New hysteresis control band of an unified power quality conditioner

Electric Power Systems Research 81 (2011) 1743–1753

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Review

New hysteresis control band of an unified power quality conditioner Mekri Fatiha a,∗ , Machmoum Mohamed b , Aït-Ahmed Nadia b a b

IRENav: Institut de Recherche de l’Ecole Navale EA3634-BP 600, 29240 BREST ARMEES, France IREENA: Institut de Recherche en Electrotechnique et Electronique de Nantes Atlantique, Bd. De l’Université, BP 406, 44602 Saint-Nazaire Cedex, France

a r t i c l e

i n f o

Article history: Received 14 September 2010 Received in revised form 14 April 2011 Accepted 4 May 2011 Available online 8 June 2011 Keywords: UPQC Hysteresis control Active filter Power flow Power quality

a b s t r a c t Power quality problems have received a great attention nowadays because of their economical impacts on both utilities and customers. The current harmonics is the most common problem of power quality, while voltage sags is the most severe. This paper deals with an Unified Power Quality Conditioner for current and voltage perturbations compensation in a power distribution network. The topology is based on two 3-phase voltage source inverters acting respectively as a parallel active power filter and a series active power filter which share two DC link capacitors. The power flow, in the Unified Power Quality Conditioner system is analysed. A novel hysteresis current and voltage control methods of Unified Power Quality Conditioner are studied with the aim to have robust control of the output voltage of series part and the output current of the parallel part. The DC voltage controller optimizes the energy storage of the DC capacitor where a fuzzy logic controller is developed. Simulation results are presented and discussed to verify the dynamical behaviour of the Unified Power Quality Conditioner and to show the effectiveness of the used perturbation identification methods and hysteresis band adaptive controllers. © 2011 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743 Power flow analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1744 Shunt active power filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1746 3.1. Current reference calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1746 3.2. DC voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1746 3.3. Hysteresis band current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1748 Series active power filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1749 4.1. Voltage reference calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1749 4.2. Hysteresis band voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1749 Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1752 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1752 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1753

1. Introduction Due to the extensive use of power electronic based equipments/loads almost in all areas, the Point of Common Coupling (PCC) could be highly distorted [1]. Also, the switching ON/OFF of high rated load connected to PCC may result into voltage sags or swells on the PCC. There are several sensitive loads, such as computer or microprocessor based AC/DC drive controller, with good voltage profile requirement, which can function improperly

∗ Corresponding author. E-mail address: [email protected] (M. Fatiha). 0378-7796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2011.05.003

or sometimes can lose valuable data or in certain cases get damaged due to these voltage sags and swells conditions. One of the effective approaches to improve power quality is to use an Unified Power Quality Conditioner (UPQC) to protect the sensitive loads. An UPQC is a combination of shunt and series Active Power Filters (APFs), sharing a common DC link [1,2]. It is a versatile device that can compensate almost all types of voltage perturbations such as voltage harmonics, unbalance, flickers, sags and swells, or current perturbations such as harmonics, current unbalance and reactive current, etc. A typical configuration of an UPQC is shown in Fig. 1. Different topologies and control strategies of the UPQC have been proposed in the past based on PWM technique. These studies are detailed and compared in [3]. The common goal of all these

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Fig. 1. Structure of unified power quality conditioner.

strategies is to improve the dynamic response of the controller in order to obtain better compensation. Most of them use conventional linear regulators [2,4,5]. Those controllers cannot provide a correct tracking of the reference, in natural or d-q frame, especially when the dynamic of current and voltage references increase. Then, it is necessary, to insure a good tracking, to use non-linear control strategies. Two control schemes models for UPQC are presented in [6]. The two control schemes have common control current control strategy, which is based on hysteresis control. The series voltage compensation is based on d-q analysis. Adaptive harmonic detection method using linear neurons is proposed in [7]. The control strategy combines hysteresis control and the one cycle current control. Fuzzy logic controller and neural network controller are also developed respectively in [8–10]. Those methods give better performance but are complex, and require significant computing time. Control strategy has a vital role in the overall performance of the power conditioner (UPQC). Rapid detection of signal disturbance with high accuracy, fast processing of the signal reference, and high dynamic response of the controllers DC voltage control, output voltage control of the series part and current control of the shunt active filter part are requirements for desired compensation. This paper is focused on voltage sags and swells along with current and voltage harmonics compensation based on adaptive hysteresis band control. Initially, the equivalent circuit of the UPQC is presented. The steady state power flow analysis is discussed in Section 2. The control algorithms for both shunt and series APFs parts are detailed, respectively in Sections 3 and 4 [4,11,12]. The UPQC performances will depend on the design of power semiconductor devices, on the modulation technique used to control the switches, on the design of coupling elements (the decoupling inductance Lf for shunt part, the filter parameters Lfs − Cfs for series part and the DC link capacitor value Cdc ), on the method used to

