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1Radial Feeder Protection
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ABSTRACT
Power distribution systems are that portion of electrical systems that connects customers to the source of bulk power (such as distribution substation). Radial distribution systems are characterized by having only one path for power to flow from the source to each customer. A typical distribution system consists of several substations which each includes one or more feeders. A three-phase primary feeder extends away from a substation, and there are many lateral feeders (three-phase, two-phase or single-phase) extending away from the primary feeder. There are loads, transformers, shunt capacitor banks, and protective devices in a distribution feeder.
There are a large number of components in distribution systems and these components age over time. An Case study of applying discrete electromechanical or electronic relays to a radial distribution circuit versus applying digital multifunction protective relays. The study will address engineering time, methods of setting each type of relay, installation time, and periodic maintenance intervals.
The purpose of this paper is to highlight the advantages of having numeric multifunction protection in place when Distributed Generation is added to a radial distribution circuit.
Advantages include extra protection elements (no new feeder relaying required for the addition of DG) and the availability of operating data through local and remote communications.
INTRODUCTION
1.1Radial Feeder Protection
Overhead lines or cables which are used to distribute the load to the customers they interconnect the distribution substations. This is an electrical supply line, either overhead or underground, which runs from the substation, through various paths, ending with the transformers. It is a distribution circuit, usually less than 69,000 volts, which carries power from the substation with the loads.
The modern age has come to depend heavily upon continuous and reliable availability 0f electricity and a high quality of electricity too. Computer and telecommunication networks, railway networks, banking and continuous power industries are a few applications that just cannot function without highly reliable power source. No power system cannot be designed in such a way that they would never fail. So, protection is required for proper working.
Power distribution systems are that portion of electrical systems that connects customers to the source of bulk power (such as distribution substation). Radial distribution systems are characterized by having only one path for power to flow from the source to each customer. A typical distribution system consists of several substations which each includes one or more feeders. A three-phase primary feeder extends away from a substation, and there are many lateral feeders (three-phase, two-phase or single-phase) extending away from the primary feeder. There are loads, transformers, shunt capacitor banks, and protective devices in a distribution feeder. There are a large number of components in distribution systems and these components age over time. Further most distribution systems are overhead systems, which are easily affected by weather, animals, etc.
These two reasons make faults in distribution systems inevitable. To reduce operating cost and outage time, fast and accurate fault location is necessary. The need to analyze protection schemes has resulted in the development of protection coordination programs.
3 Radial Protection
Whole of the power system can be subdivided in to number of radial feeders fed from one end. Generally such radial feeders are protected by over current and earth fault relays used as primary relays for 11 kV and 66 kV lines. For lines of voltage rating beyond 66 kV, distance protection is applied as a primary protection whereas over current and earth fault relays are used as back up relays. A simplified radial feeder network without transformers (in actual practice transformers do exist at substations) is shown in single line diagram of fig. 1.2 below.
5 Power System Protection
Power system protection is the process of making the production, transmission, and consumption of electrical energy as safe as possible from the effects of failures and events that place the power system at risk. It is cost prohibitive to make power systems 100 percent safe or 100 percent reliable. Risk assessments are necessary for determining acceptable levels of danger from injury or cost resulting from damage. Protective relays are electronic or electromechanical devices that are designed to protect equipment and limit injury caused by electrical failures. Unless otherwise noted, the generic term relay will be synonymous with the term protective relay throughout this text. Relays are only one part of power system protection, because protection practices must be designed into all aspects of power system facilities. Protective relays cannot prevent faults; they can only limit the damage caused by faults. A fault is any condition that causes abnormal operation for the power system or equipment serving the power system. Faults include but are not limited to: short- or low-impedance circuits, open circuits, power swings, over voltages, elevated temperature, off-nominal frequency operation.
Power system protection must determine from measurements of currents and/or voltages whether the power system is operating correctly. Three elements are critical for protective relays to be effective: measurements, data processing, and control. Figure 1.2 shows a typical application of relays to a power system. This example system contains a single source that is connected to bus S through a step-up transformer, two transmission lines that connect bus S to bus R, and a load that is connected to bus R through a step-down transformer.
Breakers A through F provide the control to isolate faulted sections of the power system. Breaker F would not be required for this example except that customer-owned generation is becoming more common and a load can change to a source. The current transformers attached to the relays at strategic points in the power system provide the necessary instrumentation for relays to determine the presence of faults. Voltage instrumentation for
RECENT SENARIO
The modern distribution system begins as the primary circuit leaves the sub-station and ends as the secondary service enters the customer's meter socket by way of a service drop. Distribution circuits serve many customers. The voltage used is appropriate for the shorter distance and varies from 2,300 to about 35,000 volts depending on utility standard practice, distance, and load to be served. Distribution circuits are fed from a transformer located in an electrical substation, where the voltage is reduced from the high values used for power transmission. Only large consumers are fed directly from distribution voltages; most utility customers are connected to a transformer, which reduces the distribution voltage to the relatively low voltage used by lighting and interior wiring systems. The transformer may be pole-mounted or set on the ground in a protective enclosure. In rural areas a pole-mount transformer may serve only one customer, but in more built-up areas multiple customers may be connected. In very dense city areas, a secondary network may be formed with many transformers feeding into a common bus at the utilization voltage.
However, multiple connections between the utility ground and customer ground can lead to stray voltage problems; customer piping, swimming pools or other equipment may develop objectionable voltages. These problems may be difficult to resolve since they often originate from places other than the customer's premises.
CONCLUSION AND FUTURE SCOPE
To verify fault location modules and distribution feeder was modeled to generate data such as fault currents, voltages, equipment parameters, protective device settings and phase distribution of line sections.
A modeling method for a distribution feeder protective device has been introduced. The problems encountered during the simulation and the solutions were discussed. A method to simulate fault cases in a batch mode was also presented. The 34-bus test feeder modified to include protective devices was simulated and tested. Two cases were presented to illustrate the modeling method was accurate.
When Distributed Generation is added to your circuit, a savings in initial cost of protective
relaying, engineering design, installation time plus conservation of physical space for mounting equipment and periodic maintenance will be realized if a multifunction protection package with both current and voltage inputs is already in place for feeder protection. The additional protective elements available in these packages will solve the protection issues addressed, regardless of the size or number of DG.s added to your circuit.
With the increasing loads, voltages and short-circuit duty of distribution substation feeders, distribution over current protection has become more important today Power systems that have evolved in the 20th century consist of generation plants, transmission facilities, distribution lines, and customer loads, all connected through complex electrical networks. In the United States, electrical energy is generated and distributed by a combination of private and public utilities that operate in interconnected grids, commonly called power pools, for reliability and marketing. Elsewhere in the world, generation is tied to load through national or privatized grids. Either way, power flows according to electrical network theory.
Interconnection improves the reliability of each pool member utility because loss of generation is usually quickly made up from other utilities. However, interconnection also increases the complexity of power networks. Power pool reliability is a function of the reliability of the transmission in the individual members. Protection security and dependability is significant in determining the reliability of electrical service for both individual utilities and the interconnected power system pool.