INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

HYBRID MULTILEVEL H-BRIDGE CONVERTER BASED STATCOM OF A NOVEL DC VOLTAGE CONTROL METHOD

USING FUZZY B.RAJA KUMARI

M.TECH (P.E.) ST.JOHNS COLLEGE OF ENGINEERING

COLLEGE AND TECHNOLOGY. Affiliated to JNTUA.

PROF. MR.A. MALLIKARJUNA PRASAD

A.MALLIKARJUNA PRASAD Associate Professor PH.D

E.RAMAKRISHNA Co-ordinator of Asst. Professor

ST.JOHNS COLLEGE OF ENGINEERING COLLEGE AND TECHNOLOGY.

Affiliated to JNTUA Abstract- This paper proposed a new dc voltage control strategy for those hybrid multilevel converters. a hybrid multilevel h-bridge converter based STATCOM is used for regulating dc voltage. This hybrid multilevel STATCOM is characterized by per-phase series connection of a high-voltage H-bridge converter operating at fundamental frequency and a low-voltage H-bridge converter operating at 5 kHz without any other circuit for dc voltage control. hybrid multilevel STATCOM compensates the reactive power there by increasing the efficiency with high performance by regulating the dc voltage. A new control strategy is proposed in this paper with focus on dc voltage regulation. Clustered equalization control is realized by injecting a zero-sequence current to the delta-loop, whereas individual voltage control is achieved by adjusting the fundamental content of ac quasi-square-waveform voltage of high-voltage convertor. Clustered equalization control is realized by injecting a zero-sequence current to the delta-loop. Here we are using the fuzzy controller to gain high performance. simulation results are shown below with high performance by maintaining dc value.

Index Terms—Cascade H-bridge, dc voltage control, hybrid multilevel, static synchronous compensator (STATCOM).

I. INTRODUCTION

STATCOM is one of the facts device which is used to regulate the un balanced voltages and here the cascaded H bride is used for the statcom application to control the dc voltage. As the H bridge multi level converter has high quality to regulate the voltage it is mostly used in applications. Compared with diode-clamped converter and flying capacitor converter, the cascaded single-phase H-bridge converter saves a large amount of clamped diodes and flying capacitors. However, further improvement of power efficiency and waveform quality is expected of cascade H-bridge topology in high-power application. Whereas individual voltage control is achieved by adjusting the fundamental content of ac

quasi-square-waveform voltage of high-voltage convertor.

To get the low distorted ac waveforms the switching frequency has to be increase which increases the switching loss there by cost. Fortunately, hybrid multilevel technology provides a good tradeoff between waveform quality and switching loss. Hybrid multi level inverters have many advantages they are: at the output level number of levels get increases there by the harmonics get reduced and quality gets increases and the overall efficiency gets increases. As the concept of “hybrid multilevel” was proposed in the literature, great attentions have been paid to this field.

A hybrid topology with the series connection of three-phase full-bridge converter (two- or three-level) and single-phase H-bridge converter is adopted in the literature. This proposed topology generates more level at the output compared to conventional one. One popular method for capacitor voltage control is selecting switching states redundancy, which introduces another problem of uncertain switching frequency for relevant devices. Additionally, the capability of compensating unbalanced load is not mentioned either. Moreover, hybrid multilevel topology based on cascaded single-phase H-bridge converter with unequal dc voltage is considered in. The mentioned control method is not suitable for STATCOM system because the dc sources are replaced by capacitors in the STATCOM system. Reactive power regulation is realized by adjusting dc voltages.

However, this control method is based on steady-state model and the dynamic performance is not discussed. The literature provides new solution with a high-voltage converter fed by dc supplies and a low-voltage converter fed by dc capacitor. In , a diode-clamped H-bridge with multi output boost rectifier functions as the high-voltage inverter. The

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

utilization of clamped diode and rectifier increases the cost of whole system. In , dc voltage ratio of 4:2:1 is arranged to the series-connected H-bridge converters. The expensive isolated dc supplies are required for ratio-4 and ratio-2 converters. The literature describes a motor drive system based on hybrid multilevel H-bridge converters with unequal dc voltage supplies. Fundamental frequency modulation is adopted in the literature for cascade hybrid H-bridge converters. In, the selective harmonic elimination method is adopted for hybrid modulation and selecting switching redundant states is applied for capacitor voltage control.

