Power Quality Improvement by Using Multipulse AC-DC Converters for Varying Loads

Power electronic devices are non-linear loads that create harmonic distortion and can be susceptible to voltage dips if not adequately protected. The most common economically damaging power quality problem encountered involves the use of variable-speed drives. Variable-speed motor drives or inverters are highly susceptible to voltage dip disturbances and cause particular problems in industrial processes where loss of mechanical synchronism is an issue. Three-Phase Ac–Dc conversion of electric power is widely employed in adjustable-speeds drive (ASDs), uninterruptible power supplies (UPSs), HVDC systems, and utility interfaces with non conventional energy sources such as solar photovoltaic systems (PVs), etc., battery energy storage systems (BESSs), in process technology such as electroplating, welding units, etc., battery charging for electric vehicles, and power supplies for telecommunication systems. Traditionally, ac–dc converters, which are also known as rectifiers, are developed using diodes and thyristors to provide uncontrolled and controlled unidirectional and bidirectional dc power. They have the problems of poor power quality in terms of injected current harmonics, resultant voltage distortion and poor power factor at input ac mains and slowly varying rippled dc output at load end, low efficiency, and large size of ac and dc filters It is well known that undesirable harmonic line currents may be generated during a transformer-rectifier combination. The rectification of AC power to DC power itself may in general produce undesirable current harmonics. These non-linear loads cause severe current harmonics that may not be tolerated by either a shutdown of the device or unacceptable powering of the devices. The great majority of power electronic equipment operates from an ac source but with an intermediate dc link. Thus, a significant opportunity exists to facilitate power electronics applications by using ac to dc rectifiers that produce low harmonic current in the ac source. Multi-pulse converters in general and non-isolated multi-pulse converters in particular can be applied to achieve clean power which is of major interest in higher power ratings. In general, by increasing the number of pulses in multi-pulse converters THD (total harmonic distortion) can be reduced and other associated performance parameters can be enhanced.This project work is an endeavor towards analyzing the different multi-pulse converters in solving the harmonic problem in a three-phase converter system. The effect of increasing the number of pulses on the performance of AC to DC converters is analyzed. Introduction : Objective Of Present Study: This project work is an endeavor towards analyzing the different multi-pulse AC to DC converters in solving the harmonic problem in a three-phase converter system. The effect of increasing the number of pulses on the performance of AC to DC converters is analyzed. For performance comparison the major factor considered is the total harmonic distortion (THD). The effect of load variations on multi-pulse AC to DC converters has been investigated. I. About HVDC transmission The history of electric power transmission reveals that transmission was originally developed with DC. However, DC power at low voltage could not be transmitted over long distances, thus it led to the development of alternating current (AC) electrical systems. Also the availability of transformers and improvement in ac machines led to the greater usage of ac transmission. Why HVDC? There are many different reasons as to why HVDC is to chosen instead of ac transmission. A few of them are listed below. Cost effective HVDC transmission requires only two conductors compared to the three wire ac transmission system. One-third less wire is used, thus readily reducing the cost of the conductors. This corresponds to reduced tower and insulation cost, thereby resulting in cheaper construction. However, the ac converters stations involve high cost for installation; thus the earlier advantage is offset by the increase in cost. If the transmission distance is long, a break-even distance is reached above which total cost of HVDC transmission is less than the ac. Asynchronous tie HVDC transmission has the ability to connect ac systems of different frequencies. Thus it can be used for intercontinental asynchronous ties. 1.1HVDC CONSTRAINTS: Even though HVDC has many advantages, the whole power system cannot be made DC, because of the fact that generation and distribution of power is ac. So HVDC technology is restricted to transmission. 1.2BASIC HVDC SYSTEM CONFIGURATIONS: There are many different configurations of HVDC based on the cost and operational requirements. Five basic configurations are shown in Fig. 1-1. The back-to back interconnection has two converters on the same site and there is no transmission line. This type of connection is generally designed for low ratings and is more economical than the long distance transmission. The converters at both the ends are identical and can be operated either in rectification or inversion mode based on the control. The monopolar link has only one conductor and the return path is through the earth International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.2, pp : 78 84 1 Feb 2014 IJSET @2014 Page 79 Fig. 1.1. Back-to-back interconnection Fig. 1.2. Monopolar link Fig. 1.3. Bipolar link Fig. 1.4. Parallel 3-terminal Fig. 1.5. Series connection 1.3COMPONENTS OF HVDC TRANSMISSION SYSTEM: 1.3.1The converter station: The converter