Fault Diagnosis Algorithm and Protection of Electric Power Systems in an Alternative Distribution System

In any power systems, protective devices will detect fault conditions and operate circuit breakers in order to disconnect the load from the fault current and limit loss of service due to failure. This fault may involve one or more phases and the ground, or may occur between two or more phases in a three-phase systems. In ground, fault’ or ‘earth fault, current flows into the earth. In a poly-phase system, a fault may affect each of the three phases equally which is a symmetrical fault. If only some phases are affected, the resulting ‘asymmetrical fault’ becomes more complicated to analyze due to the simplifying assumption of equal current magnitude in all the phases being no longer applicable. Therefore, the prospective short circuit current of the fault can be calculated for power systems analysis procedures. This will assist in the choice of protective devices like circuit breakers, current transformers and relays. This research work evaluated and analyzed the occurrence of faults in a distribution system. Fault currents were obtained and the maximum tripping time required for the protective devices to operate were determined. Hence, it was possible to select appropriate relay and circuit breaker for effective operation of a distribution


Introduction
Fault is said to occur on a power system when abnormally high current flows due to the partial or complete failure of insulation or electrical components at one or more points. The complete failure is called a short circuit. Weedy et al (2012), Carlos (2011) faults in electric power system can be categorized under several sub headings including symmetric or balanced fault affect each of the three phases equally. In transmission lines, the occurrences of faults are symmetric. This is in contrast to an asymmetric fault, where the three phases are not affected equally. In practice, most faults in power system are asymmetric (unbalanced). An asymmetric or unbalanced fault does not affect each of the three phases equally. Common types of asymmetric faults and their causes are as follows: Lineto Line, Lineto -Ground, Double Linesto -Ground Earth or Ground Faults, in three phase systems, is a fault that may involve one or more phases and ground. This type of fault is known as earth or ground fault where current flows into the earth. Transient fault is a fault that is no longer present if power is disconnected for a short time. Many faults in overhead power lines are transient in nature. At the occurrence of this type of fault, power system protection operates to isolate area of fault. A transient fault will then be Fault Current and Impedance of the Distribution Line Lerkervi and Holmes (1997) define electrical fault as an unfavorable event which usually leads to discontinuation of supply of electricity to final consumers in a particular area. Fault causes most power quality and reliability problems (Deshmukh et al, 2013;Azam et al, 2004;De Almeida, 2003). It can also lead to the damage of equipment, burning and destruction of life and properties (Schneider, 2011;Bernstein, 1991;Parise & Parise, 2013). Fault current calculation is a sensitive aspect of electrical design for electrical distribution systems in commercial and industrial installations. The value of fault current obtained determines the highest available current at a given point of fault along a distribution or transmission line (Noe & Steurer, 2007;Ekici, 2012).
In this research work, fault currents in the events of faults were determined, the maximum allowable tripping time for the protective device to trip off were also determined. Hence, overcurrent protection equipment such as instrument transformers, relays, circuit breakers and fuses can then be selected. If a breaker or fuse is not rated to handle the maximum available fault current it might receive, it may not operate normally; its internal parts could even melt or fuse together. Hence, the device may blow up under such destructive fault condition. This will lead to serious injury, and destruction of life and properties. This is the reason why the relay chosen for each protection system must be selected in accordance with the minimum and maximum values of fault current expected. Also, the relay must sense the fault, compare the fault current within the shortest possible time and initiate a trip or disconnection which will make the circuit breaker to open or close the system based on relay and auto re-closer command.

Resistance of Lagos Road Feeder
The length of each of the sections was obtained as shown below Hence, the resistance of the line per kilometer = 0.3025709 ohm.

Inductance of Lagos Road Feeder
The inductance of conductors per phase per meter can be obtained from below equation The incessant electric power supply problems facing the existence of industries in Nigeria is a pointer to the fact that there is great need for fault evaluation and protection of power system in the country. In view of this, a traditional analytical method is developed to access the occurrence of faults and outages along each of the individual consumer point in a feeder, as well as optimizes the performances of the generation, transmission and distribution system. In view of this, it will be possible to clear faults, ensuring adequate protection of the distribution system that is, bringing a steady uninterrupted power supply to consumers. The present study investigates the evaluation of the occurrence of faults on distribution lines, estimates fault currents along a distribution line, determines the maximum tripping time required for the protective devices to operate and establishes a method for selecting appropriate relay and circuit breaker for effective operation of a distribution system before the occurrence of faults and in the events of faults

Methods
The study area is the 132/33 kV, 2 X 60MVA Secondary Transmission Sub Station located in Ikorodu, Lagos State, Nigeria. The power distribution system in the area consists of the 132kV power transmission line grid which has been stepped down via two 132/33 kV, 60 MVA power transformers. The 33kV feeders were used to feed some factories and other industrial loads, while the 33 kV sub-transmission lines were further stepped down through (4)

Analysis and Calculations of Fault Currents
Fault currents were estimated based on the data collected from Ikorodu Electricity Distribution Network along 11KV Lagos Road and other feeders. From these data, calculations and analysis were made and fault currents along the distribution line were obtained. From the results obtained the ratings of the protective devices such as: current transformers, relays, circuit breakers and fuses that would be required for the protection of the distribution line and equipment were obtained. Excel software Algorithm for point of fault and fault current calculation along Lagos Road 11kV feeder will be useful for Power System Engineers in the choice of Protective devices to be installed along the feeder. This is because the protective device must be able to withstand the maximum fault current in the event of fault.

Analysis of the Maximum Permissible Disconnection Time
A 10 2 PVC Mineral insulated copper cables short circuited when connected to a 410 V supply is considered along Arisendo, Lagos Road feeder. The impedance of the short-circuit path is 0.12 Ω. A 100A B-type MCB was installed to protect the system. The maximum permissible disconnection time required for the Miniature Circuit Breaker to meet the requirement was evaluate as shown below: The results in Figure 1. shows that the device will operate maximum permissible disconnection time require. Table 1

Figure 2. Disconnection time (sec) and the Fault Current (Ampere) characteristics II
The characteristics curve in figure 1 and 2 shows that the selected protective device will operate within permissible disconnection time if the fault current is above 20 kA. Therefore, another protective device (Johnson and Philip Fuses, J and P fuses) must be selected for the point of faults (2 to 14 km along the feeder) where fault currents less than 20 kA are expected. Also, a 100 2 PVC Mineral Insulated Copper cables short circuited when connected to an 11 kV  supply along Lagos Road feeder was evaluated. The impedance of the short-circuit path varies with respect to the length. An SF6 Circuit Breaker (CB) was installed to protect the system and the maximum permissible disconnection time required for Circuit Breaker to meet the requirement was evaluate as shown below. The results show that SF6 Circuit Breaker (CB) installed to protect the system against fault current will operate within the maximum permissible disconnection time when the fault current is above 46 kA as shown in figures 4.3 to 4.6 and table 4.3 to 4.4. For fault current greater than 20 kA but less than 46 kA, 100A Btype MCB or Johnson and Philip Fuses (J and P fuses) should be installed. While Johnson and Philip Fuses (J and P fuses) will operate within the permissible disconnection time when the fault current expected is below 20 kA.    This curve shows that the protective device, 100 A, Miniature Circuit Breaker selected will not be suitable for fault current that is less than 20,000 A. This is because the disconnection time is too high. However, it will be suitable for fault current greater than 25,000 A (25 kA)