II. CONFIGURATION OF THE 100-V 3-KVA STATCOM SYSTEM

Fig. 1 shows the configuration of a three-phase STATCOM rated at 100 V and 3 kVA, which is based on hybrid cascade single-phase H-bridge cells. Table I summarizes the circuit parameters. The cascade number of N = 2 is assigned to the prototype because of the restriction of the author’s laboratory condition, resulting in six converter cells in total. In each phase cluster, one single-phase H-bridge cell is controlled as a high voltage converter with dc-link voltage of 110 V, and the other single-phase H-bridge cell acts as a low-voltage converter with dc-link voltage of 65 V.

Fig. 1. 100-V 3-kVA downscaled STATCOM. (a)

Configuration of experimental system

The explanation of this arrangement is described in the next section. Each cell is equipped with an isolating electrolytic capacitor with a capacitance value of 9400 μF. No auxiliary circuit is connected to the six split dc capacitors except for six voltage sensors.

TABLE I CIRCUIT PARAMETERS IN FIG. 1

An ac inductor is also required for each cluster to support the difference between the sinusoidal source voltage and the ac pulse width modulation (PWM) volt age of cluster, and it also makes contribution in filtering out switch ripples caused by high-frequency modulation. In experiment, switching frequencies of 50 Hz and 5 kHz are assigned to high-voltage converter and low-voltage converter, respectively. The constant dc voltages are achieved purely by the control algorithm. This design is reasonable to verify the improvement of output voltage waveform and the performance of the 100-V 3-kVA STATCOM system, because these special characteristics mainly relay on control strategy.

Fig. 2. Fully digital control system for 100-V 3-kVA

STATCOM.

As shown in Fig. 2, this system is totally controlled by a fully digital controller using a 32-bit digital signal processor (DSP) and field-programmable gate arrays (FPGA). Most of the calculation is dealt with by the DSP chip and the hybrid modulation strategy is implemented on FPGA chip. The gating signals for all the switching devices are generated by the hybrid modulation, which matches the 10-kHz sample frequency well. Hybrid modulation shown in Fig. 3 includes two parts: fundamental modulation and PWM modulation.

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

Fig. 3. Diagram for hybrid modulation.

The fundamental modulation could simply be described as follows: when the sinusoidal command is higher than a threshold value of Vcmp , the high-voltage converter outputs positive voltage; as the sinusoidal command is lower than the negative threshold value of −V cmp , the high-voltage converter outputs negative voltage; and if the sinusoidal command is in the range between –Vcmp and V cmp , the high-voltage converter outputs zero. The remaining part of sinusoidal command and the quasi-square-waveform voltage is the command voltage for low-voltage converter. It is modulated by single-polar PWM modulation technology with the carrier frequency of 5 kHz. Based on this modulation strategy, an ac waveform with higher voltage levels is produced. It brings the advantages of improving output quality, keeping high equivalent switching frequency, and reducing power loss.

A. Fundamentals of Circuit Operation

In each cluster, the high-voltage converter outputs quasi-square waveform, while the low-voltage converter outputs the remaining part between sinusoidal command and the quasi-square waveform shown in Fig. 4. Therefore, low-frequency contents in total ac voltage are completely eliminated. The total ac voltage only includes fundamental component and harmonic components around switching frequency.

Fig. 4. Three-phase equivalent circuit.

Three-phase smooth sinusoidal currents are guaranteed because the high-frequency components can be easily filtered by interface inductors. The sharp change of load can be well followed by dynamically trimming the sinusoidal command voltage, which ensures the fast speed response. As the issue with dc voltage control is also very crucial for safe operation. DC voltage control strategy is also investigated in this paper. As shown in Fig. 4, a fundamental-frequency zero-sequence current is suggested to the delta-loop for balancing of dc voltages among three phases. This clustered balancing method enables the STATCOM system to compensate unbalanced load.

Moreover, unequal dc voltages of high-voltage converter and low-voltage converter are achieved by the effect of individual voltage control, which suggests a small trim to the fundamental contents of quasi-square-waveform voltage of high-voltage converter for active-power redistribution between high-voltage and low-voltage converters. In the whole process, the low-voltage converter always compensates the remaining part to match the sinusoidal command. Additionally, the sum of all capacitor voltages is regulated by the algorithm of overall voltage control.

III. CONTROL STRATEGY

Fig. 5 shows a block diagram of the control algorithm proposed in this paper. The whole control algorithm consists of four parts, namely, decoupled current control, overall voltage control, clustered balancing control, and individual voltage control.

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

Fig. 5. Block diagram of the total control scheme for

100-V 3-kVA STATCOM.