determine active filters current and voltage references and on the dynamics and robustness of current and voltage control loops. The standard instantaneous p-q algorithm is used to determine the current references for shunt APF, and a robust three-phase digital locked loop PLL system is used to determine the voltage references for series APF in order to eliminate various perturbations existing in the supply network [5,13]. Various Pulse-Width Modulation (PWM) techniques can be used to control active power filters. The classical fixed hysteresis band control does not need any information about system parameters and has the disadvantage of uncontrolled frequency. As a result, the switching losses are increased and source currents (or voltages) contain a large amount of ripples. The proposed current/voltage controller performances can be improved by using adaptive control system theory [11,5,14]. A new technique, based on the same concept but where the hysteresis band is variable, is implemented to maintain the modulation frequency quasi constant. For the DC voltage, fuzzy logic controller is studied, to limit the harmonics distortion ratio of the source current after compensation or to minimize settling time and DC capacitor variations under transient conditions. Then, an appropriate voltage controller must be synthesized to assure high dynamic performances with reduced design of the capacitor. Simulation results will be shown and discussed in the last section, to verify the performances and the efficiency of the proposed UPQC such as voltage and current harmonics under steady-state, transient and several load asymmetrical conditions. 2. Power flow analysis Let us consider the UPQC system shown in Fig. 1, and studied in [2,15–17]. The series active filter and parallel active filter can control the load voltage and the source current respectively. The series APF acts as a controlled voltage source, compensates voltage supply

M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753

Fig. 2. Equivalent circuit of an UPQC.

Fig. 4. Active power flow during voltage sags conditions.

disturbances (including harmonics, imbalances, sags and flickers) and performs harmonics isolation through a series transformer (Lt , Rt ). In such a way, the voltage at load bus is always sinusoidal and at desired magnitude and the sensitive loads are protected. The shunt APF operates like a current source. It allows maintaining the DC link voltage at constant level. In addition to this, the shunt APF compensates current harmonics, provides the var required by the load, such that the input power factor will be unity and only fundamental active power will be supplied by the source [18,19]. The single phase equivalent circuit for an UPQC is shown in Fig. 2. The source voltage, terminal voltage at the Power Common Coupling (PCC) and load voltage are denoted by es , vs and vch , respectively. The source and load currents are denoted by is and ich , respectively. The voltage injected by series APF is denoted by vc and the current injected by shunt APF is denoted by if . By taking the load voltage, vch, as a reference phasor and suppose the lagging power factor of the load is cos ϕn and k is the factor representing the fluctuation of source voltage, then the voltage injected by series APF is defined as:

vch = Vch ∠0◦

⎫ ⎪ ⎪ ⎬

ich = Ich ∠ − ϕn vc = vch − vs = −kVch ∠0◦ ⎪ ⎪ Vs − Vch ⎭ k= Vch

(1)

The UPQC is assumed to be lossless. The active power demanded by the load is equal to the active power input at the PCC. Then, the input active power at PCC can be expressed by the following equations:

⎫

Ps = Pch ⎪ ⎪ ⎬ Vs Is = Vch Ich cos ϕn Vch (1 + k)Is = Vch Ich cos ϕn ⎪ ⎪ I ⎭ Is = ch cos ϕn 1+k

(2)

The apparent power absorbed by the series APF can be expressed as: Sc = Vc Is Pc = Vc Is cos ϕs = −kVch Is cos ϕs Qc = Vc Is sin ϕs



Fig. 3. Reactive power flow.

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(3)

Considering that the UPQC maintains the source power-factor unity (s = 0), then we have: Pc = Vc Is = −kVch Is Qc ∼ =0



(4)

The apparent power absorbed by the shunt APF can be expressed as: Sf = Vch If

(5)

The current if is the difference between the input source current and the load current, which includes the load harmonics current and the reactive current. For example: • Case 1: When the UPQC is not connected to the network, the reactive power required by the load is completely supplied only by the source. When the UPQC is connected to the network and the shunt APF becomes operational, the reactive power required by the load is now provided by the shunt APF alone as shown in Fig. 3, such that no reactive power burden is put on the mains. So as long as the shunt APF is ON, it is handling all the reactive power even during voltage sags, voltage swells and voltage harmonics compensation. • Case 2: Sag; k < 0, Vs < Vch , considering Eqs. (1) and (4), the power of series active filter will be positive. The series APF supplies the active power to the load. This condition is possible during the utility voltage sags conditions. Thus, we can say that the required active power is taken from the utility itself, by taking more current, so as to maintain the power balance in the network and to keep the DC link voltage at desired level. The active power flow during voltage sags conditions is shown in Fig. 4. Ps represents the power supplied by the source to the load during voltage sags conditions, Pc the power injected by series APF, in such way that the sum Pc + Ps will be the required load power during normal working condition and Pf is the power absorbed by shunt APF during voltage sags conditions (Pf = Pc ). • Case 3: Swell; k > 0, Vs > Vch , considering Eqs. (1) and (4), the power of series active filter will be negative. The series APF will absorb the real power from the source. This is possible during the voltage swells conditions. The source current will be less than the normal rated current. Since the supply voltage is increased, the DC link voltage can increase. To maintain the DC link voltage at constant level, the shunt APF controller reduces the current drawn from the supply. The active power flow during voltage swells condition is shown in Fig. 5.Ps represents the power supplied by the source to the load during voltage swells conditions, Pc the power absorbed by series APF, in such way that the difference Ps − Pc will be the required load power during normal working condition and Pf is the power delivered by shunt APF during voltage swells conditions Pf = Pc .

