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Abstract - This paper discusses the harmonics elimination in non-linear distribution system using a D-STATCOM, which is based on a five-level flying capacitor multilevel inverter with a superconducting magnetic energy storage (SMES) as it’s source. Power system currents are distorted due to the intensive use of power converters/switches and other non-linear loads. Harmonic distortions are the one of the main loss in the power lines, which results in greater heat losses in distribution, interference problems in communication systems etc. The D-STATCOM can provide a better protection against the harmonics problems and load balancing. The building block of a D-STATCOM is a voltage source converter (VSC) consisting of self-commutating valves and also there should be a DC bus/supply. In this proposed work the DC supply is SMES. A quite well-known topology of multilevel inverter is Flying Capacitor Multilevel Inverter, which is adopted in this system. Flying capacitor multilevel inverter (FCMLI) is a multiple voltage level inverter topology intended for high voltage and power operations with low distortion. It uses capacitors, called flying capacitors for clamping the voltage across the power semiconductor devices. A hysteresis current control technique for controlling the injected current by the FCMLI-based DSTATCOM is used in this particular work. This proposed system is analysed and simulated using MATLAB/SIMULINK.
. INTRODUCTION
All non-linear loads draw highly distorted currents from the utility system, with their third harmonics component almost as large as the fundamental. The increasing use of non-linear loads like computer SMPS, variable speed drives and arc furnaces, accompanied by an increase in associated problems concerns both electrical utilities and utility customer alike [1],[3]. STATCOM is a device used to compensate the power quality problems related to the transmission system, also it can be used for the suppressionof current related problems and harmonics. D-STATCOMisa custom power device identical to STATCOM, which is used to compensate
DSTATCOM compensate Power factor correction, current harmonic filtering, load dc offset cancellation, load balancing etc.[6,7].D-STATCOM is an active filtering device which is connected parallel with the harmonic-producing load, therefore D-STATCOM is often referred as shunt or parallel active power filter. It consists of a voltage-source converter which equipped with a dc capacitor which act as storage element, also it can inject current to the distribution system when it is required and also can withdraw current when current through the distribution system is high.Conventional D-STATCOM structures are designed on the basis of simple two-level voltage source converter (VSC) and use transformer to meet the desired voltage. A schematic of the same is shown in figure1. D-STATCOM is the best equipment to solve the power quality problem at distribution systems at cost efficiently
In this proposed system a five level flying capacitor multilevel inverter is used as D-STATCOM with SMES as dc supply. Multilevel inverter is an electrical device which converts DC power supply into an AC power supply. The DC source can come from anywhere like solar energy, wind energy or from any type of storage systems [2]. Dc-link or supply in D-STATCOM must be capable of charging or discharging to compensate for converter losses in the system [4]. During large non-linear conditions, the energy stored in the dc-link capacitor is inadequate to accomplish significant
compensation. To overcome this drawback we use substantial power supplies,soin this proposed system by introducing a new technology, withsuperconducting magnetic energy storage system (SMES) in order to satisfy the energy requirement in an effective manner. The advantages of SMES over other substantial energy supplies are high energy density, fast response, high efficiency, minimum energy loss during the conversion etc.[12].
Flying Capacitor Multilevel Inverter (FCMLI) is an efficient topology in the area of multilevel inverters. A pre charged capacitors are required for this type multilevel inverters. The basic components of FCMLI are diodes, capacitors and switching devices [5]. Although theoretically this topology has been designed to give infinite levels, but due to practical limitations and switching losses, only five levels are analysed.[11]. IGBT/MOSFET are the switching devices used in the multilevel inverters. Every inverter limb consists of cells connected as inward nested series. Every cell has a single capacitor and two power switches. Power switch is a combination of a switch connected with an anti-parallel diode. Unlike diode clamped inverter, this topology uses capacitors for clamping. An inverter with N cell will have 2N switches and N+1 different voltage levels including zero. The number of level depends upon the number of conducting switches in each limb.
II. SMES SOURCED FCML HARMONIC COMPENSATOR
A three phase three wire distribution system is realized by providing three power conductors from substation. As shown in figure 2 the compensator should be shunted with the line and consist of a three-phase inverter in order to compensate for load current. Nonlinear or unbalanced load is connected to the distribution system. For the test system a three phase diode rectifier is connected for representing a non-linear or unbalanced load. In our proposed system Flying Capacitor Multilevel Inverter is employed as D-STATCOM in distribution system. In this system a 5 level FCMLI is sourced with a SMES. The SMES is an advanced technology for the storage and transfer of energy in a reliable manner with a superconducting coil.A cryogenically cooled refrigerator is provided for cooling the coil below it’s critical temperature. The super conducting coil is protected through a protection system as shown in fig 2. A bypass switch is provided for avoiding the wastage of energy from superconducting coil, when the system does not consumes energy. Here the FCMLI is PWM controlled and the compensator is based on the conventional Hysteresis controller.