A New Multi Carrier Based PWM for Multilevel Converter

Bahr Eldin S. Mohammed and K.S.Rama Rao

Department of Electrical and Electronic Engineering Universiti Teknologi PETRONAS,

Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia

Abstract- This paper addresses a new multi-carrier modulation technique called wave shift multi-carrier modulation (WSHM), which used to control the cascade multilevel converter (CMC). The proposed switching technique generates lower voltage total harmonic distortion (THD) in comparison with conventional multi-carrier based pulse width modulation (PWM) schemes, phase-shift (PSHM) and level-shift (LSHM) modulations. To compare the performance of proposed method with PSHM and LSHM techniques, a simulation circuit of seven and nine levels for CMC is designed and simulated. To complete the comparison between the proposed method and PSHM, LSHM the dynamic behavior of dynamic voltage restorer (DVR) based seven-level CMC for power quality improvement is investigated. Digital simulations are carried out using PSCAD/ EMTDC to validate the performance of CMC.

I. INTRODUCTION

Multilevel converters (MLC) are emerging as a new breed of power converter options for power system applications. Recent advances in power switching devices enabled the suitability of MLCs for high voltage and high power applications because they are connecting several devices in series without the need of component matching. The general structure of the MLC is to synthesize a sinusoidal voltage by several levels of voltages, typically obtained from capacitor voltage sources. Three types of capacitor voltage synthesis based multilevel converters are reported [1] as follows:

1. Diode-Clamped Multilevel Converter (DCMC). 2. Flying-Capacitor Multilevel Converter (FCMC). 3. Cascaded Multilevel Converters (CMC)

Compared DCMC and FCMC converters, a CMC as shown in Fig.1 is easy to design and assemble because of the uniform circuit structure of the converter units and modularized circuit layout. Easy packaging is also possible in CMC topology as each level has the same structure, and there are no extra clamping diodes or voltage-balancing capacitors, which are required in the DCMC and the FCMC. The number of output voltage levels can then be easily adjusted by changing the number of full-bridge converters. The CMC synthesizes a desired voltage from several independent sources of DC voltages, which may be obtained from batteries, fuel cells or solar cells [2]. In general, the output voltage of CMC is controlled as follows [3]: i. By controlling the pulse width of i. the output voltage by fundamental frequency switching (FFS) method or PWM technique while keeping the magnitude of dc voltage fixed.

ii. By controlling the magnitude of dc voltage while keeping the pulse width of output voltage fixed or by controlling the modulation index in case of PWM. iii. By controlling both the magnitude of dc voltage and the pulse width of the output voltage. The basis of selecting the method of controlling the converter output voltage is dependent on the ability to control the THD. In this paper, the second method will be adopted to control the inverter output voltage. The objective of achieving minimum THD is based on two switching methods, the fundamental frequency switching (FFS) and multi-carrier-based PWM technique.

g12a

g12

g11a

g11

g14

g13a

g13

g14a

g16

g15a

g15

g16a

R=0

R=0

R=0

Vout

Fig.1. One phase of seven-level CMC

FFS modulation can be easily implemented for the CMC due to its unique structure. All switching angles can be calculated off-line and then stored in a look-up table for digital implementation. Compared with the carrier-based PWM schemes, FFS features low switching losses since all the IGBT switches operate at fundamental frequency. As the expressions for switching angles are nonlinear and transcendental, deriving a valid solution over the full range of amplitude modulation index (ma) is not always possible. Thus the switching angles should be calculated to minimize the magnitude of those harmonics that cannot be eliminated [4].Various PMW techniques applied to the multilevel converters are discussed in [5]-[7]. The PWM techniques can be classified into two categories: the triangle intersection technique and the direct

2011 IEEE Applied Power Electronics Colloquium (IAPEC)

978-1-4577-0008-8/11/$26.00 ©2011 IEEE 63

digital technique (space vector modulation). With the development of digital technology, the space vector modulation is widely used, due to not only relatively easy hardware implementation, but also its features of good dc link voltage utilization and low current ripple. But this method has a very significant drawback that if the voltage level is more than five, the control algorithm becomes too complex to implement [3]. Thus it is reasonable to adapt in this paper the triangle intersection techniques in the high level application.

II. PHASE-SHIFT MULTI-CARRIER MODULATION (PSHM)

CMC with m voltage levels requires (m – 1) triangular carriers. In the phase-shifted multi-carrier modulation, all the triangular carriers have the same frequency and the same peak-to-peak amplitude, but there is a phase shift between any two adjacent carrier waves, given by:

0360( 1)sh mϕ −= (1)

The gate signals are generated by comparing the modulating wave with the carrier wave. Fig. 2 shows the principle of PSHM for one phase of seven levels CMC presented in Fig.1 where six triangular wave carriers are required with a 60° phase displacement between any two adjacent carriers. The advantage of PSHM is that the switching frequency and conduction period is same for all devices and rotating of switching patterns is not required [4].

