UNIT-5 (EE 204)
INDUSTRIAL POWER SUPPLY
Afroz Alam & Mahboob Hassan
Department of Electrical Engineering AMU, Aligarh
Contents
• Autotransformers
• Welding Transformers
• Tariff Systems
• Power Factor Improvement
What is Autotransformer?
The autotransformer is by definition, a transformer consisting of only one winding with a part of its turns being common to both primary and secondary circuits.
Part of the load in the receiver circuit is supplied directly from the supply circuit through the primary winding, the remainder being supplied indirectly through the secondary winding by electro magnetic induction.
As such, there is no primary or secondary. Any two points on the winding can be connected to the supply and likewise a load may be connected across any two points.
A minimum of two voltage taps are required for an autotransformer to perform a useful task.
An autotransformer does not provide electrical isolation between the input and output so must not be used in safety critical applications such as portable tool transformers, arc welders or car battery chargers.
Suitable applications are in supply voltage matching where only a small difference exists between input and output voltages.
Autotransformer
The autotransformer does not differ from the ordinary transformers in its fundamental principles.
The same laws that govern the ampere-turn relations in the ordinary transformer hold good for autotransformers also.
But it differs essentially in the manner of connection to the circuits in the primary and secondary systems.
In the ordinary transformers the primary and secondary windings are magnetically interconnected but electrically separated.
Autotransformer (Contd.)
Since the change of voltage in the primary and secondary windings of transformers go through their maximum and minimum values at the same time, the result of connecting the two windings in series is to produce a voltage which is either the sum or the difference of the voltages of the windings, according to the mode of their joints. K=
N
1/N
2= V
1/V
2.
In the autotransformers, the windings are both magnetically and electrically interconnected.
Autotransformer (Contd.)
Autotransformer
Autotransformer connected for step-down operation
N
HS= Number of turns on the High Voltage Side
N
LS= Number of turns on the Low Voltage Side
Autotransformer Example
Turns ratio = a = N
HS/ N
Ls= N
A/ N
B= 80 / 20 = 4 V
LS= V
HS/ a = 120 V / 4 = 30 V
I
LS= V
LS/ Z
LOAD= 30/0.5 = 60A, I
HS= I
LS/ a = 60/4 = 15A
Note: The load resistance may be read as 0.5 Ω
How did the load current become 60A?
15A provided directly to the load by V
HS45A provided to the load by “transformer action”
Autotransformer Example
(Contd.)
Example 5.1
• A 400 turn autotransformer operating in the step-down mode with a 25% tap supplies a 4.8 kVA, 0.85 pf lagging load. The input to the transformer is 2400 V, 60 Hz. Neglecting the small losses and leakage effects, determine:
(a) the load current,
(b) the incoming line current, (c) the transformed current,
(d) the apparent power conducted and the apparent power transformed.
Example 5.1 part (a)
a = N
HS/ N
LS= 400/(0.25)(400) = 4
V
LS= V
HS/ a = 2400 / 4 = 600 V
I
LS= 4800 VA / 600 V = 8 A = I
LOAD(b) I
LINE= I
HS= I
LS/ a = 8 A / 4 = 2 A (c) I
TR= I
LS– I
HS= (8 – 2) A = 6 A
(d) S
cond= I
HSV
LS= (2 A)(600 V) = 1200 VA S
trans= I
TRV
LS= (6 A)(600 V) = 3600 VA
Example 5.1 parts (b), (c) and (d)
Two-Winding Transformer connected as an Autotransformer
Two-Winding Transformer Reconnected as Autotransformer
1 2 2
2 2 2
2
( )
( 1)
at
w
at w
S V V I
S V I
S a S
Example 5.2
• A 10 kVA, 50-Hz, 2400/240 V distribution transformer is reconnected for using as a step-up autotransformer with a 2640 V output and a 2400 V input.
• Determine
(a) the rated primary and secondary currents when connected as an autotransformer.
