Abstract

Use of power electronic devices such as variable (adjustable) speed drives, uninterruptible power supplies and electronic process control equipment has become wide spread in all types of industries including the oil and gas industry. By virtue of their switching action, the power electronic devices can cause electric power system disturbances ranging from the voltage line notching to the injection of current harmonics. Due to the finite nature (Resistance, Inductance, and Capacitance) of the power system, the injection of current harmonics gives rise to the voltage harmonics. Voltage harmonics in the power system degrade power quality and can cause equipment malfunction, while current harmonics can give rise to telephone interference. Moreover, the current harmonics do not contribute to the net ene11JY transfer, but they add to the net kVA demand which result in an increase in the demand charges. This paper presents a discussion on current harmonics injection, harmonic limit guidelines, and its impact on power quality and kVA demand.

Introduction

There are several advantages associated with the use of electronically controlled speed drives on pumps, fans, compressors, and other equipment that requires either adjustable speed or soft starting. The convenience of low maintenance, lower cost, and high efficiency are some of the virtues that make the use of these controllers imperative in industry[1,2]. The power electronic controllers used in power system for motor control and for regulated power supplies are classified as non linear load because of their switching actions. A non linear load when excited by a sinusoidal voltage of fundamental frequency (60 Hz) demands a distorted current as shown in Figure l(a). On the other hand if it were a linear load the current would be sinusoidal of the same frequency as the applied voltage [Figure 1(b)].

The distorted current of Figure l(a) can mathematically be broken into its constituent frequency sinusoidal components. The frequency components are called harmonics of the current. The term "harmonics' includes the 60 Hz component which is defined as the is order harmonic. However, in this paper, the term harmonics will be used to refer to the higher orders such as 2, 3, 4, 5, ... representing 120 Hz, 180 Hz, 240 Hz, 300 Hz,... components. Due to the symmetry of the negative and positive halves of the AC waveform, even harmonics do not exist in the AC power system. Therefore, in the context of AC power systems, harmonics are defined as the odd multiples of the fundamental frequency (60 Hz).

Consider the three phase bridge circuit shown in Figure 2(a). This circuit, and its variation obtained by replacing the front end rectifying diodes by control devices such as thyristors, is the most common configuration used in many of the power electronic controllers (adjustable speed drives, and uninterruptible power supplies). The same circuit can be U5ed for DC motor drives by replacing the transistor inverter circuit by a DC motor. The circuit shown in Figure 2(a) is also called 6-pulse controller due to the number of rectifying devices used in the front end circuit.

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