### 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.

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)