltage in p.u . x fault current in p.u . x Base MVA Unless it is given otherwise, prefault voltage shall be taken as 1.0 p.u .

7. BEEE306L: Power Systems Analysis 13 Short-circuit Capacity (SCC) The short-circuit capacity at a bus is a common measure of the strength of a bus. The short-circuit capacity or the short-circuit MVA at bus k is defined as the product of the magnitudes of the rated bus voltage and the fault current. The short-circuit MVA is used for determining the dimension of a bus bar, and the interrupting capacity of a circuit breaker. The interrupting capacity is only one of many ratings of a circuit breaker and should not be confused with the momentary duty of the breaker Based on the above definition, the short-circuit capacity or the short-circuit MVA at bus k is given by - the line-to-line voltage is expressed in kV - expressed in amperes.   - per unit prefault bus voltage - per unit reactance to the point of fault.   The base current is - base MVA - line-to-line base voltage in kV  

8. BEEE306L: Power Systems Analysis 14 Short-circuit Capacity (SCC) The fault current in amperes is If the base voltage is equal to the rated voltage, i.e.,   The prefault bus voltage is usually assumed to be 1.0 per unit, and we therefore obtain the following approximate formula for the short-circuit capacity or the short-circuit MVA.

9. BEEE306L: Power Systems Analysis 15 Short circuit MVA or Fault level

10. BEEE306L: Power Systems Analysis 16 Balanced Three-phase Fault In the analysis of symmetrical three-phase short circuits the following assumptions are generally made. Transformers are represented by their leakage reactance. The magnetizing current, and core fusses are neglected. Resistances, shunt admittances are not considered. Star-delta phase shifts are also neglected. Transmission lines are represented by series reactances . Resistances and shunt admittances are neglected. Synchronous machines are represented by constant voltage sources behind sub-transient reactances . Armature resistances, saliency and saturation are neglected. All non-rotating impedance loads are neglected. Induction motors are represented just as synchronous machines with constant voltage source behind a reactance. Smaller motor loads are generally neglected

11. BEEE306L: Power Systems Analysis 17 Balanced Three-phase Fault The procedure is summarized in the following steps. The prefault bus voltages are obtained from the results of the power flow solution. In order to preserve the linearity feature of the network, loads are converted to constant admittances using the prefault bus voltages. The faulted network is reduced into a Thevenin’s equivalent circuit as viewed from the faulted bus. Applying Thevenin’s theorem, changes in the bus voltages are obtained. Bus voltages during the fault are obtained by superposition of the prefault bus voltages and the changes in the bus voltages computed in the previous step. The currents during the fault in all branches of the network are then obtained.

12. Sym. Fault Analysis Methods to solve Sym. Fault Analysis KVL Method Thevenin’s Method Bus Impedance Matrix Method