MVA方法短路电流计算MVA Method Short Circuit Calculation

1、MVA Method Short Circuit CalculationA Short Circuit Study is an important tool in determining the ratings of electrical equipment to be installed in a project. It is also used as a basis in setting protection devices. Computer software simplifies this process however, in cases where it is not availa

2、ble, alternative methods should be used. The per-unit and ohmic method are very tedious manual calculation. These hand calculations are very prone to errors due to so many conversion required. In per unit, base conversion is a normal part of the calculation method while in ohmic method, complex enti

3、ties conversion.The easy way to do hand calculation is the MVA method.In this example, we shall be presenting a short circuit study of a power system. Motors are are already lumped with ratings 37kW and below assigned an impedance value of 25% while larger motors are 17%. A 4MVA generator is also in

4、cluded into the system to augment the utility.Figure 1Utility: 33KV, 250 MVAsc （the 250MVA in this example is just assumed. Ask your utility for the actual fault level at your point of connection）Transformer 1: 10 MVA, 33/11KV, 9% Z11KV BusGenerator: 4MVA, X"d = 0.113Transformer 2: 5 MVA, 11/6.

5、6KV, 7% ZMotor 1: 5MVA (Lumped), 17% Z6.6KV BusTransformer 3: 2 MVA, 6.6KV/400V, 6% ZMotor 3: 6.8 MVA (Lumped), 17% Z400V BusMotor 4: 300 KVA (Lumped), 17% ZMotor 5: 596 KVA (Lumped), 25% ZIn the event of a short circuit, the sources of short circuit current are1. Utility2. Generators3. MotorsStatic

6、 loads such as heaters and lighting do not contribute to short circuit.The "Equivalent MVA" are:Transformers and MotorsGeneratorsCables and ReactorsIn Figure 1, I have calculated the Equivalent MVAs of each equipment, writing it below the ratings.Figure 2Utility: MVAsc = 250MVATransformer

7、1: MVAsc = 10 / 0.09 = 111.11 MVA11KV BusGenerator: MVAsc = 4 / 0.113 = 35.4 MVATransnformer 2: MVAsc = 5 / 0.07 = 71.43 MVAMotor 1: MVAsc = 5 / 0.17 = 29.41 MVA6.6KV BusTransformer 3: MVAsc = 2 / 0.06 = 33.33 MVAMotor 3: MVAsc = 6.8 / 0.17 = 40 MVA400V BusMotor 4: MVAsc = 0.3 / 0.17 = 1.76 MVAMotor

8、 5: MVAsc = 0.596 / 0.25 = 2.38 MVAFigure 3Upstream ContributionStarting from the utility, combine MVAs writing each one above the arrows.At Transformer 1:MVAsc 33KV = 250 MVAMVAsc 11KV = 1/ (1 / 250 + 1 /111.11) = 76.87 MVAAt Transformer 2:MVAsc 11KV = 76.87 + 35.4 + 29.41 = 141.68 MVAMVAsc 6.6KV =

9、 1/ (1 / 141.68 + 1 / 71.43) = 47.49 MVAAt Transformer 3:MVAsc 6.6KV = 47.49 + 40 = 87.49 MVAMVAsc 400V = 1/ (1 / 87.49 + 1 / 33.33) = 24.14 MVAAt 400V MotorsMotor 3: MVAsc = 24.14 x 1.76 / ( 1.76 + 2.38 ) = 10.26 MVAMotor 4: MVAsc = 24.14 x 2.38 / ( 1.76 + 2.38 ) = 13.88 MVADownstream ContributionS

10、tarting from the bottom (400V Bus), I combined MVAs writing each one below the arrows. In this bus, the motor contribution to short circuit is the sum of the MVAs of the lumped motors Motor 3 and Motor 4.At Transformer 3:MVAsc 400V = 1.76 + 2.38 = 4.14 MVAMVAsc 6.6KV = 1/ (1 / 4.14 + 1 / 33.33) = 3.

11、68 MVAAt Transformer 2:MVAsc 6.6KV = 3.68 + 40 = 43.68 MVAMVAsc 11KV = 1/ (1 / 43.68 + 1 / 71.43) = 27.11 MVAAt Transformer 1:MVAsc 11KV = 27.11 + 29.41 + 35.4 = 91.92 MVANote: Two downstream plus the generator contribution.MVAsc 33KV = 1/ (1 / 91.92 + 1 /111.11) = 50.3 MVATo determine the Faults Cu

12、rrent at any bus on the power system, add the MVA values above and below the arrows. The sum should be the same on any branch.Example:11 KV Bus:From Transformer 1: MVAsc = 76.87 + 91.92 = 168.79 MVAFrom Generator : MVAsc = 35.4 + 133.39 = 168.79 MVAFrom Transformer 2: MVAsc = 141.68 + 27.11 = 168.79

13、MVAFrom Motor 1: MVAsc = 139.38 + 29.41 = 168.79 MVAThis is a check that we have done the correct calculation.Ifault 11KV = 168.79 / (1.732 x 11) = 8.86 kAAll we have done above are three phase faults, you may ask, how about single phase faults?For single phase faults, positive sequence, negative se

14、quence and zero sequence impedances need to be calculated.If = 3 (I1 + I2 + I0)Examining the circuit in Figure 1, at the 400V Bus, on Transformer 3 contributes to the zero sequence current.For transformers, the negative sequence and zero sequence impedance are equal to the positive sequence impedanc

15、e.Z1 = Z2 = Z0 orMVA1 = MVA2 = MVA0At the 400V Bus1 / MVAsc =1/3 (1 / MVAsc1 + 1 / MVAsc2 + 1 / MVAsc0)1 / MVAsc = 1/3 (1 / 28.28 + 1 / 28.28 + 1 / 33.33 )MVAsc =3 xÃâà 9.93 = 29.79Ãâà MVAIf = 29.79 / (1.732 x 0.4) = 43 kAAt 6.6KV Bus1 / MVAsc = 1/3 (1 / MVAsc1 + 1 / MVAsc2 + 1 / MVAsc0)1 / MVAsc = 1/3 (1 / 91.17 + 1 / 91.17 + 1 / 71.43)MVAsc = 3 x 27.83  = 83.49 MVAIf = 83.49 / (1.732 x 6.6) = 7.26 kAConclusion:This example illustrates that using the MVA Method of Short Circuit Calculation, it will be very easy to calculate the fault