Scholarly article on topic 'Research on Modular Multilevel Converter Suitable for Direct-drive Wind Power System'

Research on Modular Multilevel Converter Suitable for Direct-drive Wind Power System Academic research paper on "Electrical engineering, electronic engineering, information engineering"

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Abstract of research paper on Electrical engineering, electronic engineering, information engineering, author of scientific article — Yan Gangui, Liu Jigang, MU Gang, Liu Yu, Liu Yang, et al.

Abstract This paper studies Modular Multilevel Converter (MMC) which is applied to Direct-drive wind power system. With the actual situation of wind power generation system, there is a control method presented. It is appropriate for an arbitrary number of voltage levels and the frequency of the AC side waving in a larger changing range. The method of determining the capacitance parameters in submodules is also designed. The converter model is built in PSCAD/EMTDC simulation platform; the simulation results validate the rightness and feasibility of the control method.

Academic research paper on topic "Research on Modular Multilevel Converter Suitable for Direct-drive Wind Power System"

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Energy Procedia 17 (2012) 1497 - 1506

2012 International Conference on Future Electrical Power and Energy Systems

Research on Modular Multilevel Converter Suitable for Direct-drive Wind Power System

Yan Gangui, Liu Jigang, MU Gang, Liu Yu, Liu Yang, Song Wei

School of Electrical Engineering, Northeast DianLi University, Jilin 132012, Jilin Province, China

Abstract

This paper studies Modular Multilevel Converter (MMC) which is applied to Direct-drive wind power system. With the actual situation of wind power generation system, there is a control method presented. It is appropriate for an arbitrary number of voltage levels and the frequency of the AC side waving in a larger changing range. The method of determining the capacitance parameters in submodules is also designed. The converter model is built in PSCAD/EMTDC simulation platform; the simulation results validate the rightness and feasibility of the control method.

© 20121 Published by Elsevier Ltd. Selection and/or pe er-review under responsibility of Hainan University. Keyword: modular multilevel canvtitti; riitct-rrivt wind panti system; simulation

1. Introduction

Direct drive wind power system, due to eliminating the gearbox and the field winding, has a simple structure, easy maintenance,and the system can be directly coupled with the DC characteristics. At present, the MW wind turbine system has became one of the main models [1,2]. Direct drive wind power system must be full-power converter to achieve connection with power system. At present the breakdown voltage level of power switch elements is not achieved a breakthrough. The Multi-level converter is quite suitable for direct drive wind power system, because it has a lot of advantages such as smaller output harmonic content, adjustable power factor, flexible control, and weaker the voltage stress of semiconductor switch and electromagnetic disturbances. At home and abroad in recent years, there has been some literatures made a lot of research on the multilevel converters which are used in direct drive of wind power system are studied.

Nowadays multi-level converter topology which is studied more mature are as follows: Neutral -Point-Clamped Converter (NPC), Flying-Capacitor Converter (FCC) and Cascaded H-Bridges. The modular multi-level converter is a new type of multi-level converter with many traditional multi-level converter advantages. Compared with NPC and FFC, it has modular design, simpler structure, easier expansion of the number of level and faster implementation of the fault module inspection and replacement in engineering applications. What's more, the number of power switching devices will not increase with the number of levels of nonlinear growth when the modular multi-level converter is used. In comparsion of a cascade H bridge, modular multi-level converter doesn't need substantial independent DC power supply,

1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Hainan University. doi:10.1016/j.egypro.2012.02.272

bidirectional energy can flow in it, and the number of power switching devices will reduce double. So many advantages the modular multi-level converter has that the direct drive wind power system will be broadly applied.

2. The Model And Operating Principle Of Mmc

A. Operating Principle of MMC

The basic structure of permanent magnet direct drive wind power system based on modular multi-level converter is shown in Figure 1[5,6,7].