(b) the apparent power rating when connected as an autotransformer.
10 41.67 240
41.67
4.167 10
LS
HS
I kV A A
V
I A A
As a two-winding transformer
Example 5.2 (Contd.)
As an autotransformer
2
( 1) ( 2400 1) 10 110
at w
240
S a S kV A
Example 5.2 (Contd.)
Welding Transformers
Welding Transformers
A welding transformer is a step down transformer.
Primary winding has a large number of turns of thin wire.
It has less voltage and very high current in the secondary winding.
Secondary voltage is usually 15 to 45 volts but the current may be 60A to 600A.
Several taps are provided on Secondary to control the welding current.
One end of the secondary is connected to the welding electrode,
whereas the other end is connected to the pieces to be welded.
A very high current through the electrode produces I
2R heat due to the contact resistance between the electrode and pieces to be welded.
This heat is very large and melts a tip of the electrode. As a result the gap between the two pieces is filled.
Welding Transformers (Contd.)
Welding Transformers (Contd.)
Requirements of a Welding Transformer
• Drooping static volt-ampere characteristic.
• The open circuit voltage should not normally exceed 80 volts and in no case 100 volts.
• The output current should be controllable continuously over
the full available range.
V-I Characteristics
Reactors used in a Welding Transformer
• The winding used for the welding transformer is highly
reactive or a separate reactor may be added in series with the secondary.
• The welding transformer can be used with various reactors
for control of arc.
Tapped Reactor
• Output current is regulated by taps on the reactor.
• It has limited number of current settings.
Moving Coil Reactor
• Relative distance between primary and the secondary is adjustable.
• Larger the distance between the coils, lesser current is obtained.
Magnetic Shunt Reactor
• Position of central magnetic shunt can be adjusted.
• This adjusts the shunted flux
and hence output current gets
changed.
Continuous Variable Reactor
• The height of the reactor is continuously varied
• Greater the core insertion greater is the reactance and less is
the output current
Saturable Reactor
• The reactance of the reactor is adjusted by changing the value of d.c.
excitation.
• More the d.c. currents, reactor approaches to saturation.
• This changes the reactance of reactor and hence changing the current.
Tariff Systems
Power System
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• Electrical energy is most useful form of energy because it can be most conveniently transformed into other forms of energy like heat light, mechanical energy that we require in our day to day life.
• But electricity is not readily available and is required to be produced (generated) in a factory called power station.
• Like any other manufacturing process, the production (generation) of electricity also need some cost to be incurred - Plants and Equipment, Inputs (water, fuel etc.), Ash smoke disposal systems, Personnel
• Cost of Transmission and Distribution to the large number of consumers of various categories (viz. domestic, commercial, industrial, agricultural etc.)
• All these costs when added together constitutes the total cost of electricity which the consumers have to share according to the quantum of electricity consumed taking into account the nature and time of use of electricity by each category of consumers.
• The tariff is the rate at which the electrical energy is sold to the consumers.
Tariff in Power System
Types of Tariff in the Power System
The various types of tariffs followed in the market are as follows:
Simple Tariff
Flat Rate Tariff
Block Rate Tariff
Two Part Tariff
Maximum Demand Tariff
Power Factor Tariff
Three Part Tariff
When there is a fixed rate per unit of energy consumed, it is known as Simple Tariff (or Uniform Rate Tariff).
This is the most simplest of all tariffs.
In this type, the price charged per unit is constant.
It means, the price will not vary with increase or decrease in number of units used.
Disadvantages:
The cost per unit energy delivered is high.
There is no discrimination among various types of consumers.
Simple Tariff
When different types of consumers are charged at different uniform per unit rates, it is said to be Flat Rate Tariff.
In this type, the consumers are grouped into different classes.
Each class is charged at different uniform rate.
The different classes of consumers may be taken into account of their diversity and load factors.
Since this type of tariff varies according to the way of supply used, separate meters are required for lighting load, power load etc.
