Voltage and reactive power control

Voltage and reactive

power control

Presented byAhmed ALi

Instructor : Asst .Prof .Dr. Ahmed METE VURAL

power system opration

Code: EEE 574

the lamp characteristics are very sensitive to changes of voltage.

the life of the lamp may be reduced by 50%

Also the power load consisting of induction motors, the voltage

variations may cause stray operation.

Too wide variations of voltage cause excessive heating of electrical

device such as distribution transformers

Importance of Voltage and reactive power Control

both utility and customer equipment designed to operate at certain

voltage rating, Long time operation outside allowable range could

cause them damage

System stability is satisfactory

voltage levels and reactive power control have significant impact on

stability

The reactive power flow is minimized so as to reduce I 2R and I 2X

losses to a practical minimum that

ensures transmission system operates efficiently

• Reactive power does not travel very far Usually necessary to produce it

close to the location where it is needed A supplier/source close to the

location of the need is in a much better position to provide reactive

power

• Reactive power generated by the ac power source is stored in a capacitor

or a reactor during a quarter of a cycle and in the next quarter of the cycle

it is sent back to the power source. Therefore the reactive power oscillates

between the ac source and the capacitor or reactor So to avoid the

circulation between the load and source it needs to be control

Reactive power generated

alternator synchronous impedance

Transmission line

transformer impedance

feeders

These voltage variations are undesirable and must be kept within

the limits

When the load on the system increases,

the voltage drop increased in

Reactive power formulas

Q = VAR

For heavy load condition s required

+ve var C- bank

For light load condition required

-ve var L –bank

Q α

Receive power Injection

Than Total reactive power in receiving

QR = QS + QC

JQc

Ps+ JQs

PR+ J(QR +Qc )

Load Shunt compensation

Power sending

PR + JQR

Methods of control voltage Reactive PowerControl of voltage levels is accomplished by controlling the production,

and absorption, of reactive power flow at all levels in the system

Generating units provide the basic means of voltage control

Additional devices to control voltage:

o Static sources or sinks of reactive power

o Voltage regulating such as tap transformers,

o Dynamic source such as Synchronous motor

Static sources or fixed

• Shunt compensation

• Series compensation

• Static (VAR) compensators

• Static compensators

All these devices compensate reactive power using a similar operating principle.

• The device that is connected in parallel with the transmission line is

called the shunt compensator.

• A shunt compensator is widely used in transmission system to

regulate the voltage magnitude. It can be provided by either a

current source ,or voltage source

• The Shunt connected reactors are used to reduce the line over

voltages by consuming the reactive power,

• The shunt connected capacitors are used to maintain the voltage

levels by compensating the reactive power to transmission line.

Shunt compensators

Figure 1 : Transmission line with shunt compensation

Series compensators

When a device is connected in series with the transmission line or

feeder it is called a series compensator.

A series compensator can be connected anywhere in the transmission

It works as a controllable voltage source.

Series inductance exists in all AC transmission lines.

when a large current flows, this causes a large voltage drop. To

compensate, series capacitors are connected, to decreasing the

effect of the inductance.

Figure 2 : Transmission line with series compensation

V Sending

X TL

V receiving

Xc serise

A static VAR compensator (or SVC) is an electrical device for providing  reactive

power 

The term static is used to denote that there are no moving or rotating components

The SVC designed to bring the system closer to unity power factor. (usually use

 thyristor to Controlled Reactors and capacitor)

• If the power system's reactive load is capacitive (leading),

the SVC will use reactors lowering the system voltage.

• also Under inductive (lagging) conditions,

the capacitor banks are automatically switched to providing a higher

system voltage.

static VAR compensator

Figure 3 : static VAR compensator. use  thyristor switched

Advantage and disadvantage a) They are more reliable .

b) Faster in operation .

c) Smoother control and more

flexibility can be provided with

the help of thyristors.

Disadvantage

Need a transformers steps the

transmission voltage down to a much

lower level

the size and number of components.

more expansive compered with

shunt capacitive

Static Compensator

The devices use synchronous voltage sources for generating or absorbing

reactive power.

A synchronous voltage source (SVS) is constructed using a voltage source

converter (VSC).