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with = Pch + p˜ ch (t) pch (t) P6h =

2 2 I6h+1 + I6h−1 − 2I6h−1 I6h+1 cos(ϕ6h+1 − ϕ6h−1 )

tgϕ6h =

(I6h−1 sin ϕ6h−1 − I6h+1 sin ϕ6h+1 ) (I6h−1 cos ϕ6h−1 − I6h+1 cos ϕ6h+1 )

⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭

(6.b)

The components Pch and p˜ ch (t) represent respectively the average and alternating parts of instantaneous powers of nonlinear load. The voltage fluctuation of the dc capacitor due to the lowest load harmonic current components of orders 5 and 7, is obtained by:

v˜ dc (t) =

Fig. 5. Active power flow during voltage swells conditions.

v˜ dc (t) ∼ = 3. Shunt active power filter The wide use of non-linear loads, such as static power converters, is at the origin of harmonics pollution problems. These loads draw non-sinusoidal currents that cause harmonic voltage drops across the network impedance, resulting in distorted voltages. The shunt active power filters, generally based on a voltage source inverter structure, seems an attractive solution to overcome current harmonics pollution problems. It can be used to compensate unbalanced currents, current harmonics and reactive power. The main currents, obtained after compensation, are then sinusoidal and in phase with the supply voltages. The general structure of the shunt active power filter part under study is presented in Fig. 6.

1 Cdc Vdc

t ∞ 0

P6h cos(6hωt − ϕ6h ) dt

(7)

h=1

P6 sin(6ωt − ϕ6 ) 6ωCdc Vdc

(8)

In steady state, the active power provided by the supply must be equal to the power required by the load. When a variation of active power occurs in the system, because of load changing or voltage dips, the DC bus must provide the power difference between the supply and the load. This power is given by: Pf = Pch − Ps = 3Vs (Ich1 cos ϕch1 − Is ) = Vs I0

(9)

Ich1 is the fundamental current of the load and ϕch1 its phase angle. We note that for nonlinear load, for as first instance the load current can be written as follows: ich =

∞  √

In 2 sin(n(ωt) − ϕn ),

(n = 6h + 1)

(10)

n=1

3.1. Current reference calculation The function of shunt APF is to maintain the DC link voltage constant and to compensate current harmonics and fundamental reactive power due to non-linear loads. The standard p-q instantaneous power algorithm is used to determine current references as shown in Fig. 7. We have shown in [5] that the input voltage system must be sinusoidal and balanced otherwise the method of instantaneous power is not applicable in its standard form. However, the supply voltage is often unbalanced and/or distorted. To overcome this problem, a robust PLL system can be used to extract the fundamental positive sequence voltage components [5,20]. The obtained system is then used as input of the p-q algorithm for instantaneous current references extraction.

3.2. DC voltage control In the UPQC the management of dc bus concerns the role of the shunt APF. This one determines the active power necessary to keep constant the DC voltage, in steady or transient conditions. There are three principal factors which determine the fluctuations of DC voltage. The first is the alternating power created by the harmonic components of the nonlinear load current, the second is the imbalance of active power during transient state, and the third is the active power absorbed by series APF to compensate the voltage sags. The alternating power absorbed by the nonlinear load (˜pch (t)) in the case of three-phase load rectifier, detailed in [11,12], is expressed by:

p˜ ch (t) =

∞  h=1

P6h cos(6hωt − ϕ6h )

(6.a)

Pch is the DC component consumed by the nonlinear load, Ps is the active power provided by the supply, and Pf is the active power exchanged with the supply to restore the DC voltage vdc (t) at constant value, and I0 component is given by: I0 = |Ich1 cos ϕch1 − Is |

(11.a)

I0 will be determined by the voltage regulator of the DC bus. It represents the amplitude of the fundamental active current i0 (t), necessary to ensure the balance of the active power. This current is added to the reference currents of the shunt APF. i0 (t), must be sinusoidal and in phase with the supply voltage, and it consists of two fundamental components: i0 (t) = I0s sin(ωt) + I0t sin(ωt)

(11.b)

The component I0s sin(ωt) corresponds to the active power consumed by the series APF during the voltage sags (swells), and I0t sin(ωt) is referred to the power losses consumed by the power transistors in both inverters. The resulting shunt APF control block is shown in Fig. 8, using p-q algorithm for identification of the reference currents. If switching losses are neglected, the real power absorbed by DC bus can be expressed as: dv2 (t) 1 Pf = − Cdc dc 2 dt

(12)

If a Laplace transformation is made, the model for the real power analysis and the DC voltage control scheme can easily be deduced as shown in Fig. 9. The aim of the synthesized voltage regulator is to adjust DC voltage to its reference, to reject the internal disturbance dI of the system due to the variation of real power and to assure a good filtering of the external disturbance dE, relating to DC voltage ripples due to the alternating power. To realize these objectives, a PI regulator is considered, described in [11]. The cut off frequency of the PI regulator will influence its performances under transient load conditions: time response tr , DC

M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753

1747

Fig. 6. Shunt active power filter.

