A NEW CONCEPT OF MULTILEVEL STATCOM
BASED ON CASCADE TOPOLOGY
ABSTRACT
This Paper presents one way for power quality conditioning. This way means parallel connection
of the STATCOM circuits with the network, therefore it is possible to “isolate” load from source
and vice versa. Described conditioner makes possible to get: i) sinusoidal source current; ii)
reactive power compensation; iii) load voltage stabilization; iv) balanced source in conditions of
the unbalanced load. As STATCOM, the four level cascade based VSI has been used. To
confirm results of the theoretical analysis some experimental results were presented. Additional,
control algorithm, to shape six-step output voltage is proposed
1. INTRODUCTION
In professional literature, there are described many different ways to “isolate” sources from
disturbances introduced by the nonlinear loads and vice versa. For example to compensate
reactive and higher harmonics currents, produced by the nonlinear loads, STATCOM (STATic
COMpensator) can be used. In those systems (independent with control algorithm) there is need
to extract, from measured load or source currents (it depends if control algorithm is in open or
closed loop), compensating components, therefore the filtration quality is as good as well it is
possible to extract compensating components and shape them. Paper presents a one way of
power quality improvement. In presented solution, power quality improvement is possible to get
if parallel connected STATCOM acts as a sinusoidal, with fundamental frequency, voltage
source, therefore described conditioner makes possible to get:
i) Sinusoidal source current;
ii) Reactive power compensation;
iii) Load voltage stabilization;
iv) Balanced source in conditions of the unbalanced load.
Because STATCOM has to “produce” sinusoidal voltage, multilevel Voltage Source Inverters
(VSI) are the perfect solution in this case . Onto needs of the STATCOM, four-level cascade
based VSI inverter was developed.
2. FACTS
Flexible AC Transmission Systems, called FACTS, got in the recent years a well known term for
higher controllability in power systems by means of power electronic devices. Several FACTS-
devices have been introduced for various applications worldwide. A number of new types of
devices are in the stage of being introduced in practice.
In most of the applications the controllability is used to avoid cost intensive or landscape
requiring extensions of power systems, for instance like upgrades or additions of substations and
power lines. FACTS-devices provide a better adaptation to varying operational conditions and
improve the usage of existing installations. The basic applications of FACTS-devices are:
• Power flow control,
• Increase of transmission capability,
• Voltage control,
• Reactive power compensation,
• Stability improvement,
• Power quality improvement,
• Power conditioning,
• Flicker mitigation,
• Interconnection of renewable and distributed generation and storages.
Figure 2.1 shows the basic idea of FACTS for transmission systems. The usage of lines
for active power transmission should be ideally up to the thermal limits. Voltage and stability
limits shall be shifted with the means of the several different FACTS devices. It can be seen that
with growing line length, the opportunity for FACTS devices gets more and more important.
The influence of FACTS-devices is achieved through switched or controlled shunt
compensation, series compensation or phase shift control. The devices work electrically as fast
current, voltage or impedance controllers. The power electronic allows very short reaction times
down to far below one second.
Fig. 2.1. Operational limits of transmission lines for different voltage levels
The development of FACTS-devices has started with the growing capabilities of power
electronic components. Devices for high power levels have been made available in converters for
high and even highest voltage levels. The overall starting points are network elements
influencing the reactive power or the impedance of a part of the power system. Figure 2.2 shows
a number of basic devices separated into the conventional ones and the FACTS-devices.
For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some
explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices
provided by the power electronics. This is one of the main differentiation factors from the
conventional devices. The term 'static' means that the devices have no moving parts like
mechanical switches to perform the dynamic controllability. Therefore most of the FACTS-
devices can equally be static and dynamic.
Fig. 2.2. Overview of major FACTS-Devices
The left column in Figure 2.2 contains the conventional devices build out of fixed or
mechanically switch able components like resistance, inductance or capacitance together with
transformers. The FACTS-devices contain these elements as well but use additional power
electronic valves or converters to switch the elements in smaller steps or with switching patterns
within a cycle of the alternating current. The left column of FACTS-devices uses Thyristor
valves or converters. These valves or converters are well known since several years. They have
low losses because of their low switching frequency of once a cycle in the converters or the
usage of the Thyristors to simply bridge impedances in the valves.
The right column of FACTS-devices contains more advanced technology of voltage
source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated
Gate Commutated Thyristors (IGCT). Voltage Source Converters provide a free controllable
voltage in magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. High
modulation frequencies allow to get low harmonics in the output signal and even to compensate
disturbances coming from the network. The disadvantage is that with an increasing switching
frequency, the losses are increasing as well. Therefore special designs of the converters are
required to compensate this.
2.2 Configurations of FACTS-Devices
2.2.1 Shunt Devices:
The most used FACTS-device is the SVC or the version with Voltage Source Converter called
STATCOM. These shunt devices are operating as reactive power compensators. The main
applications in transmission, distribution and industrial networks are:
• Reduction of unwanted reactive power flows and therefore reduced network losses.
• Keeping of contractual power exchanges with balanced reactive power.
• Compensation of consumers and improvement of power quality especially with huge demand
fluctuations like industrial machines, metal melting plants, railway or underground train systems.
• Compensation of Thyristor converters e.g. in conventional HVDC lines.
• Improvement of static or transient stability.
Almost half of the SVC and more than half of the STATCOMs are used for industrial
applications. Industry as well as commercial and domestic groups of users require power quality.
Flickering lamps are no longer accepted, nor are interruptions of industrial processes due to
insufficient power quality. Railway or underground systems with huge load variations require
SVCs or STATCOMs.
2.2.1.1 SVC
Electrical loads both generate and absorb reactive power. Since the transmitted load varies
considerably from one hour to another, the reactive power balance in a grid varies as well. The
result can be unacceptable voltage amplitude variations or even a voltage depression, at the
extreme a voltage collapse.
A rapidly operating Static Var Compensator (SVC) can continuously provide the reactive
power required to control dynamic voltage oscillations under various system conditions and
thereby improve the power system transmission and distribution stability.
Applications of the SVC systems in transmission systems:
a. To increase active power transfer capacity and transient stability margin
b. To damp power oscillations
c. To achieve effective voltage control
In addition, SVCs are also used
1. In transmission systems
a. To reduce temporary over voltages
b. To damp sub synchronous resonances
c. To damp power oscillations in interconnected power systems
2. In traction systems
a. To balance loads
b. To improve power factor
c. To improve voltage regulation
3. In HVDC systems
a. To provide reactive power to ac–dc converters
4. In arc furnaces
a. To reduce voltage variations and associated light flicker
Installing an SVC at one or more suitable points in the network can increase transfer
capability and reduce losses while maintaining a smooth voltage profile under different network
conditions. In addition an SVC can mitigate active power oscillations through voltage amplitude
modulation.
