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Strategies and Operating Point Optimizationof STATCOM Control for Voltage UnbalanceMitigation in Three-Phase Three-Wire SystemsIEEE PAPER PRESENTATION IN THE SUBJECT OF

FACTSPREPARED BY

UTSAV YAGNIK (150430707017) & JAGDU ZUNED (150430707005) M.E. ELECTRICAL, SSEC, BHAVNAGAR

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2About the paper

Author(s):1. Kuang Li2. Jinjun Liu3. Zhaoan Wang4. Biao Wei

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007

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3Outline Abstract - 4 Introduction - 5 System Configuration - 11 Control strategies & Operating point optimization for mitigation of

unbalanced V – 15 Characteristic comparison between Modified VCVS & The VCVS –

21 Power flow analysis and DC bus voltage control – 23 Simulation and Experimental results – 29 Conclusion - 37

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4Abstract Unbalanced utility voltages affect the operation of the STATCOM

greatly and under severe conditions it may shut it down while operating due to its overcurrent protection.

So, a new control strategy has been proposed in this paper which controls the unbalanced voltages at the point of common coupling(PCC).

It is shown that proposed voltage controlled current source(VCCS) strategy and modified voltage controlled voltage source strategy are valid for unbalanced compensation.

The simulations are carried out for both heavily and lightly load conditions which shows that V and I both are under control in STATCOM.

So the compensation performance of STATCOM is proved to be satisfactory.

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5Introduction

A STATCOM can be used as a control for… Power factor Regulate voltage Stabilize power flow Improve the dynamic performance of power systems

But it faces the biggest problem of system voltage unbalance which is considered as the problem of Power Quality (PQ)

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6Introduction

As per IEC standard, the unbalance of voltage should not exceed 2 % during a long time period.

The voltage unbalance can affect various equipment's connected in the grid such as…

Induction motors Variable speed drive systems Many other power-electronic equipment.

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7Introduction

A STATCOM is treated as a positive sequence voltage source in traditional control strategies.

It works fine under balanced conditions but under unbalanced voltage mains, it will induce a negative sequence current while it passes through STATCOM because STATCOM is short circuit for negative sequence component.

The system impedance is small so a small negative sequence voltage will lead to large amount of negative sequence current.

Under such condition, the STATCOM has to be put in standby mode to avoid overcurrent protection malfunctioning.

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8Introduction

In order to keep unbalanced voltage under permitted level….. Series compensators such as… Dynamic voltage restorers (DVRs) Series power-quality conditioners (SPQCs) Active line conditioners (ALCs) Parallel compensators such as… STATCOM itself as voltage controlled voltage source(VCVS)

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9Introduction

But in the above strategy, the modified single phase transform is proposed which takes additional time delay.

Same way, three more strategies were proposed but all suffered from one or two difficulties.

This paper suggests that under unbalanced conditions, the STATCOM will operate as voltage unbalance compensation rather than its operation as reactive power compensation mode.

In this way, the function of STATCOM also gets extended. So this paper discusses “How to use STATCOM to mitigate voltage

unbalance at PCC”

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10Introduction So the paper’s flow is as below…1. Introduction of STATCOM configuration2. Three control strategies

Voltage controlled current source(VCCS) Voltage controlled voltage source(VCVS) Modified VCVS

3.For each control strategy, the principle for unbalanced voltage mitigation is analysed in detail.4.The parameter relationships are also investigated to achieve the optimum performance.5.The optimal operation point can be located from characteristic curves.6.At this point, the unbalanced voltage at PCC and STATCOM’s capacity reach at the minimum value.7.The control scheme to keep the dc bus voltage constant is put forward.8.Finally, the simulation and experimental results are shown for both resistive and diode rectifier and it is confirmed that overcurrent phenomena doesn’t occur at STATCOM anymore and unbalanced voltage magnitude is decreased to a great level at PCC.

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11System Configuration

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Rating of STATCOM:10 kV, 3 10 MVA STATCOM is directly connected in parallel with the three-phase

load via a small inductor for the attenuation of switching frequency current ripples.

The power converter of the STATCOM employs several series-connected IGBTs in each leg of the three-phase bridge circuit for medium-voltage blocking capability.

