Shunt Thyristor Controlled Reactor (TCR). (6) Statcom: The Static

Shunt Compensation:-

For shunt compensation
technique, the FACTS device is connected in parallel with the power system. Its
role is a variable current source, the two major types of shunt compensation
which are capacitive and inductive compensation.

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The shunt capacitive
compensation technique are mostly used to improve the power factor of the
system. The shunt capacitor will absorb the current leading the source voltage,
and consequently neutral out the effect of an inductive load connected to the
transmission line which causes lagging power factor. On the other hand, shunt
inductive compensation is adopted when there is insufficient load connected to
the transmission line. The shunt inductor is able to compensate the effect of
shunt capacitors where the voltage of receiving end is amplified due to the
Ferranti Effect.

Some of the examples
of shunt controllers are Static VAR Compensator (SVC), Static Synchronous
Compensator (STATCOM) and Thyristor Controlled Reactor (TCR). (6)

Statcom:

The Static
Synchronous Compensator (STATCOM) is based on the principal that a voltage
source inverter generates a controllable AC voltage source behind a transformer
reactance so that the voltage difference across the reactance produces active
and reactive power exchange between the STATCOM and the transmission network.
The STATCOM is a shunt reactive power compensating electronic device that
generates AC voltage, which in turn causes a current of variable magnitude at
the point of connection to the transmission system. This injected current is
almost in quadrature with the line voltage, thereby emulating an inductive or a
capacitive reactance at the point of connection with the transmission line.

A STATCOM installation plays
an important role in power industries to improve the stability of the system.
STATCOM in it basis is one DC-AC voltage source convertor having one storage
unit energy, usually a DC capacitor. It operating as Synchronous Voltage Source
(SVS) that connected to the line through a coupling transformer. STATCOM has a
dynamic performance far exceeding the other VAR Compensators.

Fig. 1 demonstrates a
simplified diagram of the STATCOM with an inverter voltage source, E and a tie
reactance, Xtie connected to an AC system with voltage source, Vth and a
Thevenin reactance, Xth. When the converter voltage is greater than the system
voltage, the STATCOM “sees” an inductive reactance connected at its terminal.
Hence, the system “sees” the STATCOM as a capacitive reactance and the STATCOM
is operating in a capacitive mode. The current flows from the STATCOM to the AC
system, and the device generates reactive power. In this case, the system draws
capacitive current that leads by and angle of 90? the
system voltage, assuming that the converter losses are equal to zero.

 

While the Fig. 2 below illustrates the voltage-current and
voltage-reactive power characteristic of the STATCOM compensator. It describes
the variation of STATCOM bus voltage with respect to STATCOM current or
reactive power.

(6)

 

Power factor correction:-

Fig1 shows the
single-line diagram of a power system. ? is the angle
between real axis (IRe) and imaginary
axis (IIm) of load
current so cos? is power
factor that is equal to:

Cos?=/SL

where PL and SL are real and
apparent power of load, respectively.

By using reactive power
compensator in load bus, the required reactive power is provided locally and
the following equations are achievable:

IL=Is+Icp=VL(GL+jBL)?jVLBcp,Qcp=?kcpQL

where Icp is the
injected current by compensator, Qcp is reactive
power injected by compensator for power factor correction and the coefficient kcp is
compensation gain varying between zero and one. Thus, for complete compensation (kcp=1):

Icp=?,=QL

meaning that reactive
power is completely supported by compensator. Note that depending on the
closeness of kcp to one, fewer
capacity of DG is occupied and DG will be able to support additional loads. In
addition, kcp can be tuned
based on load susceptance variation. It is achievable by using STATCOM.
Therefore, as the final deduction, kcp (STATCOM
compensation gain) is only and directly dependent on reactive power consumption
of load or power factor based on the following equation:

 

According to above
argument, it is obvious that the conventional STATCOM is not able to manage
power factors of two or more sources in different electrical distance with load.

 

According to above
argument, it is obvious that the conventional STATCOM is not able to manage
power factors of two or more sources in different electrical distance with load.

 

Voltage Fluctuation Mitigation:

 

In
non-islanded mode of operation, in absence of STATCOM, local excessive reactive
power demand is supplied by the utility grid. Sudden transients in the reactive
power demand are taken care of by utility grid and the AC bus voltage is
maintained. However, in islanded mode of operation, in absence of STATCOM,
reactive power demand is completely supplied by the converters of the power
sources such as wind power plants, solar plants and the conventional
synchronous generators of the pico-hydro plants. With limited capability to
supply the reactive power demand, islanded AC-bus of microgrid shows drastic
fluctuations in the voltage. This provokes need of AC-bus voltage regulating
control system to be embedded in STATCOM

 