A STATCOM usually contains an ( SVS) A synchronous voltage source

that is driven from a dc storage capacitor and the (SVS) is connected to

the ac system bus through an interface transformer. The transformer

steps the ac system voltage down

Figure 4 : STATCOM Static synchronous Compensator

The STATCOM can be operated in two different modes:

When the feeder voltage is lower than the voltage of the converter

the STATCOM generates reactive power.

When the converter voltage is higher than the feeder voltage,

the STATCOM absorbs reactive power

Voltage regulating tap changing transformers

Auto transformer

In this method, a number of tapping's are provided on the secondary

of the transformer.

The tap selection may be made on automatic or manual tap

changer mechanism.

the number of tapping have been provided a variation voltage on

the secondary.

When the position of the tap is varied, the number of secondary

turns is varied the voltage varied

Tap Changing transformer

Tap/changing transformer works when the movable arm makes contact with lower positions such as 1, the

secondary voltage is minimum

• this during the period of light inductive load

When the movable arm contact with higher position such as 5 ,the

secondary voltage is maximum

• This during the period of high inductive load,

During high system load conditions, network voltages are kept at

highest practical level to

• minimize reactive power requirements

• increase effectiveness of shunt capacitors to compensated reactive

power

During light load conditions, it is usually required to lower network

voltages

• avoid under excited operation of generators

Advantage of tap changing transformer

• autotransformer has a single winding with two end terminals, one or

more terminals at intermediate tap points,

• The primary voltage is applied across two of the terminals,

• the secondary voltage taken from two terminals,

• always having one terminal is common with the primary and

secondary

• the current flows directly from the input to the output, and only

smaller part inductively

• Autotransformers are frequently used in power transmission and

distribution

autotransformer

In a step down transformer the source is usually connected across

the full winding while the load is connected by a tap across the

desired voltage

In a step up transformer, the source is connected to a tap across

desired voltage ,while load is attached across the full winding

common autotransformer

Induction Regulators

There are two types of induction regulators single phase and 3 phase.

The construction it is similar to a induction motor except that the rotor is

not allowed to rotate continuously but can be adjusted in any position

either manually or by a small motor.

The adjustable output voltage by varying the inductive coupling between a

rotor and a stator winding

induction Regulators are used for voltage control of distribution primary

feeders.

Figure 5 :single induction Regulators

Single phase induction regulator.

The primary winding terminals of the stator and is connected across

the supply line.

The secondary winding is for rotor is connected in series with the line

whose voltage is to be controlled.

The primary windings either in star or delta are wound of the stator and

are connected across the supply.

The secondary windings are wound of the rotor and the six terminals are

connected in series with the line whose voltage is to be controlled.

Three phase induction regulators are used to regulate the voltage of

feeders and connection with high voltage

Three phase induction regulator

Figure 6 : Three phase induction regulator Y_ Connected

• A synchronous machine running without a prime mover or a mechanical

load

• the reactive power control depending on field excitation, Started as an

induction motor ,

• the main function is the improvement of p.f of the electrical system is

known as the synchronous condenser.

• It is installed at the receiving end of the line .

Synchronous Condenser dynamic Compensators

figure 5 : Synchronous Condenser

1 Reliability is very high.

2. No generation of harmonics.

3. Low maintenance and not affected by harmonics.

Advantages

Better efficiency of power generation, transmission and distribution

Improvement in voltage

Reduced KVA demand

Reduced system losses.

Benefit of reactive power and voltage controlled

why capacitors are used as reactive power compensation device?

A capacitor is said to be generator of reactive power. When a

capacitor is connected across a load, it provides reactive power to

the load. Its cheep type of compensation

By reactive power compensation we can control the power factor

and stability of voltage and reduce the consumption of electricity.

Summary of reactive power compaction

THANK YOU

FOR listening

any question

Reference

1. The raja Electrical Technology chapter 15 voltage controlled of

Twenty Third edition

2. REACTIVE POWER AND VOLTAGE CONTROL ISSUES IN ELECTRIC

POWER SYSTEMS Peter W. Sauer University of Illinois at Urbana-

Champaign -sauer@ece.uiuc.edu

3. www.google.com/ renewable academy

4. en.wikipedia.org