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Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and

Applications (IJERA) ISSN: 2248-9622 www.ijera.com

Vol. 2, Issue 5, September- October 2012, pp.1638-1645

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Fault Analysis And Improve Power Quality By Multilevel

Statcoms

Pragallapati Manikanta (M.Tech), K. Anand. M.E

*(Department of Electrical Engineering, GIET, JNTUK, Rajahmundry, A .P, INDIA )

ABSTRACTThe static synchronous compensator

(STATCOM) has been well accepted as a power

system controller for improving voltage

regulation and reactive compensation [1] – [5].

There are several compelling reasons to considera multilevel converter topology for the

STATCOM [6] – [8]. In this project, the method

we propose requires only that the output dc link

voltage of each phase be measured. This

measurement is typically accomplished anywayfor control purposes. If a fault is detected, the

module in which the fault occurred is then

isolated and removed from service. This

approach is consistent with the modular design of

cascaded converters in which the cells are

designed to be interchangeable and rapidlyremoved and replaced.

Keywords - Fault detection, power quality,

multilevel converter, static Synchronous

compensator (STATCOM).

I. INTRODUCTIONMany static synchronous compensators

(STATCOMs) utilize multilevel converters due tothe following: 1) lower harmonic injection into thepower system; 2) decreased stress on the electroniccomponents due to decreased voltages; and 3) lowerswitching losses. One disadvantage, however, is theincreased likelihood of a switch failure due to theincreased number of switches in a multilevelconverter. A single switch failure, however, does notnecessarily force an (2n + 1)-level STATCOMoffline. Even with a reduced number of switches, aSTATCOM can still provide a significant range of

control by removing the module of the faultedswitch and continuing with (2n − 1) levels.

This project introduces an approach todetect the existence of the faulted switch, identifywhich switch is faulty, and reconfigure theSTATCOM. This converter uses several full bridgesin series to synthesize staircase waveforms. Becauseevery full bridge can have three output voltages withdifferent switching combinations, the number of output voltage levels is 2n + 1 where n is the numberof full bridges in every phase. The converter cellsare identical and therefore modular. As higher levelconverters are used for high output rating power

applications, a large number of power switchingdevices, will be used. Each of these devices is a

potential failure point. Therefore, it is important todesign a sophisticated control to produce a fault-tolerant STATCOM. A faulty power cell in acascaded H-Bridge STATCOM can potentiallycause switch modules to explode [10] leading to thefault conditions such as a short circuit or anovervoltage on the power system resulting in anexpensive down time [11]. Subsequently, it is crucialto identify the existence and location of the fault for

it to be removed. Several fault detection methodshave been proposed over the last few years [10] –

[18]. Resistor sensing, current transformation andVCE sensing are some of the more commonapproaches. For example, a method based on theoutput current behavior is used to identify IGBTshort circuits [12]. The primary drawback with theproposed approach is that the fault detection timedepends on the time constant of the load. Therefore,for loads with a large RL time constant, the faultypower cell can go undetected for numerous cycles,potentially leading to circuit damage. Another faultdetection approach proposed in [13] is based on a

switching frequency analysis of the output phasevoltage. This method was applied to flying capacitorconverters and has not been extended to cascadedconverters. AI-based methods were proposed toextract pertinent signal features to detect faults in[14]. In [15], sensors are used to measure each IGBTcurrent and to initiate switching if a fault is detected.A fault-tolerant neutral point-clamped converter wasproposed in [16]. In [17], a reconfiguration systembased on bidirectional switches has been designedfor three-phase asymmetric cascaded H-bridgeinverters. The fundamental output voltage phaseshifts are used to rebalance a faulted multilevel

cascaded converter in [18].In this paper, the method we proposerequires only that the output dc link voltage of eachphase be measured. This measurement is typicallyaccomplished anyway for control purposes. If a faultis detected, the module in which the fault occurred isthen isolated and removed from service. Thisapproach is consistent with the modular design of cascaded converters in which the cells are designedto be interchangeable and rapidly removed andreplaced. Until the module is replaced, the multilevelSTATCOM continues to operate with slightlydecreased, but still acceptable, performance.

In summary, this approach offers thefollowing advantages:

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• No additional sensing requirements; • Additional hardwar e is limited to two by-passswitches per module;• Is consistent with the modular approach of cascaded multilevel converters; and• The dynamic performance and THD of the

STATCOM is not significantly impacted.