Fig.2. PSHM for seven-level CMC

Figs. 3 to 8 shows the simulated voltage waveforms and their harmonic content of three-phase seven and nine levels CMC using PSHM under the condition of modulation frequency, fm= 50 Hz, carrier frequency, fcr = 900 Hz, amplitude modulation index, ma = 0.9 and frequency modulation index, mf = 18. The line voltage harmonic spectrum shown in Figs. 5 and 8 is based on 50 Hz base frequency and the THD considered for the first 63 harmonics.

Fig. 3. Output phase voltage of seven-level CMC

Fig. 4. Output line-to-line voltage of seven-level CMC

Fig. 5. Seven-level CMC output line voltage harmonics (THD = 16.2264 %)

Fig.6. Output phase voltage of nine-level CMC

Fig. 7. Output line to line voltage of nine-level CMC

Fig. 8. Nine-level CMC output line voltage harmonics (THD = 10.0737 %)

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III. LEVEL SHIFT MULTI-CARRIER MODULATION (LSHM)

For m-level CMC using level-shifted multicarrier modulation scheme, (m – 1) triangular carriers are required, all having the same frequency and amplitude. The (m – 1) triangular carriers are vertically disposed such that the bands they occupy are contiguous. The amplitude modulation index is defined as:

)1( −= mVV

a cr

mm (2)

Where Vm is the peak amplitude of the modulating wave and Vcr is the peak amplitude of each carrier wave. There are three schemes for level shift multi-carrier modulation listed as follows: (i) In-phase disposition (IPD), where all carriers are in phase. (ii) Alternative phase opposite disposition (APOD), where all carriers are alternatively in opposite disposition. (iii) Phase opposite disposition (POD), where all carriers above the zero reference are in phase but in opposition with those below the zero reference. In this paper only IPD modulation scheme is addressed as it provides the best harmonic profile of all three-level shift multi-carrier modulation schemes [4]. Fig. 9 shows the principle of the IPD modulation for one phase of seven-level CMC reported in Fig.1.

Fig. 9. LSHM for seven-level CMC

A. Simulation of Three Phase Seven and Nine- Levels CMC by Using LSHM

Fig.10 to 15 shows the simulated voltage waveforms and their harmonic content of three phase seven and nine levels CMC using LSHM under the same conditions of PSHM simulation.

Fig. 10. Output phase voltage of seven-level CMC

Fig.11. Output line to line voltage of seven-level CMC

Fig. 12. Seven-level CMC output line voltage harmonics (THD = 14.5639 %)

Fig.13. Output phase voltage of nine-level CMC

Fig.14. Output line to line voltage of nine-level CMC

Fig. 15. Nine-level CMC output line voltage harmonics (THD = 8.4603 %)

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IV. PROPOSED METHOD

The proposed modulation technique is a combination of phase shift multi-carrier and level-shifted multi-carrier modulation (in-phase disposition (IPD)) schemes which overcomes the problem of rotating of switching pattern of level-shifted multi-carrier modulation and small phase displacement at phase voltage of CMC. For m level CMC in the proposed method, (m – 1) triangular carriers are required. In the carrier wave all the triangles have the same frequency, same peak to peak amplitude and are vertically disposed, but there is a phase shift between any two disposed carrier waves as in (3).

)1(43600

−= mshφ (3)

The amplitude modulation index is defined as in (4)

)1( −= mVV

a cr

mm (4)

Fig.16 shows the principle of the proposed method for one phase of seven-level CMC reported in Fig.1.

Fig. 16. Proposed method for seven-level CMC

A. Simulation of Three Phase Seven and Nine- Levels CMC by Using Proposed Method

Figs.17 to 22 shows the simulated voltage waveforms and their harmonic content of three-phase seven and nine-levels CMC using the proposed method under under the same conditions of PSHM simulation.

