Faults Analysis is power system...

Faults Analysis:

So far, we have dealt with steady state behavior of power systems under normal operating conditions. This topic is devoted to abnormal system behavior under conditions of faults. Such conditions are caused in the system accidentally through insulation failure of equipment or flashover of lines initiated by a lightning stroke or through accidental faulty operation. In high voltage networks, short circuits are the most frequent type of faults. Short circuits may be solid or may involve an arc impedance. Figure 7.1 illustrates different types of short circuits. The most frequent type of faults are single-phase earth faults, which typically constitute 50 - 80 % of all faults on transmission lines. Number of faults vary from region to region and depends on meteorological conditions, e.g. lightning intensity, and other factors. In Germany and Switzerland faults occur with a frequency of 2 - 5 faults per year and 100 km in the transmission systems. Depending on the location, the type, the duration, and the system grounding short circuits may lead to

 • electromagnetic interference with conductors in the vicinity (disturbance of communication lines),

• stability problems,

• mechanical and thermal stress (i.e. damage of equipment, personal danger)

• danger for personnel The system must be protected against flow of heavy short circuit currents by disconnecting the faulty part of the system by means of circuit breakers operated by protective relaying. The safe disconnection can only be guaranteed if the current does not exceed the capability of the circuit breaker. Therefore, the short circuit currents in the network must be computed and compared with the ratings of the circuit breakers at regular intervals as part of the normal operation planning.

Examples for different types of short circuits.

short circuit currents at network nodes are generally increasing over the years due to

• more generators,

• new lines in existing networks,

 • interconnection of isolated networks to an integrated one.

This is primarily a problem for the expansion planning, where the impacts of long-term changes on the short circuit currents have to be assessed. If the short circuit current exceeds the admissible limit at a network node, the circuit breakers have to be replaced by breakers with higher ratings. Alternatively, the impedance between feeder and fault location can be increased in order to reduce the short circuit current. This is commonly achieved by

 • introducing a higher voltage level while splitting the existing lower voltage network

Development of short circuit currents over the years.

• use of multiple busbars

• fast decoupling of busbars during faults

                Fast busbar decoupling

Since changing the circuit breakers involves very high costs, the proposed means to reduce the short circuit currents are the generally preferred solution. However, this results in a more complex network structure. It also leads to more possibilities to reconfigure the network topology during operation. When a multiple busbar is introduced, for example, a line can be switched from one busbar to another. Switching actions have a significant influence on the short circuit currents, but not all possible topologies can be studied during network planning. Therefore, calculating the short circuit currents has become more and more a problem for the network operation planning. Prior to each switching action all short circuit currents of the new topology must be calculated in order to decide if the switching action may be carried out. This requires computation algorithms that are sufficiently fast for real time applications. The majority of system faults are not three-phase faults but faults involving one line to ground or occasionally two lines to ground. These are unsymmetrical faults requiring special computational methods like symmetrical components. Though symmetrical faults are rare, symmetrical short circuit analysis must be carried out, as this type of fault generally leads to the most severe fault current flow against which the system must be protected. Symmetrical fault analysis is, of course, simpler to carry out. A power network comprises synchronous generators, transformers, lines, and loads. Though the operating conditions at the time of fault are important, the loads can usually be neglected during short circuits, as voltages dip very low so that currents drawn by loads can be neglected in comparison with short circuit currents.

The synchronous generator during short circuit has a characteristic time varying behavior. In the event of a short circuit, the flux per pole undergoes dynamic change with associated transients in damper and field windings. The reactance of the circuit model of the machine changes in the first few cycles from a low sub transient reactance to a higher transient value, finally settling at a still higher synchronous (steady state) value. Depending upon the arc interruption time of the circuit breakers, an appropriate reactance value is used for the circuit model of synchronous generators for the short circuit analysis. In a modern large interconnected power system, heavy currents flowing during a short circuit must be interrupted much before the steady state conditions are established. Furthermore, from the considerations of mechanical forces that act on the circuit breaker components, the maximum current that a breaker has to carry momentarily must also be determined. For selecting a circuit breaker we must, therefore, determine the initial current that flows on occurrence of a short circuit and also the current in the transient that flows at the time of circuit interruption. 

We distinguish between two different approaches to calculate the short circuits in a power system:

 • Calculation of transient currents.

• Calculation of stationary currents First, we will focus on the calculation of transient currents since this will help us to understand the physical phenomena during short circuits. However, for large power systems, the computation of transient currents is not feasible. For this reason simplified techniques for short circuit current computation will be presented that are based on stationary models.