A Modified Capacitor Voltage Control Algorithm for ...

A Modified Capacitor Voltage Control Algorithm for Suppressing the Effect of Measurement Noise on Grid-Connected ZSource Inverters Controllers Ahmed A. Hakeem, A. Elserougi, Amr El Zawawi Electrical Dept., Alexandria University, Egypt

Abstract—Inverters are considered the orbit of research objectives for safe and reliable grid interface. Amongst the conventional inverters known as suitable candidates for gird connection in the last two decades, Z-source inverter (ZSI) emerged with buck/boost voltage capabilities compared to the counterpart voltage source inverter (VSI) and current source inverter (CSI). Therefore, ZSI introduces the merit of single stage grid interfacing of DG especially with renewable energy sources. Proper control of key parameters in single stage grid interface is essential. In the ZSI operation, the capacitor voltage is the most vital key parameter which can be controlled throughout the boosting factor. In this paper, the conventional capacitor voltage control is modified to enhance the performance of grid-connected ZSI and increase its immunity against the measurement noise. The proposed modification is verified throughout simulation. Furthermore, the experimental results obtained from a grid-connected ZSI prototype substantiate the proposed improvement. Index Terms—Capacitor voltage control, constant boost, synchronous reference frame, and Z-source inverter (ZSI).

Vdc Vdco V c* Vc Vac Vac* M ' M TST Vp Vn P* Q* idq* idq

Input DC voltage Voltage across semi-conductor switches Reference capacitor voltage Actual capacitor voltage Amplitude of inverter's output phase voltage Amplitude of reference output phase voltage. Modulation index Boosting index Shoot-Through period Positive envelop Negative envelop Active power reference Reactive power reference Inverter's reference d and q current components Inverter's actual d and q current components

978-1-4799-0224-8/13/$31.00 ©2013 IEEE

A.M. Massoud

ECEN Dept., Texas A&M University at Qatar



NOMENCLATURE

S. Ahmed

Vdq Vg Li

Electrical Dept., Qatar University and Alexandria University

Actual d and q voltage components of the grid Amplitude of grid phase voltage Interfacing inductance I. INTRODUCTION

I

most of distributed generation (DG) systems, especially with renewable energy sources, a conditioning stage for regulating the output voltage is necessary before the inverter stage. Boosting the DC voltage before the inversion stage as in fuel cell and photovoltaic applications is crucial. If a voltage source inverter (VSI) is employed, a DC-DC converter with boosting capability is required. Current source inverter (CSI) and Z-source inverter (ZSI) can commonly achieve the boosting function within the inversion stage, but the CSI operates only as a boost converter and it requires enough overlap which is sensitive to any EMI noise from the surrounding power circuits and might burn out the inverter [1]. ZSI shown in Fig.1, proposed and modeled in [1-4], overcomes the demerits of the VSI and the CSI. The ZSI accomplishes the combined tasks, boosting up and DC-AC conversion, in a single stage fashion by adding an extra zero state to the conventional eight states of the VSI. This extra state, Shoot-Through (ST) state, is achieved with the aid of the impedance network shown in Fig.1. The complete design of the impedance network is proposed in [5], where the design of impedance network affects the ripples in the inductor current and capacitor voltage. N

Impedance network L + Renewable Energy Source Fig. 1. Z-source inverter

204

+

via

C vib C

L

-

vic

The allowable extra state in the ZSI, ST state, is fulfilled by gating the upper and lower switches in the same leg at the same time. This could be accomplished using seven different possibilities through gating one leg per time, gating two legs per time or gating the three legs at the same time [1]. Different topologies of ZSI, compared to the conventional ZSI shown in Fig.1, exist for various applications such as quasi ZSI (qZSI) [6] and current fed qZSI [7-9]. This new family of qZSI is developed to increase the boosting range. Similar to multilevel VSI, multilevel ZSI has been reported in literature [10-13]. Switched inductor ZSI and switched inductor qZSI, are introduced in [14, 15]. Semi-ZSI is another topology of the ZSI where only two active switches to achieve the same output voltage as in the traditional full-bridge [16]. ZSI in different applications has been reported extensively in literature such as distributed generation applications [17], UPS applications [18], PV applications [16], and electric drives [19-21]. ZSI pulse width modulation (PWM) techniques can be namely classified into three main techniques; simple boost, maximum boost, and constant boost techniques [1, 22, and 23]. Other PWM techniques exist like modified space vector, sine carrier, and double carrier PWM techniques proposed in [24-28]. In [29-32] an exhaustive survey is established between different PWM techniques of the ZSI. Similar to VSI carrier-based PWM but with additional two boundaries for ST state generation, the ZSI PWM can be obtained. In the simple boost technique, the two boosting boundaries are two constant lines as shown in Fig.2a. For the maximum boost and constant boost techniques the two boosting waves are as shown in Fig.2b and Fig.2c respectively. 1