Voltage regulation can be defined as
regulating DG output voltage based on load changes (for example, no load to
full load). Similar to power factor analysis, Eq. (5) can
be easily deduced as voltage deviation at load bus:

where Zs=Rs+jXs. It is clear
that voltage deviation is dependent on both real and reactive power consumption
of load. Since compensator is added to the system (in parallel with load), it
tries to reduce voltage deviation (?V?0). Therefore, QL in Eq. (5) should
be replaced with Qs=QL+Qcr where Qcr might be fixed (passive compensator) or it might
change (Qcr=kcrQL) based on voltage deviation and using
active compensators such as STATCOM. In the latter case, Qcr is tuned so that |Vs|=|VL|. As a result,
the roots of the following equation for the variable Qs when |Vs|=|VL| is
desired to find required Qcr for voltage
regulation:

In an ideal STATCOM, Qcr is generated
automatically so that |Vs|=|VL|. According to Eq. (6), reactive power compensation of STATCOM to regulate voltage
is dependent on both real power and reactive power of load (in comparison with
power factor correction that STATCOM compensation rate is only dependent on QL). Furthermore,
STATCOM effort is directly dependent on the line impedance Zs; meaning that
STATCOM is able to regulate voltage at a certain electrical distance from
source. In other words, the conventional STATCOM is unable to handle voltage
regulation of a system with different loads in different electrical distance
from source(s). That’s why STATCOMs are usually installed at the middle of line
to improve voltage profile.

 

Design of statcom:-

 

1.   
Power circuit:-

The PCB of Power Circuit is shown in (fig.
1). PCB was made using EAGLE Software. Power circuit contains the main topology
of DC-AC conversion. The power circuit consists of three parallel legs, each
leg consisting of two IGBTs(FGA25N120NTD) which are switched using the
switching pulses obtained from the driver circuit. A Driver circuit is
interfaced with the power circuit to ensure required driving characteristics of
the IGBTs. The IGBTs are switched at a frequency of 2kHz. This leads to problem
of high voltage spikes across the switch due to circuit inductance and also it
leads to ringing. To eliminate this, RC snubber circuit is used in the STATCOM
circuit. When the switch gets open, circuit eliminates the voltage transients
and ringing , as it provides alternate path for the current flowing through
circuit’s intrinsic leakage inductance5. Also it dissipates the energy in
resistor and thus junction temperature is reduced.

 

2.   
Control system:-

STATCOM includes a 2-level voltage source
inverter with a capacitor bank in DC link. The voltage source inverter is
driven by 3 phase SPWM waves. SPWM waves are equipped with dead band
programming in high side and low side IGBT circuit. Frequency, power angle and
voltage magnitude of STATCOM can be all controlled by controlling the SPWM
waves. STATCOM is synchronized to the utility grid using synchronizing control
systems67. The synchronizing control systems are shown in (fig. 2) It
includes,

 

A) 
Frequency control:-

A
feedback of line to line voltage of grid is fed to the frequency measurement
unit. The measured frequency is then given to the SPWM generator. Response time
of frequency control systems is crucial for us to avoid power instability

 

B)  
Phase-lock control system:-

Feedback
of grid voltage is fed to SPWM generator and SPWM is held in a constant phase
relation (power angle) with respect to the grid voltage. Reference given to
phase control decides real power transaction with the grid.

 

C)  
Charging and maintaining capacitor
voltage:-

With
no active source on DC side, charging of DC link capacitor is done by consuming
real power from the grid (fig. 3). Power angle is deliberately kept lagging so
as to charge the capacitor. Under steady state conditions, power angle is
constant and lagging just sufficient for the STATCOM to supply real power
losses in the power circuit and filter circuit. The job of charging and
maintaining the DC link capacitor voltage is done by the DC link voltage
regulating control systems.

 

D) 
Supply and consumption of reactive
power:-

The
STATCOM delivers reactive power or absorbs reactive power based on the formula
6

where,
Q = reactive power V = Voltage of the grid E = Voltage at inverter side X =
reactance ? = power angle

For
positive VAR (supply of reactive power), STATCOM voltage has to be higher than
the grid voltage. Increasing the modulation index of the SPWM waves serves the
purpose. Reactive power flow out of the STATCOM can directly be controlled by
controlling the modulation index of SPWM waves. The actual control systems are
configured to maintain the AC bus voltage constant to the specified reference;
which itself is indirectly done by controlling the modulation index i.e. by
controlling the AC bus voltage (fig. 4).7

 

(3)

 

Reactive power comensation for
statcom:-

The STATCOM is a
shunt connected reactive power compensation device. It is capable of generating
or absorbing reactive power. The output voltage of the STATCOM can be varied to
control the specific parameters of an electrical power system. The voltage
source inverter is employed turn off capability semiconductor switches. It is
an important part in the STATCOM because it can operate at high switching
frequencies.