Figure 1. Direct-drive permanent magnetic wind power system based on MMC

The upper and lower arms of the each phase of the converter include a current-limiting reactance and N submodules. Current-limiting reactance plays an important role: Firstly DC voltage requires each phase cascade submodule voltage to support. But the capacitor voltage of each submodule has been in a change in the operation of the state.As a result the change will lead to interphase circulation and phase circulation. while the current-limiting reactance is able to limit the circulation in a small phase range. Secondly when the converter internal or external malfunction happens, limiting reactance can limit short-circuit current rate. The converter includes a current-limiting reactance and N submodules. The upper and lower arm of the structure of submodule are shown in the enlarged part of the figure 1. Each submodule is consisted of two IGBT switches, two reverse diodes and a capacitor component. Through balance controlling of submodule capacitor voltage, the capacitor can be considered as an ideal voltage source, then each submodule of the input or removal is equivalent to a DC voltage source's input or removal. In order to prevent the direct short-circuit in submodule DC capacitor, the upper and lower semiconductor switches should work in complementary state, i.e. the upper and lower signals gh and gl have to meet gh+gl=1 and ghgl=0. There are two states existing in the working process of the converter submodules, and the relationship between output voltage and submodule capacitor voltage is v(f=ghvdh as shown in Table 1. The submodule state can switch between s = 0 and s = 1 by controlling the IGBT state.

Tab. 1 Relationship between the different set of switching states and output of the submodule

Mode gh gl vo im Vdc

S=0 0 1 0 +

S=1 1 0 Vdc + Î

B. Converter Model

MMC n-level converter's main circuit is shown in figure 2. The converter must satisfy two constraints[5,6,7] to make sure of its operation stability: 1) The submodule capacitor voltage waves in a

smaller area. 2) The voltage of DC bus is stable. The control strategy of capacitor voltage balance is used to reduce submodule capacitor voltage fluctuation.While the power balance which flows in or out of the DC capacitor can be made sure by the stability of capacitor voltage of DC bus. In addition, in order to ensure the stability of DC voltage, it's necessary to keep the same number of inputed submodules in each phase at any time, namely, when a submodule is inputed in the upper bridge arm, then the one in lower bridge arm should be removed and vice versa. Otherwise it will cause interphase circulation and capacitor voltage fluctuations within the submodule[6]. Therefore, at any time it must be met (1):

IN = Co N + Na = N

(,i=a,b,c)

In the above formula: Ni represents the number of module which is required to be inputted in i-phase; Nih and Nu represent the number of module which is required to be inputted in the upper or lower arm of i-phase; C0 is a constant. It's can be seen from (1), when the number of submodules which is required to be inputted in upper arm is got, the number of lower arm can be known.

Figure 2. Diagram of n-level modular converter

For the reason of symmetry of the three-phase, a-phase is used as an example. List KVL equations according to upper and lower bridge arm and DC bus capacitor circuit of a-phase, the conclusion is as follows.

Udc = NaVo + 4 d{lah lal ) (2)

In the above formula: udc is the DC voltage of converter; Ls is the current-limiting reactance, iah is the upper bridge arm current of a-phrase, iai is the lower bridge arm current of a-phase. List KVL equations according to upper and lower bridge arm and DC circuit of a-phase and then simplify them. After that expression of AC side voltage in a-phase converter can be shown as follows:

u - h _ Lda_ a 2 s dt When the converter are operating steadily, the fluctuations of DC bus voltage and the voltage drop on current-limiting reactance can be ignored, and the error caused by them can be adjusted by the closed-loop link of control system. At this time (3) can be reduced to:

Udc = NaV

Ua =— - nahV0

The converter is connected directly with the permanent magnet wind-driven generator instead of going through a transformer. And the relationship between the positive pole voltage of DC bus and the state of three-phase module are:

ah ^ Nbh

+ Nch y

As a conclusion the relationship between the phase voltage of converter AC side and per arm submodule state is as shown in (6):

"Nah ' -1 0 1/2" Ua

Nbh = 0 -1 1/2 Ub

_ NNch _ 1 1 1/2 u c

It is can be seen from the above equation, when the converter runs, reference voltage of each phase on converter AC side can be obtained from the Operation Control System. And it's very simple to get the number of modules on the arm of each phase from (6). The number of lower bridge arms can be shown from the number of upper bridge arms and their constraints.