SVC installations consist of a number of building blocks. The most important is the
Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide
controllability. Air core reactors and high voltage AC capacitors are the reactive power elements
used together with the Thyristor valves. The step up connection of this equipment to the
transmission voltage is achieved through a power transformer.
Fig 2.3 SVC building blocks and voltage / current characteristic
In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switched or
Controlled Reactors (TSR / TCR). The coordinated control of a combination of these branches
varies the reactive power as shown in Figure. The first commercial SVC was installed in 1972
for an electric arc furnace. On transmission level the first SVC was used in 1979. Since then it is
widely used and the most accepted FACTS-device.
SVC
SVC USING A TCR AND AN FC
In this arrangement, two or more FC (fixed capacitor) banks are connected to a TCR
(thyristor controlled reactor) through a step-down transformer. The rating of the reactor is chosen
larger than the rating of the capacitor by an amount to provide the maximum lagging vars that
have to be absorbed from the system. By changing the firing angle of the thyristor controlling the
reactor from 90° to 180°, the reactive power can be varied over the entire range from maximum
lagging vars to leading vars that can be absorbed from the system by this compensator.
Fig 2.4 SVC of the FC/TCR type.
The main disadvantage of this configuration is the significant harmonics that will be
generated because of the partial conduction of the large reactor under normal sinusoidal steady-
state operating condition when the SVC is absorbing zero MVAr. These harmonics are filtered in
the following manner. Triplex harmonics are canceled by arranging the TCR and the secondary
windings of the step-down transformer in delta connection. The capacitor banks with the help of
series reactors are tuned to filter fifth, seventh, and other higher-order harmonics as a high-pass
filter. Further losses are high due to the circulating current between the reactor and capacitor
banks.
Fig 2.5 Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators
and synchronous condenser.
These SVCs do not have a short-time overload capability because the reactors are usually
of the air-core type. In applications requiring overload capability, TCR must be designed for
short-time overloading, or separate thyristor-switched overload reactors must be employed.
SVC USING A TCR AND TSC
This compensator overcomes two major shortcomings of the earlier compensators by reducing
losses under operating conditions and better performance under large system disturbances. In
view of the smaller rating of each capacitor bank, the rating of the reactor bank will be 1/n times
the maximum output of the SVC, thus reducing the harmonics generated by the reactor. In those
situations where harmonics have to be reduced further, a small amount of FCs tuned as filters
may be connected in parallel with the TCR.
Fig 2.6 SVC of combined TSC and TCR type.
When large disturbances occur in a power system due to load rejection, there is a possibility for
large voltage transients because of oscillatory interaction between system and the SVC capacitor
bank or the parallel. The LC circuit of the SVC in the FC compensator. In the TSC–TCR
scheme, due to the flexibility of rapid switching of capacitor banks without appreciable
disturbance to the power system, oscillations can be avoided, and hence the transients in the
system can also be avoided. The capital cost of this SVC is higher than that of the earlier one due
to the increased number of capacitor switches and increased control complexity.
2.2.1.2 STATCOM
In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic
COMpensator) went into operation. The STATCOM has a characteristic similar to the
synchronous condenser, but as an electronic device it has no inertia and is superior to the
synchronous condenser in several ways, such as better dynamics, a lower investment cost and
lower operating and maintenance costs. A STATCOM is build with Thyristors with turn-off
capability like GTO or today IGCT or with more and more IGBTs. The static line between the
current limitations has a certain steepness determining the control characteristic for the voltage.
The advantage of a STATCOM is that the reactive power provision is independent from the
actual voltage on the connection point. This can be seen in the diagram for the maximum
currents being independent of the voltage in comparison to the SVC. This means, that even
during most severe contingencies, the STATCOM keeps its full capability.
In the distributed energy sector the usage of Voltage Source Converters for grid interconnection
is common practice today. The next step in STATCOM development is the combination with
energy storages on the DC-side. The performance for power quality and balanced network
operation can be improved much more with the combination of active and reactive power.
Fig 2.7 STATCOM structure and voltage / current characteristic
STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either
Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices. The
STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the
STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus
voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller then Es then lagging
or inductive VARS are produced.
Fig 2.8 6 Pulse STATCOM
The three phase STATCOM makes use of the fact that on a three phase, fundamental
frequency, steady state basis, the instantaneous power entering a purely reactive device must be
zero. The reactive power in each phase is supplied by circulating the instantaneous real power
between the phases. This is achieved by firing the GTO/diode switches in a manner that
maintains the phase difference between the ac bus voltage ES and the STATCOM generated
voltage VS. Ideally it is possible to construct a device based on circulating instantaneous power
which has no energy storage device (ie no dc capacitor).
A practical STATCOM requires some amount of energy storage to accommodate
harmonic power and ac system unbalances, when the instantaneous real power is non-zero. The
maximum energy storage required for the STATCOM is much less than for a TCR/TSC type of
SVC compensator of comparable rating.
Fig 2.9 STATCOM Equivalent Circuit
Several different control techniques can be used for the firing control of the STATCOM.
Fundamental switching of the GTO/diode once per cycle can be used. This approach will
minimize switching losses, but will generally utilize more complex transformer topologies. As an
alternative, Pulse Width Modulated (PWM) techniques, which turn on and off the GTO or IGBT
switch more than once per cycle, can be used. This approach allows for simpler transformer
topologies at the expense of higher switching losses.
The 6 Pulse STATCOM using fundamental switching will of course produce the 6 N1
harmonics. There are a variety of methods to decrease the harmonics. These methods include the
basic 12 pulse configuration with parallel star / delta transformer connections, a complete
elimination of 5th and 7th harmonic current using series connection of star/star and star/delta
transformers and a quasi 12 pulse method with a single star-star transformer, and two secondary
windings, using control of firing angle to produce a 30phase shift between the two 6 pulse
bridges. This method can be extended to produce a 24 pulse and a 48 pulse STATCOM, thus
eliminating harmonics even further. Another possible approach for harmonic cancellation is a
multi-level configuration which allows for more than one switching element per level and
therefore more than one switching in each bridge arm. The ac voltage derived has a staircase
effect, dependent on the number of levels. This staircase voltage can be controlled to eliminate
harmonics.
Fig 2.10 Substation with a STATCOM
2.2.2 Series Devices
Series devices have been further developed from fixed or mechanically switched compensations
to the Thyristor Controlled Series Compensation (TCSC) or even Voltage Source Converter
based devices.
