Download Selection of scheme

LAB 3
1.
2. Sketch a Single Line Diagram (SLD) of the power plant on the basis of data provided in
Lab 2. Show all the basic structure of the power plants: Generators, Generator Neutral
Grounding Method, Generator Leads, Medium Voltage Bus Bar, Generator Circuit Breakers,
High Voltage Bus Bar, Transformer Connection, Station Supply Connection, Head Works
Supply, Low Voltage Distribution System, HV Switchyard etc.
3. Prepare an AUTOCAD drawing and submit the drawing in A3 paper size
The following attachment consists of the single line diagram of the power plant showing all
the basic structure of it like generator, Generator Neutral Grounding Method, Generator Leads,
Medium Voltage Bus Bar, Generator Circuit Breakers, High Voltage Bus Bar, Transformer
Connection, Station Supply Connection, HV Switchyard.
Almost all the hydraulic turbine-driven generators used are synchronous alternating current
machines, which produce electrical energy by the transformation of the rotational mechanical
energy. The electrical and mechanical design of each generator must confirm to the electrical
requirements of the power distribution system to which it will be connected, and also to the
hydraulic requirements of its specific plant. The electrical characteristics of the generator depend
on the following factors:
a) Capacity and the power factor
b) Generator power output rating
c) Generator voltage
d) Insulation
e) Short-circuit ratio
f) Line charging and condensing capacities
g) Power factor
h) Reactance
i) Amortisseur windings
j) Efficiencies
The Specification of a Generator used in our Power Plant :
Salient Pole Synchronous Generator .
Size/capacity
- 27.390 MVA.
Efficiency
- 90%.
Power Factor
- 0.85.
Generation Voltage
-66 KV.
Duty
-Continuous.
Alignment
-Vertical.
Insulation type
-Class F.
Cooling type
-Forced Air.
No. of poles
Synchronous speed
-8
-690.71322RPM
Our hydropower project consists of 3 units each of 23.281MW. So we have selected
salient pole synchronous generator of 66 KV, MVA with unit transformer scheme. That is the
power is generated at 66 KV level and stepped up to 132 KV and then the two units are
synchronized at 132 KV bus and the generated power is evacuated.
Drawing baki cha three unit bhako
EARTHING SELECTION:
The various grounding system in practice used for the earthing of generator is as follows:
1. Solid Neutral grounding.
2. Reactor Neutral grounding.
3. Resistor Neutral grounding.
4. Distribution Transformer Secondary Resistor Neutral grounding.
Among these methods we choose the distribution transformer secondary resistor neutral
grounding method for our power plant generator. It is high resistance neutral earthing. It
consists of a low ohm resistor shunted across the secondary of a step down transformer
whose primary is connected in the generator neutral circuit.
In this method of grounding the generator neutral current, during a phase to earth fault is
limited to a very low value between 5 to 15 A.
The primary advantage of this method is it limits the fault current during the short circuit
condition. With solid grounding the fault current is very high which cause various problems
hence to limit the current we use this method. This method is widely use in many
hydropower plant such as Kaligandaki, Marsyangdi, Khimti etc.
GENERATOR GROUNDING METHOD:
The main reasons for grounding the neutrals of synchronous generators are to limit over
voltages on the generators and connected equipment under phase to ground fault conditions,
and to permit the application of suitable ground fault relaying. We choose Distribution
transformer-secondary resistor neutral grounding, which is in effect high-resistance
neutral grounding. With this method of grounding, the generator neutral current, during a
phase-to-ground fault, is limited to a very low value, usually between 5A and 15A, by the use
of a relatively low-ohm resistor shunted across the secondary of a conventional step-down
transformer whose primary is connected in the generator neutral circuit. The possible damage
at the fault is therefore least of any of the various grounding methods.
Fig :-Distribution transformer-secondary resistor neutral grounding
We preferred to select the resistance grounding method. Its value is selected such that the
value of Xcg >> Rn or Xcg/Rn>>1.
The Phase Capacitance to Ground
Co=0.13µF.
Surge Capacitance
Cs=.25µF.
The total capacitance to ground
Ccg=3(.13+.25) µF=1.14 µF
Xcg=1/(2pf Ccg)=2792.190 Ω
Resistance in secondary=2792.19/(11/.41)2=3.90 Ω.
The cost of grounding devices and switch gear for other grounding methods with compared
to this grounding method is excessive due to large values of ground fault current.
Hence this method of earthing is suitable for our plant.
EXCITATION SYSTEM:
The excitation system used in this hydropower plant is brushless
DC excitation system since to avoid the cost of regular maintenance of brush replacement
due to wear and tear of the brushes during operation and since our unit is small there is no
stability concern. It consists of dry type transformer. A solid state rectifier rotates with the
shaft converting the AC output from the generator into the DC which is the supplied to the
rotating field coils of the main generator without the need of brushes.
GENERATOR LEADS
The term “generator leads” applies to the circuits between the generator terminals and the
low voltage terminals of the GSU (generator step-up) transformers. The equipment selected
depends upon the distance between the generator and transformer, the capacity of the generator,
types of generator breaker employed,the economics of installation. There are three types of
generator leads:



Non segregated
Segregated
Isolated
We have,
Generator output = 27.390 MW
Generated Voltage =66KV
So current in each line is I = (27.390)/(√3*66*.85) = 0.28188 KA.
= 281.88A
Types of bus bar to be used baki cha
Selection of scheme
We use unit generator-transformer scheme
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This is a conventional scheme preferred for medium sized power plants in a small power
system where the plant size is significant for the system.
With the use of individual transformer for each generator unit, unit protection of
generator and transformer becomes easy. Overall protection system becomes simple
and easy to locate the faults.
The fault level at the generator bus is less as compared to other schemes. ( refer to
relevant section)
This scheme makes maintenance outage simpler to arrange. In case of fault in a unit,
other units can run independent of it and the plant’s availability is higher.
It requires higher number of switchgear.
Drawing baki cha
Voltage bus bar pani baki cha