TABLE II CONTROL GAINS AND PARAMETERS

A. Decoupled Current Control

Referring to Fig. 1, the set of voltage–current equation can be obtained as follows:

퐿 + 푅 푖 = 푣 − 푣

퐿 + 푅 푖 = 푣 − 푣

퐿 + 푅 푖 = 푣 − 푣 (1)

Where RL is the equivalent series resistance of the inductor. Applying the d–q transformations, the equations in d–q axis are derived

퐿 − 휔퐿 . 푖 + 푅 푖 = 푣 − 푣

퐿 − 휔퐿 . 푖 + 푅 푖 = 푣 − 푣 (2)

The proportional and integral fuzzy regulators with parameters of k ip and kii are introduced for closed-loop current control. Parameters of kip and kii are given in Table II. The command voltages in d-axis and q-axis are given by

푣 = 휔퐿 . 푖 + 푣 − 푘 + (푖∗ − 푖 )

푣 = −휔퐿 . 푖 + 푣 − 푘 + 푖∗ − 푖 (3)

Here, id and iq are the feedback currents in d-axis and q-axis, respectively. i∗d and i∗q are the d-axis and q-axis reference currents. The three-phase command voltages viu∗ , viv∗ , and viw∗ can be obtained by applying the inverse d–q transformations to vid and viq, as shown in Fig. 6. The command current generating algorithm intended for detecting reactive and negative-sequence load current includes two parts: the reactive current algorithm and the negative-sequence current algorithm.

Fig. 6. Block diagram of decoupled control.

These two parts are all based on d–q transformations and moving average low-pass filter. The main difference between them is that the negative-sequence current algorithm needs to change the position of b-phase current and c-phase current when applying d–q transformations.

Fig. 7. Block diagram of command current generating

algorithm.

As shown in Fig. 7, the upper algorithm is for reactive current detection and the lower algorithm is for negative-sequence current detection. The three-phase line currents i ∗la , i ∗lb, i ∗lc ought to be transformed into phase current because of the delta configuration of the three clusters. Fig. 8 shows the diagram for this transformation.

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

Fig. 8. Diagram for the transformation from line

current to phase current

One of the solutions, as described in the following equation, is preferred because of its independence from the zero-sequence current [23]:

푖 = − (푖∗ − 푖∗ )3

푖 = − (푖∗ − 푖∗ )3

푖 = − (푖∗ − 푖∗ )3 (4)

B. Overall Voltage Control

Fig. 9 shows the overall voltage control diagram. Vdc ref is the reference value for the sum of all the dc capacitors’ voltage. vdc sum acts as the feedback, which is obtained by summing up all the dc capacitors’ voltage. The fuzzy regulator is preferred for overall control. The regulator design process for kvp and kvi is similar to that in the literature [24]. The output of fuzzy regulator is the active component of command current.

Fig. 9. Block diagram of overall voltage control

C. Clustered Balancing Control

Fig. 10 shows the block diagram of clustered balancing control. Fuzzy regulators with constant parameters are adopted for calculating the amount of power for redistribution. The reference zero-sequence current is synthesized based on (1) and (6).

Fig. 10. Clustered balancing control among the three

clusters by introducing a zero-sequence current, where each of the three clusters is considered as

single-phase H-bridge converters.

Referring to (3) and (5), the closed-loop control is formed in Fig. 11, where Vcu is the total dc voltage of u-phase cluster at steady-state operating point; vˆcu is the small voltage change around V cu . The closed-loop transfer function is obtained by

푣푣

=푅푘 푠 + 푅푘

푅퐶푉 푠 + 2푉 + 푅푘 푠 + 푅푘 (5)

Fig. 11. Block diagram of clustered balancing

control.

Based on the parameters of kop and koi for fuzzy regulators are designed and the design processing can be found in the literature. The parameters are given in Table II.

D. Individual Voltage Control

Fig. 12 shows the block diagram of the individual voltage control. Let Δucu1 be the difference between the reference voltage and the capacitor voltage of high-voltage converter

∆푣 = 푣 _ − 푣 (6)

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

Fig. 12. Individual voltage control, taking u-phase

cluster for example.

After introducing the proportional regulator kp, the closed loop control based on (20) is formed in Fig. 13, where vˆcu1 is the small voltage change around Vcu1. The closed-loop transfer function is obtained by

= .

. (7)

Based on (29), the regulator kp is designed and the design processing can be found in the literature.

Fig. 13. Block diagram of individual voltage control.