The main applications are:
• reduction of series voltage decline in magnitude and angle over a power line,
• reduction of voltage fluctuations within defined limits during changing power transmissions,
• improvement of system damping resp. damping of oscillations,
• limitation of short circuit currents in networks or substations,
• avoidance of loop flows resp. power flow adjustments.
2.2.2.1 TCSC
Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems in
transmission systems. Firstly it increases damping when large electrical systems are
interconnected. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR), a
phenomenon that involves an interaction between large thermal generating units and series
compensated transmission systems.
The TCSC's high speed switching capability provides a mechanism for controlling line power
flow, which permits increased loading of existing transmission lines, and allows for rapid
readjustment of line power flow in response to various contingencies. The TCSC also can
regulate steady-state power flow within its rating limits.
From a principal technology point of view, the TCSC resembles the conventional series
capacitor. All the power equipment is located on an isolated steel platform, including the
Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the
control and protection is located on ground potential
together with other auxiliary systems. Figure shows the principle setup of a TCSC and its
operational diagram. The firing angle and the thermal limits of the Thyristors determine the
boundaries of the operational diagram.
Fig. 2.11 . Principle setup and operational diagram of a Thyristor Controlled Series Compensation
(TCSC)
Advantages
Continuous control of desired compensation level
Direct smooth control of power flow within the network
Improved capacitor bank protection
Local mitigation of sub synchronous resonance (SSR). This permits higher levels of
compensation in networks where interactions with turbine-generator torsional vibrations
or with other control or measuring systems are of concern.
Damping of electromechanical (0.5-2 Hz) power oscillations which often arise between
areas in a large interconnected power network. These oscillations are due to the dynamics
of inter area power transfer and often exhibit poor damping when the aggregate power
tranfer over a corridor is high relative to the transmission strength.
2.2.2.3 SSSC
While the TCSC can be modeled as a series impedance, the SSSC is a series voltage source. The
principle configuration is shown in Figure 2.12, which looks basically the same as the
STATCOM. But in reality this device is more complicated because of the platform mounting and
the protection. A Thyristor protection is absolutely necessary, because of the low overload
capacity of the semiconductors, especially when IGBTs are used. The voltage source converter
plus the Thyristor protection makes the device much more costly, while the better performance
cannot be used on transmission level. The picture is quite different if we look into power quality
applications. This device is then called Dynamic Voltage Restorer (DVR). The DVR is used to
keep the voltage level constant, for example in a factory in feed. Voltage dips and flicker can be
mitigated. The duration of the action is limited by the energy stored in the DC capacitor. With a
charging mechanism or battery on the DC side, the device could work as an uninterruptible
power supply.
Fig. 2.12. Principle setup of SSSC and implementation as DVR for power quality applications
2.2.3 Shunt And Series Devices
2.2.3.1 Dynamic Power Flow Controller
A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC).
The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series
compensation.
A functional single line diagram of the Dynamic Flow Controller is shown in Figure 2.13 The
Dynamic Flow Controller consists of the following components:
• a standard phase shifting transformer with tap-changer (PST)
• series-connected Thyristor Switched Capacitors and Reactors (TSC / TSR)
• A mechanically switched shunt capacitor (MSC). (This is
optional depending on the system reactive power requirements)
Fig. 2.13. Principle configuration of DFC
Based on the system requirements, a DFC might consist of a number of series TSC or TSR. The
mechanically switched shunt capacitor (MSC) will provide voltage support in case of overload
and other conditions. Normally the reactances of reactors and the capacitors are selected based
on a binary basis to result in a desired
stepped reactance variation. If a higher power flow resolution is needed, a reactance equivalent
to the half of the smallest one can be added.
The switching of series reactors occurs at zero current to avoid any harmonics. However, in
general, the principle of phase-angle control used in TCSC can be applied for a continuous
control as well. The operation of a DFC is based on the following rules:
• TSC / TSR are switched when a fast response is required.
• The relieve of overload and work in stressed situations is handled
by the TSC / TSR.
• The switching of the PST tap-changer should be minimized
particularly for the currents higher than normal loading.
• The total reactive power consumption of the device can be
optimized by the operation of the MSC, tap changer and the
switched capacities and reactors.
In order to visualize the steady state operating range of the DFC, we assume an inductance in
parallel representing parallel transmission paths. The overall control objective in steady state
would be to control the distribution of power flow between the branch with the DFC and the
parallel path. This control is accomplished by control of the injected series voltage.
The PST (assuming a quadrature booster) will inject a voltage in quadrature with the node
voltage. The controllable reactance will inject a voltage in quadrature with the throughput
current. Assuming that the power flow has a load factor close to one, the two parts of the series
voltage will be close to collinear. However, in terms of speed of control, influence on reactive
power balance and effectiveness at high/low loading the two parts of the series voltage has quite
different characteristics. The steady state control range for loadings up to rated current is
illustrated in Figure 2.14, where the x-axis corresponds to the throughput current and the y-axis
corresponds to the injected series voltage.
Fig. 2.14. Operational diagram of a DFC
Operation in the first and third quadrants corresponds to reduction of power through the DFC,
whereas operation in the second and fourth quadrants corresponds to increasing the power flow
through the DFC. The slope of the line passing through the origin (at which the tap is at zero and
TSC / TSR are bypassed) depends on the short circuit reactance of the PST.
Starting at rated current (2 kA) the short circuit reactance by itself provides an injected voltage
(approximately 20 kV in this case). If more inductance is switched in and/or the tap is increased,
the series voltage increases and the current through the DFC decreases (and the flow on parallel
branches increases). The operating point moves along lines parallel to the arrows in the figure.
The slope of these arrows depends on the size of the parallel reactance. The maximum series
voltage in the first quadrant is obtained when all inductive steps are switched in and the tap is at
its maximum.
Now, assuming maximum tap and inductance, if the throughput current decreases (due e.g. to
changing loading of the system) the series voltage will decrease. At zero current, it will not
matter whether the TSC / TSR steps are in or out, they will not contribute to the series voltage.
Consequently, the series voltage at zero current corresponds to rated PST series voltage. Next,
moving into the second quadrant, the operating range will be limited by the line corresponding to
maximum tap and the capacitive step being switched in (and the inductive steps by-passed). In
this case, the capacitive step is approximately as large as the short circuit reactance of the PST,
giving an almost constant maximum voltage in the second quadrant.
2.2.3.2 Unified Power Flow Controller
The UPFC is a combination of a static compensator and static series compensation. It acts as a
shunt compensating and a phase shifting device simultaneously.