3. Design The Parameter Of Submodule Capacitor

In the modular converter, capacitor is an important factor which determines the total cost and the area size. The design of capacitor parameter reasonable or not has a directly effect on the economy and performance of the converter. Based on capacitive voltage periodic control [6,7], the method of confirming capacitance is designed.

From the topology of MMC, each phase has a strict symmetry and shares the same DC bus voltage and impedance. Three-phase power is divided equally by the three arms. Similarly, because of the symmetry of MMC, the phase current is divided equally between the upper and lower arm. The voltage and current expressions of upper arm in a-phase can be shown as:

1 ah ~ 3 ' d + 2 '

=2 vd -J2^sxnat

= 3' d

Im sin(®t -p0)

In the above formula: vah represents the voltage of a-phase upper arm; iah is the current of a-phase upper arm; vdc is the DC bus voltage, id is the DC bus current, Vm and Im are the RMS of voltage and current in a-phase AC side of the converter, w stands for angular frequency of AC system and p0 is the initial phase of the current in a-phase. There is instantaneous power flowing in the upper arm of a-phase, and it can be expressed as formula (8):

P V^ ahah

=1 vdid [1 - kv sin cut] * [1 + mt sin(©i - (p0 )] 6

= 6 vd'd [(1"

kmt cos^o) + 1

A *sin(®t + -—kvmt cos2®t]

In the above formula:

Vdc /2

stands for Voltage modulation ratio,

42im /2.

m =-m-is the

current modulation ratio. A = ^(m. cos (p0 - kv)2 + (m. sin (p0 )2

(pl = arctan -

m t sin <p0

mi cos (p0 - kv

Known from (8), the current of upper arm of a-phase is made from DC component, fundamental

frequency component and 2 times frequency components. When the current of upper arm is much larger than zero in every exchange cycle, the capacitance of it is in the state of charge and corresponding time bucket is kT+t!~kT+t2. So the largest energy fluctuations of the capacitance in one exchange cycle can be expressed as (9):

£t+t2

t p(a>x)dx (9)

In one AC cycles, the voltages of the capacitor can be expressed by (10), at the beginning and ending time of the capacitor charging.

[v(kT + O = (1 + £)v.

In the above formula: £ stands for coefficient of Capacitor voltage fluctuation, vs=kmv0, km represents

the deviation coefficient of Capacitor voltage and km^ 1, so the energy fluctuations in each capacitor can be expressed as:

= 1 CoV(to + Tc)2 -1 CoV(to)2

= 2 Co[(1 + £)v, + (1 -][(1 + £)v, - Q-&V, ] (11)

= 2co^Vt

For the AC cycle is much longer than capacitor balance cycle, energy fluctuations of per submodule is equivalent Aw = ns Awper, after the charging of one AC cycle. And then the equation is got:

Co = -(1 - (^)2)i (12)

3nsk^a>gvs cos^ 2

4. Converter Control System

A. Converter Pulse Width Control

The equivalent PWM waveform of voltage reference wave can be got from the AC side of the converter through coordinated controlling the switch capacity between the various submodules in the converter. Ignoring the pressure drop of current-limiting reactance in the converter, the voltage of converter AC side reference waveform can be expressed as: ^ N ud ud . ,

uk,ref (t) = - m-^srn(®? + n) (13)

In the above formula: m =-(0<m<1), udc is the DC voltage of converter, m is the angular

frequency of AC system, p0 is the initial phase angle. The timeline was divided in accordance with the PWM control cycle, and then the average of reference voltage in each control cycle is obtained according to the equivalence principle as [6]:

vk ,av = —77 f uk rf ^

' (14)

= 5L{C0S(atb + Po) - C0s(^ta + AOX

"2 2K üitb - ta) ; In the above formula: vk,av is the average of reference voltage of k-phase in a PWM control cycle.