The STATCOM has multiple compensation functions and the control strategies must be embedded into the control unit. In Figure in the next slide, the block diagram of the control system is also shown.

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These signals are sent into a digital signal processor (DSP) control unit.

In this paper, the control strategy is fully digitally implemented based on a TMS320F2812 DSP. And it does not require any additional analog-to–digital or digital-to-analog conversion chips.

The cost of the whole control system is very low, but the reliability is comparatively high. Simultaneously, a touch panel is introduced in the control system, and it is very convenient to operate the STATCOM and monitor the system status.

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15CONTROL STRATEGIES AND OPERATING POINTOPTIMIZATION FOR MITIGATION OF VOLTAGE

UNBALANCE In three phase three wire systems die to absence of zero

sequence components, only the negative sequence components are considered at the PCC.

In this paper, all of the negative- and positive-sequence quantities are denoted by subscripts and , respectively, and the formulation is based on phasor analysis.

A. VCCS Voltage Unbalance Mitigation Strategy and Operating Point Optimization

In the VCCS voltage unbalance mitigation strategy, the single-phase equivalent circuit of the power system is shown in Figure in the next slide.

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UNBALANCE

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UNBALANCE With this control scheme, the STATCOM is controlled as a current

source. Its amplitude is determined by the unbalanced voltage and an equivalent amplifying quantity of the STATCOM . Here, a rotating factor is introduced for the phase-angle control of the compensating current.

The unbalanced voltage at the PCC and the current of the STATCOM are as follows:

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UNBALANCE The unbalanced voltage can be mitigated by in large value,

which is realized with the STATCOM. It is clear that only the unbalanced current flows into the STATCOM. One merit of VCCS control is that the current of the STATCOM is fully under control whenever the voltage of the utility grid is balanced or not. Therefore, the STATCOM could theoretically be operated online safely at all times.

for the VCCS voltage unbalance mitigation strategy, the rating of the STATCOM is

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UNBALANCE B. VCVS Voltage Unbalance Mitigation Strategy and

Operating Point Optimization

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UNBALANCE denotes the connecting reactor of the STATCOM with the utility

grid in the figure of last slide. In this strategy the rating of the STATCOM becomes…

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21Characteristic comparison between modified VCVS and the

VCVS

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22Characteristic comparison between modified VCVS and the

VCVS The capacity of the STATCOM with the VCCS strategy is still a little smaller than that with the modified VCVS strategy under the same unbalanced voltage level.

If the linking reactor could be too small to be neglected, these two lines could overlap each other. It means that from the outside characteristic, these two control strategies have no difference when reactance is very small.

But in application, it should have a certain value (e.g., 0.1 p.u.). Therefore, in general, the VCCS mitigation strategy is a little better than the modified VCVS control strategy.

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23POWER-FLOW ANALYSIS AND DC BUS VOLTAGE CONTROL

In the VCCS strategy, only the negative-sequence current flows into the STATCOM.

With proper control, the dc bus voltage of the STATCOM could be stable.

Similarly, in the VCVS strategy, only the negative-sequence voltage is generated by the STATCOM. Although there is positive-sequence current, the positive-sequence voltage is zero.

Thus, for the positive-sequence component, the active power is also zero.

From the mentioned analysis with the three control strategies, the STATCOM is shown to consume only a small amount of active power besides absorbing or generating almost all of the reactive power necessary for unbalance compensation. Therefore, the STATCOM could operate normally.

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The figure in the above slide shows Detection of the fundamental negative-sequence component in PCC voltages and control algorithm for constant dc voltage with System voltages without harmonic distortion.

If the utility voltage contains a lot of harmonics, the method in Figure above does not work effectively. The detected signal would also be harmonic contaminated. Therefore, another unbalanced voltage detection method is introduced as shown in Figure below. In this case, the transformation matrix is correspondingly modified so that only the negative-sequence voltage of the utility becomes dc component in the d– q frame.

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In order to ensure that the STATCOM is working properly, the dc bus voltage has to be controlled to a certain stable level.