Fig. 2.15. Principle configuration of an UPFC
The UPFC consists of a shunt and a series transformer, which are connected via two voltage
source converters with a common DC-capacitor. The DC-circuit allows the active power
exchange between shunt and series transformer to control the phase shift of the series voltage.
This setup, as shown in Figure 2.15, provides the full controllability for voltage and power flow.
The series converter needs to be protected with a Thyristor bridge. Due to the high efforts for the
Voltage Source Converters and the protection, an UPFC is getting quite expensive, which limits
the practical applications where the voltage and power flow control is required simultaneously.
OPERATING PRINCIPLE OF UPFC
The basic components of the UPFC are two voltage source inverters (VSIs) sharing a common
dc storage capacitor, and connected to the power system through coupling transformers. One VSI
is connected to in shunt to the transmission system via a shunt transformer, while the other one is
connected in series through a series transformer.
A basic UPFC functional scheme is shown in fig.2.16
Fig 2.16 UPFC
The series inverter is controlled to inject a symmetrical three phase voltage system (Vse), of
controllable magnitude and phase angle in series with the line to control active and reactive
power flows on the transmission line. So, this inverter will exchange active and reactive power
with the line. The reactive power is electronically provided by the series inverter, and the active
power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to
demand this dc terminal power (positive or negative) from the line keeping the voltage across the
storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is
equal only to the losses of the inverters and their transformers. The remaining capacity of the
shunt inverter can be used to exchange reactive power with the line so to provide a voltage
regulation at the connection point.
The two VSI’s can work independently of each other by separating the dc side. So in that case,
the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to
regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as
SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power
flow on the transmission line.
The UPFC has many possible operating modes. In particular, the shunt inverter is operating in
such a way to inject a controllable current, ish into the transmission line. The shunt inverter can
be controlled in two different modes:
VAR Control Mode: The reference input is an inductive or capacitive VAR request. The shunt
inverter control translates the var reference into a corresponding shunt current request and
adjusts gating of the inverter to establish the desired current. For this mode of control a feedback
signal representing the dc bus voltage, Vdc, is also required.
Automatic Voltage Control Mode: The shunt inverter reactive current is automatically regulated
to maintain the transmission line voltage at the point of connection to a reference value. For this
mode of control, voltage feedback signals are obtained from the sending end bus feeding the
shunt coupling transformer.
The series inverter controls the magnitude and angle of the voltage injected in series with the line
to influence the power flow on the line. The actual value of the injected voltage can be obtained
in several ways.
Direct Voltage Injection Mode: The reference inputs are directly the magnitude and phase angle
of the series voltage.
Phase Angle Shifter Emulation mode: The reference input is phase displacement between the
sending end voltage and the receiving end voltage.
Line Impedance Emulation mode: The reference input is an impedance value to insert in series
with the line impedance.
Automatic Power Flow Control Mode: The reference inputs are values of P and Q to maintain
on the transmission line despite system changes.
3. THE STATCOM
The STATCOM (or SSC) is a shunt-connected reactive-power compensation device that is
capable of generating and/ or absorbing reactive power and in which the output can be varied to
control the specific parameters of an electric power system. It is in general a solid-state switching
converter capable of generating or absorbing independently controllable real and reactive power
at its output terminals when it is fed from an energy source or energy-storage device at its input
terminals. Specifically, the STATCOM considered in this chapter is a voltage-source converter
that, from a given input of dc voltage, produces a set of 3-phase ac-output voltages, each in phase
with and coupled to the corresponding ac system voltage through a relatively small reactance
(which is provided by either an interface reactor or the leakage inductance of a coupling
transformer). The dc voltage is provided by an energy-storage capacitor.
A STATCOM can improve power-system performance in such areas as the following:
1. The dynamic voltage control in transmission and distribution systems;
2. The power-oscillation damping in power-transmission systems;
3. The transient stability;
4. The voltage flicker control; and
5. The control of not only reactive power but also (if needed) active power in the connected line,
requiring a dc energy source.
Furthermore, a STATCOM does the following:
1. it occupies a small footprint, for it replaces passive banks of circuit elements by compact
electronic converters;
2. it offers modular, factory-built equipment, thereby reducing site work and commissioning
time; and
3. it uses encapsulated electronic converters, thereby minimizing its environmental impact.
A STATCOM is analogous to an ideal synchronous machine, which generates a balanced set of
three sinusoidal voltages at the fundamental frequency with controllable amplitude and phase
angle. This ideal machine has no inertia, is practically instantaneous, does not significantly alter
the existing system impedance, and can internally generate reactive (both capacitive and
inductive) power.
The Tennessee Valley Authority (TVA) installed the first ±100-MVA STATCOM in 1995 at its
Sullivan substation. The application of this STATCOM is expected to reduce the TVA’s need for
load tap changers, thereby achieving savings by minimizing the potential for transformer failure.
This STATCOM aids in resolving the off-peak dilemma of over voltages in the Sullivan
substation area while avoiding the more labor- and space-intensive installation of an additional
transformer bank. Also, this STATCOM provides instantaneous control and therefore increased
capacity of transmission voltage, providing the TVA with greater flexibility in bulk-power
transactions, and it also increases the system reliability by damping grids of major oscillations in
this grid.
To summarize, a STATCOM controller 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.
3.1 The Principle of Operation
A STATCOM is a controlled reactive-power source. It provides the desired reactive-power
generation and absorption entirely by means of electronic processing of the voltage and current
waveforms in a voltage-source converter (VSC). A single-line STATCOM power circuit is
shown in Fig. 3.1(a), where a VSC is connected to a utility bus through magnetic coupling. In
Fig. 3.1(b), a STATCOM is seen as an adjustable voltage source behind a reactance meaning that
capacitor banks and shunt reactors are not needed for reactive-power generation and absorption,
thereby giving a STATCOM a compact design, or small footprint, as well as low noise and low
magnetic impact. The exchange of reactive power between the converter and the ac system can
be controlled by varying the amplitude of the 3-phase output voltage, Es, of the converter, as
illustrated in Fig. 3.1(c). That is, if the amplitude of the output voltage is increased above that of
the utility bus voltage, Et, then a current flows through the reactance from the converter to the ac
system and the converter generates capacitive-reactive power for the ac system. If the amplitude
of the output voltage is decreased below the utility bus voltage, then the current flows from the
ac system to the converter and the converter absorbs inductive-reactive power from the ac
system. If the output voltage equals the ac system voltage, the reactive-power exchange becomes
zero, in which case the STATCOM is said to be in a floating state.
