The plant capacity of a 3-phase generating station consists of two 10,...
Problem Statement:
The plant capacity of a 3-phase generating station consists of two 10,000 kVA generators of reactance 12% each and one 5000 kVA generator of reactance 18%. The generators are connected to the station bus-bars from which load is taken through three 5000 kVA step-up transformers each having a reactance of 5%. Determine the maximum fault MVA which the circuit breakers on (i) low voltage side and (ii) high voltage side may have to deal with.
Solution:
Given data:
- Two 10,000 kVA generators of reactance 12% each
- One 5000 kVA generator of reactance 18%
- Three 5000 kVA step-up transformers each having a reactance of 5%
Calculation of Fault MVA:
We can calculate the maximum fault MVA which the circuit breakers on low voltage and high voltage side may have to deal with by using the following steps:
Step 1: Calculate the total generator capacity
- Total generator capacity = 2 x 10,000 kVA + 1 x 5000 kVA = 25,000 kVA
Step 2: Calculate the total generator reactance
- Total generator reactance = 2 x 12% + 1 x 18% = 42%
Step 3: Calculate the total transformer capacity
- Total transformer capacity = 3 x 5000 kVA = 15,000 kVA
Step 4: Calculate the total transformer reactance
- Total transformer reactance = 3 x 5% = 15%
Step 5: Calculate the fault MVA on low voltage side
- Maximum fault MVA on low voltage side = Total transformer capacity / Total transformer reactance
- Maximum fault MVA on low voltage side = 15,000 kVA / 15% = 100,000 MVA
Step 6: Calculate the fault MVA on high voltage side
- Maximum fault MVA on high voltage side = Total generator capacity / Total generator reactance
- Maximum fault MVA on high voltage side = 25,000 kVA / 42% = 59,524 MVA
Conclusion:
Hence, the maximum fault MVA which the circuit breakers on low voltage side may have to deal with is 100,000 MVA and the maximum fault MVA which the circuit breakers on high voltage side may have to deal with is 59,524 MVA.
The plant capacity of a 3-phase generating station consists of two 10,...
. The plant capacity of a 3-phase generating station consists of two 8 MVA generators of reactance 14·5%
each and one 4 MVA generator of reactance 9·5%. These are connected to a common bus-bar from
which loads are taken through a number of 3 MVA step-up transformers each having 4% reactance.
Determine the MVA rating of the circuit breakers on (i) L.V. side and (ii) H.V. side. Reactances given
are based on the MVA of each equipment.
[(i) 15·24 MVA (ii) 50·25 MVA]
4. The 33 kV bus-bar of a station are in two sections A and B separated by a reactor. Section A is fed from
four 10 MVA generators each having 20% reactance and section B is fed from the grid through 50 MVA
transformer of 10% reactance. The circuit breakers have rupturing capacity of 500 MVA. Find the
reactance of the reactor to prevent the circuit breakers being overloaded if a symmetrical short-circuit
occurs on an outgoing feeder connected to it.
[1·45 ΩΩ
Ω]
5. A generating station has five section bus-bar connected with a tie-bar through 7·5% reactors rated at
3000 kVA. Each generator is of 3000 kVA with 10% reactance and is connected to one section of the
bus-bar. Find the total steady input to a dead short-circuit between the lines on one of the sections of the
bus-bars (i) with and (ii) without reactors.
[(i) 55·3 MVA (ii) 150 MVA]
6. A generating station has four bus-bar sections. Each section is connected to tie-bar though 20% reactors
rated at 200 MVA. Generators of total capacity 100 MVA and 20% reactance are connected to each busbar section. Calculate the MVA fed to a fault under short-circuit condition one one of the bus-bars.
[1000 MVA]
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