Vp

p.u.

0.5 0 -0.5 -1

Vn

0

0.005

0.01 Time (s)

0.015

0.02

(a) Simple boost technique 1

Vp

p.u.

0.5 0 -0.5 -1

Vn

0

0.005

0.01 Time (s)

0.015

0.02

(b) Maximum boost technique. 1

To obtain a high performance of the ZSI during any disturbance, the capacitor voltage should be well controlled [33] as in Fig.3. The capacitor voltage control will maintain the output voltage of the impedance network constant by controlling the shoot-through period. The main disadvantage of the conventional control scheme is its sensitivity to measurement noise. This paper proposes a modification for the conventional capacitor voltage control, to overcome the noise problem. In addition to employment of modified modulation scheme proposed in [34] to enhance the inverter response and reduce the voltage stress on the semiconductor switches. The scheme involves a separation between the modulation index and the boosting factor, to get two control parameters (the modulation index, M, and the boosting index, M’). This paper is comprised of five sections, following the introduction. Section II introduces the proposed modification for the capacitor voltage control to suppress the effect of measurement noise on the system performance. Simulation and experimental results are presented in Section III and IV respectively to ensure the validity of the proposed concept. Finally, section V provides the main work contributions. II. MODIFIED CONTROL SCHEME OF GRID-CONNECTED ZSI A. Decoupled Modulation Index and Boosting Factor For better performance of the ZSI grid-connected system, the capacitor voltage control technique must be considered. As in [34] a separation between the modulating and boosting waves is employed. Hence, two control parameters exist. The first parameter is the modulation index (M) which is a ratio of the modulating waves peak to the carrier waves peak and the ' second is the boosting index (M ) which is a ratio of the boosting waves peak to the carrier waves peak. This separation could be applied for simple, maximum and constant PWM techniques. Constant boost PWM is employed in this work due to its advantages over the simple boost and maximum boost techniques, as it gives lower stresses on the semi-conductor switches and outputs less harmonics in the output current [23, 34]. The relation between the output fundamental maximum phase voltage (Vac) and the input DC voltage (Vdc) for the common PWM techniques before and after separation is shown in TABLE I for the three commonly used PWM techniques. The accomplishment of this separation introduces a separate control of the boosting index; hence capacitor voltage control technique can be applied for any PWM technique.

Vp

TST

p.u.

0.5

Max

0

TST

-0.5 -1

Min

+ Error

Vn

0

0.005

0.01 Time (s)

0.015

0.02

Min

(c) Constant boost technique. Fig. 2. Modulating waves and boosting waves of the commonly used PWM techniques

Max

Fig. 3. Conventional capacitor voltage control

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Vc

Vc*

TABLE I

Constant boost

M Vdc (2M '1) 2 M Vdc Vac  (3 3M ' ) 2

Vac 

Vdc ( 3M  1) 2 M

Vac 

Vac 

(1) (2)

(3)

where ∆V is set to allow more reactive power to be supplied and maintain the boosting index larger than the modulation index so that the ST state will not affect the active states [34].