Fig. 7. Block diagram of implementing p-q algorithm.

Fig. 9. Close loop DC voltage control using PI regulator. Fig. 8. Block diagram of shunt APF control.

voltage variations Vdc under the step change of the nonlinear load current and the Total Harmonic Distortion, THD, of the mains currents. A trade off must consequently be found between two criterions: to limit the THD of the source current after compensation or to minimize Vdc and tr . It is obvious that the bandwidth of this external loop must be low, enough for minimal distortion introduced. For this scope, fuzzy regulator is used [11]. In our application, the fuzzy controller is based on processing the voltage error (E) and ˙ as shown in Fig. 10. We note: its derivate E, 2 2 E = Vdcref − Vdc

(13)

The determination of the membership functions depends on the designer’s experiences and experts’ knowledge. It is not trivial to choose a particular shape that is better than others. Triangle shaped membership function has the advantages of simplicity and easier implementation and is chosen in this application. Fig. 11 shows the membership functions of the input and the output linguistic variables [21–23]. In the design of a fuzzy control system, the formulation of its rule set plays a key role in improvement of system performances. The rule table is constructed to contain the 49 rules as shown in Table 1, where linguistic codes are: LP: large positive; MP: medium positive; SP: small positive; ZE: zero; LN: large negative; MN: medium negative; SN: small negative.

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Fig. 10. Close loop of DC voltage control with fuzzy regulator.

˙ and output variables. Fig. 11. Membership functions for input variables (E, E)

Table 1 Inference rules. ˙ E/E LN MN SN ZE SP MP LP

LN LN LN LN LN MN SN ZE

MN LN LN LN MN SN ZE SP

SN LN LN MN SN ZE SP MP

ZE LN MN SN ZE SP MP LP

SP MN SN ZE SP MP LP LP

MP SN ZE SP MP LP LP LP

LP ZE SP MP LP LP LP LP

Fig. 12. Typical PWM phase voltage and current waves in modulation cycle.

By adding the total current: 2HB = Various inference mechanisms have been developed to defuzzify the fuzzy rules. In this paper, we applied max–min inference method to get implied fuzzy set of the turning rules. The imprecise fuzzy control action generated from the inference engine must be transformed to a precise control action in real applications. The center of mass method is used to defuzzify the implied fuzzy control variables.



HB

with a = 0, 1/3 or 2/3.



n

1 (aVdc − vs (t)) Lf

di∗ (t) 1 vs (t) + s Lf dt

− t1n

+

Lf



dis∗ (t) dt

1

t aVdc n 1n

⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭ (15)

The general expression of incremental current fall during T1  ON is given by: di∗ (t) t2n (aVdc − vs (t)) − t2n s Lf dt

Then, HB is:

(14)



t1n

= t1

3.3. Hysteresis band current control

di∗ (t) t1n (aVdc − vs (t)) − t1n s HB = Lf dt



n

−HB =

The fixed hysteresis current control band is very simple and easy to implement, but has the disadvantage of an uncontrollable high switching frequency. On the other hand, the hysteresis current control adaptive band allows operating at a fixed switching frequency and is usually performed by software using the system parameters. In this case, the operating conditions must be known to meet sufficient and accurate control. To determine adaptive hysteresis band, as shown in [24,25], we based on typical PWM phase voltage and current waves during a modulation cycle as shown in Fig. 12. It shows that the increment of current around its reference for a period modulation, the voltage vs is considered positive and is constant, switches T1 and T1 close respectively for periods t1 and t2 . The general expression of incremental current rise HB during T1 ON, is given by:

=



−2HB = −t2

1 di∗ (t) vs (t) + s Lf dt

−

1 Lf

(16)



(t2n aVdc )

(17)

n

The average currents, increase and decrease during the period t1 and t2 respectively, is shown by the weighted average supply voltage. The second term of Eqs. (15) and (17) can be expressed as follows:

 n 

t1n aVdc = t1 a Vdc

⎫ ⎪ ⎬ (18)

t2n aVdc = t2 a Vdc ⎪ ⎭

n

with a =

 t1n a n

t1

and

a =

 t2n a n

t2

a and a represent the voltage coefficients and are the average values of a in the two time intervals. For computational simplicity,

M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753

1749

where Ih (s) is the load current harmonic and s is the Laplace operator. According to Eqs. (20) and (21), the error signal to the input of the regulator has the same dynamics whatever the variable to track (if or is ). The controller is the same for both references. We just have to reverse the command, to switch control mode. In this work, we chose the command of the source current (is ). Fig. 13 shows the block diagram of the adaptive hysteresis band current control using Eq. (19). The Hysteresis Band (HB) can be modulated at different points of fundamental frequency of the cycle, to control the PWM switching pattern of the inverter. For symmetrical operation of all the three phases, it is expected that the band profiles, HB1 , HB2, HB3 , will be identical but the phases will be displaced.

