Power Quality Improvement in Three-Phase Four Wire System using a Shunt APF with Predictive Current Control Fahmy, A, Hamad, M. S, Abdelsalam, A K., and Lotfy, A Arab Academy for Science, Technology and Maritime Transport (AASTMT), Alexandria, Egypt Correspondence author email:
[email protected] Abstract- In this paper, a four-wire capacitor midpoint shunt active power filter (APF) with a predictive control technique is used to mitigate both of the supply current harmonics and the neutral current, hence achieving balanced supply current. The proposed strategy provides a simple controller incorporating Phase Locked Loop (PLL) independency, minimized number of sensors, ease of practical implementation, and reduced system size and cost. The proposed system's performance is investigated using a MATLAB simulation model. In addition, a prototype is implemented to experimentally validate the proposed system effectiveness.
Index Terms- power quality, three-phase four-wire system, shunt active power filter, predictive current control, harmonic compensation.
I.
INTRODUCTION
Hinfluence
armonics is one of the power quality issues that to a great extent transformer overheating, rotary machine vibration, voltage quality degradation, destruction of electric power components, and malfunctioning of medical facilities [1]. Power quality improvement has been given considerable attention due to the intensive use of non linear loads and the limitations required by international standards such as IEEE519-1992, IEClOOO-3-2, and IEClOOO-3-4 [2]. Those limitations were set in order to limit the disturbances and avoid major problems in power system. Since linear and/or non-linear single-phase loads are rapidly increasing, zero sequence component and current harmonics are generated. This causes system unbalance and overheating of the associate distribution transformers that may lead to a system failure, especially in weak networks [3]-[5]. The harmonic current can be suppressed by using a passive or active power filter (APF) [6]. Passive filter are used due to their simplicity, ease of maintenance and low cost. However, it has several disadvantages like the risk of series and parallel resonances, system impedance dependency and aging effect of the filter passive components. Generally, APFs sort out the classical problems of passive filters [7], Shunt APF can be used to mitigate both of the line current harmonics and the neutral current in order to improve the system power quality and enhance the grid connection [8] [10], Four-wire shunt APF has been utilized in the three-phase four-wire systems in three main typologies [11-17] namely; (i) Capacitor midpoint/split-capacitor, (ii) Four-pole/four-leg and
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Three single-phase bridge configuration / three full bridge topology. Four-pole type offers better controllability but requires more semiconductor devices than capacitor midpoint, in addition to the need of a complicated controller to control the forth-leg [16]. Three full-bridge topology allows reduced filter-side voltage but uses higher number of semiconductor devices and necessary three single-phase high frequency isolation transformers [17]. Capacitor midpoint configuration is used because it offers lower number of power semiconductor devices and accordingly a simpler associated control strategy. However, the entire neutral current flows through dc-bus capacitors [15]. The APF has two main aspects; harmonics currant extraction and current control technique. Among the harmonics current extraction techniques; the instantaneous reactive power theory [18-19], the synchronous reference frame (SRF) [16], p-q-r theory [20-21] Fast Fourier Transform (FFf) [22], Discrete Fourier Transform [23], Adaptive control algorithm [24], and capacitor voltage control [25-28]. Different current controllers can be used such as hysteresis [29], PI-controller [30], or predictive controller [31-40]. In this paper, a four-wire capacitor midpoint shunt APF with a simple predictive current controller is proposed to mitigate the line current harmonics and the neutral current hence achieving balanced supply current. The proposed technique does not require a Phase Locked Loop (PLL) and utilizes less number of sensors. (iii)
II.
PROPOSED FOUR-WIRE SHUNT
APF WITH
PREDICTIVE CONTROLLER.
This paper focuses on harmonics, neutral current mitigation and system balance in three phase four wire system where the load is non-linear and unbalanced. A three-phase four-wire supply feeds three-phase four-wire non-linear unbalanced load.
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Fig. 1: A three-phase four-wire supply feeding a three-phase four-wire load
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The system block diagram is shown in Fig. 1. This system suffers from supply current harmonics, supply current unbalance, and the current flowing in the neutral wire. To sort out these issues, a four-wire capacitor midpoint shunt APF is connected to the Point of Common Coupling (PCC) between the supply and the load terminals as show in Fig. 2. The APF is controlled with a simple predictive control strategy. The proposed APF control system block diagram is shown in Fig. 3. It requires measurement of the supply voltage and supply current at PCC, in addition to voltage of the APF's two DC-link capacitors. Measurement of the load current and the injected filter current are not required. The APF reference current is extracted using DC-link capacitor voltage control method [31, 36, 41]. The two capacitors' voltages, Vdc1 and Vdc2 0 are subtracted from the reference voltage,
Vd:' A PI-controller acts on the resultant error. The
dc-link voltage is maintained constant and the power balance between the supply, APF, and the load is achieved as the capacitor compensates instantaneously the difference between the supply and the load power [42-43]. The multiplication of the PI-controller output with the PCC per unit voltage forms the supply current reference. No supply voltage harmonics is considered. The measured supply current, PCC voltage and reference current are used to predict the required inverter reference voltage, or the modulating signals, necessary to generate the inverter pulse width modulation (PWM), consequently the switching decision that forces the actual current to track its reference. The relation between filter current, ie' inverter output voltage, Vc , and PCC voltage, vs, is defined in the discrete form by:
v;(k+I)= L{(k+I�-i,(k)J+V,(k) Vc (k 1) (k
(1)
where L; is the)nterfacing inductance, T, is the sampling time, + are the filter reference current and the + 1) and predicted reference output voltage at sampling instant respectively. The filter current at sampling instant k is
Where iL is the load current. Since the sampling instant (k 1) is not available, i; (k + 1) is assumed to be equal to ( k) .