Adjusting the phase shift between the converter-output voltage and the ac system voltage
can similarly control real-power exchange between the converter and the ac system. In other
words, the converter can supply real power to the ac system from its dc energy storage if the
converter-output voltage is made to lead the ac-system voltage. On the other hand, it can absorb
real power from the ac system for the dc system if its voltage lags behind the ac-system voltage.
A STATCOM provides the desired reactive power by exchanging the instantaneous
reactive power among the phases of the ac system. The mechanism by which the converter
internally generates and/ or absorbs the reactive power can be understood by considering the
relationship between the output and input powers of the converter. The converter switches
connect the dc-input circuit directly to the ac-output circuit. Thus the net instantaneous power at
the ac output terminals must always be equal to the net instantaneous power at the dc-input
terminals (neglecting losses)
Figure 3. 1 The STATCOM principle diagram: (a) a power circuit; (b) an equivalent
circuit; and (c) a power exchange.
Assume that the converter is operated to supply reactive-output power. In this case, the real
power provided by the dc source as input to the converter must be zero. Furthermore, because the
reactive power at zero frequency (dc) is by definition zero, the dc source supplies no reactive
power as input to the converter and thus clearly plays no part in the generation of reactive-output
power by the converter. In other words, the converter simply interconnects the three output
terminals so that the reactive-output currents can flow freely among them. If the terminals of the
ac system are regarded in this context, the converter establishes a circulating reactive-power
exchange among the phases. However, the real power that the converter exchanges at its ac
terminals with the ac system must, of course, be supplied to or absorbed from its dc terminals by
the dc capacitor.
Although reactive power is generated internally by the action of converter switches, a dc
capacitor must still be connected across the input terminals of the converter. The primary need
for the capacitor is to provide a circulating-current path as well as a voltage source. The
magnitude of the capacitor is chosen so that the dc voltage across its terminals remains fairly
constant to prevent it from contributing to the ripples in the dc current. The VSC-output voltage
is in the form of a staircase wave into which smooth sinusoidal current from the ac system is
drawn, resulting in slight fluctuations in the output power of the converter. However, to not
violate the instantaneous power-equality constraint at its input and output terminals, the
converter must draw a fluctuating current from its dc source. Depending on the converter
configuration employed, it is possible to calculate the minimum capacitance required to meet the
system requirements, such as ripple limits on the dc voltage and the rated-reactive power support
needed by the ac system.
The VSC has the same rated-current capability when it operates with the capacitive- or
inductive-reactive current. Therefore, a VSC having a certain MVA rating gives the STATCOM
twice the dynamic range in MVAR (this also contributes to a compact design). A dc capacitor
bank is used to support (stabilize) the controlled dc voltage needed for the operation of the VSC.
The reactive power of a STATCOM is produced by means of power-electronic equipment of the
voltage-source-converter type. The VSC may be a 2- level or 3-level type, depending on the
required output power and voltage. A number of VSCs are combined in a multi-pulse connection
to form the STATCOM. In the steady state, the VSCs operate with fundamental-frequency
switching to minimize converter losses. However, during transient conditions caused by line
faults, a pulse width–modulated (PWM) mode is used to prevent the fault current from entering
the VSCs. In this way, the STATCOM is able to withstand transients on the ac side without
blocking.
3.2 The V-I Characteristic
A typical V-I characteristic of a STATCOM is depicted in Fig. 3.2. As can be seen, the
STATCOM can supply both the capacitive and the inductive compensation and is able to
independently control its output current over the rated maximum capacitive or inductive range
irrespective of the amount of ac-system voltage. That is, the STATCOM can provide full
capacitive-reactive power at any system voltage even as low as 0.15 pu. The characteristic of a
STATCOM reveals strength of this technology: that it is capable of yielding the full output of
capacitive generation almost independently of the system voltage (constant-current output at
lower voltages). This capability is particularly useful for situations in which the STATCOM is
needed to support the system voltage during and after faults where voltage collapse would
otherwise be a limiting factor.
Figure 3.2 also illustrates that the STATCOM has an increased transient rating in both the
capacitive- and the inductive-operating regions. The maximum attainable transient over current
in the capacitive region is determined by the maximum current turn-off capability of the
converter switches. In the inductive region, the converter switches are naturally commutated;
therefore, the transient-current rating of the STATCOM is limited by the maximum allowable
junction temperature of the converter switches.
Figure 3.2 The V-I characteristic of the STATCOM.
In practice, the semiconductor switches of the converter are not lossless, so the energy stored in
the dc capacitor is eventually used to meet the internal losses of the converter, and the dc
capacitor voltage diminishes. However, when the STATCOM is used for reactive-power
generation, the converter itself can keep the capacitor charged to the required voltage level. This
task is accomplished by making the output voltages of the converter lag behind the ac-system
voltages by a small angle (usually in the 0.18–0.28 range). In this way, the converter absorbs a
small amount of real power from the ac system to meet its internal losses and keep the capacitor
voltage at the desired level. The same mechanism can be used to increase or decrease the
capacitor voltage and thus, the amplitude of the converter-output voltage to control the var
generation or absorption.
The reactive- and real-power exchange between the STATCOM and the ac system can be
controlled independently of each other. Any combination of real power generation or absorption
with var generation or absorption is achievable if the STATCOM is equipped with an energy-
storage device of suitable capacity, as depicted in Fig. 3.3. With this capability, extremely
effective control strategies for the modulation of reactive- and real-output power can be devised
to improve the transient- and dynamic-system-stability limits.
Figure 3.3 The power exchange between the STATCOM and the ac
system.
Figure 3.4 An elementary 6-pulse VSC STATCOM.
3.3 Harmonic Performance
An elementary 6-pulse VSC STATCOM is shown in Fig. 3.4, consisting of six self-commutated
semiconductor switches (IGBT, IGCT, or GTO) with anti parallel diodes. In this converter
configuration, IGBTs constitute the switching devices. With a dc-voltage source (which may be
a charged capacitor), the converter can produce a balanced set of three quasi-square voltage
waveforms of a given frequency by connecting the dc source sequentially to the three output
terminals via the appropriate converter switches.
The power quality embraces issues such as voltage flicker, voltage dip, and voltage rise, as well
as harmonic performance and high-frequency noise. Power electronic devices distort voltage and
current waveforms in a power network, influencing power facilities and customer equipment in a
diverse manner. Harmonic currents induce abnormal noise and parasitic losses, and harmonic
voltages cause a loss of accuracy in measurement instruments and the faulty operation of relays
and control systems. Electromagnetic noise, caused by the noise of the high-frequency
electromagnetic waves emitted from power-electronic circuits, affects electronic devices used in
business and industry and often induces interfering voltage in communication lines. The
corrective measure generally recommended for mitigating harmonics and high-frequency noise is
to limit their generation at the source.