Nu = int(-^) (15)

Nkh = Nki +1 (16) int(x) is the integral function, in every PWM control cycle (7c), the equivalent of Nk,av can be expressed as N,i and N». N,i and N,h in each control cycle time can be got according to the following formula.

DkTcNkl + (1 - Dk )TcNkh = TcNKav (17)

Nka„ - N, ,

The formulation was as follows after reduction: Dk =

k,av kh

Nkl - Nkh

B. Capacitor Balance Control Strategy

Converter model and mode of analysis in the front of this paper, both of them take the submodule as an ideal voltage source into consideration. However, in the actual operation, with charging and discharging in submodule, the capacitor voltage of submodule also changes all the time. If we cannot effectively control the voltage fluctuations of submodule capacitor, among phases circulation will occur in converter, and the capacitor voltages among phases of each submodule would wave largely, furthermore which can make the unbalanced-voltage-division of the power switch tube, as a result, the converter couldn't work as usual.

The method of periodical capacitor voltage control[6,7] is used in this paper, then testing the current of upper and lower arm in each phase at every beginning of the capacitance balance control cycle. The current is divided into charging current iah>0, ial<0 and discharging current iah<0, ial>0 (others similarly), Measures the capacitance-voltage, and give an order to the capacitor voltage of upper and lower arm of every phase in submodule. The submodules are inputted to the bridge arm in series, the direction of module current is the same as the bridge arm's. When the bridge arm is the charging, put Nkl modules which have smaller capacitors voltage into operation, and then charge the capacitor of them. When the bridge arm is discharging, put Nkl modules which have larger capacitors voltage into operation, and then discharge the capacitor of them. The reference of submodule capacitor voltage can be made sure to wave around v0 in accordance with this control strategy of capacitor voltage balance.

C. Transmission Power Control

In order to analyze conveniently, ignore the higher harmonic produced by the converter. Employ the theory of voltage vector oriented. If d-axis is defined coincidence with voltage vector, q-axis would advance it 90 degrees of electric angle. The dynamic model of converter under the dq coordinate frame can be expressed as in (18)

did R i . ™ Ed ± Ud — =--ld + Wl„---\--

dt L q L L (18)

^ - R>q -<*<+u,

dt L d L

Here, R is the line equivalent resistance, L is the line equivalent reactance, CO is the Electrical angular frequency of the ac system, ud and uq are the dq-axis components of voltage converter, Ed is the d-axis component of ac system.

From (20), we can see that Modular Multilevel Converter can be expressed by second order nonlinear system, there id and iq are the state variable, Ed and Eq are the known quantity, ud and uq is the control variable. id and iq can be solved when the control variable is given.

After the power system voltage oriented, d-axis voltage component ud=ug and the q-axis voltage component uq=0, modular multilevel converter transport active power and the reactive power under the dq coordinate frame can be expressed by (19)

p=- udh

< 23 (19)

q = - 3 udiq

From the mathematics of reactive power and active power, we can see that the active power is controlled by id, and reactive power is controlled by iq, so that active power and reactive power decoupling control can realize. The power outer loop and current inner loop control are employed in power transmission system. In the inner loop control system, id and iq reference pass by the PI regulator then the dq-axis voltage component and feed forward compensation of the network voltage are added, the reference of voltage ud and uq can be got. Through the follow-up pulse width modulation of converter the operation and withdraw single of submodule can be got and power transmission can realize. Control rate can be expressed by (20)

ud = -(id,ref - id )(kp1 +—) + ™Liq - Ed

k (2°) uq = (iq ref ~ iq )(kp 2 + ~f) + ™Lid

Here, kpi, kn, kp2 and ki2 are proportional coefficient and integral coefficient of PI regulator. From the above analysis, control theory of the converter is illustrated by Figure 3.

