Reactive Power Control in Electrical Power Transmission System

7/28/2019 Reactive Power Control in Electrical Power Transmission System

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Sub: APS-1

Topic: Effect on Power Transfer Capacity


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A simple system analysis can be performed to

develop a basic understanding of the effect of

Shunt & Series Compensation on Power

transmission Capacity.

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It is desirable both economically and technically

to operate the electric power systems at near unity

power factor (u.p.f).

Usually power factor correction means to generate

reactive power as close as

possible to the load which it requires rather than

generating it at a distance and transmit it to

the load, as it results in not only need of

a large sized conductor but also increased losses

thereby reducing transmission efficiency.

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Artificial injection of reactive power at

the loads may relieve the transmission

network from reactive power flow and

improves both transmission efficiency andoperating power factor where as artificial

injection of negative reactance in the

lines may relieve the lines from excessive

voltage drop and improves the voltageregulation.

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The methods available for the injection of

both are static compensation and

synchronous compensation. Static

compensation involves capacitors andreactors where as synchronous

compensation involves synchronous phase

modifier.

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In all cases it is not required to satisfy

both the objectives of:

increasing the power level at which the voltage

profile is flat; and decreasing electrical length in order to improve

power transfer level

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Short lines may require voltage support, i.e.,

increase natural load

This may be achieved by shunt capacitors, provided does not become excessive as a result

Lines longer than 500 km cannot be loaded up

to natural load because of excessive In such cases, reduction of is the first priority

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To control voltage and/or improve maximumpower transfer capability

Achieved by modifying effective lineparameters:

-characteristic impedance,

-electrical length, = l

The voltage profile is determined by ZC

The maximum power that can be transmitteddepends on ZC as well as .

C

LZ

C

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Used to compensate the undesirable

voltage effects associated with line

capacitance

limit voltage rise on open circuit or light loadShunt compensation with reactors:

increases effective ZC reduces the effective natural load , i.e., voltage

at which flat voltage profile is achieved

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Effect on Power Transfer Capacity


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They are connected either:

directly to the lines at the ends, or

to transformer tertiary windings; conveniently

switched as var requirements varyLine reactors assist in limiting switching

surges

In very long lines, at least some reactors

are required to be connected to lines

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Used in transmission systems to

compensate for I 2X losses

Connected either directly to H.V. bus or

to tertiary winding of transformersNormally distributed throughout the

system so as to minimize losses and

voltage drops

Usually switched: a convenient means of

controlling voltage



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Shunt capacitor compensation of transmission

lines in effect

decreases ZC increases , i.e., electrical length

Advantages: low cost and flexibility of

installation and operating

Disadvantages: Q output is proportional to

square of the voltage; hence Q output reduced

at low voltages

Shunt capacitors are used extensively indistribution systems for power factor correction

and feeder voltage control



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Connected in series with the line

Used to reduce effective inductive reactance of

line

increases maximum power reduces I 2X loss

Series capacitive compensation in effect

reduces both:

characteristic impedance ZC, and

electrical length

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Reactive power produced increases with increasing

power transfer

Self regulating

Typical applications

improve power transfer compatibility

alter load division among parallel lines voltage regulation



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(a) Power transfer as a function of transmission angle

(b) Midpoint voltage as a function of power transfer


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With shunt capacitor compensation (chosen to

keep midpoint voltage at 1.0 pu when P = 1.4

Po

)

maximum power transfer capability increased to 1.58 pu

of natural power (SIL); represents an increase of 0.16 pu

over the uncompensated case

voltage regulation is poor, i.e., the voltage magnitude is

very sensitive to variations in power transfer



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With series capacitor compensation (chosen to keep

mid point voltage at 1.0 pu when P = 1.4 Po)

maximum power transfer capability increased to 2.65 pu

voltage regulation significantly improved



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Alternating current transmission systemsincorporating power electronics-based and

other static controllers to enhance

controllability and increase power transfer

capability



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Without fundamental research in this area, very

little use will be made with full confidence of the

real opportunities offered by FACTS devices. For the time being we only have limited examples,

entirely based on simulation, which demonstrate

that fast regulation of reactive compensation on a

transmission grid could be very useful in the future.

Because of this, there may exist an immediate

danger of uncoordinated system-wide fast

regulation via FACTS devices which could become

detrimental to system integrity under certain

operating conditions.



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Dynamic: Transient and dynamic stability

Subsynchronous oscillations

Dynamic overvoltages and undervoltages Voltage collapse

Frequency collapse



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Static: Uneven power flow

Excess reactive power flows

Voltage capability Thermal capability



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Long-term Control

-Power Flow control

-FACTS scheduling

-Economics

Dynamic Control

-System oscillation damping

-

Voltage stability-FACTS ringing

Local Control

-Control target acquisition

-Power electronics topology

-Modulation strategies

time

Is there a

one-size-fits-all

controller?



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UPFC

SSSC

TCSC

TCPAR

These devices can affect active power flow



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Sensitivity analysis

Where is the change in power transfercapacity in response to an addition of tcompensation in line i-j with admittance bij+j gijand b and g are sensitivity parameters

t

t

gt

t

b ijg

ij

b



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Optimization (optimal power flow) with genetic

algorithms to minimize some cost function

Generation costs

Congestion

Problem is nonlinear, non-smooth, and non-convex

Max-flow (graph theory) uses forward and backward

labeling from source to sink to dynamically determineline flows



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Dynamic Coordination of FACTS settings Security

Economics

Droop

Hierarchical or local control of FACTS?



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transient stability improvement

inter-area oscillation damping

voltage collapse avoidance

subsynchronous resonance mitigation

Each control objective will (possibly)

require a different FACTS placement



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Most dynamic control development has

concentrated on SMIB or very small two-

area systems

How is control implemented in a largenonlinear interconnected dynamic

network?

FACTS-FACTS interaction

FACTS-generator interaction

Hardware/field verification limited



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Unbalanced operation

Harmonics

Integration of Energy Storage (BESS,

SMES, flywheels)Power electronic topologies

Power electronics devices



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A. Edris et al., Proposed Terms and

Definitions for Flexible AC Transmission

System (FACTS), IEEE Transactions on

Power Delivery, Vol. 12, No. 4, October1997, pp. 18481853.

R. Mohan Mathur and Rajiv K. Varma -

Thyristor-Based FACTS Controllers for

Electrical Transmission Systems


Reactive Power Control in Electrical Power Transmission System

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