Power generation systems are power generators and related systems that are used in power plants or large-scale facilities. When applying an AC drive and motor to an application; the characteristics of the drives load such as horsepower, torque and speed occur.
Electric power supply system in a country comprises of generating units that produce electricity; high voltage transmission lines that transport electricity over long distances; distribution lines that deliver the electricity to consumers; substations that connect the pieces to each other; and energy control centers to coordinate the operation of the components.
Cascade Efficiency
Single line representation of power system
Trying to represent a practical power system where a lot of interconnections between several generating stations involving a large number of transformers using three lines corresponding to R, Y, and B phase will become unnecessary clumsy and complicated. To avoid this, a single line along with some symbolical representations for generator, transformers substation buses are used to represent a power system rather neatly.
Distribution System
It is usual to express voltage, current, volt-amperes and impedance of an electrical circuit in per unit (or percentage) of a base or reference values of these quantities. The Per Unit System Definition value of any quantity is defined as:
Consider first a single-phase system. Let
Base Volt-Ampere = (VA)B VA
Base Volt = (V)B
So Base Current Ib = (VA)B/(V)B Amp
Base Impedance Zb= (V)B/Ib = {(V)B}2/(VA)B Ohm
If the actual impedance is Z (ohms), its Per Unit Value value is given by
For a power system, practical choice of base values are:
Base Megavolt-Ampere = (MVA)B
OR Base Kilovolt-Ampere = (kVA)B
Similarly Base kilovolt = (kV)B
In a three-phase system rather than obtaining the Per Unit System values using per phase base quantities, the per unit values can be obtained directly by using three-phase base quantities. Let
Three-phase base Mega Volt-amperes = (MVA)B
Line-to-line base kilovolts = (kV)B
Assuming star connection (equivalent star can always be found),
When MVA base is changed from Old (MVA)B, to New (MVA)B, B, new, and kV base is changed from (KV)B old, Old to (kV)B new, new, the new per unit impedance
Per Unit Representation of a Transformer
For a transformer
Z1(p.u) = Z2(p.u) = Z(p.u)
which shows that the per unit transformer impedance of a transformer will remain same in either side of the transformer.
Per Unit Impedance Diagram of a Power System
From a one-line diagram of a power system we can directly draw the impedance diagram by following the steps given below:
Advantages of Per Unit System
There are mainly two advantages of using the Per Unit System.
Hydro Power Plant
The generated electric power (P) in a hydropower plant is given by
where, Q = Discharge in m3/S
H = Water head in metre
η = Efficiency of plant
Power output of tidal scheme
P = QpgH in watt
where, Q = Quantity of water flow in m3/s
p = Density of sea water = 1025 kg/m3
g = Acceleration due to gravity = 9.81 m/s2
H = Water head in meter
Specific Speed of Turbine
Specific speed of a turbine is the speed of a scale model of turbine which develops 1 metric horse power under head of 1 m.
where, Ns = Specific speed in metric unit
N = Speed of turbine in rpm
P1 = Output in metric horse power
H = Effective head in meter
A Classification of Turbines According to Range of Head and Specific Speed
Nuclear Power Plant
The nuclear power plant is only the source which can supply the future energy demands of the world. The nuclear power plant use nuclear fission process in which atomic weight material nucleus splits into metals of lower atomic weight and releases the huge amount of energy
There are following main parts of nuclear plant
Fuel the radioactive element are used as fuel in the reactor. Generally, U235 and Pu239 are used as fuel.
Moderator Graphite, heavy water and beillium can be used as moderator. Control rods The most common used materials for control rods are cadmium, boron and helium.
The load characteristics depends upon the following factors
Demand Factor = Maximum demand / Total connected load
The word “demand” itself says the meaning of Demand Factor. The ratio of the maximum coincident demand of a system, or part of a system, to the total connected load of the system.
Average Load or Average Demand
Load Factor
Group Diversity Factor
Utilisation Factor
Capacity Factor
or
Load Factor
Plant Capacity
Electric-power transmission practically deals in the bulk transfer of electrical energy, from generating stations situated many kilometers away from the main consumption centers or the cities. For this reason the long distance transmission cables are of utmost necessity for effective power transfer, which in-evidently results in huge losses across the system. Minimizing those has been a major challenge for power engineers of late and to do that one should have a clear understanding of the type and nature of losses. One of them being the corona effect in power system, which has a predominant role in reducing.
Factors Affecting Corona
The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends :
Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs. Consider two conductors of radii r cm and spaced d cm apart. If V is the phase-neutral potential, then potential gradient at the conductor surface is given by
where
g0 = breakdown strength of air at 76 cm of mercury and 250C
= 30 kV/cm (max) or 21.2 kV/cm (r.m.s)
∴ Critical disruptive voltage,
Correction must also be made for surface condition of the conductor. This is accounting for multiplying the above expression by irregularity factor mo.
∴ Critical disruptive voltage
where mo = 1 for Polished Conductor
= 0.98 to 0.92 for Dirty Conductor
= 0.87 to 0.80 for Stranded Conductor
Visual Critical Voltage
It is the minimum phase neutral voltage at which corona glow appears all along the line conductor. It has been seen that in case of parallel conductor the corona glow does not glow at the Disruptive Voltage Vc but at higher voltage called Visual Critical Voltage.
Power Loss Due to Corona
f = frequency of the Supply
Vc = Critical Disruptive Voltage(r.m.s)
V = Phase to Neutral Voltage (r.m.s)
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1. What is a power system supply system? |
2. What is the per unit system in electrical engineering? |
3. How does a power system supply system work? |
4. What are the main components of a power system supply system? |
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