A fault in a power system is defined as a condition which adds a disturbance to the proper operation and the stability of the system. In simple words, a fault is referred to any abnormal condition, which has occurred in a power system, has properly operated prior to the abnormal condition.
A fault may occur due to various reasons such as environmental conditions, such as lighting, storms, etc., improper maintenance of the power system; insulation failures etc. They can cause the system instability. Therefore it is very important to study the faults and get necessary actions to overcome such situations.
It is essential to ensure the protection of the power system because a fault may cause severe damages to the expensive equipments that connected to the system, even life hazards. When in design phase of a power system, the designer should pay his attention to the protection scheme of the system, should implement the system as a system which has high degree of protection. The method of fault analyze by the aid of sequence components, is a great tool to the designer, to calculate the fault level at any point of the system prior to the implementation as well as after. The results of the fault study will be helping the designer to select the correct ratings for the protective devices such as breakers and relays.
The faults in power systems can mainly be classified as Series faults and Parallel faults. This classification is done in a broader sense. Series faults are referred as the faults that occur along the transmission line serially such as conductor aging, breaking etc. Here we consider only the Parallel faults. Parallel faults may subdivide in to two categories as Symmetrical faults and Asymmetrical faults, by the appearance of them to the system.
Assumptions made in the experiment
In order to reduce the complexity and for a trouble-free implementation in the DC network analyzer the following assumptions were made.
The pre-fault bus voltage is 1 p.u.
Fault currents are very much higher than the load currents and therefore the load currents were neglected
Line resistance is smaller compared to the reactance therefore it can be neglected.
The bus voltages kept in nominal values, therefore it is considered as the pre fault bus voltage is 1 in per unit basis. It is a valid assumption to neglect load currents compared with fault currents because fault currents are normally hundreds of times than the load currents. Normally the lines are made of good conductor materials to minimize the losses and voltage drops. So there resistance compared with reactance is very much smaller. Hence the third assumption is also a valid one and it also eases the analysis.
Generally, the majority of faults in power systems are asymmetrical in nature. Single Line to Ground, Line to Line, Double lines to Ground are the possible combinations of faults that can occur within the system. When this type of fault occurs, it gives rise to unsymmetrical currents. Unsymmetrical currents have different magnitudes in the three lines with unequal phase displacement.
The asymmetrical faults can be analyzed using symmetrical component method. This method resolves the unbalanced three phase system in to three systems, which have same phase sequence as the power system, opposite sequence to the power system, and independent in sequence to the power system, which known as positive sequence, negative sequence and zero sequence respectively. (Actually this is a theorem which can apply for any n number of unbalanced vector systems, which states that they can resolve in to n-1 number of balanced systems and an independent system to the source).
By using this theorem, we can derive the sequence networks to the unbalanced system. Then we can change the interconnections between the sequence networks to represent any type of fault that occur within the system.The DC network analyser can be used for the simulation purpose by using proportional resistances to represent sequence impedances of generators, transformers and transmission lines. DC power supply represents the generators in this case. For a unsymmetrical fault, three sequence networks can be connected according to the fault the measurements can be taken. For a symmetrical fault equivalent single phase network can be easily implemented using suitable resistances.
Facts, which should consider when deriving, sequence networks
The positive and negative sequence impedances of a line are the same and it is the normal impedance of the line. However the zero sequence impedance is much grater than the positive or negative sequence impedance.
The positive sequence impedance of a synchronous generator is equal to the synchronous impedance of the machine. The negative sequence impedance is much less than the positive sequence impedance. The zero sequence impedance is a variable.
The positive and negative sequence impedances of a transformer are equal and it is the impedance of the transformer. But the zero sequence impedance depends on the earth connection.