26-09-2013, 12:27 PM
HIGH-EFFICIENCY VOLTAGE REGULATOR FOR RURAL NETWORKS
HIGH-EFFICIENCY VOLTAGE .docx (Size: 1.93 MB / Downloads: 22)
Abstract
This paper presents a high-efficiency voltage regulator, which combines robustness, low costs and easy maintenance without power electronics components. 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.
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. Field test results are presented to show the operation of the voltage regulator.
INTRODUCTION
LONG-duration voltage variation (undervoltage and over- voltage) is a central issue in distribution network power quality. Supply voltage and power quality are regulated by cer- tain standards, such as the European EN 50160 [1] or the Amer- ican ANSI C84-1 [2]. These standards are complemented in each country or state by specific codes and rules [3]. The Euro- pean 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 [4] 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 un- dervoltages, 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 overvoltages 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 varia- tions 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.
POWER QUALITY
The contemporary container crane industry, like many other industry segments, is often enamored by the bells and whistles, colorful diagnostic displays, high speed performance, and levels of automation that can be achieved. Although these features and their indirectly related computer based enhancements are key issues to an efficient terminal operation, we must not forget the foundation upon which we are building. Power quality is the mortar which bonds the
foundation blocks. Power quality also affects terminal operating economics, crane reliability, our environment, and initial investment in power distribution systems to support new crane installations. To quote the utility company newsletter which accompanied the last monthly issue of my home utility billing: ‘Using electricity wisely is a good environmental and business practice which saves you money, reduces emissions from generating plants, and conserves our
natural resources.’ As we are all aware, container crane performance requirements continue to increase at an astounding rate. Next generation container cranes, already in the bidding process, will require average power demands of 1500 to 2000 kW – almost double the total average
THE BENEFITS OF POWER QUALITY
Power quality in the container terminal environment impacts the economics of the terminal operation, affects reliability of the terminal equipment, and affects other consumers served by the same utility service. Each of these concerns is explored in the following paragraphs.
1. Economic Impact
The economic impact of power quality is the foremost incentive to container terminal operators. Economic impact can be significant and manifest itself in several ways:
a. Power Factor Penalties
Many utility companies invoke penalties for low power factor on monthly billings. There is no industry standard followed by utility companies. Methods of metering and calculating power factor penalties vary from one utility company to the next. Some utility companies actually meter kVAR usage and establish a fixed rate times the number of kVAR-hours consumed. Other utility companies monitor kVAR demands and calculate power factor. If the power factor falls below a fixed limit value over a demand period, a penalty is billed in the form of an adjustment to the peak demand charges.
Equipment Reliability
Poor power quality can affect machine or equipment reliability and reduce the life of components. Harmonics, voltage transients, and voltage system sags and swells are all power quality problems and are all interdependent. Harmonics affect power factor, voltage transients can induce harmonics, the same phenomena which create harmonic current injection in DC SCR Variable speed drives are responsible for poor power factor, and dynamically varying power factor of the same drives can create voltage sags and swells. The effects of harmonic distortion, harmonic currents, and line notch ringing can be mitigated using specially designed filters.
On-and-off control
In an on-and-off controller, thyristors are used to switch on the circuits for a few cycles of voltage and off for certain cycles, thus altering the total RMS voltage value of the output and acting as a high speed AC switch. The rapid switching results in high frequency distortion artifacts which can cause a rise in temperature, and may lead to interference in nearby electronics.[2][4] Such designs are not practical except in low power applications.[6]
Phase angle control
In phase angle control, thyristors are used to halve the voltage cycle during input. By controlling the phase angle or trigger angle, the output RMS voltage of the load can be varied. The thyristor is turned on for every half-cycle and switched off for each remaining half-cycle. The phase angle is the position at which the thyristor is switched on. TRIACs are often used instead of thyristors to perform the same function for better efficiency.[7] If the load is a combination of resistance and inductance, the current cycle lags the voltage cycle, decreasing overall power output.[6]
CONCLUSION
This paper has presented a high-efficiency low voltage regulator specially designed for rural networks operation. The voltage regulator is robust, economical, easily maintained, and has five voltage compensation steps. These characteristics make it suitable for use in rural distribution networks. According to the analysis presented in the paper, the main advantages of the proposed design are the following.