Fig. 17. Output phase voltage of seven-level CMC

Fig.18. Output line to line voltage of seven-level CMC

Fig. 19. Seven-level CMC output line voltage harmonics (THD = 13.512 %)

Fig.20. Output phase voltage of nine-level CMC

Fig.21. Output line to line voltage of nine-level CMC

Fig. 22. Nine-level CMC output line voltage harmonics (THD = 8.2846 %)

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Load 1

B1

DVR Controller

Vs 13 kV 100 MVA

13/115/115 kV

DVR

115/11 kV

115/11 kV

Load 2

Fault

V. DYNAMIC VOLTAGE RESTORER (DVR)

The power electronic converter based series compensator that can protect critical loads from all supply side disturbances other than outages is called a DVR. The DVR is capable of generating or absorbing independently controllable real and reactive power at its ac output terminals. The main focus in this paper is to compare the performance of the proposed switching technique with multi-carrier-based PWM schemes, phase-shift and level-shifted modulation. Based on the pervious analysis of seven and nine level CMC, a seven level CMC is selected as a candidate design for DVR. This paper analyzed the dynamic behavior of the DVR and the controlling system based on the proposed method as well as phase-shift and level-shifted modulation techniques.

Fig. 23. Schematic representation of the DVR The DVR injects a set of three-phase ac output voltages in series and in synchronism with the distribution feeder voltages. The amplitude and phase angle of injected voltages are variable there by allowing control of the real and reactive power exchange between the DVR and the distribution system. The reactive power exchange between the DVR and the distribution system is internally generated by the DVR without ac passive reactive components. The DVR can protect the

sensitive load by injecting voltage of controllable amplitude, phase and frequency into the distribution feeder via a series injection transformer. Thus DVR can only supply partial power to the load during very large variations (sag or swells) in source voltage [8]-[12]. The real power exchanged at the DVR Output ac terminals are provided by the DVR input dc terminals or by an external energy source or energy storage system. Fig. 23 shows a typical DVR connected in series with 11 kV distribution feeder that supplies a sensitive load. A test system comprising of 13.0 kV, 100 MVA, 3-phase transmission line, represented by Thevenin equivalent, feeding into the primary side of a three-winding transformer is selected as shown in Fig. 24. Two (R-L) loads are connected to the tertiary windings via two two-winding transformers. The DVR is connected in series with a sensitive load 2 through a coupling transformer, with a leakage reactance of 10 %. A unity transformer turn ratio is used for the DVR coupling transformer with no booster capabilities. A DVR based CMC is connected to the 11 kV tertiary winding to provide instantaneous voltage support at the load point. Two different simulation studies are carried out using PSCAD/EMTDC. In the first case the system is tested without the DVR for a three-phase fault via a fault resistance of 0.36Ω. The voltage sag is observed from 0.3 s to 0.6 s as shown in Figs. 25. The second simulation is carried out using the same scenario as above but now with the DVR based seven-level CMC in operation in three different modes as follows: (i) DVR control system based on PHSM. (ii) DVR control system based on LHSM. (iii) DVR control system based on proposed technique. The simulation results are presented as shown in Figs. 26 to 28. The DVR based seven level CMC is simulated to be in operation only for the duration of the fault of 0.3 s and the total simulation period is observed to be 0.8 s. As the DVR is in operation, the voltage sag is mitigated almost completely at the end of 0.6 s. The sag mitigation is performed with smooth, stable and rapid DVR controls based on the proposed method. Acceptable overshoots are observed when the DVR comes in and out of operation.

Non-Sensitive load

Sensitive load

Coupling Transformer

Voltage Source Converter

DC Energy Storage

Fig.24. Schematic diagram of test system including DVR

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Fig. 25. Load voltage during three-phase fault without DVR

Fig. 26. Load voltage during three-phase fault with DVR based on PSHM

Fig. 27. Load voltage during three-phase fault with DVR based on LSHM-IPD

Fig. 28. Load voltage during three-phase fault with DVR based on the

proposed method

VI. COMPARSION BETWEEN WSHM AND PSHM,LSHM SCHEMES

The line voltage THD profile of nine-level CMC with the WSHM method and the other two PSHM and LSHM methods under different values of mf are shown in Figs. 29(a) and 29(b) based on 50 Hz base frequency and the THD considered for 255 harmonics. It can be seen that the THD produced by WSHM method lower than by PSHM and LSHM techniques.

(a)

(b)

Fig.29 THD profile of line voltage produced by the nine-level CMC (a) ma = 1, (b) ma = 0.85

CONCLUSIONS

This paper presents simulation of seven and nine levels CMC and one of the custom power equipment namely, DVR based seven-level CMC in a distribution system. Three types of multicarrier based PMW techniques were considered to control the output voltage of CMC. Among those three modulation techniques, it has been found and proved that the proposed method is better than others in terms of THD reduction. The simulation results have demonstrated excellent voltage regulation capabilities of the seven levels CMC based DVR using the proposed new multi-carrier based PWM technique.

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