POWER FLOW CONTROL

POWER FLOW CONTROL

power flows:
1. Prime mover and excitation control of generators.
2. Switching of shunt capacitor banks, shunt reactors, and static var
systems.
3. Control of tap-changing and regulating transformers.
4. FACTS based technology.
A simple model of a generator operating under balanced steady-state
conditions is given by the Thévenin equivalent of a round rotor synchronous
machine connected to an infinite bus as discussed in Chapter 3. V is the
generator terminal voltage, E is the excitation voltage, δ is the power angle, and
X is the positive-sequence synchronous reactance. We have shown that:

P =(EV/X)sinδ
Q = (V/X)(Ecosδ −V)

The active power equation shows that the active power P increases
when the power angle δ increases. From an operational point of view, when the
operator increases the output of the prime mover to the generator while holding
the excitation voltage constant, the rotor speed increases. As the rotor speed
increases, the power angle δ also increases, causing an increase in generator
active power output P. There is also a decrease in reactive power output Q,
given by the reactive power equation. However, when δ is less than 15°, the
increase in P is much larger than the decrease in Q. From the power-flow point
of view, an increase in prime-mover power corresponds to an increase in P at
the constant-voltage bus to which the generator is connected. A power-flow
program will compute the increase in δ along with the small change in Q.
The reactive power equation demonstrates that reactive power output Q
increases when the excitation voltage E increases. From the operational point of
view, when the generator exciter output increases while holding the primemover
power constant, the rotor current increases. As the rotor current
increases, the excitation voltage E also increases, causing an increase ingenerator reactive power output Q. There is also a small decrease in δ required
to hold P constant in the active power equation. From the power-flow point of
view, an increase in generator excitation corresponds to an increase in voltage
magnitude at the infinite bus (constant voltage) to which the generator is
connected. The power-flow program will compute the increase in reactive
power Q supplied by the generator along with the small change in δ.
The effect of adding a shunt capacitor bank to a power-system bus can
be explained by considering the Thévenin equivalent of the system at that bus.
This is simply a voltage source VTh in series with the impedance Zsys. The bus
voltage V before connecting the capacitor is equal to VTh. After the bank is
connected, the capacitor current IC leads the bus voltage V by 90°. Constructing
a phasor diagram of the network with the capacitor connected to the bus reveals
that V is larger than VTh. From the power-flow standpoint, the addition of a
shunt capacitor bank to a load bus corresponds to the addition of a reactive
generating source (negative reactive load), since a capacitor produces positive
reactive power (absorbs negative reactive power). The power-flow program
computes the increase in bus voltage magnitude along with a small change in δ.
Similarly, the addition of a shunt reactor corresponds to the addition of a
positive reactive load, wherein the power flow program computes the decrease
in voltage magnitude.
Tap-changing and voltage-magnitude-regulating transformers are used
to control bus voltages as well as reactive power flows on lines to which they
are connected. In a similar manner, phase-angle-regulating transformers are
used to control bus angles as well as real power flows on lines to which they are
connected. Both tap changing and regulating transformers are modeled by a
transformer with an off-nominal turns ratio. From the power flow point of view,
a change in tap setting or voltage regulation corresponds to a change in tap ratio.
The power-flow program computes the changes in Ybu bus voltage magnitudes
and angles, and branch flows.
FACTS is an acronym for flexible AC transmission systems. They use
power electronic controlled devices to control power flows in a transmission
network so as to increase power transfer capability and enhance controllability.
The concept of flexibility of electric power transmission involves the ability to
accommodate changes in the electric transmission system or operating
conditions while maintaining sufficient steady state and transient margins.
A FACTS controller is a power electronic-based system and other static
equipment that provide control of one or more ac transmission system
parameters. FACTS controllers can be classified according to the mode of their
connection to the transmission system as:
1. Series-Connected Controllers.
2. Shunt-Connected Controllers.
3. Combined Shunt and Series-Connected Controllers.The family of series-connected controllers includes the following
devices:
1. The Static Synchronous Series Compensator (S3C) is a static,
synchronous generator operated without an external electric energy
source as a series compensator whose output voltage is in
quadrature with, and controllable independently of, the line current
for the purpose of increasing or decreasing the overall reactive
voltage drop across the line and thereby controlling the transmitted
electric power. The S3C may include transiently rated energy
storage or energy absorbing devices to enhance the dynamic
behavior of the power system by additional temporary real power
compensation, to increase or decrease momentarily, the overall real
(resistive) voltage drop across the line.
2. Thyristor Controlled Series Compensation is offered by an
impedance compensator, which is applied in series on an ac
transmission system to provide smooth control of series reactance.
