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Faults in Power System and the Role of Protection Schemes
The modem society has come to depend heavily upon continuous and reliable availability of electricity-and a high quality of electricity too. Modern-day usage includes industrial, commercial, domestic, and residential usage. A power system is the largest interconnected machine ever devised by humankind. With an increasing demand for electric supply, power systems grew larger and more complex. Previously only, central power stations such as thermal power plants, hydroelectric power plants, or gas turbine power plants were used to convert some other resources of energy into electric power. However, nowadays, renewable energy resources are used as distributed generators thus increasing the complexity of the power system. Due to its complexity, faults in the power system are inevitable.
With the increase in complexity, the power system protection has also affected negatively and it’s more difficult than ever to completely protect a power system and all of its equipment against all types of faults and abnormal conditions. No power system can be designed in such a way that it would never fail. In the language of protection engineers, these failures are called faults. Power System Protection deals with, how to prevent faults, and how to mitigate the consequences of the faults.
Owning to the importance of power system protection in the system, here is a complete compendium of articles on the fundamentals of power system protection. In this article, we discuss various types of electrical faults, some abnormal conditions, the impact of arcing on the system. I hope that this easy to understand article will be helpful for you in understanding the basics of power system protection.
Types of Power System Faults
Series faults are nothing but a break in the path of current. Normally such faults do not result in catastrophes except when the broken conductor touches other conductors or some grounded part. However, there are some instances where an open circuit can have dangerous consequences, For example, the secondary circuit of a current transformer and the field circuit of a dc machine if open-circuited can have dangerous consequences.
Shunt Faults (Short Circuits)
When the path of the load current is cut short because of the breakdown of insulation, we say that a ‘short circuit’ has occurred. The insulation can break down for a variety of reasons.
Causes of Shunt Faults
Shunt faults are basically due to the failure of insulation. The insulation may fail because of its own weakening, or it may fail due to overvoltage. The weakening of insulation may be due to one or more of the factors such as aging, temperature, weather conditions i.e. rain, hail, snow, etc., chemical pollution, foreign objects, and some other causes. The overvoltage may be either internal (due to switching) or external (due to lightning).
Effects of Shunt Faults
In an isolated power system, the steady-state fault currents would not be much of a concern as they are too small to cause any damage. However, in an interconnected power system all the generators (and even motors) will contribute towards the fault current, thus building up the value of the fault current to a couple of tens of times the normal full-load current. Faults cause heavy currents to flow. If these fault currents persist even for a short time, they will cause extensive damage to the equipment that carries these currents.
Over-currents, in general, cause overheating and attendant danger of fire. Overheating also causes deterioration of the insulation, thus weakening it further. Transformers are known to have suffered mechanical damage to their windings, due to faults.
Impact of Faults in Interconnected Power System
In an interconnected system, there is another dimension to the effect of faults. The generators in an interconnected power system must operate in synchronism at all instants. The electrical power output from an alternator near the fault drops sharply. However, the mechanical power input remains substantially constant at its pre-fault value. This causes the alternator to accelerate. The rotor angle δ starts increasing. Thus, the alternators start swinging with respect to each other. If the swing goes out of control, the alternators will have to be tripped out. Thus, in an interconnected power system, the system stability is at stake. Therefore, the faults need to be isolated as selectively and as speedily as possible.
What is Reclosure Operation?
Such faults due to insulation flashover are many times temporary, i.e. if the arc path is allowed to de-ionize, by interrupting the electrical supply for a sufficient period, then the arc does not re-strike after the supply is restored. This process of interruption followed by intentional re-energization is called ‘reclosure’.
In low-voltage systems, up to three reclosures are attempted, after which the breaker locked out. The repeated attempts at reclosure, at times, help in burning out the object which is causing the breakdown of insulation. In EHV systems, where the damage due to short circuit may be very large and the system stability at stake, only one reclosure is allowed.
A short circuit may be Dead Short Circuit, Completely coupled short circuit of conductors. A fault that bypasses the entire load current through itself, is called a metallic fault or a Partial Short Circuit; a partial short circuit can be modeled as a non-zero resistance (or impedance) in parallel with the intended path of the current. Most of the time, the fault resistance is nothing but the resistance of the arc that is formed as a result of the flashover.
Modelling of Arc Resistance
The arc resistance is highly nonlinear in nature. Early researchers have developed models of the arc resistance. One such widely used model is due to Warrington, which gives the arc resistance as:
Rarc = 8750 (S + 3 ut)/(I^1.4)
Where, S is the spacing in feet, u is the velocity of air in mph, t is the time in seconds and I is the fault current in amperes.
Classification of Shunt Faults in Power System
Phase Faults and Ground Faults in Power System
Those faults, which involve only one of the phase conductors and ground, are called ground faults. Faults involving two or more phase conductors, with or without ground, are called phase faults. Single line to ground faults (L-G) are the most likely whereas the fault due to simultaneous short circuit between all three lines, known as the three-phase fault (L-L-L), is the least likely. Further, the probability of faults on different elements of the power system is different.
The transmission lines which are exposed to the vagaries of the atmosphere are the most likely to be subjected to faults. Indoor equipment is least likely to be subjected to faults. The severity of the fault can be expressed in terms of the magnitude of the fault current and hence it’s potential for causing damage. In the power system, the three-phase fault is the most severe whereas the single line-to-ground fault is the least severe.
Abnormal Operating Conditions
The boundary between the normal and faulty conditions is not crisp. There are certain operating conditions inherent to the operation of the power system which is definitely not normal, but these are not electrical faults either. Some examples are the magnetizing inrush current of a transformer, starting current of an induction motor, and the conditions during power swing.
Protective Relays Operation during Abnormal Operating Conditions
Some examples of abnormal operating conditions are starting currents of motors, inrush currents of transformers, and stable power swings. Magnitude wise, these currents may qualify as faults, but there is no need to provide protection from them. Thus, the protective system must be able to discriminate between the normal operating conditions, abnormal operating conditions, and faults.
Can Protective Relays Prevent Faults in Power System?
It can be seen from the above discussion that protective relays cannot prevent faults. To a certain extent, faults can be prevented by using properly designed and maintained equipment. However, it is not possible to totally prevent the occurrence of faults.
What are Protective Relays Supposed to Do?
The protective relays are supposed to detect the fault with the help of current and voltage transformers, and selectively remove only the faulty part from the rest of the system by tripping an appropriate number of circuit breakers.
I hope you’ve liked this article on the various types of faults in the power system, their causes, arcing phenomena, and the role of power system protection relays. You may also like our detailed article on the working and construction of a power system and Load flow analysis in ETAP software.
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