Dynamic Voltage Restorer Adopting 150o Conduction Angle VSI

Ahmed A. Helal, Member, IEEE, and Mohamed H. Saied, Member, IEEE

Abstract--This paper presents a dynamic voltage restorer based on a new firing control strategy for the three-phase six-switch voltage source inverter. In this firing control strategy, one of the three inverter legs is to be intentionally opened, one per time in a pre-planned sequence. This strategy combines the commonly used 180o and 120o conduction modes to generate a new operating mode, defined as 150o conduction mode. The performance of the proposed dynamic voltage restorer with 150o

conduction angle inverter control is evaluated using dynamic transient simulation in order to illustrate the advantages of the new control scheme. The simulation results show good improvement in the magnitude and time response of the dynamic voltage restorer output voltage. This outcome results in more accurate and faster voltage restoring capability. Also, a great reduction in the total harmonic distortion of the inverter output voltage is attained and hence a consequent reduction of the voltage harmonics in the supply side.

Index Term--Dynamic voltage restorer, power quality, voltage

sag, voltage source inverters.

I. INTRODUCTION

Electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric power. This concern is mainly due to the development of many load equipment, with microprocessor based controls and power electronic devices, is more sensitive to power quality variations than was equipment used in the past. Power quality could be defined as the concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment [1]. Voltage sag is considered one of the most important power quality disturbances due to its high occurrence frequency compared with other power quality disturbances and its severe impact on sensitive loads. Voltage sag is defined as a momentary decrease in voltage rms value between 10% to 90% of the nominal voltage level, for duration of half a cycle to one minute [2]. Voltage sags are caused by remote faults such as a single line to ground fault on the power system or due to starting of large induction motors. A. A. Helal is with Arab Academy for Sciences and Technology, Alexandria, Egypt (ahmedanas@aast.edu). M. H. Saied is with Abu Qir Fertilizers and Chemical Industries Company, Alexandria, Egypt (e-mail: Mohamed.saied@alexandria.cc).

Different custom power devices are introduced to solve this problem. The most common of which is the dynamic voltage restorer (DVR). This power electronics converter-based compensator is connected in series with the distribution feeder between the supply and the loads. Its main function is to mitigate any supply voltage disturbance, especially voltage sag, by inserting a voltage with the required magnitude, frequency and phase shift in order to restore the load voltage to its rated value [3].

II. DVR POWER CIRCUIT

DVR is a series connected custom power device. Its main function is the protection of sensitive loads from any voltage disturbances except voltage outage. It is basically consists of power circuit and control circuit. The power circuit of the DVR has four main parts; voltage source inverter (VSI), voltage injection transformer, DC energy storage device and low pass filter [4], [5] as shown in Fig. 1.

Fig. 1. Power circuit of the DVR

A. Voltage Source Inverter (VSI) VSI is used to convert the DC voltage supplied by the energy storage device to an AC voltage. This voltage is boosted through injection transformer to the main system. Usually the rating of the VSI is low voltage and high current due to the use of step up injection transformers.

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2008 IEEE Electrical Power & Energy Conference

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B. Voltage Injection Transformer

Its basic function is to step up the ac low voltage supplied by the VSI to the required voltage. In case of three phase DVR type, three single phase injection transformers are usually used. The maximum voltage sag the DVR can compensate depends mainly on the rating of the inverter and the injection transformer. C. DC energy storage device

It provides the real power requirement of the DVR during compensation. Lead-acid batteries, Flywheels, Super conducting Magnetic Energy Storage (SMES) and Super capacitors can be used as the storage devices. For Batteries and SMES, DC to AC conversion (inverters) is necessary, while for flywheels AC to AC conversion is required [5]. D. Passive Filter

A passive low pass filter consists of an inductor and a capacitor. It can be placed either at the high voltage side or the inverter side of the injection transformer. It is used to filter out the switching harmonic components from the injected voltage. By placing the filter at the inverter side, the higher order harmonics are prevented from penetrating into transformer, thereby it reduce the voltage stress on the injection transformer. When the filter is placed on the high voltage side, since harmonics can penetrate into the high voltage side of the transformer, a higher rating transformer is required [4], [5].

