Harmonic elimination, reactive power compensation and load balancing in three-phase, four-wire electric distribution systems supplying non-linear loads

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Abstract

In this paper, a new control scheme of a three-phase active power filter (APF) is proposed to eliminate harmonics, to compensate reactive power and neutral currents and to remedy system unbalance, in a three-phase four-wire electric power distribution system, with unbalanced non-linear loads. The APF is realized using three single phase IGBT based PWM-VSI bridges with a common dc bus capacitor. A hysteresis rule based carrierless PWM current controller is used to derive gating signals for the IGBTs. Three-phase balanced reference supply currents are estimated using load currents, dc bus voltage and supply voltages. The reference currents of the APF are derived by taking the difference of corresponding reference supply current and load current. The APF forces the compensating currents to conform to the desired reference currents. The APF is found effective to compensate reactive power and neutral current while eliminating harmonics with load balancing.

Introduction

In modern electric power supply distribution systems, there is a sharp rise in the use of single phase and three-phase non-linear loads such as computer power supplies [1], commercial lighting [2], rectifier equipment in telecommunication networks, domestic equipments like TVs, ovens, adjustable speed drivers (ASD) and asynchronous ac–dc links as in wind, and wave electric power generation systems. These non-linear loads generally have solid state control of electric power and draw non-sinusoidal unbalanced currents from ac mains resulting in harmonic injection, reactive power burden, excessive neutral currents and unbalanced loading of ac mains. Further, they cause poor power-factor, low efficiency, neutral conductor bursting and interference with nearby communication networks. Conventionally loss-less L-C filters are used to reduce harmonics and power capacitors are employed to improve the power factor of the ac supply but they have drawbacks of fixed compensation characteristics, resonance and large size. Because of increased pollution of the supply system, this field has attracted the attention of power electronics experts in the last two decades and a number of attempts 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 have been made on the analysis, design and development of equipment generally named as active power filter (APF) to provide a dynamic and adjustable solution to eliminate harmonics and reactive power burden on the ac mains.

Initially APFs were developed using BJTs [3] but nowadays other devices such as GTO, MOSFET, SIT and IGBT are being employed in a number of APF configurations. Large number of APF circuit concepts are reported with shunt 8, 9, 10, 11, series or hybrid configurations 14, 15 generally named as unified power quality conditioners [16]. Many approaches such as notch filter 20, 21, instantaneous reactive power theory [11], synchronous detection method [19], synchronous d–q frame method [14], flux based control [15] and closed loop P–I 14, 15 and sliding mode control 13, 17 are used to improve the performance of the APFs. These approaches have become feasible because of the advent of new microelectronic devices such as DSPs and micro-controllers, and availability of fast and accurate hall effect sensors.

The problem of unbalanced load is well known to power engineers for a long time and thyristor based compensators 5, 6 were developed but they introduced harmonics in the ac mains. In the literature, numerous attempts are reported on individual aspects of three-phase, three-wire APFs 8, 9, 10, 11, 12, 13, 14, 15 and single phase APFs 23, 24, 25, 26, to cancel neutral currents 21, 22 and harmonic elimination of unbalanced non-linear loads 18, 19, 20. It is worthwhile to investigate a new control method of an APF which can compensate harmonics, reactive power burden, load unbalance and neutral currents. This paper presents studies on the new control scheme of such an APF.

Section snippets

System configuration and control scheme

Fig. 1 shows the basic APF scheme including a set of non-linear loads on a three-phase, four-wire electric supply system. The load may be either single phase, two-phase or three-phase and non-linear in nature. In the present case, three single phase uncontrolled diode bridge rectifiers with resistive-capacitive loading are considered as non-linear unbalanced loads on four-wire, three-phase ac mains. This load draws a non-sinusoidal currents from ac mains.

An IGBT based voltage source inverter

Modeling of the APF system

Different components of the system are modeled separately and integrated to develop the overall model for the simulation of steady state and transient behavior.

Performance of APF system

Performance characteristics of the APF system with new control method are shown in Fig. 3Fig. 4 illustrating the steady state and transient behavior. The harmonic spectrum of supply, load and neutral currents for single phase, two-phase and three-phase non-linear loads are also presented.

The essential parameters of the system are given in the Appendix A. From these results, the following observations are made.

Fig. 3 shows the supply, load, neutral and APF currents for a pattern of load

Conclusions

The new control approach for the three-phase APF has been demonstrated to result in sinusoidal, unity power-factor, balanced supply currents. Performance of the APF is observed to be excellent as it leads to reduced harmonics, reactive power burden and neutral currents. The unbalancing caused by unbalanced non-linear loads is also remedied at the supply mains. The supply currents always remain below load currents resulting in increased loading capability of the distribution system. The APF

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