IV. FUZZY LOGIC CONTROLLER

In FLC, basic control action is determined by a set of linguistic rules. These rules are determined by the system. Since the numerical variables are converted into linguistic variables, mathematical modeling of the system is not required in FC. The FLC comprises of three parts: fuzzification, interference engine and defuzzification. The FC is characterized as i. seven fuzzy sets for each input and output. ii. Triangular membership functions for simplicity. iii. Fuzzification using continuous universe of discourse. iv. Implication using Mamdani’s, ‘min’ operator. v. Defuzzification using the height method.

Fig.14 Fuzzy logic controller

Fuzzification: Membership function values are assigned to the linguistic variables, using seven fuzzy subsets: NB (Negative Big), NM (Negative Medium), NS (Negative Small), ZE (Zero), PS (Positive Small), PM (Positive Medium), and PB (Positive Big). The partition of fuzzy subsets and the shape of membership CE(k) E(k) function adapt the shape up to appropriate system. The value of input error and change in error are normalized by an input scaling factor

TABLE III FUZZY RULES

In this system the input scaling factor has been designed such that input values are between -1 and +1. The triangular shape of the membership function of this arrangement presumes that for any particular E(k) input there is only one dominant fuzzy subset. The input error for the FLC is given as

E(k) = ( ) ( )

( ) ( ) (8)

CE(k) = E(k) – E(k-1) (9)

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

Fig.15 Membership functions

V. SIMULATION RESULTS

The below figures shows the simulation results verifying the effect of hybrid modulation. As the dc voltage of the high-voltage converter maintains at 110 V and those of the low-voltage converter stays at 65 V, nine-level voltage waveform is produced. The voltage steps of 45 and 65 V are close to each other, so it looks like a seven-level voltage waveform. The quasi-square-waveform voltage of the high-voltage converter and the three-level PWM waveform voltage of low-voltage converter are also generated as expected. This hybrid modulation scheme is effective in both producing high-quality low-harmonic output voltage and reducing the power loss of high-voltage converters.

Fig.16. Matlab model of proposed system

Fig.17. Fuzzy logic controller

Fig.18. Matlab model of multi level inverter.

Fig.19. Matlab model of control algorithm.

Fig. 20. Simulational waveforms testing for the

hybrid modulation. CH1: source voltage vsab; CH2: output voltage of u-phase cluster viu;CH3: output voltage of cell u2viu2; CH4: output voltage of cell

u1viu1.

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

(a)

(b)

Fig. 21. Simulational waveforms testing for individual balancing control with high dc-link

voltage 110 V and low dc-link voltage 65 V. (a) CH1: source voltagevsa; CH2: STATCO Mline current ica (1 A/10 mV); CH3: capacitor voltage

vcu2; CH4: capacitor voltage vcu1. (b) CH1: capacitor voltage vcv1; CH2: capacitor voltagevcv2; CH3: capacitor voltage vcu2; CH4: capacitor voltage

vcu1.

(a)

(b)

Fig. 22. Simulational waveforms testing for clustered balancing control. (a) CH1: source voltagevsa; CH2: capacitor voltage vcu2;CH3:STATCOM line current

ica(1 A/10 mV); CH4: capacitor voltagevcu1. (b) CH1: capacitor voltagevcu2; CH2: capacitor

voltagevcv2; CH3: capacitor voltagevcv1;CH4: capacitor voltage vcu1.

Fig. 23. Simulational waveforms in a transient state from inductive to capacitive operation at 2.2 kVA.

CH1: source voltage vsab; CH2: output voltage viu; CH3: STATCOM phase current icu(1 A/10 mV);

CH4: reactive power q ∗ (kVA/div).

(a)

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

(b)

Fig. 24. Simulational waveforms verifying the effect

of compensating balance load. (a) CH1: source voltage vsa; CH2: source side current isa(1 A/10 mV); CH3: load current ila(1 A/10 mV); CH4:

STATCOM line current ica(1 A/10 mV). (b) CH1: source voltage vsa; CH2: capacitor voltagevcu2;

CH3: capacitor voltage vcu1; CH4: STATCOM line current ica(1 A/10 mV).

(a)

(b)

(c)

Fig. 25. Simulational waveforms verifying the effect of compensating serious unbalance load. (a) CH1: source voltage vsa; CH2: STATCOM line current i

cb(1 A/10 mV); CH3: source side current isa(1 A/10 mV); CH4: STATCOM line current ica(1 A/10 mV).

(b) CH1: capacitorvcu2; CH2: capacitor voltage vcu1;CH3: STATCOM line current i cb (1 A/10

mV); CH4: STATCOM line current ica(1 A/10 mV). (c) CH1: source voltage vsa; CH2: source current i

sb(1 A/10 mV); CH3: source current isa(1 A/10 mV); CH4: STATCOM line current ica(1 A/10 mV).