III. DVR COMPENSATION TECHNIQUES

There are three main compensation techniques applied in the DVR operation, namely pre-sage compensation, in-phase compensation and energy optimization compensation [6]. A. Pre-sag compensation

In this compensation technique the DVR supplies the difference between the sagged and pre-sag voltage and restores the voltage magnitude and the phase angle to the nominal pre sag condition. Fig. 2 describes the pre-sag compensation. The main drawback of this technique is it requires a higher capacity energy storage device as well as a large voltage injection capability. Therefore, the pre-sag strategy is applied only in cases where the load is exceptionally sensitive to changes of the supply voltage phase angle (for example some applications of thyristor converters). If the load is not sensitive to the changes of the supply-voltage phase angle then the most often applied strategy is the in-phase strategy [7].

Fig. 2. Per-sag compensation technique

B. In-phase compensation Only the voltage magnitude is compensated in this control technique. The compensated voltage is in phase with the sagged source side voltage, as shown in Fig. 3. The main advantage of this technique is that the magnitude of an inserted voltage is minimal. Therefore, the rated power of the DVR is minimized for the existing current and terminal-supply voltage. In addition, application of the in-phase strategy decreases the demand for injection of active power however; it does not minimize its value as the case with energy optimization technique [8].

Fig. 3 In-phase compensation technique

C. Energy optimization technique

This technique compensates with minimized energy requirement. In order to minimize the use of real power the voltages are injected at 90o phase angle to the supply current, as shown in Fig. 4. Therefore the DVR supplies only the reactive power. It is advised to apply energy optimization technique when the value of injected active power is critical due to the capacity of the energy storage [9].

Fig. 4 Energy optimization technique

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IV. 150O CONDUCTION ANGLE VSI

This section explains a new switching strategy controlling the most-common, simple, and well-known three-phase six-switch voltage source inverter (VSI) [10]. The proposed strategy is referred to as; open-leg switching strategy (OLSS), which intentionally opens one of the three inverter-legs per-time, in a pre-planned sequence. This increases the number of allowed switching state combinations originally obtained by two-level inverters from 8 (23) to 27 (33). Consequently, each one of the six transistors constructing the VSI could conduct for 150o, instead of the known 180o or 120o conduction-modes.

For a wye-connected load fed from the inverter, the known 6 active and 2 zero space-voltage-vectors (SVVs), coincided in the origin, construct the classical 7 space vector diagram (7-SVD). Using the OLSS, new different 6 non-zero voltage-vectors are primary introduced to the literature and added to the former 7, constituting an innovated 13 space vector diagram (13-SVD) for the inverter, compared to the 7-SVD originally generated using other modulation techniques. The output phase-voltage becomes a 7-level, 12-step waveform, and a 50% reduction of the total harmonic distortion is obtained.

A. The Open-Leg Switching Strategy

The simplified schematic of the classical three-phase two-

level six-switch VSI is shown in Fig. 5. The mid-point (p) is considered the voltage-reference of the dc-bus (Vd). Thus, the inverter generates three voltage-levels (0, ±Vd/2) instead of only two-levels (0, +Vd) [11]. A three-phase wye-connected balanced load is considered.

Fig. 5. Classical three-phase six-switches VSI

The OLSS intentionally opens one of the three inverter-legs per-time, in a pre-planned sequence. Thus, the line-to-centre voltage (e.g. Vap) has three states: i) Switched-up; only the upper transistor T1 is turned on (referred as +). ii) Switched-down; only the lower transistor T4 is turned on, (referred as −). iii) Floated; neither of the transistors (T1 and T4) concerning this leg is turned on (referred as *). In this way, each of the 3 inverter-legs has 3 switching states and the two-level VSI presents 27 (33) switching patterns.

This introduces new six non-zero voltage-vectors to the

literature, as explained in the following section, and increases the redundancy of the passive or zero voltage-vectors (ZVVs).