Fig. 13. Simplified model for an adaptive current control.

4. Series active power filter The main function of a series active power filter is the protection of sensitive loads from supply voltage perturbations such as sags or voltage harmonics.

Fig. 14. Simplified model of series active filter.

4.1. Voltage reference calculation There are various methods of the determination of voltage references for series active power filters [5,18]. The used method is based on a robust PLL system and is able to detect quickly any voltage drop due to dips or flickers besides voltage harmonics in the network. The synthesized regulator must answer the following objectives: the control of the direct voltage component vsd must be carried out without bias and with a fast dynamics, the predominant undesirable frequencies present on the direct voltage component vsd under unbalanced mode (100 Hz) or distorted conditions (300 Hz and 600 Hz) must be filtered by the regulator. For this purpose, several regulators are studied and compared to satisfy these constraints. In our study, the determination of voltage references for series active power filter is based on a robust three-phase digital locked loop (PLL) system using RST regulator. This regulator has negligible oscillations and ensures a fast locking of the PLL as presented in [5,26].

Fig. 15. Simplified model for fixed hysteresis-band control.

we take a = a . By combining Eqs. (15), (17) and (18) the hysteresis band is given by: a Vdc HB = 4fm Lf



1−



Lf2 2 a 2 Vdc

di∗ (t) 1 vs (t) + s Lf dt

2 

(19)

where fm is the switching frequency, is∗ is the source reference current and dis∗ /dt represents its slope. We can note that the load current can be expressed by the reference currents and the measured currents, as follows: I(s) = Ich1 (s) + Ih (s) = Is (s) + If (s)

(20)

4.2. Hysteresis band voltage control

(21)

The series APF is realized with a new control strategy related to hysteresis control method for the voltage inverter. The control section must be able to derive the reference voltage waveforms

Therefore: Ich1 (s) − Is (s) = −(Ih (s) − If (s)) = ε = error

500

500

vref, vc(V)

vref, vc(V)

0

0

-500

e

-500 0

0.02

0.04

0.06

0.08

0.1

0.12

e

200

0

0.02

0.04

0.06

0.08

0.1

0

0.02

0.04

0.06

0.08

0.1

0.12

0 -50

0

-100 -200 -400

-150 0

0.02

0.04

0.06

0.08

a) with fixed hysteresis band control

0.1

0.12

t(s)

0.12

t(s)

b) with adaptive hysteresis band control

Fig. 16. Output voltage of the series APF and voltage error e(t).

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M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753 70

with matching ripples contained in the line voltage. To establish the series APF voltage controller, the simplified equivalent diagram presented in Fig. 14 is first deduced and the system is modelled by the following equations:

50

Vc (V)

dvc (t) I + Ich = Cfs dt

60

(22)



30



20

d2 vc (t) dvc (t) dI (t) Vt (t) = vc (t) + Lfs Cfs + Rfs Cfs − Lfs ch + Rfs Ich (t) dt dt dt 2 (23)

10 0

0

1000

1.5

2

0.03

is

0

0.04

0.05

3

3.5

*

is

-500

-1000 0.02

2.5

is

500

-500

-1000 0.02

0.06

0.03

0.04

0.05

0.03

0.04

0.05

0.06

100

100

δ

1

50

50

δ

0

0 -50

-50

-100

-100 0.02

0.03

0.04

0.05

t(s)

0.06

0.02

0.06

t(s)

a) with fixed hysteresis band control

b) with adaptive hysteresis band control

Fig. 18. Source current and current error ı(t).

v s(V)

500 0 -500 500

v ch (V)

0 -500 200

v c(V)

0 -200 1000

ich (A)

is(A)

v dc (V)

0 -1000 1000 0 -1000 750 700 650 0.04

0.05

0.06

0.07

0.08

4

4.5

5

x 104

to maintain the mean value of the control state at the required value. The instantaneous value of the output voltage is compared with the reference voltage, when the sensed output signal deviates from the reference, by more than a prescribed value, the inverter is operated to reduce the deviation. This means that the

*

is

0.5

Fig. 17. Spectrum of the output filter voltage control.

