Comparison and Analysis on Common Modulation Strategies for the Cascaded Multilevel STATCOM
Chunyan Zang
1,2, Zhenjiang Pei
2, Junjia He
1, Guo Ting
1, Jing Zhu
2, Wei Sun
21 Institute of Electric and Electronic Engineering, Huazhong University of Science and Technology, CHINA
2 Xi’an Electric Manufacturing Corp., CHINA
Abstract- With the application of multilevel inverter in the high voltage and large power occasions in recent years, its modulation strategy has become a research hot point in the field of power electronics. As STATCOM is a typical application of multilevel converter, its common modulation strategies such as Step Wave modulation, Specific Harmonic Elimination modulation, Carrier PWM modulation, Space Vector modulation are summarized in this paper. It also points out the particularity of the most popular cascaded multilevel structure to other main circuit topologies of STATCOM and pays heavy attentions on two typical modulation strategies: SHE-PWM and CPS-SPWM. Simulations on these two algorithms in a cascaded 5-level STATCOM are done. By results discussion and comparison, it is known that these two methods are both suitable for the cascaded multilevel topology. The harmonic elimination ability of SHE-PWM is better than that of CPS-SPWM, while its calculation work becomes tremendous and difficult with the level number increasing. CPS-SPWM is easy to realize and its calculation speed is fast even under a very high level number. The proper modulation strategy of a cascaded multilevel STATCOM will be chosen based on the switch frequency limitation, STATCOM volume and its main circuit topology requirements.
I. INTRODUCTION
With the development of power devices and relative theories in many years, the theory and material foundation for the application of multilevel converter have been completed [1]. There are a lot of various products widely used in the high voltage and large power cases, such as variable frequency speed (VVVF), active power filter devices (APF), HVDC systems and reactive power compensation of power system, etc. STATCOM is one of the typical applications, which has less harmonics, high efficiency and reliability. The advantages of STATCOM have been shown in the reactive power compensation field of power system day by day. With its increasing application in the power grid, controlling the performance of STATCOM becomes an important goal. Generally speaking, the waveform quality, switching losses, voltage utilization are some of the principal indicators for estimating its modulation strategy. With the switching frequency of modern high power device increasing, the waveform quality has been improved much, while the
switching loss problem has become highlighted increasingly.
When solving the switching losses problem, the solution ways of improving the circuit topology, which is represented by the soft-switching technology, also brings about the problem about circuit structure complication. For some large and complex installations such as STATCOM, it is particularly true.
Therefore, how to decrease the switching loss of STATCOM from the view of modulation strategies optimization is a question worth discussing.
At present, the common modulation strategies for STATCOM are Step Wave modulation, Carrier PWM modulation, Specific Harmonic Elimination modulation, Space Vector modulation and so on[2]-[5]. Among them, CPS-SPWM technique is an excellent modulation strategy for high power switch inverter, and it is able to achieve higher switching frequency equivalent effect at a lower switching frequency. This method not only makes it possible for the SPWM technology applying in the high power field, but also effectively enhances the capacity of devices, reduces the output harmonics and improves the entire signal transmission bandwidth of devices at the same time. In addition, the technology also has advantages such as good linearity of the input and output, superior control performance, which strengthen the application base for the CPS-SPWM technique in STATCOM. This paper summarizes several common modulation strategies for the cascade multilevel structure of STATCOM, does MATLAB simulation experiment of CPS-SPWM method, as well as compares the CPS-SPWM method to SHE-PWM method. At last some useful conclusions are given out.
II. MAIN CIRCUIT STRUCTURE
Early STATCOM generally used the zigzag transformer in the main circuit topology as voltage source inverter. However, the zigzag transformer has some difficulties, which is hard to overcome in terms of cost, transformer loss as well as the control. When the cascaded multilevel inverter is used as the main circuit topology, STATCOM has a little floor space, easy to split-phase control, high reliability, and easy expansion of capacity, etc. In addition, because it has no use for the zigzag transformer, it makes the existing STATCOM free of the most serious problems such as device over-voltage caused by the
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magnetic remanence saturation and nonlinearity in the transformer excitation circuit .So the cascaded multilevel inverter structure for large capacity STATCOM is concerned by a large number of engineering designers more and more.
Fig.1 shows the main circuit of a cascaded STATCOM with 5-level voltage. Each phase chain inverter is made up of 2 single-phase bridges (known as H-bridge) with independent DC capacitors in series. The detailed unit connection style has two ways: Y-connection or △-connection. And each H-bridge can work in two-level or three-level inverter mode. As the output voltage of the cascaded inverter is the superposition of each H-bridge output voltage, increasing the level number can make the harmonic content of the output voltage reduce further, or reduce the switching frequency under the premise of meeting the harmonic requirements of the main switch devices.
