lecture notes for sub station and power system operation & maintenance course

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Lecture Notes for Sub Station and Power System Operation

& Maintenance Course

By

U Tin Maung Tun

Chief Engineer

Thermal Generation Department

Myanmar Electric Power Enterprise

20th

July 2009

Subject - Power System

1. Introductory

1-1 Importance of Electrical EnergyCivilizations of mankind are closely engaged with energy everyday. In the

future our existence will be more and more dependent upon energy.

Electric Energy occupies the top position in the energy hierarchy. It finds

numerous uses in home, industry agriculture and even in transport. Improvement of

the quality of life of the people depend so much upon the supply of electrical energy.

1-2 Superiority of Electrical EnergyElectrical energy is considered to all other forms of energy such as, (Chemical,

Heat, Light, Sound, and Mechanical) due to the following reasons.

(a) Cheapness (much cheaper than other)

(b) Convenient and Efficient Transmission

(c) Easy to control

(d) Cleanness

(e) Greater Flexibility (very easy to use)

Now 25% of the total energy used is electricity in the world, but because of the

numerous advantages this portion will be greater and greater in future.

1-3 Generation of Electrical Energy

(a) Conventional methods

By using engines, steam turbines, gas turbines, hydraulic turbines as prime-

movers for the electrical machines (Generators or Alternators).

(b) Non Conventional methods

- Generation without using prime-movers

- Solar cells, Fuel cells thermo electric, Geothermal tidal, Wind



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2. Power Systems

2-1 In the early days a generation plant supplies electricity to an area.

Now demand of electricity is greater and greater, the capacity of power plant is

larger and larger. The power plants are connected with High Voltage Transmission

Lines and transported to cities, towns and Industrial Areas by using Primary Sub

Station and medium voltage line. The interconnected network of Generation Plants,

Sub Stations and Loads are called Power System.

2-2 Elements of Power System

(a) Generators

(b) High and Extra High Voltage Transmission Lines

(c) Sub-Stations

(d) - Switch gears and measuring instruments

- Power transformers

(e) Protection and Control and Communication Systems, Reactors, Capacitors

(f) Consumer equipments (Loads)

2-3 Advantages of Power System

(a) Economical

(b) Reliable (Less Interruption)

(c) Low capital investment

(d) Low operation and maintenance cost

(e) Flexibility of operation

(f) Less reserve capacity (Spinning and Stand-by)

3. Power system studies

The following three studies should be done for proper and reliable power

system.

1. Load flow study

2. Short circuit study

3. Stability study

Without above mentioned studies a power system cannot be operated with

proper voltage regulations, minimum loss and less outages.



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3-1 Load flow study

In a 3AC power system active and reactive power flows from the generating

stations to the load through different network buses and branches (Transmission

Lines). Active power (P) and reactive Power (Q) supplied by generators at generator

buses. Reactive power (Q) is supplied or drawn from load buses by shunt

compensation elements (Shunt Capacitor, Reactors, Static VAR systems) and also

load elements such as motors (Synchronous and Induction) chokes on Lamps,

Welding transformers, Arc furnaces.

The flow of P and Q in a power system is called load flow.

In load flow study the voltage of buses and their phase angles are affected by

the power flow and vice-versa.

3-1-1 Advantages of load flow study

(a) Planning and Operation of existing power system.

(a) Future extension of generating plants, transmission lines, sub-stations and new

branches.

(c) Localizations of shunt power capacitors, reactor s, fixed series capacitors,

thyristor controlled series capacitors, static VAR compensators (FSC, TCSC,

and SVC)

(d) Selection of the voltage ranges and number of taps in On Load Tap Changers

(OLTC) of power transformers.

3-1-2 Methods of load flow study

For many years load flow studies were carried out by means of a special

purpose analog computer called AC network analyzer but the advent of high speed

digital computers replace their use for large system studies in greater flexibility,

economy, accuracy and speed of operation. Now we can get load flow study software

operable with modern personal computers.3-2 Short circuit study

In a AC 3-phase power system the faults or short circuit occurrences can not be

avoided, mostly on transmission lines, bus bars, Current transformer, Potential transformer,

Circuit breakers, etc. The amount of short circuit current is calculated by special software

with high speed personal computers. It well takes a lot of time to calculate manually by

mathematic equations.



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3-2-1 Types of Short Circuits

(a) Single line to ground

(b) Line to Line

(c) Double Line to ground

(d) Three phase (Symmetrical)

3-2-2 Advantages of short circuit studies

(a) Replacement of aged equipments (Circuit Breakers, CT, Bus, etc.)

(b) Define the setting of protective relays (Over current, Earth Fault) for fault

discrimination and reduce the blackout areas.

(c) Predetermine the short circuit capacity of future power system extension

equipments.

(d) Support system stability

(e) Support power quality assurances

t(f) Support power system earth methods to limit Earth Fault Current

3-3 Stability Study

In all power system, the larger machines are of the synchronous type these

include substantially of the generators and condenser and considerable parts of the

motors. On such system it is necessary to maintain synchronism.

Transient disturbances are caused by change in loads switching operations and

particularly faults and loss of excitation.

Thus maintenance of synchronism during study state conditions and regaining

of synchronism or equilibrium after disturbance are very important to the electrical

power producers (Utilities).

Stability = Maintenance of Synchronism

Stability Limit = Maximum power flow of part of the (or) point of the

System

3-3-1 Types of stability

(a) Steady state stability

(b) Transient stability

(c) Dynamic stability



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(a) Steady state stability

Capability of an electric power system to maintain synchronism

following the slow disturbance (Normal Load fluctuations, action of automatic

voltage regulator and generator governor).

(b) Transient stability

Large disturbances such as load thrown out line faults fault clearing,

load thrown in.

In this case the synchronism is maintained and the maximum amount of

power at that time is transient stability limit.

(c) Dynamic StabilityAbility of power system to maintain the synchronism after the

initializing until the system settle down to the new study state equilibrium

condition. This actually occurs in about (1-1.5) seconds after disturbance.

Ps =X

VsVrSin

3-3-2 Methods to improve stability

(a) Increase the system voltages

(b) Reduce the transfer reactance

(c) Use high speed circuit breaker

(d) Auto re-closing (3 poles for feeders, Single pole for tie line)

(e) - Grounding resistors instead of solid grounding

(f) - Series capacitors in the mid way of the line

Load angle

Ps

lecture notes for sub station and power system operation & maintenance course

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