Since the common-mode-voltage CMV (Vnp) is defined as VCMV= (Vap+Vbp+Vcp)/3, where Vap, Vbp, and Vcp are the line-to-center voltages, it is not zero for all of the switching states.

B. Classical Conduction-Modes of the VSI

1. 180-Degree conduction-mode:

This is the most common type of transistors firing, in which, one transistor, per inverter leg, conducts for 180o. So, three transistors remain on at any instant of time. Six of the available 8 switching states and the corresponding voltage components are shown in Table 1 [12]. The corresponding classical 7-SVD containing the six active voltage-vectors (AVVs) V1-V6 and the ZVVs forming sextants boundaries is shown in Fig. 6.

TABLE 1

THE 180O CONDUCTION-MODE; SIX AVVS, SWITCHING STATES, LINE-TO-CENTER (VAP, VBP, & VCP), CM (VNP), PHASE (VAN, VBN, &

VCN), AND LINE-TO-LINE (VAB, VBC, & VCA) VOLTAGES.

Fig. 6. The classical 7-SVD (6 active voltage vectors (V1-V6) and 2 coincided ZVVs in the origin) of the two-level six-switch VSI (180o conduction-mode). 2. 120-Degree conduction-mode:

Referring to the OLSS, the same inverter bridge can be controlled with each switch conducting for 120o. As a result, at any instant, only two switches conduct [12]. The simplified schematic diagram of the VSI corresponding to the pattern (+ * −) is considered, from where voltage components can be obtained. Similar circuits can be easily obtained for the other 5 states: {(* + −), (− + *), (− * +), (* − +), and (+ − *)}.

V2 (+ + −)

V5 (− − +)

V3 (− + −)

V6 (+ − +)

V1 (+ − −) V7 (+ + +) V0 (− − −)

V4 (− + +)

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Table 2 shows the voltage components and the SVVs (V12-V61), which can be calculated and proved as follows:

TABLE 2 VOLTAGE COMPONENTS OF THE NEW SIX PAVVS GENERATED BY

THE TWO-LEVEL SIX-SWITCH INVERTER.

(1) This means that the switching state (+ * −), results in a space

voltage-vector has 0.707*Vd voltage amplitude and 30o counter clockwise phase-angle. In the d-q plane, this voltage vector lies in the middle of the 1st sextant (the 60o sextant with the adjacent AVVs, V1 and V2). This vector is defined as the partial active voltage vector (PAVV) V12. Similarly, it can be shown that the other PAVVs (V23-V61) have similar criteria. Fig. 7 shows the new six PAVVs (V12-V61) generated by the VSI considering the instantaneous opening of an inverter leg (in a pre-planed time).

Fig. 7. The primarily introduced to the literature, 30o counter clockwise phase-shifted, 7-SVD (6 (PAVVs) (V12-V61) and 13 coincided ZVVs in the origin) generated by the two-level six-switch VSI (120o conduction-mode).

C. The Emerging 150o Conduction-Mode of the VSI

Here, twelve switching patterns are presented per cycle; with each pattern duration is 30o. Three transistors conduct in one interval, while only two transistors conduct in the next one [10]. Table 3 shows the 12 non zero voltage vectors (NZVVs) of the 27 proposed switching states generated by the inverter controled by the OLSS (150o conduction-mode control). It is clear that each transistor conducts for 150 degrees.

Fig. 8 shows the innovated 13-space-vector-diagram

generat-ed by the two-level six-switch VSI controled via the 150o conduction-mode. The vertices of the major six AVVs constit-ute the outer hexagon, while the non-major six PAVVs constit-ute the inner hexagon; the total of 12 NZVVs, in addition to the ZVVs lie in the origin.

TABLE 3 VOLTAGE COMPONENTS OF THE MAJOR-AVV AND NON-MAJOR-

PAVV GENERATED BY SIX-SWITCH INVERTERS USING THE PROPOSED 150O CONDUCTION-MODE.

Fig. 8. The innovated 13-SVD (6 AVVs, 6 PAVVs, and 15 ZVVs) generated by the two-level six-switch VSI using the OLSS (150o conduction-mode).