Fig. 1. A 5-level cascaded STATCOM in △-connection way III. COMMON MODULATION STRATEGIES A. Step Wave Modulation
Step wave modulation is a simple method by using the staircase waveform to approach the sine wave, which is a comparatively visual way. In this method, the duration time of each level can be selected in order to eliminate some low-order harmonics, while the more the voltage level has, the better spectral characteristics can be gained. The advantages of this strategy are easy hardware implementation, minimum switching frequency and little switching loss. However, this method would bring a few low-order harmonic components into the output waveform, which makes the wave quality poor. On the other hand, it is difficult to achieve the dynamic adjustment on the output waveform, which means it is not able to build an effective closed-loop control.
Fig. 2. 9-level step wave modulation scheme B. Carrier PWM Modulation
The carrier PWM modulation strategy for multilevel inverter
is the spread application of two-level carrier SPWM technology into the multilevel field. Its basic theory is as below: in order to get a sine voltage waveform with a preset frequency, SPWM is realized by comparing a sine control signal at that frequency with a triangular wave, whose crossing points are used to make sure the switch change-over moments (Fig.3). So it is also called as “Triangle Carrier Method”. For the multilevel carrier PWM modulation, if the level number of the converter is N, it requires N-1 carriers.
Fig. 3. Sine modulation wave and triangle carrier waves
The multilevel carrier modulation can change a lot due to the various control freedom degrees. On the hand of the carrier wave, the degree of freedom includes the carrier wave type, amplitude, frequency, phases, offset and phase relationship between carriers. On the hand of the modulation wave, the degree of freedom also includes modulation wave type, amplitude, frequency, whether containing the superposition of zero-sequence component or not, and the phase relationship of multiphase system between modulation waves. Besides that, there are some other degrees of freedom such as the phase between the carrier and modulation wave. By choosing these different freedom degrees, it can generate various methods for the specific topology and implementation of specific objects control. Carrier modulation methods for the multilevel STATCOM in the existing literature include Sub Harmonic PWM, Switching Frequency Optimal PWM and Carrier Phase Shifting PWM (CPS-SPWM). The first two are mainly used in diode-clamped multilevel converter, while the last one is mainly used for cascaded multilevel converter.
C. Selective Harmonic Elimination Modulation
Fig. 4. The “groove” scheme of SHE-PWM algorithm
It has also been called as the “Switch-point Preset PWM”
which based on the multilevel step wave modulation strategy. Its principle is the selective elimination of specific harmonics by presetting the appropriate “groove” on the step wave, which can improve the quality of output waveform as well as reduce the output THD (Fig.4). This method is similar to the staircase wave harmonic elimination method, while the only difference is the coefficients of output voltage waveform after the Fourier analysis. Low switching frequency, high efficiency, low harmonic content and high utilization of voltage are the
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prominent advantages of SHE-PWM. However, there are very complicated nonlinear transcendental equations need to be solved when seeking the particular switch points for the multilevel SHE-PWM method, which limits its industrial application. So its application is commonly limited in the 3-level structure STATCOM at present.
D. Space Vector Modulation
Space vector modulation has been widely studied and applied because of its advantages such as wide range of linear modulation, high DC voltage utilization, little storage space, simple structure, convenient control, easy realization and so on.
Like the two-level SVM, multilevel SVM is a vector-based synthesis modulation method. Take a three-phase three-level structure for example, the output voltage of each phase has three kinds of status: P, 0, or N, so the total output of three-phase has 27 kinds of status. Its space vector scheme is shown in Fig.5. In 27 kinds of status, there are 3 zero-vectors, 24 non-zero vectors (including 6 vectors whose spatial location are overlapped). So totally there are 19 independent space vectors, which are called the “Basic Vectors”.
Fig. 5. 3-level SVPWM algorithm vector scheme
SVM algorithm usually consists of four steps: ① Projecting the reference vector into the selected coordinate system; ② Taking the integer value of the projector component; ③ Identifying the neighboring vector and calculating the operation ratio; ④ Mapping the switch status. For an M-level inverter, the number of the switching status is
m
3 and the number of the basic vector is 3m(m-1)+1. The calculation work of the conventional SVM algorithm will increase dramatically with the increase of level number, as well as the algorithm complexity.IV. ALGORITHM SIMULATION
As it has been improved by some literatures that the SHE-PWM is an improvement method on the base of the Step Wave modulation and the mathematic theory of Space Vector modulation is equivalent to that of Carrier PWM [6]-[7], this paper pays heavy attention on two typical modulation strategies:
Carrier PWM and SHE-PWM.
