10-11-2012, 01:23 PM
HIGH-EFFICIENCY VOLTAGE REGULATOR FOR RURAL NETWORKS
HIGH-EFFICIENCY VOLTAGE.doc (Size: 1.55 MB / Downloads: 229)
ABSTRACT
This paper presents a high-efficiency voltage regulator, which combines robustness, low costs and easy maintenance without power electronics components. Power quality is the combination of voltage quality and current quality. Quality of supply is a combination of voltage quality and the non-technical aspects of the interaction from the power network to its customers.
These characteristics make it suitable for rural networks, where investments and operational cost in power quality improvement are limited. The regulator consists of a multi winding reduced-power transformer, and provides serial voltage compensation.
This paper presents a new voltage regulator that fulfills the rural networks needs: high efficiency, robustness, easy maintenance and low cost. Section II presents the design of the voltage regulator, describing its power circuit and control system. Some practical considerations regarding the design of the voltage regulator are presented in Section III. And finally, Section IV presents the operation experience data of voltage regulators installed in the distribution network.
Different voltage compensation steps are obtained by modifying the connection and the polarity between the primary and secondary windings. The transformer design has been optimized to obtain a high-efficiency and low-cost regulator. An automatic controller monitors the output voltage and sets the optimal compensation step. At present more than 400 units of the voltage regulator are in operation.
INTRODUCTION
Long-duration voltage variation (under voltage and overvoltage) is a central issue in distribution network power quality. Supply voltage and power quality are regulated by certain standards, such as the European EN 50160 or the American ANSI C84-1. These standards are complemented in each country or state by specific codes and rules. The European EN 50160 stipulates that the maximum voltage amplitude variation accepted is 10%, while the American ANSI C84-1 defines a normal operating range of 120 V 5%. National rules usually define more restrictive voltage ranges; for instance, the Spanish rule for voltage quality sets the maximum variation of the voltage at the load connection point at 230 V 7%. The value of voltage amplitude is an important quality issue, because loads are designed to work correctly within a specific voltage range. Several problems in domestic and industrial equipment are associated with long duration under voltages, such as malfunctioning in relays and contactors, incandescent lighting dim, switch-off of discharge lighting, failure of nonlinear loads (e.g., computer power supplies), and torque reduction in induction machines.
On the other hand, long duration over voltages usually result in the overheating of loads (motors and transformers), and hence a reduction in their expected durability. Low voltage rural distribution networks compared with urban networks are more susceptible to long-term voltage variations, due to the dispersed configuration of customers. Voltage variations in rural areas are usually associated with long distances between the loads and the distribution transformer. Nowadays, the integration of non-controllable dispersed generation in these networks is a new potential source of voltage variation problems. To minimize long-term voltage variations in rural networks, distribution companies have traditionally performed different actions: 1) tap change control in the main distribution transformer; 2) installation of compensation equipment, such as capacitor banks, voltage regulators, boosters, or auto-boosters; and 3) as a last resort, because it is the most expensive alternative, the distribution company upgrades the low voltage network (increasing the line capability, or changing the network rated voltage). In rural areas, the ratio of contracted power per connection point is much smaller than for urban areas; therefore, investments to solve specific voltage problems are limited.
POWER DISTRIBUTION CONTROL
DISTRIBUTION SYSTEM
Electrical power is transmitted by high voltage transmission lines from sending end substation to receiving end substation. At the receiving end substation, the voltage is stepped down to a lower value (say 66kV or 33kV or 11kV). The secondary transmission system transfers power from this receiving end substation to secondary sub-station. A secondary substation consists of two or more power transformers together with voltage regulating equipments, buses and switchgear. At the secondary substation voltage is stepped down to 11kV. The portion of the power network between a secondary substation and consumers is known as distribution system.
The distribution system can be classified into primary and secondary system. Some large consumers are given high voltage supply from the receiving end substations or secondary substation.
The area served by a secondary substation can be subdivided into a number of sub- areas. Each sub area has its primary and secondary distribution system. The primary distribution system consists of main feeders and laterals. The main feeder runs from the low voltage bus of the secondary substation and acts as the main source of supply to sub- feeders, laterals or direct connected distribution transformers. The lateral is supplied by the main feeder and extends through the load area with connection to distribution transformers. The distribution transformers are located at convenient places in the load area. They may be located in specially constructed enclosures or may be pole mounted.