The introduced sampling time delay is less significant if the sampling frequency is increased [44]. The filter reference current is (3) Hence, proposed predicted APF inverter output voltage can be expressed in terms of the reference and actual supply currents by
v;(k+l)= Lf(k);,(k)J+V,(k)
N four-wire supply
(4)
Where the supply reference current
i;(k)in
(4) represents
three-phase balanced quantities. This achieves sinusoidal and balanced supply currents. The load neutral current is given by (5) In addition to compensate supply current harmonics, the APF is controlled to achieve the balance of the phase currents and injects the compensating current, ic", which compensates load neutral current, iLn, resulting in a compensated supply neutral current, is", as illustrated in Fig. 2 and can be represented by (6) Therefore, the predictive control method proposed for the four-wire shunt APF can compensate both of the supply current harmonics and unbalance, thus mitigating the neutral current. This method has several advantages as it provides simple control algorithm with less computational burden, it does not require a PLL, minimizes the number of sensors; (load and filter currents are not measured), provides ease of practical implementation, and reduces both of the system's size and cost.
(k+ 1), III.
PERFORMANCE INvESTIGATION OF THE PROPOSED SYSTEM
(2)
Three-phase
+
Three-phase, fOllfwire, nonlinear, unbalanced load
The proposed system shown in Fig. 2 is simulated using a MATLAB/Simulink model to investigate its performance. The PCC voltage is 380 V. The non-linear load is represented by a three-phase diode rectifier feeding an inductive load acting as a harmonic current producing source. The current unbalance is presented by connecting unbalanced three single-phase loads in parallel to the non-linear load. The resistance and the inductance of the shunt APF coupling inductor, are R; 0.02 Q and L; 2.2 mH respectively. The capacitors' midpoint is connected to the neutral wire through an inductor of LI1 2.2 mHo Two similar DC-link capacitors of 2.2 mF each are used. The reference voltage for this loop is set at 750 V and the inverter switching frequency, Is, is 5 kHz. The simulation results for system before and after compensation are shown in Fig. 4. The three- phase supply voltage waveforms at the PCC are shown in Fig. 4(a). =
=
=
Fig 2: Four-wire capacitor midpoint shunt APF connected to a three-phase four-wire system
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Ln icn Fig. 3: Block diagram for controlling APF using predictive control
Typical non-linear load current, iL, is shown in Fig. 4(b). It is shown that the load current is distorted and unbalanced because of the bridge rectifier loading effect and the parallel unbalanced three single-phase resistive loads. The neutral current, iu" is shown in Fig. 4(c). The filter current, in shown in Fig. 4(d) is injected at the PCC. As a result, sinusoidal and balanced supply current, is> is achieved as shown in Fig. 4(e), The filter current, icl1, shown in Fig. 4(f) is injected. As a result, the supply neutral current as shown in Fig. 4(g). is mitigated
Fig. 5(a) shows the two capacitors dc voltages, Vdc1 and, Vdc2 which are nearly balanced and Fig. 5(b) shows total dc link voltage, Vdc which has almost no variation. The supply current rms value and the current balance are compared before and after compensation as depicted in Fig 6. The balance has improved from 57% to 97%. Also, the supply current total harmonic distortion, (THD), is compared before and after compensation as illustrated in Fig. 7. The APF improves the THD of the line currents from 30.7%, 30.7%, and 17.6 to 3.2%, 3.2%, and 2.9% which comply with the IEEE std. 519-1992 [2]. From the obtained simulation results, the validity of the proposed control system is provided. The four-wire midpoint shunt APF with the introduced predictive current control improves the power quality and the supply current THD by compensating the current harmonics generated by the non-linear load and achieves the balance of the three-phase supply current. Accordingly, it compensates the supply neutral current.
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Fig 5: APF capacitors voltage simulation results: (a) the two-capacitor voltages, Vdc/ andYdc' and (b) total voltage. Vdc
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After compensation Balance 97.19% 8.76
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Fig. 4: Simulation results: (a) supply voltage, v" (b) load current, h, (c) load neutral current, iLn, (d) filter current, i" (e) supply current after compensation, i" (f) filter neutral current, i"" and (g) supply neutral current after compensation, isn.
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RMS values of supply currents, A
Supply current rms value and balance before and after compensation
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