The 150o conduction-mode has the following advantages [10]: i) Combines the classical 7-SVD generated by the 180o conduction-mode with another new 7-SVD generated by the 120o mode, to generate a primarily introduced to the literature 13-SVD.

V2 (+ + −)

V5 (− − +)

V3 (− + −)

V6 (+ − +)

V12 (+ * −) V34 (− + *)

V45 (− * +) V61 (+ − *)

V7 &V0 +13 Coincided ZVVs

V56 (* − +)

V23 (* + −)

V1 (+ − −) V4 (− + +)

13 Coincided ZVVs

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ii) Increases the RMS values of output voltages, compared to 120o mode, to almost those obtained by 180o mode. iii) Provides a 30o safety margin period, which is large enough, to avoid short-circuit on dc-bus. iv) Produces 7-level phase-voltage waveforms, (0, ± Vd/3, ± Vd/2, ±2Vd/3), compared to only 4 or 3 -levels in 180o and 120o modes, respectively.

v) Highly reduces the THD and DF of output voltage wave-forms, by presenting 12-step waveforms, which are closer to the sinusoidal waveform compared to the original 6-step ones.

vi) Almost eliminates the low order harmonics, that has 1/n of fundamental magnitude in previous modes, by improving the 1/n undesired magnitude relation.

V. APPLICATION OF 150O CONDUCTION ANGLE VSI ON DVR In order to examine the effectiveness of the 150o conduction angle strategy applied to VSI used in power electronics based custom power devices, a DVR test system, as shown in fig. 1, has been built up based on the MATLAB/SIMULINK environment. The system parameters are presented in table 4.

TABLE 4 TEST SYSTEM PARAMETERS

The simulation results are shown in Figures 9-11. Fig. 9 shows the output voltage from the VSI utilized in the DVR to mitigate voltage sag condition due to symmetrical three phase fault at the supply side. The voltage obtained as a result of application of the 150o conduction angle strategy.

Fig. 9. VSI output with 150o conduction angle strategy

The output waveform using this novel conduction angle strategy results in more voltage levels compared to other conduction angle modes, therefore the total harmonics distortion will be reduced. The output voltage of the VSI is filtered using passive low pass filter and then applied to the injection transformer in order to boost the load voltage to its nominal value before voltage sag occurs. The rms of the source voltage due to three phase fault is shown in Fig. 10 in p.u. values. It shows about 35% voltage sag at the start of fault condition (0.4s) and last for the whole fault duration (0.2s).

Fig. 10. Source voltage due to symmetrical three phase fault With the application of the DVR based on the 150o conduction angle strategy applied to VSI, the load voltage restores its nominal value as in pre-fault condition. The time response of the DVR restoring action depends on the fault circuit parameters as well as on the efficiency of the DVR controller design. As the 150o conduction angle strategy applied to VSI has more controllability on the VSI output voltage, compared with other strategies, the efficiency of the DVR controller in applying the required voltage at the required time to the injection transformer will increased. The load voltage after the application of DVR is shown in Fig. 11 which proofs the capability of the DVR in protecting the load from the supply voltage disturbance.

Fig. 11. Load voltage with the application of DVR

Parameter Value Load voltage

380V

Load apparent power

5kVA

Load power factor

0.8 lag

Injection transformer ratio

1:4

Common DC link voltage

100V

Common DC link capacitor

470 µF

Inductance 40 µH Low pass filter Capacitance 47 µ F

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VI. CONCLUSION

This paper utilizes the novel 13-space-vector-diagram generated by the emerging 150o conduction-mode controlling three-phase six-switch VSI, applying to a dynamic voltage restorer. Using the novel control method, Open Leg Switching Strategy, each transistor of the six conducts for 150o instead of the known 180o or 120o. Thus, a seven–level, 12-step output voltage waveforms, which resemble the sinusoidal wave-shape, are primarily obtained by the inverter. Consequently, the harmonic contents involved in the injecting voltage waveforms, is highly reduced. The DVR with this new control strategy gain more voltage boosting controllability in both voltage level and time response.