• Case 1: Fixed hysteresis. The principal diagram of classical fixed hysteresis band control is shown in Fig. 15. The hysteresis value band governs the inverter switching pattern in such a manner as

500

0

f(Hz)

Vt is the output inverter voltage, vc is the output voltage of the series active filter, Vdc is the dc bus voltage and Ich is the load current witch can be considered as a disturbance. To understand the principle of the new Hysteresis Band control HB, we will treat 2 cases, as follows:

1000

40

0.09

0.1

0.11

0.12

0.13

0.14

t(s) Fig. 19. Performances of the UPQC for voltage sags compensation with nonlinear load.

M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753

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500 0

vs(V)

-500 500

vch(V)

0 -500

vc(V)

ich(A)

200 0 -200 1000 0 -1000 1000 0 -1000

is(A)

750

vdc(V)

700 650 0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.14

t(s) Fig. 20. Performances of the UPQC for voltage sags and harmonics compensation.

switching occurs whenever the output voltage crosses the value of HB. The output voltage signal of the series APF is given by:

vc = Vref + HB in rising case vc = Vref − HB in decreasing case



(24)

Thus the output wave is restricted to flow within a channel of width (HB) that follows the reference wave. The frequency of inverter depends of the width of the hysteresis band. The output voltage will contain significant ripples. To solve this problem, it is necessary to improve the hysteresis control.

500 0

vs(V)

-500 0.1 500

vch(V)

0.15

0.2

0.25

0.3

0.35

0.15

0.2

0.25

0.3

0.35

0.15

0.2

0.25

0.3

0.35

0.15

0.2

0.25

0.3

0.35

0.15

0.2

0.25

0.3

0.35

0.15

0.2

0.25

0.3

0 -500 0.1 100 0

vc(V)

-100 0.1 1000

ich(A)

0

-1000 0.1 1000

is(A)

0 -1000 0.1

vdc(V)

710 700 690 0.1

Fig. 21. Compensation of flicker phenomenon by the UPQC.

t(s)

0.35

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M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753

• Case 2: An adaptive hysteresis band. The fixed hysteresis band is very simple and easy to implement, but it has the disadvantage of an uncontrollable high switching frequency. An adaptive hysteresis band method allows operating at nearly constant frequency and is usually performed by software using the system parameters. An adaptive hysteresis band voltage control PWM technique can be programmed as a function of reference voltage and the parameters of active filter to minimize the influence of voltage distortions on modulated waveform. Based on Eq. (23), and by identification to adaptive current band Eq. (15), (17) and (18), the hysteresis voltage band can be written as: a Vdc HB = 4fm Rfs Cfs



1−

2 C2 Rfs fs



2 a 2 Vdc

dVref (t) 1 V (t) + Rfs Cfs sl dt

2 

(25)

with Vsl = Vref + Lfs Cfs

d2 Vref dt 2



− Lfs

dIch + Rfs Ich dt



5. Simulation results The system (UPQC) shown in Fig. 1 is simulated using Matlab/Simulink software. The parameters of the UPQC are: Vs = 220 V, f = 50 Hz, Rs = 0.5 m, Ls = 11 ␮H, Lf = 150 ␮H, Rf = 5 m, Vdc = 700 V, Cdc = 8.3 mF, Rfs = 0. 02 , Lfs = 4 × 10−4 H, Cfs = 5.7 × 10−4 F, series transformer brought to the primary Rt = 6.6 m, Lt = 21.1 ␮H, and the rectifier feed an R-L load Rch = 0.66 , Lch = 4 mH. The THD of the nonlinear load current is 29.5%. The switching frequency of inverters is fixed to 8 kHz. In Fig. 16, the series active power filter has been simulated (voltage sags). The output compensation voltage Vc presents significant ripples, which depend on the value of the fixed bandwidth. This band influences the value of amplitude of the ripples, the switching frequency and the recovery time is very important. To solve these problems (to minimize the ripples and the recovery time. . .) especially if the disturbance is important as show in Fig. 16(a) and (b) illustrates the results obtained with the adaptive band. The transient mode is compensated and the ripples are reduced. Fig. 17 presents the spectrum of the output filter voltage control in case of voltage sags and shows that the switching frequency is nearly constant and equal to 8 KHz. To illustrate the improvements of the proposed control, operating conditions are chosen at the limits of its tracking capability. The shunt active power has been simulated. In Fig. 18, the error, ␦, of the total current converted is shown together with is and is∗ the reference current. It should be put in evidence that both controls are able to follow the reference current, the performance difference consisting in the ability to maintain a constant switching frequency and to limit ripples and peak current. It is usual to evaluate the error of a current control in terms of RMS value of current ripples. Simulation results show that we have good tracking for two cases of control Fig. 18(a and b). The worse case is given by hysteresis fixed band, Fig. 18(a). The current error is more important than that given by adaptive band control and the switching frequency varies during the fundamental period. As a result, the switching losses are increased and current source contains excessive harmonics. The line current and current error given by adaptive band control is shown in Fig. 18(b), for this technique switching frequency is kept constant. The performance of the proposed control algorithm of the active power filter is found to be excellent and the source current is practically sinusoidal. The source current THD decreases from 29.5% before compensation to less than 3% after compensation.