Flat Rate Tariff
When a given block of energy is charged at a specified rate and the
succeeding blocks of energy are charged at progressively reduced/increased rates is called as block rate tariff.
In this type, the energy consumption is divided into many blocks and price per unit is fixed in each block.
Block Rate Tariff
When the rate of electrical energy is charged on the basis of maximum demand of the consumer and the units consumed, it is called two-part tariff.
In this type, the total charge to be made from the consumer is split into two components i.e. fixed charges and running charges.
The running charges depend upon the number of units (kWh of energy )
consumed by the customer. Thus the consumer is charged at a certain amount per kW of maximum demand + a certain amount per kWh of energy
consumed.
Total charges = ₹ (X * kW + Y * kWh)
It is easily understood by the consumer.
It recovers fixed charges which depend upon the maximum demand of the consumer independent of the units consumed.
Disadvantages
Consumer has to pay the fixed charges irrespective of the fact whether he has consumed the electrical energy or not.
There is always error in assessing the maximum demand of the consumer.
Two Part Tariff
It is similar to two-part tariff.
The only difference is the maximum demand of the consumer is calculated by installing a maximum demand meter at his premises.
This type of tariff is mostly applied to the bulk consumers.
Maximum Demand Tariff
The tariff in which the power factor of the consumers is taken into account is known as power factor tariff.
Power Factor Tariff
When the total charges to be made from the consumer is split into three parts, fixed charge, semifixed charge and running charge, it is known as three-part tariff.
This type of tariff is applied to big consumers.
The principle objective of this type of tariff is that the charges are split into three components ( fixed charge, charge per kW of maximum demand, charge per kWh of energy consumed).
Three Part Tariff
Power Factor Improvement
What is Power Factor?
Power factor
AC power has two components:
Real power or active power (P), expressed in watts (W).
Reactive power ( Q), expressed in reactive volt-amperes (VAR).
These are combined to the Complex power/Apparent Power ( S)
expressed volt-amperes (VA).
The power factor is defined as the ratio of real power to apparent
power.
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Power factor (P.F.) is the ratio of actual power to the apparent power.
P.F.=Actual power(kW) / Apparent power (kVA).
For a purely resistive load the power factor is unity. Active and reactive power are designated by P &Q respectively. The average power in a circuit is called active power and the power that
supplies the stored energy in reactive elements is called reactive power.
Definition Of Power Factor
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Active Power:
Also known as “real power” or simply “power.” Active power is the rate of producing, transferring, or using electrical energy. It is
measured in watts and often expressed in kilowatts (kW) or
megawatts (MW). The terms “active” or “real” power are used in place of the term “power” alone to differentiate it from “reactive power.
Apparent Power:
The product of the voltage (in volts) and the current
(in amperes). It comprises both active and reactive power . It is measured in “volt-amperes” and often expressed in
“kilovolt-amperes” (kVA) or “megavolt-amperes” (MVA).
Definition Of Power Factor (Contd.)
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• Inductive loads cause the current to lag behind the voltage. The wave forms of voltage and current are then "out of phase" with each other. The more out of phase they become then the lower the Power Factor. Power Factor is usually expressed as Cos Phi (Ø).
Definition Of Power Factor (Contd.)
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Consider a canal boat being pulled by a horse.
If the horse could walk on water then the angle (Phi) Ø would be zero and COSINE Ø=1. Meaning all the horse power is being used to pull the load.
However the relative position of the horse influences the power. As the horse gets closer to the barge, angle Ø1
increases and power is wasted, but, as the horse is positioned further away, then angle Ø2 gets closer to zero and less
power is wasted.
Understanding Power Factor
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Causes Of Low Power Factor
A poor power factor can be the result of either a significant phase difference between the voltage and current at the load terminals or it can be due to a high harmonic content or
distorted/discontinuous current waveform.
Poor load current phase angle is generally the result of an inductive load such as an induction motor power transformer, lighting ballasts, welder or induction furnace, Induction
generators wind mill generators and high intensity discharge lightings.
