Jg2516381645

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
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 )

ABSTRACT
The static synchronous compensator potential failure point. Therefore, it is important to
(STATCOM) has been well accepted as a power design a sophisticated control to produce a fault-
system controller for improving voltage tolerant STATCOM. A faulty power cell in a
regulation and reactive compensation [1]–[5]. cascaded H-Bridge STATCOM can potentially
There are several compelling reasons to consider cause switch modules to explode [10] leading to the
a multilevel converter topology for the fault conditions such as a short circuit or an
STATCOM [6]–[8]. In this project, the method overvoltage on the power system resulting in an
we propose requires only that the output dc link expensive down time [11]. Subsequently, it is crucial
voltage of each phase be measured. This to identify the existence and location of the fault for
measurement is typically accomplished anyway it to be removed. Several fault detection methods
for control purposes. If a fault is detected, the have been proposed over the last few years [10]–
module in which the fault occurred is then [18]. Resistor sensing, current transformation and
isolated and removed from service. This VCE sensing are some of the more common
approach is consistent with the modular design of approaches. For example, a method based on the
cascaded converters in which the cells are output current behavior is used to identify IGBT
designed to be interchangeable and rapidly short circuits [12]. The primary drawback with the
removed and replaced. proposed approach is that the fault detection time
depends on the time constant of the load. Therefore,
Keywords - Fault detection, power quality, for loads with a large RL time constant, the faulty
multilevel converter, static Synchronous power cell can go undetected for numerous cycles,
compensator (STATCOM). potentially leading to circuit damage. Another fault
detection approach proposed in [13] is based on a
I. INTRODUCTION switching frequency analysis of the output phase
Many static synchronous compensators voltage. This method was applied to flying capacitor
(STATCOMs) utilize multilevel converters due to converters and has not been extended to cascaded
the following: 1) lower harmonic injection into the converters. AI-based methods were proposed to
power system; 2) decreased stress on the electronic extract pertinent signal features to detect faults in
components due to decreased voltages; and 3) lower [14]. In [15], sensors are used to measure each IGBT
switching losses. One disadvantage, however, is the current and to initiate switching if a fault is detected.
increased likelihood of a switch failure due to the A fault-tolerant neutral point-clamped converter was
increased number of switches in a multilevel proposed in [16]. In [17], a reconfiguration system
converter. A single switch failure, however, does not based on bidirectional switches has been designed
necessarily force an (2n + 1)-level STATCOM for three-phase asymmetric cascaded H-bridge
offline. Even with a reduced number of switches, a inverters. The fundamental output voltage phase
STATCOM can still provide a significant range of shifts are used to rebalance a faulted multilevel
control by removing the module of the faulted cascaded converter in [18].
switch and continuing with (2n − 1) levels. In this paper, the method we propose
This project introduces an approach to requires only that the output dc link voltage of each
detect the existence of the faulted switch, identify phase be measured. This measurement is typically
which switch is faulty, and reconfigure the accomplished anyway for control purposes. If a fault
STATCOM. This converter uses several full bridges is detected, the module in which the fault occurred is
in series to synthesize staircase waveforms. Because then isolated and removed from service. This
every full bridge can have three output voltages with approach is consistent with the modular design of
different switching combinations, the number of cascaded converters in which the cells are designed
output voltage levels is 2n + 1 where n is the number to be interchangeable and rapidly removed and
of full bridges in every phase. The converter cells replaced. Until the module is replaced, the multilevel
are identical and therefore modular. As higher level STATCOM continues to operate with slightly
converters are used for high output rating power decreased, but still acceptable, performance.
applications, a large number of power switching In summary, this approach offers the
devices, will be used. Each of these devices is a following advantages:

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• No additional sensing requirements;
• Additional hardware is limited to two by-pass
switches 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-based
power converter version of the SVC. Operating as a
shunt-connected SVC, its capacitive or inductive
output currents can be controlled independently from
its terminal AC bus voltage. Basically, STATCOM
is comprised of three main parts (as seen from
Figure below): a voltage source converter (VSC), a
step-up coupling transformer, and a controller. In a
very-high-voltage system, the leakage inductances of UT is the STATCOM terminal voltage; Ueq
the step-up power transformers can function as is the equivalent Thevenin voltage seen by the
coupling reactors. The main purpose of the coupling STATCOM; Xeq is the equivalent Thevenin
inductors is to filter out the current harmonic reactance of the power system seen by the
components that are generated mainly by the STATCOM.
pulsating output voltage of the power converter. The
STATCOM is connected to the power system at a
PCC (point of common coupling), through a step-up
coupling transformer, where the voltage-quality
problem is a concern. The PCC is also known as the
terminal for which the terminal voltage is UT. All
required voltages and currents are measured and are
fed into the controller to be compared with the
commands. The controller then performs feedback
control and outputs a set of switching signals (firing
angle) to drive the main semiconductor switches of
the power converter accordingly to either increase
the voltage or to decrease it accordingly. A
STATCOM is a controlled reactive-power source. It
provides voltage support by generating or absorbing
reactive power at the point of common coupling
without the need of large external reactors or
capacitor banks. III. MULTILEVEL STATCOM
The charged capacitor Cdc provides a DC A cascaded multilevel STATCOM contains
voltage, Udc to the converter, which produces a set of several H-bridges in series to synthesize a staircase
controllable three-phase output voltages, U in waveform. The inverter legs are identical and are
synchronism with the AC system. The synchronism therefore modular. In the eleven-level STATCOM,
of the three-phase output voltage with the each leg has five H-bridges. Since each full bridge
transmission line voltage has to be performed by an generates three different level voltages (V, 0, −V)
external controller. This reactive power exchange is under different switching states, the number of
the reactive current injected by the STATCOM, output voltage levels will be eleven. A multilevel
which is the current from the capacitor produced by configuration offers several advantages over other
absorbing real power from the AC system. converter types [19].
1) It is better suited for high-voltage, high-power
applications than the conventional converters since
the currents and voltages across the individual
switching devices are smaller.
2) It generates a multistep staircase voltage
Where Iq is the reactive current injected by the waveform approaching a more sinusoidal output
STATCOM voltage 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, Where n is the number of cells in each phase. Figure
the voltages across all of the dc capacitors should illustrates the carrier and reference waveforms for a
maintain a constant value. Variations in load cause phase leg of the eleven-level STATCOM. In this
the dc capacitors to charge and discharge unevenly figure, the carrier frequency has been decreased for
leading to different voltages in each leg of each better clarity. Normally, the carrier frequency for
phase. However, because of the redundancy in PWM is in the range of 1–10 kHz.
switching states, there is frequently more than one
state that can synthesize any given voltage level. IV. FAULT ANALYSIS FOR THE
Therefore, there exists a “best” state among all the MULTILEVEL STATCOM
possible states that produces the most balanced A converter cell block, as shown in Figure,
voltages [20]. can experience several types of faults. Each switch
in the cell can fail in an open or closed state. The
closed state is the most severe failure since it may
lead to shoot through and short circuit the entire cell.
An open circuit can be avoided by using a proper
gate circuit to control the gate current of the switch
during the failure [23]. If a short circuit failure
occurs, the capacitors will rapidly discharge through
the conducting switch pair if no protective action is
taken. Hence, the counterpart switch to the failed
switch must be quickly turned off to avoid system
collapse due to a sharp current surge. Nomenclature
for the proposed method is given in Table I.

The staircase voltage waveform shown in
Fig. 3 is synthesized by combining the voltages of
the various cells into the desired level of output
voltage. At the middle levels of the voltage
waveform, due to the switching state redundancy,
there are more than one set of switching
Since there are multiple possible switching combinations that may be used to construct the
states that can be used to synthesize a given voltage desired voltage level. Therefore, by varying the
level, the particular switching topology is chosen switching patterns, the loss of any individual cell
such that the capacitors with the lowest voltages are will not significantly impact the middle voltages of
charged or conversely, the capacitors with the the output voltage. However, the peak voltages
highest voltages are discharged. This redundant state require that all cells contribute to the voltage;
selection approach is used to maintain the total dc therefore, the short circuit failure of any one cell will
link voltage to a near constant value and each lead to the loss of the first and (2n + 1) output levels
individual cell capacitor within a tight bound. and cause degradation in the ability of the
Different pulse width modulation (PWM) techniques STATCOM to produce the full output voltage level.
have been used to obtain the multilevel converter Consider the simplified eleven-level converter
output voltage. One common PWM approach is the shown in Figure. The process for identifying and
phase shift PWM (PSPWM) switching concept [21]. removing the faulty cell block is summarized in
The PSPWM strategy causes cancellation of all Figure. The input to the detection algorithm is ˆEout
carrier and associated sideband harmonics up to the for each phase, where ˆEout is the STATCOM
(N − 1)th carrier group for an N-level converter. filtered RMS output voltage.
Each carrier signal is phase shifted by