II. STATCOM

The STATCOM is a solid-state-basedpower converter version of the SVC. Operating as ashunt-connected SVC, its capacitive or inductiveoutput currents can be controlled independently fromits terminal AC bus voltage. Basically, STATCOMis comprised of three main parts (as seen fromFigure below): a voltage source converter (VSC), astep-up coupling transformer, and a controller. In avery-high-voltage system, the leakage inductances of the step-up power transformers can function as

coupling reactors. The main purpose of the couplinginductors is to filter out the current harmoniccomponents that are generated mainly by thepulsating output voltage of the power converter. TheSTATCOM is connected to the power system at aPCC (point of common coupling), through a step-upcoupling transformer, where the voltage-qualityproblem is a concern. The PCC is also known as theterminal for which the terminal voltage is UT. Allrequired voltages and currents are measured and arefed into the controller to be compared with thecommands. The controller then performs feedbackcontrol and outputs a set of switching signals (firing

angle) to drive the main semiconductor switches of the power converter accordingly to either increasethe voltage or to decrease it accordingly. ASTATCOM is a controlled reactive-power source. Itprovides voltage support by generating or absorbingreactive power at the point of common couplingwithout the need of large external reactors orcapacitor banks.

The charged capacitor Cdc provides a DCvoltage, Udc to the converter, which produces a set of controllable three-phase output voltages, U insynchronism with the AC system. The synchronismof the three-phase output voltage with thetransmission line voltage has to be performed by anexternal controller. This reactive power exchange isthe reactive current injected by the STATCOM,which is the current from the capacitor produced byabsorbing real power from the AC system.

Where Iq is the reactive current injected by theSTATCOM

UT is the STATCOM terminal voltage; Ueq

is the equivalent Thevenin voltage seen by the

STATCOM; Xeq is the equivalent Theveninreactance of the power system seen by theSTATCOM.

III. MULTILEVEL STATCOM

A cascaded multilevel STATCOM containsseveral H-bridges in series to synthesize a staircasewaveform. The inverter legs are identical and aretherefore modular. In the eleven-level STATCOM,each leg has five H-bridges. Since each full bridge

generates three different level voltages (V, 0 , −V) under different switching states, the number of output voltage levels will be eleven. A multilevelconfiguration offers several advantages over otherconverter types [19].1) It is better suited for high-voltage, high-powerapplications than the conventional converters sincethe currents and voltages across the individualswitching devices are smaller.2) It generates a multistep staircase voltagewaveform approaching a more sinusoidal outputvoltage by increasing the number of levels.3) It has better dc voltage balancing, since each

bridge has its own dc source.

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To achieve a high-quality output voltage waveform,the voltages across all of the dc capacitors shouldmaintain a constant value. Variations in load causethe dc capacitors to charge and discharge unevenlyleading to different voltages in each leg of eachphase. However, because of the redundancy in

switching states, there is frequently more than onestate that can synthesize any given voltage level.Therefore, there exists a “best” state among all the

possible states that produces the most balancedvoltages [20].

Since there are multiple possible switchingstates that can be used to synthesize a given voltagelevel, the particular switching topology is chosensuch that the capacitors with the lowest voltages arecharged or conversely, the capacitors with the

highest voltages are discharged. This redundant stateselection approach is used to maintain the total dclink voltage to a near constant value and eachindividual cell capacitor within a tight bound.Different pulse width modulation (PWM) techniqueshave been used to obtain the multilevel converteroutput voltage. One common PWM approach is thephase shift PWM (PSPWM) switching concept [21].The PSPWM strategy causes cancellation of allcarrier and associated sideband harmonics up to the( N − 1)th carrier group for an N -level converter.Each carrier signal is phase shifted by

Where n is the number of cells in each phase. Figureillustrates the carrier and reference waveforms for aphase leg of the eleven-level STATCOM. In thisfigure, the carrier frequency has been decreased forbetter clarity. Normally, the carrier frequency forPWM is in the range of 1 – 10 kHz.

IV. FAULT ANALYSIS FOR THE

MULTILEVEL STATCOM

A converter cell block, as shown in Figure,can experience several types of faults. Each switchin the cell can fail in an open or closed state. Theclosed state is the most severe failure since it maylead to shoot through and short circuit the entire cell.An open circuit can be avoided by using a propergate circuit to control the gate current of the switchduring the failure [23]. If a short circuit failureoccurs, the capacitors will rapidly discharge throughthe conducting switch pair if no protective action is

taken. Hence, the counterpart switch to the failedswitch must be quickly turned off to avoid systemcollapse due to a sharp current surge. Nomenclaturefor the proposed method is given in Table I.

The staircase voltage waveform shown inFig. 3 is synthesized by combining the voltages of the various cells into the desired level of outputvoltage. At the middle levels of the voltagewaveform, due to the switching state redundancy,there are more than one set of switchingcombinations that may be used to construct thedesired voltage level. Therefore, by varying theswitching patterns, the loss of any individual cellwill not significantly impact the middle voltages of the output voltage. However, the peak voltages

require that all cells contribute to the voltage;therefore, the short circuit failure of any one cell willlead to the loss of the first and (2n + 1) output levelsand cause degradation in the ability of theSTATCOM to produce the full output voltage level.Consider the simplified eleven-level convertershown in Figure. The process for identifying andremoving the faulty cell block is summarized inFigure. The input to the detection algorithm is ̂E outfor each phase, where ̂E out is the STATCOMfiltered RMS output voltage.