POWER FLOW
For distribution system the power flow analysis is a very important and fundamental tool. Its results play the major role during the operational stages of any system for its control and economic schedule, as well as during expansion and design stages. The purpose of any load flow analysis is to compute precise steady-state voltages and voltage angles of all buses in the network, the real and reactive power flows into every line and transformer, under the assumption
of known generation and load.
During the second half of the twentieth century, and after the large technological developments in the fields of digital computers and high-level programming languages, many methods for solving the load flow problem have been developed, such as Gauss-Siedel (bus impedance matrix), Newton-Raphson’s (NR) and its decoupled versions. Nowadays, many improvements have been added to all these methods involving assumptions and approximations of the transmission lines and bus data, based on real systems conditions.
MODERN DISTRIBUTION SYSTEM
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. A variety of methods, materials, and equipment are used among the various utility companies, but the end result is similar. First, the energy leaves the sub-station in a primary circuit, usually with all three phases. The most common type of primary is known as a Wye configuration (so named because of the shape of a "Y".) The Wye configuration includes 3 phases (represented by the three outer parts of the "Y") and a neutral (represented by the centre of the "Y".) The neutral is grounded both at the substation and at every power pole. The other type of primary configuration is known as delta. This method is older and less common.
Delta is so named because of the shape of the Greek letter delta, a triangle. Delta has only 3 phases and no neutral. In delta there is only a single voltage, between two phases (phase to phase), while in Wye there are two voltages, between two phases and between a phase and 27 neutral (phase to neutral). Wye primary is safer because if one phase becomes grounded, that is, makes connection to the ground through a person, tree, or other object, it should trip out the circuit breaker tripping similar to a household fused cut-out system. In delta, if a phase makes connection to ground it will continue to function normally. It takes two or three phases to make connection to ground before the fused cut-outs will open the circuit. The voltage for this configuration is usually 4800 volts.
REQUIREMENT OF DISTRIBUTION SYSTEM
A considerable amount of effort is necessary to maintain an electric power supply within the requirements of various types of consumers. Some of the requirements of a good distribution
system are: proper voltage, availability of power on demand, and reliability.
PROPER VOLTAGE
One important requirement of a distribution system is that voltage variations at consumers’ terminals should be as low as possible. The changes in voltage are generally caused due to the variation of load on the system. Low voltage causes loss of revenue, inefficient lighting and possible burning out of motors.
High voltage causes lamps to burn out permanently and may cause failure of other appliances. Therefore, a good distribution system should ensure that the voltage variations at consumers’ terminals are within permissible limits. The statutory limit of voltage variations is +10% of the rated value at the consumers’ terminals. Thus, if the declared voltage is 230 V, then the highest voltage of the consumer should not exceed 244 V while the lowest voltage of the consumer should not be less than 216 V.
AVAILABILITY OF POWER DEMAND
Power must be available to the consumers in any amount that they may require from time to time. For example, motors may be started or shut down, lights may be turned on or off, without advance warning to the electric supply company. As electrical energy cannot be stored, therefore, the distribution system must be capable of supplying load demands of the consumers. This necessitates that operating staff must continuously study load patterns to predict in advance those major load changes that follow the known schedules.
RELIABILITY
Modern industry is almost dependent on electric power for its operation. Homes and office buildings are lighted, heated, cooled and ventilated by electric power. This calls for reliable service. Unfortunately electric power, like everything else that is man-made, can never be absolutely reliable. However, the reliability can be improved to a considerable extent by (a) inter-connected system, (b) reliable automatic control system and © providing additional
reserve facilities.
RADIAL DISTRIBUTION SYSTEM
In this system, separate feeders radiate from a single sub-station and feed the distributors at one end only. Figure (a) shows a single line diagram of a radial system for d.c. Distribution where a feeder OC supplies a distributor AB at point A. Obviously, the distributors are fed at one point only i.e. point A in this case. Figure (b) shows a single line diagram of radial system for a.c. distribution. The radial system is employed only when power is generated at low voltage and the sub-station is located at the centre of load. This is the simplest distribution circuit and has the lowest initial cost.