Power factor decreases with the installation of non resistive
loads such as induction motors, Transformers etc.
Methods for Power Factor Improvement
The following devices are mainly used for Power Factor Improvement:
• Static Capacitor
• Synchronous Condenser
• Static VAR Compensator (SVC)
• Static Synchronous Compensator (STATCOM)
Static Capacitor
Most of the industries and power system loads are inductive hence draws lagging current which decrease the system power factor.
For Power factor improvement, static capacitors are connected in parallel with those devices which work on low power factor.
These static capacitors provides leading current which neutralize the lagging inductive component of load current hence improving the power factor of the load circuit.
These capacitors are installed in vicinity of large inductive load (e.g.
Induction motors, transformers etc.) and improve the load circuit
power factor to improve the system efficiency.
Fig.1 shows a single phase inductive load drawing a lagging current (I), and the load power factor is Cosθ .
Static Capacitor (Contd.)
In second case, a capacitor (C) has been connected in parallel with the load as shown in fig 2.
Static Capacitor (Contd.)
• Now a current (Ic) is flowing
through Capacitor which leads 90°
from the supply voltage.
• The load current is (I). The Vectors combination of (I) and (Ic) is (I’) which is lagging from voltage at θ2 as shown in Fig 3.
Static Capacitor (Contd.)
• It can be seen from Fig 3 that θ2 < θ1. Therefore Cosθ2 > Cosθ1.
Hence the load power factor is improved by capacitor.
• Also note that after the power factor improvement, the circuit current would be less than from the low power factor circuit current.
• Also, before and after the power factor improvement, the active component of current would be same in that circuit because capacitor eliminates only the reactive component of current.
• Also, the Active power (in Watts) would be same after and before power factor improvement.
Static Capacitor (Contd.)
Advantages:
Capacitor bank offers several advantages over other methods of power factor improvement:
• Losses are low in static capacitors
• There is no moving part, therefore need low maintenance
• It can work in normal atmospheric conditions
• Do not require a foundation for installation
• They are lightweight so it is can be easy to installed
Static Capacitor (Contd.)
Disadvantages:
• The age of static capacitor bank is less (8 – 10 years)
• With changing load, we have to ON or OFF the capacitor bank, which causes switching surges on the system
• If the rated voltage increases, then it causes damage it
• Once the capacitors spoiled, then repairing is costly
Static Capacitor (Contd.)
Synchronous Condenser
• Synchronous condenser is generally used to improve the power factor in large industries.
• When a Synchronous motor operates at No-Load and over-exited then it is called a synchronous condenser.
• Whenever a Synchronous motor is over-exited, its armature current leads voltage and hence power factor becomes leading. This leading armature current cancels lagging current from other sources and gives very high power factor.
• When a synchronous condenser is connected across supply voltage (in parallel) then it draws leading current and partially eliminates the reactive component and this way, power factor is improved.
Synchronous Condenser (Contd.)
Advantages:
• Long life (almost 25 years)
• High Reliability
• Step-less adjustment of power factor.
• No generation of harmonics of maintenance
• The faults can be removed easily
• It is not affected by harmonics
• Require low maintenance
Disadvantages:
• It is expensive (maintenance cost is also high) and therefore mostly used by large power users.
• An auxiliary device has to be used for this operation because synchronous motor has no self starting torque.
• It produces noise.
Synchronous Condenser (Contd.)
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Synchronous Condenser
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Static VAR Compensator (SVC)
The Static VAR Compensator (SVC) is a shunt device of the Flexible AC Transmission Systems (FACTS) family using power electronics to control power flow and improve transient stability on power grids.
The SVC regulates voltage at its terminals by controlling the amount of reactive power injected into or absorbed from the power system.
When system voltage is low, the SVC generates reactive power (SVC capacitive). When system voltage is high, it absorbs
reactive power (SVC inductive).
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