Vdc ( 3M '1) 2 M

B. Grid-Connected ZSI Fig.4 shows the block diagram of the grid-connected ZSI. P* and Q* are the reference active and reactive power respectively, from which the reference current components are calculated. These current components are limited using saturation blocks to meet the limited overload capability of the inverter. Voltage oriented control has inner current control loops for both id and iq. The current controller is used to obtain the inverter reference voltage. An improvement has been added to the conventional capacitor voltage control algorithm as shown in Fig.5. In case of measurement noise, the conventional capacitor voltage control technique will deal with this noise as a deviation from the reference point, and then it will modify its output to adjust the measured value to the reference set point. The improved algorithm gives better performance as it does not deal directly with the value of the error as the conventional one but deals only with the sign of the error. As shown in Fig.5, two values have to be tuned; the maximum and the minimum values of the boosting index is tuned based on the maximum and minimum boosting required from the PWM technique. Also the maximum and minimum of the error is tuned based on the acceptable range of the error of the capacitor voltage. Also the value of the constant K in the integrator block (K/S) is selected to control the steady state peak to peak ripples on the capacitor voltage. When K is increased the steady state peak to peak ripples will increase but the settling time will be lower and vice versa if it is decreased, as the steady state peak to peak ripples will decrease and the settling time will be higher. The most appropriate value for K is less than or equal 1.0 to get very low steady state peak to peak ripples on the capacitor voltage. One more feature could be added to this improved algorithm is to control the value of K. Hence it would be high at the beginning to get lower settling time then decreased at steady state to get lower steady state peak to peak ripples, but this is out of the scope of this paper. As proposed in [34]; instead of setting any capacitor reference voltage (Vc*) (e.g. up to the maximum value the capacitor would withstand), for the constant boost technique, an estimation of the required reference is given by (1). This reference is calculated based on the desired boosting index ' (M ) calculated from (2). Saturation block with maximum limit of capacitors voltage rating must be used as shown in Fig.5, to avoid overvoltage problem across the capacitors.

III. SIMULATION RESULTS A MATLAB/Simulink model is built to verify the proposed concept. The model parameters are as listed in TABLE II. The DC voltage is set to 48 V and the grid peak phase voltage is set to 40 V. A random noise of +2 V (which might exist in the practical system due to any EMI from the surrounding power circuits) is added to the measured capacitor voltage. The simulation results for the conventional and the proposed techniques are shown from Fig.6, where ∆V is set to 10 V which is selected to maintain the active states not affected by the shoot through state [34], then from (1), (2), and (3), Vc*=86.6 V. Fig.6a and 6b show the capacitor voltage of both techniques. For the conventional technique, the noise appears on the capacitor voltage. While from Fig. 6b, the proposed control does not greatly affect by the noise. Similarly for the input DC current shown in Fig. 6c, 6d, the inductor current shown in Fig. 6e, 6f, the output current of the inverter shown in Fig.6g, 6h, and the output active power of the inverter shown in Fig. 6i, 6j. If this noise is increased to +4 V, the voltage across the semi-conductor switches and the output line current of both algorithms will be as shown in Fig.7. Three-phase bridge inverter + v

L C

Energy Source

C Vc Capacitor voltage controller Vc*

Grid

Li

ia

vib vic

L

vabc

Max. boost

With separation

M Vdc Vac  (2M  1) 2 Vdc M Vac  (3 3M   ) 2



iabc

Simple boost

Without separation



3M ' / 2 Vdc ( 3M ' )  1 1 M'  3  (0.5Vdc Vac* ) * where the term V ac is given by (3) Vac*  Vg  V Vc* 

SEPARATION APPLIED TO CONVENTIONAL PWM TECHNIQUES OF ZSI

M’*

PWM generation

Estimator

M, θref

Voltage Oriented Control vdq

i*dq

θ

Threeidq θ Phase - abc/dq PLL + i* dq vdq

P*,Q* vdq

Fig. 4. Active and reactive power control of grid-connected ZSI.

M’* M’*

Max Min Error Min

K/S

Max

Fig. 5. Improved capacitor voltage control algorithm.