VII. REFERENCES

[1] R.C. Dugan, M.F. McGranaghan, H.W. Beatty, Electrical power systems quality, McGraw-Hill, 1996. [2] IEEE Recommended Practice for Monitoring Electric Power Quality, IEEE Standard 1159-1995. [3] A. Ghosh, G. Ledwich, "Compensation of distribution system Voltage using DVR," IEEE Trans. Power Delivery, vol. 17, pp. 1030-1036, Oct. 2002. [4] C. Zhan, V.K. Ramachandaramurthy, A. Arulampalam, C. Fitzer, S. Kromlidis, M. Barnes, N. Jenkins, "Dynamic voltage restorer based on voltage space vector PWM control" in Proc. 2001 Applied Power Electronics Conference and Exposition, pp. 1301- 1307. [5] V.K. Ramachandaramurthy, C. Fitzer, A. Arulampalam, C. Zhan, M.Barnes, N. Jenkins “Control of a battery supported dynamic voltage restorer” in Proc. 2002 IEE Generation, Transmission and Distribution, Volume 149, Issue 5, pp. 533 – 542. [6] Antonio Moreno-Muoz, Power Quality Mitigation Technologies in a Distributed Environment, Springer-Verlag London Limited, 2007. [7] J. Nielsen, G. Blabjerg F., and N. Mohan, "Control Strategies for Dynamic Voltage Restorer Compensating Voltage Sags with Phase Jump", in Proc .2001 of IEEE, APEC , Vol. 2, pp. 1267- 1273. [8] K. Perera, A. Atputharajah, S. Alahakoon, and D. Salomonsson “Automated control technique for a single phase Dynamic Voltage Restorer” in Proc. 2006 of the International Conference on Information and Automation, , Colombo, Sri Lanka, pp. 103-108. [9] D.M. Vilathgamuwa, A. Perera, S. Choi, “Voltage sag compenation with energy optimized dynamic voltage restorer.” IEEE Trans. Power Delivery, Vol. 18, No. 3, pp. 928–936, 2003.. [10] M. H. Saied, M. Z. Mostafa, T. M. Abdel-Moneim, and H. A. Yousef, “On three-phase six-switches voltage source inverter: A 150o conduction mode,” in Proc. 2006 IEEE ISIE , July 9-12, Montreal, Quebec, Canada, pp. 1504–1509. [11] J. Rodriguez, J. Lai, and F. Peng, “Multilevel Inverters: A Survey of Topologies, Controls, and Applications,” IEEE Trans. Ind. Electron., vol. 49, pp. 724-738, Aug. 2002. [12] M. H. Rashid, Power Electronics: Circuits, Devices, and Application, Prentice Hall Inc., 2nd Edition, 1993.

VIII. BIOGRAPHIES

A. A. Helal (S’97 M’2004) was born in Alexandria, Egypt, in 1972. He received the B.Sc. and M.Sc. degrees, both in electrical engineering, from Alexandria University, Alexandria, Egypt, in 1994 and 1998, respectively. He received the Ph.D. degree from Heriot-Watt University, Edinburgh, U.K., in 2004. He is currently a Lecturer in the Electrical and Control Engineering Department, Faculty of Engineering and Technology, Arab

Academy for Sciences, Technology (AAST), Alexandria, Egypt. His areas of interest include electrical machine design, modeling, and simulation; power quality, and power electronics applications in power systems.

Mohamed Saied (M’2007) was born in Kafr-Al-Sheikh, Egypt, on January 1, 1973. He graduated from Electrical Engineering Dept., Faculty of Engineering, Alexandria University, Egypt in July 1995. By July 2000, he got his Master of Science in the dead beat control area. He obtained the Ph.D. from the Electrical Engineering Dept., Faculty of Engineering, Alexandria University in July 2007. His Ph.D. specialty is in the direct torque control of

induction machines using multilevel inverters. Dr. Mohamed Saied is currently working for Abu-Qir Fertilizers & Chemical Industries Co., Alexandria Egypt as the managing director of the Electrical Engineering Dept. I. His special fields of interest included Multi-level inverters, AC-drives, direct torque control, power quality and deadbeat control.

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