27-02-2013, 04:47 PM
three-phase four-wire distribution static compensator
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Abstract
In this paper, a new three-phase four-wire distribution static compensator (DSTATCOM) based on a T-connected transformer and a three-leg voltage source converter (VSC) is proposed for power quality improvement. The T-connected transformer connection mitigates the neutral current and the three-leg VSC compensates harmonic current, reactive power, and balances the load.
Two single-phase transformers are connected in T-configuration for interfacing to a three-phase four-wire power distribution system and the required rating of the VSC is reduced. The insulated gate bipolar transistor (IGBT) based VSC is supported by a capacitor and is controlled for the required compensation of the load current. The dc bus voltage of the VSC is regulated during varying load conditions. The DSTATCOM is tested for power factor correction and voltage regulation along with neutral current compensation, harmonic elimination, and balancing of linear loads as well as nonlinear loads. The performance of the three-phase fourwire DSTATCOM is validated using MATLAB software with its Simulink and power system blockset toolboxes.
Introduction
Three-PHASE four-wire distribution systems are facing severe power quality problems such as poor voltage regulation, high reactive power and harmonics current burden, load unbalancing, excessive neutral current, etc. Three-phase four-wire distribution systems are used in commercial buildings, office buildings, hospitals, etc. Most of the loads in these locations are nonlinear loads and are mostly unbalanced loads in the distribution system. This creates excessive neutral current both of fundamental and harmonic frequency and the neutral conductor gets overloaded. The voltage regulation is also poor in the distribution system due to the unplanned expansion and the installation of different types of loads in the existing distribution system. In order to control the power quality problems, many standards are proposed, such as the IEEE-519 standard. There are mitigation techniques for power quality problems in the distribution system and the group of devices is known by the generic name of custom power devices (CPDs) . The distribution static compensator (DSTATCOM) is a shunt-connected CPD capable of compensating power quality problems in the load current. Some of the topologies of DSTATCOM for threephase four-wire system for the mitigation of neutral current along with power quality compensation in the source current are four-leg voltage source converter (VSC), three single-phase VSCs, three-leg VSC with split capacitors, three-leg VSC with zig-zag transformer, and three-legVSC with neutral terminal at the positive or negative of dc bus . The voltage regulation in the distribution feeder is improved by installing a shunt compensator . There are many control schemes reported in the literature for control of shunt active compensators such as instantaneous reactive power theory, power balance theory, synchronous reference frame theory, symmetrical components based, etc.The synchronous reference frame theory is used for the control of the proposed DSTATCOM.
In this paper, a new topology of DSTATCOM is proposed for a three-phase four-wire distribution system, which is based on three-leg VSC and a T-connected transformer. The T-connected transformer is used in the three-phase distribution system for different applications. But the application of T-connected transformer for neutral current compensation is demonstrated for the first time. Moreover, the T-connected transformer is suitably designed for magnetic motive force (mmf) balance. The T-connected transformer mitigates the neutral current and the three-leg VSC compensates the harmonic current and reactive power, and balances the load. The insulated gate bipolar transistor(IGBT) based VSC is self-supported with a dc bus capacitor and is controlled for the required compensation of the load current. The DSTATCOM is designed and simulated using MATLAB software with its Simulink and power system blockset (PSB) toolboxes for power factor correction and voltage regulation along with neutral current compensation, harmonic elimination, and load balancing with linear loads as well as nonlinear loads.
T-Connected Transformer
Introduction
The Power Partners, Inc. "T-connected," overhead distribution transformer can be used to serve most three phase applica- tions up to 500 kVA. "T-connected" trans- formers consist of two single-phase core/ coil assemblies. This type of connection is also referred to as a "Scott-T" connection.
"T-connected", overhead distribution transformers can be used to serve most three-phase applications. However, Triplex needed for some applications where large motors are the load and these motors are frequently started. Oil field pumping loads and some irrigation pumping loads should use only the triplex designs.
The proposed DSTATCOM consisting of a three-leg VSC and a T-connected transformer, where the T-connected transformer is responsible for neutral current compensation. The windings of the T-connected transformer are designed such that the mmf is balanced properly in the transformer.
A three-leg VSC is used as an active shunt compensator along with a T-connected transformer, as shown in Fig. 2, and this topology has six IGBTs, three ac inductors, and one dc capacitor. The required compensation to be provided by the DSTATCOM decides the rating of the VSC components. The data of DSTATCOM system considered for analysis is shown in the Appendix. The VSC is designed for compensating a reactive power of 12 kvar, as decided from the load details. The selection of interfacing inductor, dc capacitor, and the ripple filter are given in the following sections.
the connection of two single-phase transformers in T-configuration for interfacing with a three-phase four-wire system. The T-connected windings of the transformer not only provide a path for the zero-sequence fundamental current and harmonic currents but also offer a path for the neutral current when connected in shunt at point of common coupling (PCC). Under unbalanced load, the zero-sequence load-neutral current divides equally into three currents and takes a path through the T-connected windings of the transformer. The current rating of the windings is decided by the required neutral current compensation. The voltages across each winding are designed as shown shortly. The phasor diagram shown in Fig. 3(b) gives the following relations to find the turn’s ratio of windings. If Va1 and Vb1 are the voltages across each winding and Va is the resultant voltage.
Advantages
• Easier, more economical and cleaner installations are provided by three phase over head transformers compared to three single-phase units.
• Lighter weight and lower cost provided by "T-connected" design compared to conventional three-phase design.
• Elimination of overloads from system unbalance by operating "T-connected" transformers without primary grounds.