In this paper, a single voltage control loop is adopted. The error signal between the dc bus voltage and its reference is regulated with a PI controller. The output of the PI regulator is added to the d-channel signal in the d–q frame.

Therefore, the certain positive-sequence component is generated at the fundamental frequency with the same phase angle of the PCC positive- sequence voltage after inverse transformation.

Since the detected signal is used as the reference for the STATCOM current in the VCCS control strategy, it will then generate certain positive- sequence currents besides the currents for voltage unbalance mitigation.

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The generated positive-sequence currents along with the same phase angle as the PCC positive-sequence voltage will produce certain active power, which is used to adjust the dc-side voltage of the STATCOM.

For the case of utility voltages with severe harmonics or the case of using VCVS control strategies, the control diagram is shown in Fig.(b). The principle of adjusting the necessary active power flow in the STATCOM to control the dc-side voltage does not change.

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29SIMULATION AND EXPERIMENT RESULTS

In order to verify the three control strategies for voltage unbalance mitigation, detailed computer simulation investigations in the time domain have been carried out with PSIM6.0. For each control strategy, two kinds of loads are studied

Resistive Load: R = 10 Ω Rectifier Load: R = 10 Ω & L = 2 mHWhere Usp = 220 V, Usn = 88 V & Ls = 4 mH Figure in the next slide shows the simulation waveforms with the STATCOM

under the VCCS control. After the STATCOM is put into operation at 0.04 s, the PCC voltage becomes

more balanced. The negative-sequence voltage approaches a very small extent, even zero. More evidence is that the load current is also symmetrical after compensation. A corresponding result is the increase of the current in the STATCOM and the power supply.

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VCCS control for (a) Resistive load & (b) diode rectifier load

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VCVS control for (a) Resistive load & (b) diode rectifier load

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Modified VCVS control for (a) Resistive load & (b) diode rectifier load

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Furthermore, experiment investigations have been carried out on an established reduced scale prototype STATCOM with 3.2-kHz switching frequency. The prototype is rated at 380 V/50 kVA. The unbalanced supply voltage is obtained through a programmable a.c. source Chroma 6590, where Usp = 160 V, Usn = 64 V.

Fig. in the next slide shows the PCC voltage and load current waveforms before and after compensation.

It is seen in Fig. (a) that the voltage at the PCC is little distorted but highly unbalanced originally. After the STATCOM is put into operation, the three-phase PCC voltage becomes almost balanced as shown in Fig. (b).

It means that the negative-sequence voltage was almost fully compensated by the STATCOM with the proposed VCCS control strategy. Fig. (c) and (d) are the load current waveforms before and after the STATCOM works.

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Fig. (a) in next slide is the voltage waveforms at the PCC with the VCVS control strategy. Compared with Fig. (b) in last slide, it is clearly shown that the amplitude of the voltage under the VCVS control has greatly decreased. This phenomenon is caused by the linking reactor between the STATCOM and the grid. The smaller the reactor is, the larger the positive-sequence current of the STATCOM is, and the smaller the positive-sequence voltage at the PCC is.

Fig. (b) in next slide is the voltage waveforms at the PCC with the modified VCVS control strategy, which is similar to the results in Fig. (b) last slide. Because of the similarity to those in the VCCS control strategy, the load current waveforms in the VCVS and the modified VCVS control strategies are not shown here.

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From the experimental results, it is obvious that the supply voltage unbalance Problem can be mitigated effectively by the STATCOM with the proposed VCCS and modified VCVS control strategies.

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37Conclusion Novel control strategies are proposed for STATCOM to compensate for the

utility voltage unbalance either caused by the asymmetric load or upstream supply source.

Through extensive theoretical analysis, computer simulation, and experimental tests on a hardware prototype of STATCOM, the following conclusions are obtained.

I. The STATCOM can compensate V unbalance at PCC.II. The VCCS & VCVS strategies are proven for V unbalance mitigation using

STATCOM.III. The STATCOM shows best compensation characteristics at the Optimum

point when realizing the two control strategies. When compensating utility voltage unbalance, the STATCOM can be

controlled to absorb almost only reactive power. Only a small amount of active power is consumed for stabilizing the dc capacitor voltage.

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