Fig. 26. Simulational waveforms confirm the control effect when source voltage sag. CH1: source voltage vsa; CH2: output voltage viu; CH3: capacitorvcu1;

CH4: STATCOM line current ica(1 A/10 mV).

V. CONCLUSION

A new control strategy is presented in this paper for dc voltage regulation. Hybrid multi level inverters have many advantages they are: at the output level number of levels get increases there by the harmonics get reduced and quality gets increases and the overall efficiency gets increases. This paper has analyzed the fundamentals of dc voltage control based on cascaded hybrid multilevel H-bridge converters. The control system introduced this paper to maintain the dc voltage at rated value as well as by the ability of compensating serious unbalanced load. This control strategy has taken full advantages of the available switching devices by operating the high-

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VI /Issue 4 / JULY 2016

IJPRES

voltage device at low switching frequency and low-voltage device at high frequency. The control system with the STATCOM increases the overall waveform quality by reducing switching loss, and improving whole system’s efficiency Analyzed the fundamentals of dc voltage control based on cascaded hybrid multilevel H-bridge converters and hybrid modulation for multilevel converter is verified by using MATLAB/SIMULINK.

REFERENCES

[1] W. Song and A. Q. Huang, “Fault-tolerant design and control strategy for cascaded H-bridge multilevel converter-based STATCOM,” IEEE Trans. Ind. Appl., vol. 57, no. 8, pp. 2700–2708, Aug. 2010.

[2] C. Han, A. Q. Huang, M. E. Baran, S. Bhattacharya, W. Litzenberger, L. Anderson, A. L. Johnson, and A.-A. Edris, “STATCOM Impact study on the integration of a large wind farm into a weak loop power system,” IEEE Trans. Energy Convers., vol. 23, no. 1, pp. 226–233, Mar. 2008.

[3] H. Akagi, S. Inoue, and T. Yoshii, “Control and performance of a transformerless cascade PWM STATCOM with star configuration,” IEEE Trans. Ind. Appl., vol. 43, no. 4, pp. 1041–1049, Jul./Aug. 2007.

[4] N. Hatano and T. Ise, “Control scheme of cascaded h-bridge STATCOM using zero-sequence voltage and negative-sequence current,” IEEE Trans. Power Del., vol. 25, no. 2, pp. 543–550, Apr. 2010.

[5] Q. Song and W. Liu, “Control of a cascade STATCOM with star configuration under unbalanced conditions,” IEEE Trans. Power Electron., vol. 24, no. 1, pp. 45–58, Jan. 2009.

[6] R. Sternberger and D. Jovcic, “Analytical modeling of a square-wave controlled cascaded multilevel STATCOM,” IEEE Trans. Power Del., vol. 24, no. 4, pp. 2261–2269, Oct. 2009.

[7] A. J. Watson, P. W. Wheeler, and J. C. Clare, “A complete harmonic elimination approach to DC link voltage balancing for a cascaded multilevel rectifier,” IEEE Trans. Ind. Electron., vol. 54, no. 6, pp. 2946–2953, Dec. 2007.

[8] F. Z. Peng, J.-S. Lai, J. W. McKeever, and J. Van Coevering, “A multilevel voltage-source inverter with separate DC sources for static var generation,” IEEE Trans. Ind. Appl., vol. 32, no. 5, pp. 1130–1138, Sep./Oct. 1996.

[9] Y. S. Lai and F. S. Shyu, “Topology for hybrid multilevel inverter,” Proc. Inst. Elect. Eng.—Elect. Power Appl., vol. 149, no. 6, pp. 449–458, Nov. 2002.

[10] M. Manjrekar and T. Lipo, “A hybrid multilevel inverter topology for drive applications,” in Proc. IEEE Appl. Power Electron. Conf., Feb. 1998, vol. 2, pp. 523–529.

RAJA KUMARI.B Completed B.E. in Electrical & Electronics Engineering in 2011from Kottam college of Engineering Kurnool Affiliated to JNTUA, Anantapuram and M.Tech in Power Electronics in 2013 from St.Johns college of Engineering College Affiliated to JNTUA,Anantapuram Working as Assistant Professor at ST.JOHNS COLLEGE OF ENGINEERING MR.A. Mallikarjuna Prasad yerrakota.yemmignur-518360 kurnool (dist) A.P India. Area ofinterest includes Power Electronics.

E-mail id: rajakumare001@gmail.com