3. Thyristor Switched Series Compensation is offered by an
impedance compensator, which is applied in series on an ac
transmission system to provide step-wise control of series
reactance.
4. The Thyristor Controlled Series Capacitor (TCSC) is a capacitive
reactance compensator which consists of a series capacitor bank
shunted by thyristor controlled reactor in order to provide a
smoothly variable series capacitive reactance.
5. The Thyristor Switched Series Capacitor (TSSC) is a capacitive
reactance compensator which consists of a series capacitor bank
shunted by thyristor controlled reactor in order to provide a
stepwise control of series capacitive reactance.
6. The Thyristor Controlled Series Reactor (TCSR) is an inductive
reactance compensator which consists of a series reactor shunted
by thyristor controlled reactor in order to provide a smoothly
variable series inductive reactance.
7. The Thyristor Switched Series Reactor (TSSR) is an inductive
reactance compensator which consists of a series reactor shunted
by thyristor controlled reactor in order to provide a stepwise
control of series inductive reactance.
Shunt-connected Controllers include the following categories:
1. A Static Var Compensator (SVC) is a shunt connected static var
generator or absorber whose output is adjusted to exchange
capacitive or inductive current so as to maintain or control specific
parameters of the electric power system (typically bus voltage).
SVCs have been in use since the early 1960s. The SVC application
for transmission voltage control began in the late 1970s.
2. A Static Synchronous Generator (SSG) is a static, self-commutated
switching power converter supplied from an appropriate electricenergy source and operated to produce a set of adjustable multiphase
output voltages, which may be coupled to an ac power
system for the purpose of exchanging independently controllable
real and reactive power.
3. A Static Synchronous Compensator (SSC or STATCOM) is a
static synchronous generator operated as a shunt connected static
var compensator whose capacitive or inductive output current can
be controlled independent of the ac system voltage.
4. The Thyristor Controlled Braking Resistor (TCBR) is a shuntconnected,
thyristor-switched resistor, which is controlled to aid
stabilization of a power system or to minimize power acceleration
of a generating unit during a disturbance.
5. The Thyristor Controlled Reactor (TCR) is a shunt-connected,
thyristor-switched inductor whose effective reactance is varied in a
continuous manner by partial conduction control of the thyristor
valve.
6. The Thyristor Switched Capacitor (TSC) is a shunt-connected,
thyristor-switched capacitor whose effective reactance is varied in
a stepwise manner by full or zero-conduction operation of the
thyristor valve.
The term Combined Shunt and Series-Connected Controllers is used to
describe controllers such as:
1. The Unified Power Flow Controller (UPFC) can be used to control
active and reactive line flows. It is a combination of a static
synchronous compensator (STATCOM) and a static synchronous
series compensator (S3C) which are coupled via a common dc link.
This allows bi-directional flow of real power between the series
output terminals of the S3C and the shunt output terminals of the
STATCOM, and are controlled to provide concurrent real and
reactive series line compensation without an external electric
energy source. The UPFC, by means of angularly unconstrained
series voltage injection, is capable of controlling, concurrently or
selectively, the transmission line voltage, impedance, and angle or,
alternatively, the real and reactive power flow in the line. The
UPFC may also provide independently controllable shunt reactive
compensation.
2. The Thyristor Controlled Phase Shifting Transformer (TCPST) is a
phase shifting transformer, adjusted by thyristor switches to
provide a rapidly variable phase angle.
3. The Interphase Power Controller (IPC) is a series-connected
controller of active and reactive power consisting of, in each phase,
of inductive and capacitive branches subjected to separately phaseshifted
voltages. The active and reactive power can be set
independently by adjusting the phase shifts and/or the branch
impedances, using mechanical or electronic switches. In the
particular case where the inductive and capacitive impedancesform a conjugate pair, each terminal of the IPC is a passive current
source dependent on the voltage at the other terminal.
The significant impact that FACTS devices will make on transmission
systems arises because of their ability to effect high-speed control. Present
control actions in a power system, such as changing transformer taps, switching
current or governing turbine steam pressure, are achieved through the use of
mechanical devices, which impose a limit on the speed at which control action
can be made. FACTS devices are capable of control actions at far higher
speeds. The three parameters that control transmission line power flow are line
impedance and the magnitude and phase of line end voltages. Conventional
control of these parameters is not fast enough for dealing with dynamic system
conditions. FACTS technology will enhance the control capability of the
system.
A potential motivation for the accelerated use of FACTS is the
deregulation/competitive environment in contemporary utility business. FACTS
have the potential ability to control the path of the flow of electric power, and
the ability to effectively join electric power networks that are not well
interconnected. This suggests that FACTS will find new applications as electric
utilities merge and as the sale of bulk power between distant exchange partners
becomes more wide spread.