Fig. 19 shows the behaviour of the UPQC to compensate a 50% three-phase voltage sags during 40 ms in the presence of a nonlinear load. The upper traces show the network voltages with faults (vs ) and the voltage across the load (vch ). The voltage injected by the series APF (vc ), the non-linear load current (ich ), the source current (is ) and the DC bus voltage variations before, during and after compensation are also shown in this figure. The UPQC reacts quickly and allows both compensating current harmonics and supplying voltage sags. The current supply and the voltage of the load are sinusoidal and in phase after compensation. The voltage of the DC bus reduces to 670 V when dip occurs, but this value is enough for the series filter to compensate the perturbation. The response of the DC bus voltage regulator is fast and after a half period the DC capacitor value is restored to the set point (700 V). Fig. 20 shows the behaviour of the UPQC under distorted supply voltages conditions. The network voltage perturbations consists of harmonics h5 = 20%, h7 = 15%, h13 = 3%, h17 = 7% and h19 = 5%. Results show that the source current is sinusoidal and in phase with the supply voltage. The THD of the source current decreases from 29.5% before filtering to 3% while the THD of the load voltage decreases from 27.5% to less than 1%. At t = 0.06 s, a 50% voltage sags is introduced on the system, both series and shunt APFs are operating together as UPQC. The UPQC is maintaining the load voltage sinusoidal and at desired constant level even during the sag. While series APF is providing the required real power to the load, the shunt APF is maintaining the DC link voltage at constant level and the source delivered more current. This extra power flows from source to shunt APF, shunt APF to series APF via DC link and from series APF to the load. Fig. 21 describes the response of the UPQC for compensating flicker phenomenon. The amplitude of the voltage fluctuation is considered to be 20% at frequency of 8 Hz. The action of the series APF allows the compensation of the terminal voltage of the load which is found sinusoidal and balanced, and the action of the shunt APF makes it possible to have sinusoidal current in the network. We note that the DC bus voltage presents variations of 1.2% around its nominal value with the same frequency. The source current is sinusoidal and follows the evolution of network voltage frequency during flicker.

6. Conclusion This paper presents a steady state power flow analysis of an UPQC and demonstrates the validation of a simpler control approach for the series and shunt APFs based on hysteresis adaptive band. In an adaptive hysteresis current and voltage control methods, the band can be chosen to improve the quality of the output signal. We have demonstrated that the modified control signal makes the output ripples under control, compensates the transient mode, and the bands are modulated with the system parameters to maintain the modulation frequency nearly constant. The simulation results confirm the viability of the proposed approach and prove that the UPQC, thanks to robust voltage and current controllers, allows improving power quality by compensating current harmonics, voltage harmonics, voltage sags and flicker phenomenon. In all cases, the UPQC is maintaining the supply current and the load voltage sinusoidal at desired levels. Therefore, the proposed control can easily be adapted to others more severe constraints. The fuzzy regulator of DC capacitor voltage has been studied to improve the UPQC performances and to reduce the design of energy storage capacitor [11]. The future step in this work is to build a practical experiment (6 KVA platform) and to validate the proposed three phase UPQC control based on Fuzzy regulators.

M. Fatiha et al. / Electric Power Systems Research 81 (2011) 1743–1753

Nomenclature Lf The decoupling inductance of shunt APF Lfs , Cfs, Rfs Inductance, capacitor and resistance of series APF Capacitor of DC link Cdc Lt , Rt Inductance and resistance of series transformer es Source voltage vs , Vs Instantaneous value and rms of the voltage at PCC vsd The direct voltage component at PCC vch , Vch Instantaneous value and rms of the load voltage Vdc , v˜ dc (t) The DC bus voltage and its fluctuation Instantaneous value and rms value of the source current is , Is ich , Ich Instantaneous value and rms value of the load current Ich1 The fundamental current of the load The fundamental phase angle of the load ϕch1 vc , Vc Instantaneous value and rms value of the voltage injected by series APF Vref Reference voltage of series APF if , If Instantaneous value and rms value of the current injected by shunt APF cos ϕn Power factor of the load Ps , Pch , Pc ,Pf Active power input at the PCC, of the load, of series APF, shunt APF respectively p˜ ch (t) The alternating parts of instantaneous powers of nonlinear load. Sc , Sf Apparent power absorbed by the series and shunt APFs, respectively E, E˙ Voltage error and its derivate References [1] B. Han, B. Bae, H. Kim, S. Baek, Combined operation of unified power quality conditioner with distributed generation, IEEE Transactions on Power Delivery 21 (1) (2006) 330–338. [2] V. Khadkikar, A. Chandra, A.O. Barry, T.D. Nguyen, Steady state power flow analysis of unified power quality conditioner (UPQC), in: IEEE International Conference on Industrial Electronics and Control Applications (ICIECA), November 29–December 2, 1–6, Quito, Ecuador, 2005. [3] R. Rezaeipour, A. Kazemi, Review of novel control strategies for UPQC, International Journal of Electrical and Power Engineering 2 (3) (2008) 185–191. [4] M. Machmoum, N. Bruyant, M.A.E. Alali, S. Saadate, Compensateur actif série d’harmoniques, de déséquilibre et de creux de tension des réseaux électriques, Revue Internationale de Génie Electrique 4 (3–4) (2001) 317–332. [5] F. Mekri, M. Machmoum, N. Aït-Ahmed, B. Mazari, A comparative study of voltage controllers for series active power filter, Electric Power Systems Research 80 (6) (2010) 615–626. [6] B. Malabika, P.D. Shyama, K.D. Gopal, Comparative evaluation of two models of UQPC for suitable interface to enhance power quality, Electric Power Systems Research 77 (2007) 821–830. [7] Y. Rong, C. Li, Q. Ding, An adaptive harmonic detection and a novel current control strategy for unified power quality conditioner, Simulation Modelling Practice and Theory 17 (2009) 955–966.