Figure 3. Schematic of control theory of three-level modular converter 5. Analysis Of Converter Operration

Three-level modular multilevel converter is applied in direct-drive wind power system which is established under the software package PSCAD/EMTDC. The voltage source whose frequency and voltage can be controlled is equal to the permanent magnet generator. Simulation system parameters are as follows: there 4 submodules per phase, the grid voltage is w/=1kV, DC-link voltage is udc=2kV, submodule DC reference voltage is Vo=1kV, the line equivalent reactance is Z=5mH, the line equivalent resistance is ^=0.001Q, current limiter reactance is Ls=3mH, submodule capacitance is C0=3300 ¡jF, switching frequency is f=5kHZ and balance cycle of capacitance is Tc=0.4ms.

From the beginning of the simulation until the instant t=1s, the equivalent source line voltage is Ml=690v and frequency is f=35 Hz, reference active power of the transmission system is Pref=0.4MW. When the simulation time is t>2s, equivalent source line voltage is M;=900v, f=45 Hz, Pref=1.4MW. In the simulation reference reactive power is constant Qrej=0. The result of simulation is illustrated in Fig.5~10.

"1'§.5 1 1.5 2 2.5 3

Figure 4. Waveform of phase current ia of converter

THD=2.268%

Figure 5. Harmonic spectrum of phase current ia when simulation time t>2s

Figure 4 shows current simulation result of a-phase waveform of ac output current of modular multilevel converter. Figure 5 gives the FFT analysis the current of a-phase when simulation time is t>2s. The simulation results have low sinusoidal distortion figure 5 shows current simulation result of a-phase waveform of ac output current of modular multilevel converter, figure 6 gives the FFT analysis the current of a-phase when simulation time is t>2s. The simulation results have low sinusoidal distortion and THD=2.268%.

Figure 6. Capacitor voltages of each submodule

Figure 6 shows simulation results of capacitor voltages of each submodule of a-phase. From the result, we can see that voltage fluctuating is less than ±10%, voltage fluctuating increase with the transmission

power rising. All the modules capacitor voltage balance has been effectively controlled.

Figure 7. Output line-to-line voltage uab of the converter

^ 0.8 ^ 0.4

1 1.5 2 2.5

Figure 8. Active power of converter

Figure 9. Reactive power of converter

Figure 10. Voltage of DC -bus

Figure 7 shows the uab waveform of line voltage of converter AC output. The 5-level waveform is close to sine wave and voltage stress of the IGBT is decreased effectively. Figure 8 shows the simulation of the process of active and reactive reference, and figure 9 shows the measured value waveform of system simulation. Active power from the simulation results shows that the fast response time of the dynamic response is less than 0.1s and has small overshoot, and then there is no static error in the steady-state operation. From the result of simulation we know that there is fluctuating reactive power produced when the reference power changed. There is no static errors exist when system steady-state operating. Modular multilevel converter operates under unity power factor; voltage fluctuated of DC-bus is less than 5%.

6. Conclusion

This paper establishes a direct drive wind power system which is suitable for a modular three-level PWM converter model, analyzes the work of modular converter operating principle, builds converter control strategy and derives a pulse width modulation algorithm. Because of applying the method of simple control and achieving easily, the PI controller was used in the operation control system. The paper builds a modular three-level converter based on PSCAD/EMTDC simulation platform; the simulation results show that the three-level converter can effectively reduce the stress of voltage switching device stand, reduce the current harmonic content, improve the power factor converter, and the dynamic performance of the entire system to follow well. Simulation results confirm the correctness of PWM converter system algorithms and the effective controllability of strategy of submodule capacitor voltage balancing.

Biographies

Yan Gangui received his Ph. D. degree from Tsinghua University in 2003. He is a Member of IEEE. His areas of research interest are power electronic, power system and wind power generation.

Liu Jigang received his B.Eng. degree from Zhongyuan University of Technology and now a M.Eng candidate. His areas of research interest are power electronic and wind power generation.

References

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