16th International Middle- East Power Systems Conference -MEPCON'2014 Ain Shams University, Cairo, Egypt, December 23 - 25, 2014
Mitigation of Harmonic Distortion in VFD-based Industrial Distribution System with PFC Abdullah Elsawalhy
Ebrahim A. Badran, IEEE Member
Sayed Nagy
Technical Planning Department South Delta Elect. Dist.Co – EGYPT
[email protected]
Electric Engineering Department MansouraUniversity - EGYPT
[email protected]
Electric Power & Machines Department Al-Azhar Univ. – EGYPT
[email protected]
Abstract – Industry developments may cause problems for the electrical power systems. The non-linear loads are considered the main reason of these problems. Harmonics are the major power quality problems in industrial and commercial power systems. This paper presents the harmonic problem in a petrochemical plant in Egypt. This industrial distribution system contains variable frequency drive (VFD) loads. The impact of power factor correction (PFC) capacitors on the harmonic distortion and system resonance is studied. The reduction of the harmonic distortion via multi-stages single tuned filter design is proposed. ETAP is used for the simulation of the investigated test system. The results show a highly reduction in 5th and 7th harmonic orders and the total harmonic distortion. Also, the resonance is eliminated. Index Terms - Harmonic distortion, Industrial Distribution system, Power factor correction, Harmonic filters.
I. INTRODUCTION Power systems are designed to operate at fundamental frequencies. However, certain types of loads produce currents and voltages with frequencies that are integer multiples of the fundamental frequency. These higher frequencies are a form of electrical pollution known as power system harmonics. Power electronic loads control the flow of power by drawing currents only during certain intervals of the 50/60Hz period. Thus, the current drawn by the load is no longer sinusoidal and appears chopped or flattened. The non-sinusoidal current can interact with system impedance to give rise to voltage distortion and, in some cases, resonance [1]. Harmonic pollution has become a serious problem that affects the quality of power in both transmission and distribution of power systems because the proliferation of nonlinear loads. The problems caused by harmonics include malfunctioning of fuses or circuit breaker relays, heating of conductors and motors, insulation degradation, and communication interference [2-4]. With the proliferation of VFDs, electronic device need to be adjusted because amounts of 5th and 7th order harmonic currents are being injected into power distribution system [5].
VFDs may take one of two forms. One form is voltage source inverter drives which use large capacitors to regulate the DC voltage of the inverter [6]. Variable frequency is provided when the inverter chops the DC voltage up to the required waveform. The other form is current source inverters which use a similar principle but with a large inductance instead of a capacitor to regulate the current input to the inverter. Harmonic currents produced by VFDs depend heavily on the type of the drive, the loading upon the drive, and the characteristics of the system supplying the drive [7, 8]. The characteristic current harmonics are produced by the rectifier and are determined by the pulse number h = kq ±1, where h is the harmonic number (integer multiple of the fundamental), k is any positive integer, and q is the pulse number of the converter. This means that a 6-pulse (or 3phase) rectifier will exhibits harmonics at the 5th, 7th, 11th, 13th, 17th, … etc. multiples of the fundamental. VFDs also produce harmonic currents at the output of the inverter which are seen by the motor. Most of these harmonics are integer multiples of the inverter operating frequency and not the power supply frequency, but little generalization can be made about their magnitude since this varies greatly with the type of drive and the switching algorithm for the inverter semiconductors [9]. Therefore, in this paper the harmonic problem in an industrial distribution system which containing VFDs is presented. Also, the impact of power factor correction capacitors on the harmonic distortion and system resonance is disscussed. Furthermore, the reduction of the harmonic distortion via multi-stages single tuned harmonic filter design is proposed. ETAP is used for simulating the investigated industrial test system. II. HARMONIC DISTORTION EVALUATION AND ASSESSEMNT According to IEEE 519 standard, which summarized in Table 1 and Table 2, the customer should be responsible for limiting harmonic currents according to the voltage level. The IEEE limits for voltage and current harmonics dependent on several variables such as point of common coupling (PCC), available short circuit current (ISC), load current (IL) and total demand distortion (TDD) [10]. Table 1 : Current Distortion Limits (% IL) for Distribution Systems [10]
1
11≤h