In principle, the STATCOM-output voltage wave is a staircase-type wave synthesized from the
dc-input voltage with appropriate combinations of converter switches. For example, the 6-pulse
converter shown in Fig. 3.4 is operated typically with either a 1200 or 1800 conduction sequence
for converter switches. For a 1800 conduction sequence, three switches conduct at a time; for a
1200 conduction sequence, two switches conduct at a time. Figure 3.5 shows the 3-step staircase-
line voltage, vab, along with the fundamental component, Vfund, for a conduction sequence of 1800.
The line voltage vab, in terms of its various frequency components, can be described by the
following Fourier-series equation;
…….(3.1)
Figure 3.5 An ac line-voltage output of a 6-pulse voltage-source inverter
for a 1808
conduction sequence.
where coefficients a0, ah, and bh can be determined by considering one fundamental period of vab.
The vab waveform is symmetrical, so the average voltage a0 = 0. It also has odd-wave symmetry;
therefore, ah= 0. The coefficient bh is determined as
….(3.2)
…..(3.3)
Therefore
…….(3.4)
For 1800 conduction sequence, α = 300; hence the triplen harmonics are zero
Figure 10.6 The output voltage of a 48-pulse STATCOM that generates reactive power.
in the line voltage, as seen from Eq. (3.4). It is also noted that the converter has harmonic
components of frequencies (6k ± 1) f 0 in its output voltage and 6k f0 in its input current, where f 0
is the fundamental-output frequency and k=1, 2, 3, . . . As is evident, the high harmonic content
in the output voltage makes this simple converter impractical for power-system applications.
To reduce harmonic generation, various converter configurations and converter-switching
techniques are suggested in the literature. For example, the first installed commercial
STATCOM has a 48-pulse converter configuration so that the staircase ac-line output-voltage
waveform has 21 steps, as shown in Fig. 10.6, and approaches an ideal sinusoidal waveform with
a greatly reduced harmonic content. Switching strategies, such as selective harmonic elimination
techniques, also aid in limiting harmonic generation at its source.
3.4 Steady-State Model
A STATCOM is always connected in shunt with the ac system through some magnetic coupling,
namely, the coupling transformer or interface reactor. A typical STATCOM connection is shown
in Fig. 3.7; it consists of a VSC using either a GTO or IGBT as a switching device, and a
capacitor, Cs, on the dc side as an energy-storage device. The resistance, Rp, in parallel with Cs
represents both the capacitor losses and switching losses. The STATCOM is connected to the ac
system through magnetic coupling, represented by leakage inductance, Ls, and resistance, Rs.
The STATCOM improves the desired power-system performance, including dynamic
compensation, mitigating the SSR by modulating the reactive power at the common-coupling
point, and so forth.
Figure 3.7 A typical STATCOM connection to the ac system.
The first-order differential equations for the ac-side circuit of the STATCOM circuit in Fig. 10.7
can be written as
…….(3.5)
These equations are converted on R-I frame of reference (the synchronously rotating frame of
reference) as follows:
…….(3.6)
The STATCOM dc-side-circuit equation can be written as
…….(3.7)
The instantaneous powers at the ac and dc terminals of the converter are equal, giving the
following power-balance equation:
……..(3.8)
where the constant 3/2 is the reference-frame transformation constant. Based on the phasor
diagram given in Fig. 3.7, EsR and EsI can be defined as follows
……(3.9)
where Kcs is the constant relating the ac and dc voltage. For example, in a 12-pulse VSC, Kcs =
2√6π. Therefore, Eq. (3.8) becomes
…..(3.10)
, …….(3.11)
Substituting the value of Idc in Eq. (3.7)
..(3.12)
Also, substituting the values of EsR and EsI from Eq. (10.9) into Eq. (3.6),
…..(3.13)
From Eqs. (3.12) and (3.13), the state-space model in the R-I frame for the STATCOM circuit in
Fig. 3.7 can be written as follows:
……(3.14)
The steady-state solution of the STATCOM circuit represented by Eq. (3.14) using Rs = 0.01 pu,
Xs = 0.15 pu, Rp =128 pu, Cs = 0.013 pu, and Kcs = 2√6π is plotted in Fig. 10.8 as a function of
the phase-difference angle, vd. In this plot, IsR and IsI are, respectively, the active and reactive
components of the STATCOM current, Is. The reactive-power output from STATCOM is
controlled by varying vd. It should be noted that IsI varies almost linearly with vd, and the range
of vd for controlling IsI within ±1 pu is very small.
Figure 3.8 The steady-state characteristics of a STATCOM.
Figure 3.9 The IEEE First Benchmark System with a STATCOM for SSR
damping studies.
3.5 A Multilevel VSC–Based STATCOM
The harmonic contamination of the power-system network by the addition of STATCOM into
the power system can be reduced by employing multilevel VSC configurations.
The multilevel converters usually synthesize a staircase-type voltage wave from several levels of
dc-voltage sources (typically capacitor-voltage sources). The multilevel VSC schemes studied
and tested so far include the diode clamp, the flying capacitor, and the cascaded, separate dc-
source converter types. Multilevel converters can reach high voltages and reduce harmonic
distortion because of their structure. To increase the voltage rating, many single-phase full-
bridge converters (FBCs) can be connected in series, automatically leading to a desirable
reduction of harmonic distortion. However, the need to balance capacitor voltages, the
complexity of switching, and the size of the capacitors all limit the number of levels that can be
practically employed.
Figure 3.11 shows the 3-phase star-connected arrangement of the separate dc-source, 3-level
binary VSC commonly referred to as a BVSI. It consists of three single-phase FBCs, each with
its own dc source, connected in series. However, the magnitude of each dc source is in binary
proportion of Vdc, 2Vdc, and 4Vdc, where Vdc is chosen to get the desired fundamental ac-voltage
output for a normalized 1-pu modulation index. The switches are turned on and off to generate a
15-step ac-voltage output over one fundamental cycle. In general, n-level BVSI would produce a
(2 n + 1 −1)–step ac-voltage output versus a (2n + 1)–step output generated by a conventional n-
level, separate dc-source VSC configuration.
Figure 3.12 illustrates the various voltages in the 3-level BVSI STATCOM. The resulting ac-
phase voltage, vav, and the fundamental-output voltage, va, of the 3-level converter are also
shown in Fig. 3.12. The output-phase voltage is
Figure3.11 The 3-phase, star-connected 3-level BVSI.
