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.

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