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If the STATCOM RMS output voltagedrops below a preset threshold value ( E_), then, afault is known to have occurred (see Fig. 6). Once afault has been detected to have occurred, then, thenext step is to identify the faulty cell. By utilizingthe switching signals in each converter cell, (i.e., S 1and S 2), it is possible to calculate all of the possiblevoltages that can be produced at any given instant asillustrated in Table II (terminology adopted from[23]): Thus, the output voltage of a cell is and sincethe cells of the STATCOM are serially connected,the total output voltage per phase is

Where „n‟ is the number of blocks.

If there is a faulted cell, only one fi will benear the actual STATCOM output phase voltage E out; all of the others will be too high. Therefore, todetermine the location of the fault cell, each fi iscompared against E out to yield

The smallest xi indicates the location of thefaulted block because this indicates the fi whichmost closely predicts the actual E out. The choice of

threshold voltage E_ depends on the number of cellsin the converter. The ideal output voltage is

During a fault, E out will decrease by V dc0 yielding

Therefore, the threshold voltage E_ shouldbe chosen such that (n − 1 /n) E out ,0 ≤ E _ ≤ E out ,0.In an eleven-level converter, n = 5 and the faulted

RMS voltage will decrease by roughly 20%.Therefore, a good choice for E_ is 85% of the ratedoutput STATCOM voltage.

V. METHOD OF COMPARISON Each fault detection method has its

own advantages and disadvantages. Most of themethods in the literature are applicable to neutralpoint-clamped converters and are therefore notdirectly applicable to cascaded converters. In this

section, each applicable approach is succinctlysummarized and compared with other methods. Tworecent methods are briefly reviewed below.1) Voltage Frequency Analysis [23]. In thisscheme, the basic approach is to use SPWM toproduce the converter output voltage. By usingSPWM, voltages with different phase angles will beproduced at each cell of the multilevel converter.The sum of the three phase voltages is zero innormal operation, but that is not zero if there is afaulty cell. This condition is used as the criteria foridentifying the faulty cell. The phase angle of thevoltage sum indicates the location of the fault.

2) AI-BASED FAULT DETECTION [14]. This scheme is built around a neural

network (NN) classification for fault diagnosis of a

multilevel cascaded converter. Multilayer perceptionnetworks are used to identify the type and location

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of occurring faults. The principal componentanalysis is utilized in the feature extraction processto reduce the NN input size. Since these methods areall designed to detect and then bypass the faultedcell, the hardware requirements are identical. Thesemethods are compared and contrasted to the

proposed method in Table III. Each method has itsown advantages and disadvantages. For example, thevoltage frequency method detects and clears thefaulty cell rapidly, but requires complex frequencyanalysis and may not be suitable for implementationin all applications. The proposed method does notrespond as rapidly, but only requires simplecalculations and can be implemented easily in mostDSPs. Furthermore, the proposed method onlyrequires voltage magnitude measurements which areeasily obtained.

VI. EXPERIMENTAL RESULTSTo confirm the operation of the fault

detection algorithm for cascaded H-bridge multilevelconverters, an experimental prototype is constructedfor applying and detecting different type of faults.The experimental rack consists of 36 Power exCM75Du-24F IGBTs rated at 1200 V and 75 A formain switching devices. Passive components includea 1.2-mH, 45-A reactor and 18 electrolytic

capacitors rated at 3900 μF and 450 V. The IGBTsare driven by Concept 6SD106E1 gate drivers andcontrolled by a 320F2812 fixed-point digital signalprocessor (DSP).

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The output voltage of the converter duringthe normal operation, during the fault, and afterremoving the faulty cell is depicted in Figure. Afault is applied to the second cell at point “F” as

shown in the figure with the dashed line. Afterdetecting the fault and bypassing the faulty H-bridge, the modulation index is increased tocompensate for the lost voltage levels in the output.In addition, the PWM switching patterns aremodified based on existence of two cascaded H-

bridges instead of three. This causes significant

improvement in the output waveform of theconverter.

The above both output of ExperimentalSTATCOM dynamics shows the same fault as inFigure, except the fault bypass signal is intentionallydelayed by several cycles to demonstrate the effectof changing the PWM pattern. Note that after thefault and discharge of the corresponding capacitor,the output waveform contains considerable

distortion. However, modifying the PWM switchingsignals based on two cascaded H-bridges, the THDof the output waveform can be significantlydecreased and the filtered output waveform becomesinusoidal again.

VII. CONCLUSION In this paper, a fault detection and

mitigation strategy for a multilevel cascadedconverter has been proposed. This approach requiresno extra sensors and only one additional by bypassswitch per module per phase. The approach has beenvalidated on a 115-kV system with a STATCOMcompensating an electric arc furnace load. Thisapplication was chosen since the arc furnace

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provides a severe application with its non sinusoidal,unbalanced, and randomly fluctuating load. Theproposed approach was able to accurately identifyand remove the faulted module. In addition, theSTATCOM was able to remain in service andcontinue to provide compensation without exceeding

the total harmonic distortion allowances.

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