…..(3.16)
Where
……..(3.17)
The phase voltage given by Eq. (10.16) is obtained by varying the voltage output of each FBC
level in Eq. (3.17) by appropriately switching various devices and their combinations. From Fig.
3.12, the a-phase converter-output voltage for n-level BVSI is given by
Figure 3.12 Typical voltages of the 3-level BVSI.
……(3.18)
From Eq. (3.18), the fundamental root mean square (rms) voltage, Va, the harmonic rms voltage,
V2k1, and the maximum fundamental-phase voltage,Va max, can be determined as
…….(3.19)
And
.(3.20)
And
…….(3.21)
4. MULTI-LEVEL VSI
It is possible to notice more and more publications concerning modernization and development,
one of the basic directions in building DC/AC converters, which there are multi-level voltage
inverters, formulating step voltages using few supply sources both isolated as sectioned. Absence
in such inverters transformers takes off limitations in output voltage frequency control in range
of low frequencies. In result it is possible to distinguish three basic solution directions of multi-
level voltage inverters topologies:
- multi-level voltage inverters with levelling diodes (DC- Diode Clamped);
- multi-level voltage inverters with levelling capacitors (CC- Capacitor Clamped);
- multi-level voltage inverters as Isolated Series H-Bridges (ISHB), also called multi-level
cascade inverters;
On the base of above been mentioned structures, it is possible to create group of the new inverter
topologies as connection of the standard three-phase inverters with one-phase bridge inverters
All above mentioned structures makes possible obtainment quasi-sinusoidal output voltages, in
result
Fig.4. 1. Phase-to-phase output voltage and its spectrum:
a) standard VSI inverter; b) cascade topology
multi-level VSI(without PWM).
Basic blocks of this type of inverter there are conventional three-phase inverter (T5-T6; T5’-T6’;
T5’’-T6’’), as well as tree one-phase bridges (T1- T4), (T1’-T4’), (T1’’-T4’’) from which every
one is connected in series with half-bridge of the three phase inverter. Individual modules require
isolated supply source. During registration even supply volt of what, it is possible to reduce or
even to resign from applying additional filtering arrangements. It is a huge advantage mainly in
refer to use of them in drive and telecommunication, etc. Besides those inverters can be built on
higher voltages than conventional (with two voltage steps), what in case of devices working, e.g.
in industrial average voltage systems can lessen whole arrangement about fitting transformer.
Multi-level VSI are created among others to improve output voltage wave shape. Because multi-
level voltage (reminds more sinusoidal) it contains less higher harmonics, also extorted load
current is more sinusoidal (Fig.4. 1a,b).
4.1. Proposed topology multi-level VSI
Fig 4.2 presents proposed inverter, which is a series connection of one-phase transistor bridges
with three-phase voltage inverter. Proposed inverter can work both in three- as well as four-line
nets in last case supply source on inverter input contains divider from two capacitors, creating
zero point.
Fig.4. 2. Cascade topology based multi-level voltage inverter (experimental circuit)
Voltage values were accepted Udc2 and Udc1. All three one-phase bridges with unipolar
modulation are shaping three-step output voltage (VSI 3L), meanwhile three-phase bridge with
bipolar modulation shapes two-step phase voltage (UVSI 2L). Fig 4.3 presents formation of the
phase-to-phase output voltage UL1-2. It is a sum of voltages on one-phase of the inverter and
phase-to-phase voltage of the three-phase inverter (UL1-2=UVSI 3L2-UVSI 2L-UVSI 3L1).
Number of levels in the phase-to-phase output voltage, in three line net, carries out N=2n-1,
where: n- number of levels in phase voltage for four line net. In this case 7-step output voltage in
cascade topology based inverter is generated
4.2. Control algorithm
In system presented in Fig 4.4. difference signal between current reference value iZ and real
value iL is given to proportional-integrating (PI) regulator. Exit signal of this regulator is
compared with three triangular signals with frequencies of the commutating switches and with
even amplitudes. Triangular signals are shifted in relation to itself with amplitude value as it is in
Fig. 5. Result of comparison is given to the comparator, which forms steering impulses with
modulated widths. Arrangements possess constant switching frequency.
Fig.4.3. Voltage curves presenting phase-to-phase voltage construction (from above: Ref2 – two
step inverter phase-to-phase output voltage VSI 2L, Ch2- three-level inverter output voltage VSI
3L2, Ch4 three-level inverter output voltage VSI 3L1, Ch1 – cascade multi-level inverter
phaseto- phase output voltage UL1-2)
Fig.4. Arrangement for load current course formation
with constant switching frequency
Fig.4.5 presents inverter output voltage for one phase, which is sum of output voltages first (VSI
2L) and second (VSI 3L1) inverter with bipolar and unipolar modulations and in result of this it
is for-even-level quasi-sinusoidal curve (when Udc1=Udc2).
4.3. Experimental model
Experimental investigations (Fig4.6 - Fig.4.9) were made with the following parameters:
Udc1=Udc2= 50V; load resistance R=20Ω and inductance L=2mH. Analog pwm followup
modulator with 12kHz frequency was applied. level cascade topology inverter.
Fig 4.5. Inverter bridges voltages summation to show
formulation of the four-level phase voltage
Fig.4.6. Experimental model view of the multi-level
cascade topology inverter.
Fig.4. 7. Reference signal and load current, RL load.
Fig 4.8. Phase voltages of the cascade four-level VSI.
Fig.4. 9. Phase-to-phase voltages of the proposed fourlevel
VSI a) with PWM, b) without PWM.
4.4. Extension of control algorithm
So far there was considered multi-level cascade topology inverter, in which supply voltage
values on individual inverter bridges were even Udc1=Udc2. Then phase output voltage was sum
of voltages on one-phase bridge and half-bridge of the three-phase inverter (Fig.4.5). Founding,
that Udc1Udc2 as well as applying control algorithm, which both makes possible summation
as well as subtraction of voltage values, it is possible on four level inverter topology to shape six-
level phase voltage. Proposed diagram of the modified control algorithm presents Fig.4.10.
Modulation in this control algorithm was made on five comparators where there was compared
sinusoidal modulating signal with five triangular signals with even amplitudes and frequencies.
Triangular signals are shifted in relation to itself with value of amplitude how it shows
Fig.4.11.a). Principle of operation of the control algorithm is similar how in Fig.4.4, with this
that additionally on exit of comparator logical arrangement was applied.