1753

[8] P. Kirawanich, R.M. O’Connell, Fuzzy logic control of an active power line conditioner, IEEE Transactions on Power Electronics 6 (19) (2004) 1574–1585. [9] L.H. Tey, P.L. So, Y.C. Chu, Neural network-controlled unified power quality conditioner for system harmonics compensation, in: Transmission and Distribution Conference and Exhibition Asia Pacific. IEEE-PES, 2, 2002, pp. 1038–1043. [10] L.H. Tey, P.L. So, Y.C. Chu, Unified power quality conditioner for improving power quality using ANN with hysteresis control, International Conference on Power System Technology 2 (2004) 1441–1446. [11] F. Mekri, B. Mazari, M. Machmoum, Control and optimization of shunt active power filter parameters by fuzzy logic, Canadian Journal of Electrical and Computer Engineering 31 (3) (2006) 127–134. [12] H. Akagi, New trends in active filters for power conditioning, IEEE Transactions on Industry Applications 32 (6) (1996) 1312–1322. [13] A. Sannino, J. Svensson, A series connected voltage source converter for voltage sag mitigation using vector control and a filter compensation algorithm, in: Proceeding of the 35th annual IEEE Industry Applications Society annual meeting, IAS, Italy, 2476-2481, 2000. [14] J. Zeng, C. Yu, Q. Qi, A novel hysteresis current control for active power filter with constant frequency, Electric Power Systems Research 68 (2004) 75–82. [15] V. Khadkikar, A. Chandra, A.O. Barry, T.D. Nguyen, Application of UPQC to protect a sensitive load on a polluted distribution network, in: IEEE-PES General Meeting, Canada, 2006, p. 6. [16] H. Fujita, H. Akagi, The unified power quality conditioner: the integration of series and shunt active filters, IEEE Transactions on Power Electronics 13 (2) (1998) 315–322. [17] M. Shafiee Khoor, M. Machmoum, Simplified Analogical Control of a Unified Power Quality Conditioner, in: conference CD-ROM IEEE Power Electronics Specialist Conference, PESC, Brazil, 2005. [18] H. Akagi, E.H. Watanabe, M. Aredes, Instantaneous Power Theory and Applications to Power Conditioning, Wiley–IEEE Press, 2007 (April), Series on power engineering. [19] H.R. Mohammadi, A.Y. Varjani, H. Mokhtari, Multiconverter unified powerquality conditioning system: MC-UPQC, IEEE Transactions on Power Delivery 24 (3) (2009) 1679–1686. [20] H. Awad, J. Svensson, M. Bollen, Phase locked loop for static series compensator, in: conference CD-ROM of European Power Electronics Conference, EPE’03, France, 2003. [21] S.S. Min, K.C. Lee, J.W. Song, K.B. Cho, A Fuzzy current controller for fieldorientated controlled induction machine by fuzzy rule, in: Proceeding of IEEE-PESC, 1992, pp. 265–270. [22] Y.Y. Tzou, S.Y. Lin, Fuzzy-tuning current-vector control of a three phase PWM inverter-performance AC drives, IEEE Transactions on Industrial Electronics 45 (8) (1998) 782–791. [23] M.B.B. Sharifian, Y. Ebrahimi, R. Rahnavard, Improving of power system transient stabilization based on fuzzy gain scheduling PID, in: International Conference on Electrical Engineering, ICEE, Hong Kong, 2007. [24] B.K. Bose, An adaptive hysteresis—band current control technique of a voltage fed PWM inverter for machine drive system, IEEE Transactions on Industry Electronics 37 (5) (1990) 402–408. [25] S. Casoria, G. Sybille, P. Brunelle, Hysteresis modelling in Matlab/power system blockset, Mathematics and Computer in Simulation 63 (2003) 237–248. [26] F. Mekri, M. Machmoum, B. Mazari, N. Aït Ahmed, Determination of voltage references for series active power filter based on a robust PLL system, in: Conference CD-ROM IEEE International Symposium on Industrial Electronics, ISIE, Spain, 2007.