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+

Sign(Vc*-Vc)

Vc*

Vc

90

(V)

(V)

90

85

80 1.5

1.6

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85

80 1.5

2

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15

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10 5

10

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1.95

2

(f) 10

Fundamental= 5.142 A, and THD=4.93%

Fundamental= 5.18 A, and THD=4.42%

5

(A)

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(h)

400

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(A)

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200 100

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(i) (j) Fig. 6. Comparison between the conventional and proposed capacitor voltage controller assuming random measurement noise of +2 V, and 300 W and 0 VAR reference power: (a) and (b) capacitor voltage for conventional and proposed controller respectively , (c)and (d) input DC current for both cases, (e) and (f) inductor current for both cases, (g) and (h) inverter output AC current for both cases, (i) and (j) inverter output active power for both cases. 90

85

(V)

(V)

90

80 1.5

1.6

1.7

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80 1.5

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Time (s)

10

Fundamental= 2.627 A, and THD=8.43%

2

Fundamental=5.171 A, and THD=4.43%

5 (A)

5 (A)

1.9

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0

-5

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2

(c) (d) Fig. 7. Comparison between the conventional and proposed capacitor voltage controller assuming random measurement noise of +4 V: (a) and (b) capacitor voltage for conventional and proposed controller respectively , (c) and (d) inverter output AC current for both cases.

It is obvious from simulation results that the effect of increasing the noise is significant with the conventional control technique but nothing changed in the proposed control

technique, (i.e. immunity against measurement noise has been gained by employment the proposed concept).

207

IV. PRACTICAL RESULTS A ZSI prototype shown in Fig.8 was built in the laboratory to verify the proposed improvement with the following parameters given in TABLE II. The DC voltage is set to 24 V and the grid peak phase voltage is set to 15 V. Fig. 9 shows the measured capacitor voltage internally in the DSP memory while the system is turned off. This measured voltage is a noise due to EMI which will affect the system as shown in Fig. 10 using the conventional capacitor voltage control algorithm, where this technique deals with this noise as an error in the capacitor voltage. The effect of this noise is showed in the output current and the input DC link current. The improved algorithm helps in the mitigation of this noise as shown in Fig. 11. The experiments were conducted for two different active power references using the improved algorithm as shown in Fig. 11, where the upper trace shows the voltage across the semi-conductor switches and the DC link voltage, the middle trace shows the line current and the last one shows the DC link current. For the different active power references, the noise in the measurements didn't affect the performance of the ZSI using the improved capacitor voltage control technique as shown in Fig. 11a and 11c. This emphasizes the simulation's results where the noise got no effect on the performance of the ZSI using the improved technique. Comparing between Fig. 10 and Fig. 11b, for the same reference power, the measured noise shown in Fig. 9 affected the conventional technique while nothing affected the proposed technique. This emphasizes the simulation results introduced in the previous section. V. CONCLUSION Noise in the measurements is expected and may be time variant. This highly affects the performance of the ZSI especially in the grid-connected mode. Therefore mitigation of this effect is the main contribution of this paper. The effect of the noise on the conventional technique of controlling the capacitor voltage is significant especially when its amplitude increases as shown in the simulation results. The conventional technique is so sensitive to any small deviation in the measured capacitor voltage. The improved technique proposed in this paper overcomes the problems of the conventional technique, where the performance of the ZSI is enhanced as shown in the simulation results and verified experimentally. TABLE II PARAMETERS OF THE ZSI FOR THE MATLAB/SIMULINK MODEL AND THE EXPERIMENTAL PROTOTYPE L-Z network 0.01 Ω, 5 mH C-Z network 1500 µF Interfacing impedance 0.01 Ω, 5 mH Switching frequency (fs) 3 kHz Fundamental frequency (fo) 50 Hz

ACKNOWLEDGMENT This publication was made possible by NPRP grant (NPRP 4 250 - 2 – 080) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. Interfacing transformer with the grid

Batteries

DSP and gate drives

R-load

Power meters and current transformers

IGBTs Oscilloscope

Li Sensors

Impedance network

Fig. 8. Grid-connected ZSI prototype.

Fig. 9. Measured noise while the system is turned off due to EMI.

Fig. 10. Experimental output results and measured noise for 0 VAR and 25 W power references using the conventional algorithm.

(a) Reference power of 25 W and 0 VAR

(b) Reference power of 50 W and 0 VAR Fig. 11. Experimental results for different power references for the improved algorithm.

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[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

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