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V. METHOD OF COMPARISON
Each fault detection method has its
own advantages and disadvantages. Most of the
methods in the literature are applicable to neutral
point-clamped converters and are therefore not
directly applicable to cascaded converters. In this
section, each applicable approach is succinctly
summarized and compared with other methods. Two
recent methods are briefly reviewed below.
1) Voltage Frequency Analysis [23]. In this
scheme, the basic approach is to use SPWM to
produce the converter output voltage. By using
If the STATCOM RMS output voltage SPWM, voltages with different phase angles will be
drops below a preset threshold value (E_), then, a produced at each cell of the multilevel converter.
fault is known to have occurred (see Fig. 6). Once a The sum of the three phase voltages is zero in
fault has been detected to have occurred, then, the normal operation, but that is not zero if there is a
next step is to identify the faulty cell. By utilizing faulty cell. This condition is used as the criteria for
the switching signals in each converter cell, (i.e., S1 identifying the faulty cell. The phase angle of the
and S2), it is possible to calculate all of the possible voltage sum indicates the location of the fault.
voltages that can be produced at any given instant as
illustrated in Table II (terminology adopted from
[23]): Thus, the output voltage of a cell is and since
the 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 be
near the actual STATCOM output phase voltage
Eout; all of the others will be too high. Therefore, to
determine the location of the fault cell, each fi is
compared against Eout to yield

The smallest xi indicates the location of the
faulted block because this indicates the fi which
most closely predicts the actual Eout. The choice of
threshold voltage E_ depends on the number of cells
in the converter. The ideal output voltage is

During a fault, Eout will decrease by Vdc0 yielding

Therefore, the threshold voltage E_ should 2) AI-BASED FAULT DETECTION [14].
be chosen such that (n − 1/n) Eout,0 ≤ E_ ≤ Eout,0. This scheme is built around a neural
In an eleven-level converter, n = 5 and the faulted network (NN) classification for fault diagnosis of a
RMS voltage will decrease by roughly 20%. multilevel cascaded converter. Multilayer perception
Therefore, a good choice for E_ is 85% of the rated networks are used to identify the type and location
output STATCOM voltage.

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of occurring faults. The principal component
analysis is utilized in the feature extraction process
to reduce the NN input size. Since these methods are
all designed to detect and then bypass the faulted
cell, the hardware requirements are identical. These
methods are compared and contrasted to the
proposed method in Table III. Each method has its
own advantages and disadvantages. For example, the
voltage frequency method detects and clears the
faulty cell rapidly, but requires complex frequency
analysis and may not be suitable for implementation
in all applications. The proposed method does not
respond as rapidly, but only requires simple
calculations and can be implemented easily in most
DSPs. Furthermore, the proposed method only
requires voltage magnitude measurements which are
easily obtained.

VI. EXPERIMENTAL RESULTS
To confirm the operation of the fault
detection algorithm for cascaded H-bridge multilevel
converters, an experimental prototype is constructed
for applying and detecting different type of faults.
The experimental rack consists of 36 Power ex
CM75Du-24F IGBTs rated at 1200 V and 75 A for
main switching devices. Passive components include
a 1.2-mH, 45-A reactor and 18 electrolytic
capacitors rated at 3900 μF and 450 V. The IGBTs
are driven by Concept 6SD106E1 gate drivers and
controlled by a 320F2812 fixed-point digital signal
processor (DSP).

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improvement in the output waveform of the
converter.

The above both output of Experimental
STATCOM dynamics shows the same fault as in
Figure, except the fault bypass signal is intentionally
delayed by several cycles to demonstrate the effect
of changing the PWM pattern. Note that after the
fault and discharge of the corresponding capacitor,
the output waveform contains considerable
distortion. However, modifying the PWM switching
signals based on two cascaded H-bridges, the THD
of the output waveform can be significantly
The output voltage of the converter during decreased and the filtered output waveform become
the normal operation, during the fault, and after sinusoidal again.
removing the faulty cell is depicted in Figure. A
fault is applied to the second cell at point “F” as VII. CONCLUSION
shown in the figure with the dashed line. After In this paper, a fault detection and
detecting the fault and bypassing the faulty H- mitigation strategy for a multilevel cascaded
bridge, the modulation index is increased to converter has been proposed. This approach requires
compensate for the lost voltage levels in the output. no extra sensors and only one additional by bypass
In addition, the PWM switching patterns are switch per module per phase. The approach has been
modified based on existence of two cascaded H- validated on a 115-kV system with a STATCOM
bridges instead of three. This causes significant compensating an electric arc furnace load. This
application was chosen since the arc furnace

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