stations at each end are identical and can be operated either as an inverter or rectifier based on the control. This is accomplished in two ways: 1) With phase isolated bus bars where the bus conductors are housed within insulated bus ducts with oil or SF6 as the insulating medium, 2) With wall bushings and these require care to avoid external or internal breakdown [1]. Fig. 1.6. HVDC substation configuration 1.3.2Converter Transformer: The arrangement of the transformer windings depends on the converter configuration. For example the 12-pulse converter configuration can be obtained with any of the following transformer arrangements [2]:  Six single-phase, two winding  Three single-phase, three winding  Two three-phase, two winding 1.3.3 Smoothing Reactors: The main purpose of a smoothing reactor is to reduce the rate of rise of the direct current following disturbances on either side of the converter [2]. 1.3.4AC Filters: Filters are used to control the harmonics in the network. 1.3.5DC Filters: The harmonics created by the convertercancausedisturbancesintelecommunication systems and specially designed DC filters are used in order to reduce the disturbances. International Journal of Scientific Engineering and Technology (ISSN : 2277-1581) Volume No.3 Issue No.2, pp : 78 84 1 Feb 2014 IJSET @2014 Page 80 1.3.6Transmission medium: HVDC cables are generally used for submarine transmission and overheads lines are used for bulk power transmission over the land. 1.3.7HVDC TECHNOLOGY: The fundamental process that occurs in an HVDC is the conversion of electrical current from ac to DC (rectifier) at the transmitting end and from DC to ac at the receiving end. 1.4 SELECTION OF CONVERTER CONFIGURATION: Introduction: A DC system can be operated with constant voltage or with constant current. In this the selection of converter configuration Fig. 1.7. Current converter Fig. 1.8. Voltage converter The voltage can still be regulated using the devices and the current ratio remains unaltered. The converter configuration as shown in fig.1.4, has an impedance Z connected across Again there are three different combinations of voltage and current source converters. a) Voltage source converters on both ends b) Voltage source converter on one end and current source on other end c) Current source converters on both ends. a) Natural Commutated Converters: These are most used in the HVDC systems as of today. The component that enables this conversion process is the thyristor. b) Capacitor Commutated Converters: The capacitors are connected in series between the converter transformer and the thyristor valves. c) Forced Commutated Converters: This type of converters is advantageous in many ways. For the control of active and reactive power, high power quality etc., 1.5 CONVERTER OPERATION: The six-pulse converter bridge shown in the fig.1.5 is used as the basic converter unit of HVDC transmission rectification where electric power flows from the ac side to the DC side and inversion where the power flow is vice versa. Thyristor valves conduct current on receiving a gate pulse in the forward biased mode. The thyristor has unidirectional current conduction control and can be turned off only if the current goes to zero in the reverse bias. This process is known as line commutation. Inadvertent turn-on of a thyristor valve may occur once its conducting current falls to zero when it is reverse biased and the gate pulse is removed. Too rapid an increase in the magnitude of the forward biased voltage will cause the thyristor to inadvertently turn on and conduct [1]. The design of the thyristor valve and converter bridge must ensure such a condition is avoided for useful inverter operation. Fig. 1.9. HVDC operation a) Commutation: Commutation is the process of transfer of current between any two-converter valves with both valves carrying current simultaneously during this process [1]. b) Converter Bridge Angles: The electrical angles, which describe the converter bridge operation, are shown in fig. 1.9. c) Delay angle alpha (α): This angle is controlled by the gate firing pulse and if less than 90 degrees, the converter bridge is a rectifier and if greater than 90 degrees, it is an inverter. This angle is often referred to as the firing angle. d) Advance angle beta (β): The time expressed in electrical angular measure from the starting instant of forward current conduction to the next zero crossing of the idealized sinusoidal commutating voltage. The angle of advance β is related in degrees to the angle of delay ‘α’ by: β = 180-α e) Overlap angle (μ): The duration of commutation between two converter valves expressed in electrical angular measure. f) Extinction angle gamma (γ):Gamma (γ) depends on the angle of advance β and the angle of overlap μ and is determined by the relation γ=β–μ 1.6 Control and Protection: HVDC transmission systems involve (must transport) very large amounts of electric power and the desired power transfer is achieved by precisely controlled DC current and voltage across the system. Also in DC transmission the power-flow direction is determined by the relative voltage magnitudes at the converter terminals which can be controlled by adopting a firing-angle control scheme. Therefore it is very important and necessary to continuously and precisely measure system quantities which include at each converter bridge, the DC current, its DC side voltage the delay angle α and for an inverter, its extinction angle γ. Each converter station is assumed to be provided with constant current and constant extinction-angle controls for equidistant firing angle control.