A. CPS-SPWM Method
Because there are a variety of SPWM modulation methods in practical applications, such as Unipolar SPWM, Bipolar SPWM and Unipolar Dual-Frequency SPWM, it leads to several CPS-SPWM methods on that base[8]-[10]. Among them, the positive half-period of the output voltage only has positive pulses and the negative half-period only has negative pulses,
which modulation way is called as “Unipolar Dual-Frequency SPWM”. This paper introduces Unipolar Dual-Frequency CPS-SPWM method. Set the frequency modulation ratio Mf as 10, the amplitude modulation ratio Ma as 0.8, and the frequency of carrier waveform as 250Hz, the simulation results in MATLAB are as below:
Fig. 6. Modulation waveform (A-5-level) Fig. 7. Output waveform (A-5-level) Fig.6 is the modulation waveform and Fig.7 is the three-phase output waveforms of CPS-SPWM under the condition that the simulation STATCOM model has 2 H-bridges per phase. Do FFT analysis and we can get the frequency spectrum of the STATCOM output voltage by CPS-SPWM modulation strategy, which is shown in Fig.8. From the Fig.8 it can be seen that the amplitude value of the fundamental wave is 160V and the harmonics concentrates near the 20th order. If the number of H-bridge per phase adds one, the main harmonic contents move to the 30th order nearby.
Fig. 8. Frequency spectrum (A-5-level) Fig. 9. Frequency spectrum (A-7-level) B. SHE-PWM Method
In order to comparing with the results of CPS-SPWM, the switching frequency of SHE-PWM is chosen as 250Hz, Ma is 0.8 and each H-bridge has 5 switch angles. Its simulation results in MATLAB are as below:
Fig.10. Modulation waveform (B-5-level) Fig.11. Output waveform (B-5-level) Fig.10 is the modulation waveform and Fig.11 is the three-phase output waveforms of CPS-SPWM under the condition that the simulation STATCOM model has 2 H-bridges per phase. Do FFT analysis and we can get the frequency
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spectrum of the STATCOM output voltage by SHE-PWM modulation strategy, which is shown in Fig.12. From the Fig.12 it can be seen that the amplitude value of the fundamental wave is also 160V and the harmonics under the 30th order basically approach to zero, which means the harmonic elimination result of SHE-PWM is better than that of CPS-SPWM under 5-level condition. If the number of H-bridge per phase adds one, the harmonic contents under the 35th order basically approach to zero, whose superiority on harmonic elimination is not very obvious at that time.
Fig.12. Frequency spectrum (B-5-level) Fig.13. Frequency spectrum (B-7-level) Comparing Fig.8 and Fig.12, Fig.9 and Fig.13, it can be seen that the obvious superiority of SHE-PWM on the waveform quality decreases with the H-bridge number increasing. Now more and more STATCOM prototypes use the CPS-SPWM modulation strategy. From the on-the-spot running experience of the exist STATCOM projects, early projects especially whose volume over 50MVar have taken the SHE-PWM method, as well as the present projects especially whose volume under 10MVar are inclined to adopt the CPS-SPWM method. With the development of the switching frequency and other performances of power electronics device, it can be predicted that the application of CPS-SPWM will increase more. Of course, with some inherent advantages, the SHE-PWM method is the unique at some certain cases. And it will also have wide applications and perspective if those non-linear equations can be accurately solved by some shortcut algorithms.
V. CONCLUSIONS
Multilevel inverter modulation strategy has been one of the core contents of the study on multilevel inverter. Multilevel inverter modulation strategy is closely linked its topology.
Different topology has different characteristics and different performance requirements. However, it can be summed up into two main control objectives: The first is the output voltage control, that is, the output pulse sequence is equivalent to the reference voltage waveform in the volt-second sense. The second is the inverter operation status control, which includes the capacitor voltage balance control, output harmonic control, switching device power output balance, device loss control and so on.
At present, STATCOM in the multilevel cascade structure is popular in high voltage, high capacity industrial fields and its modulation strategy is constantly optimized. Aimed at this kind of STATCOM, this paper introduces several common modulation strategies and compares their characteristics. Theory analysis and simulation results show that CPS-SPWM method
can get the higher system equivalent switching frequency under lower device switching frequency, as well as reduce the inductance values in AC side and increase the transmission bandwidth of STATCOM. Using this modulation strategy can reduce the size of STATCOM, lower costs, and help to improve the dynamic response of devices. However, we ought to see that the voltage utilization ratios of SHE-PWM are higher than that of CPS-SPWM, while the algorithm complexity of SHE-PWM will increase with the level number increasing. In practice application, it needs to choose the proper PWM modulation strategy based on the switch frequency limitation, STATCOM volume and its main circuit topology requests. And the application trends of these modulation strategies may be changed with the development of hardware and software in the relative field.
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