Fig.4. 10. Modified control algorithm of the proposed
multilevel VSI, where it is possible to shape sixlevel
phase output voltage
Fig. 4. 11. Voltage time base wave shapes presenting
phase voltage level formulation for proposed topology:
a) signal representing PWM; b) control
signals (for one branch); c) voltages summation
and 4-level voltage UL1(4L) for Udc1=Udc2; d) 5-
level voltage UL1(5L) for Udc1=2Udc2 and 6-level
voltage UL1(6L) for Udc1=4Udc2.
Fig.4.12. shows voltage vectors in one-phase inverter bridge and in one leg of the cascade
inverter, which illustrate formation of levels in output voltage. From analysis of voltage vectors
it results, that at maintenance of condition Udc1=Udc2, proposed topology VSI shapes 4 level
phase voltage (Fig.4.11c; Fig.4.12b), at maintenance of condition Udc1=2Udc2; five-level
(Fig.11d, Fig.12c), meanwhile at Udc1=4Udc2 – sixlevel (Fig.4.11d, Fig.4. 12d).
Fig.12. Voltage vectors presenting phase vol-tage
level formulation for cascade topology from
Fig.2: a) with 4levels for standard control from
Fig. 4 and condition Udc1=Udc2; b) with 4 levels
for modified control from Fig.10 and condition
Udc1=Udc2; c) with 5 levels for modified control
from Fig.10 and condition Udc1=2×Udc2; d) with
6 levels for modified control from Fig.10 and
condition Udc1=4×Udc2
In proposed method of voltages formation with 4, 5,6 levels, in cascade topology inverter
with supply condition Udc1≠Udc2 there are voltage "stresses” on switches. Analysing one
branch of the cascade inverter’s, for case from Fig.11b) voltages on transistors of the inverter
VSI 3L1 are two times larger than on transistors of the three-phase inverter’s VSI 2L; what leads
to larger commutation losses. For case from Fig.11c) voltages on transistors are the same,
meanwhile for case from Fig.4.11d) larger voltage stresses are n transistors of the one-phase
inverter VSI 3L1 o. In this of case losses of the VSI 3L1 inverter, are larger than those of the
three-phase bridge inverter’s.
6. RESULTS IMPLEMENTATION PROPOSED VSI FOR STATCOM
To verify results of the theoretical investigations a down scale multilevel VSI hardware model,
with parameters presented in Tab.1, was developed. During investigations DC link voltages were
even UDC1=UDC2=UDC3=UDC4 and on output of the cascade based four level VSI a couple
choke was implemented
Fig.(6.1- 6.5) present experimental waveforms, during steady state operation of the STATCOM
VPQC, for two different load types, linear (resistive-inductive load) and non-linear (six pulse
rectifier with resistive- inductive load). Fig.6.1 illustrates investigated conditioner’s behaviour in
situation of linear R-L load, R=20 [] , L=72 [mH]. It is seen from this figure that multilevel
STATCOM has meaningful influence on the source current, distortions, in which, mostly come
as result of the distorted supply voltage ( Fig.6.2 ).
Above figure illustrates also the reactive power compensation capability. Fig.6.3. demonstrates
conditioner’s possibility for balancing the unbalanced loads in conditions of balanced source.
Fig.6.4. demonstrates the filtering capabilities of the multilevel STATCOM. As one can see from
those figures, the load current contains a large amount of harmonics due to the six pulse rectifier
with resistive-inductive load, however the source current is almost sinusoidal, see Fig.6.4. and
Tab.2. As it was told earlier, in the paper, STATCOM, with described control algorithm, is
“sensitive” on supply voltage variations (sags, dips), one can see from Fig.6.4. that those
variations have impact on nature of the source current, in our case, because of source voltage
magnitude is over it’s nominal value, becomes more inductive. Additionally Tab.2 presents the
THD coefficients in characteristic points of the investigated STATCOM and Fig.6.5
Fig.6.1. Symmetrical RL load: a) load; b) source (Ch-
1: source voltage (phase L1); Ch-2, Ch3, Ch4 –
load/source currents in three phases.
[13] demonstrates, in conditions of the non-linear load, four level cascade based VSC’s DC link
voltages
.
Fig.6.2. From above: Ch3 source voltage; Ch4 - multilevel
VSI output voltage; Ch2=Ch3-Ch4
Fig.6.3. Linear no symmetrical RL load: a) load side;
b) source side (Ch-2, Ch3, Ch4 – load/source
currents in three phases)
Fig.6.4 . Non-linear load, source voltage magnitude
over it’s nominal value (3%): a) P2=0.8 [kW];
b), c) P2=1.2 [kW]. Ch-1: multilevel VSI output
voltage; Ch-2: source current; Ch3- source voltage;
Ch4- load current.
Fig.6.5 . DC link voltages. From above: R-1: UDC1; R-
2:UDC2; R-3: UDC3; R-4: UDC4.
7. CONCLUSIONS
Paper presents three phase STATCOM based on the four level cascade VSI, which permits to
fulfill various tasks. To verify properties of the proposed conditioner’s a down scale hardware
model was developed. On the base of experimental investigations one can say that:
- conditioner can free from higher harmonics source current, even in situation of strongly
deformed load current;
- conditioner stabilizes load voltage in situation of source voltage magnitude variations;
- conditioners possess the reactive power compensation capability;
- conditioner possess the capability of balancing the unbalanced loads in conditions of balanced
source;
- load voltage stabilization in conditions of the source voltage magnitude variations leads to the
input reactive power growth;
- to avoid problem of the source voltage shape influence on the filtration quality, control
algorithm has to be equipped with low pass filter to checksource voltage harmonics.
8. REFERENCES
1. Ghosh A., Ledwich G: Power Quality Enhancement Using Custom Power Devices. Kluwer
Academic Publishers, Boston, 2002.
2. H. Fujita, Y. Watanabe, H. Akagi: Control and analysis of a unified power flow controller,
IEEE Trans. Power Electronics, 14, 6, 1999, pp.1021-1027.
3. F. Peng, H. Akagi, H. Nabae: Compensation characteristics of the combined system of shunt
passive and series active filters, IEEE Trans. on Industry Applications, 1993, Vol.29, No.1,
pp.144-15.
4. R. Strzelecki, H. Supronowicz: Power factor in AC supply systems and improvements
methods, Publishing house of the Technical University of Warszawa, Warszawa 2000.
5. R. Strzelecki, J. Rusiński, G. Benysek: Voltage source power quality conditioner,
Electromagnetic phenomena in Nonlinear Circuits - EPNC 2002, XVII Symposium. Leuven,
Belgia, 2002, pp. 179-182.
Top Related