The main reason for reactive
power compensation in a system: 1) the voltage regulation; 2) increased system
stability; 3) better utilization of machines connected to the system; 4)
reducing losses associated with the system; 5) to prevent voltage collapse as
well as voltage sag. The impedance of transmission lines and the need for
lagging VAR by most machines in a generating system results in the consumption
of reactive power 3 4. The unnecessary voltage drops lead to increased
losses which need to be supplied by the source.(4)

 

Working Principle of Statcom:-

 

VSC
is the backbone of STATCOM and it is a combination of self-commutating
solid-state turn-off devices (viz. GTO, IGBT, IGCT and so on) with a reverse
diode connected in parallel to them. The solid-state switches are operated
either in square-wave mode with switching once per cycle or in PWM mode
employing high switching frequencies in a cycle of operation or selective harmonic
elimination modulation employing low switching frequencies. A DC voltage source
on the input side of VSC, which is generally achieved by a DC capacitor and
output, is a multi-stepped AC voltage waveform, almost a sinusoidal waveform.
The turn-off device makes the converter action, whereas diode handles rectifier
action. STATCOM is essentially consisting of six-pulse VSC units, DC side of
which is connected to a DC capacitor to be used as an energy storage device,
interfacing magnetics (main coupling transformer and/or
inter-mediate/inter-phase transformers) that form the electrical coupling
between converter AC output voltage (Vc) and system voltage (Vs) and a
controller. The primary objective of STATCOM is to obtain an almost harmonic
neutralised and controllable three-phase AC output voltage waveforms at the
point of common coupling (PCC) to regulate reactive current flow by generation
and absorption of controllable reactive power by the solid-state switching
algorithm. As STATCOM has inherent characteristics for real power exchange with
a support of proper energy storage system, operation of such controller is
possible in all four quadrants of Q–P plane 2 and it is governed by the
following power flow relation

where
S is the apparent power flow, P the active power flow, Q the reactive power
flow, Vs the main AC phase voltage to neutral (rms), Vc the STATCOM fundamental
output AC phase voltage (rms), X (¼ vL, where, v ¼ 2pf ), the leakage
reactance, L the leakage inductance, f the system frequency and a the phase
angle between Vs and Vc. Active power flow is influenced by the variation of a
and reactive power flow is greatly varied with the magnitude of the voltage
variation between Vc and Vs. For lagging a, power (P) flows from Vc to Vs, for
leading a, power (P) flows from Vs to Vc and for a ¼0, the P is zero and Q is
derived from (1) as follows

The
AC voltage output (Vc) of STATCOM is governed by DC capacitor voltage (Vdc) and
it can be controlled by varying phase difference (a) between Vc and Vs (and
also by m, modulation index for PWM control). The basic twolevel and
three-level VSC configurations and respective AC output voltage (Vc) waveforms
corresponding to a squarewave mode of operation are illustrated in Figs. 1 and
2, respectively. Functionally, STATCOM injects an almost sinusoidal current (I)
in quadrature (lagging or leading) with the line voltage (Vs), and emulates as
an inductive or a capacitive reactance at the point of connection with the
electrical system for reactive power control, and it is ideally the situation
when amplitude of Vs is controlled from full leading (capacitive) to full
lagging (inductive) for a equals to zero (i.e. both Vc and Vs are in the same
phase). The magnitude and phase angle of the injected current (I) are determined
by the magnitude and phase difference (a) between Vc and Vs across the leakage
inductance (L), which in turn controls reactive power flow and DC voltage, Vdc
across the capacitor. When Vc . Vs, the STATCOM is considered to be operating
in a capacitive mode. When Vc , Vs, it is operating in an inductive mode and
for Vc ¼ Vs, no reactive power exchange takes place. In the high rating STATCOM
operated under fundamental frequency switching, the principle of phase angle
control (a) is generally adopted in control algorithm to compensate converter
losses by active power drawn from AC system and also for power flows in or out
of the VSC to indirectly control the magnitude of DC voltage with charging or
discharging of DC bus capacitor enabling control of reactive power flow into
the system. Phasor diagrams on the operating principle are illustrated in
(Figs. 3a– 3g).  (1)

 

 

Most
Optimal statcom installation location:-

After finding
out that STATCOM possess better ability to improve the overall bus voltage
profile, the next step carried out was to figure out the best location to
install the STATCOM for the 14-bus test system. By manually incorporating the
STATCOM at each bus and then proceed by running the simulation, all the results
are then gathered and tabulated to compare the result. The Fig.
14illustrates the average value of all buses voltage magnitude for easier
comparison.

It was found that bus 9 is
the best location for STATCOM to be installed. This is due to bus 9 has the
worst initial voltage profile as compared to the other buses. Hence, it will
benefit greater from the STATCOM as compared to installing at other buses.

Most
optimal statcom MVAR rating:-

After
identifying the most optimal location to install STATCOM is bus 9 for the
14-bus test system, the MVAR capacity of STATCOM was then determined. The value
was changed manually from ?60MVAR to ?120MVAR with 10MVAR step interval. The
result was then tabulated in the Table 4.1 below and Fig. 15 below
was plotted accordingly using the average value of the data. The negative sign
indicates that reactive power is absorbed from the bus 9 to increase the bus
voltage magnitude.

From the graph, it was found
that the most ideal value would be ?98 MVAR in order to obtain the best overall
and desired 1.0pu voltage magnitude. (6)

 

 

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