MVA Method calculation short circuit

[jonner]

Using MVA Mehod fault calculation
if network feeder uses impedance 100MVA base and sync motor have x''=15.38%, x1=15.38%, x0= 15.38%


how to find MVAzero contributed by
1. Network Feeder
2. sync. motor,

if detail Network Feeder and motor as shown figure below (ATTACH).
any information you could supply would be very wellcome

 

[rcwilson]

In the MVA method, calculate the short circuit MVA of each element as if it was connected to an infinite bus.

For a feeder with data given on a 100 MVA base, the MVAsc will be 100 MVA divided by the per unit impedance. MVAsc = MVA base/ Zpu.

For a synchronous motor, it is the motor rating in MVA (not kw) divided by its sub-transient reactance Xd"
MVAsc =(KW *pf )/(Xd" x 1000).

If by "MVAo" you mean the zero sequence MVA, just use the zero sequence per unit impedance in the equation.

Note that unless the synchronous motor has its winding's neutral point connected to ground, it will be not contribute any zero sequence current, Xd" zero = infinite, and MVAo= 0.

 

[rcwilson]

From your data screens:

For feeder, Zpu = Squareroot(R^2+X^2) = 19.91% = 0.1999. MVAsc = 100 MVA/0.1999 = 500 MVA.

Since the feeders positive and negative impedance are shown as identical, the MVAsc1= MVAsc2 = 500 MVA.
The feeder zero sequence impedance calculates out to 45.71% on 100 MVA base. So MVAo = 100/.4571 = 218 MVA.


The synchronous motors nameplate MVA is 2000 kW/(.9278pf x 1000) = 2.156 MVA. Its short circuit MVA is 2.156/0.23 = 9.37 MVA. The zero sequence MVA for the motor is 0.

 

[rcwilson]

For a synchronous motor, it is the motor rating in MVA (not kw) divided by its sub-transient reactance Xd"
MVAsc =(KW *pf )/(Xd" x 1000).

My equation in first post is wrong. Should be: MVAsc = KW/(Xd" x pf x 1000).

 

[jonner]

element contributes Zero sequence MVA

element contributes Zero sequence MVA

Thank you very much for your reply

but in your calculation of short circuit sync.motor, MVAsc = MVA/0.23


1. so what the x''=15.38%, x1=15.38%, x0= 15.38%
are used for? why you are using 0.23 (23%) ?
2. correct me if I wrong, but should the zero sequence MVA of motor (MVAsc0) be .....0?
in a figure before, motor
will contribute to earth fault through the netral connected to earth?

3. By the way...please refer to fig.(DIAGRAM A,B) (Attach), are there something wrong of calculation MVAsc of 3 winding trafo?

thank you in advance

 

[rcwilson]

[

but in your calculation of short circuit sync.motor, MVAsc = MVA/0.23
1. so what the x''=15.38%, x1=15.38%, x0= 15.38%
are used for? why you are using 0.23 (23%) ?

I got the 0.23 off the screenshot of the motor data input form. I see now that it doesn't match your data: Xd?% = 15.38%. MVAsc for positive and negative sequence should be 2MW/(pf x 0.1538). Assume nameplate pf is 0.95, then MVAsc1&2 = (2MW / (0.95 x 0.1538) = 13.69 MVA. But check the data, if the 15.38% is given based on 2000kW rating, use that.

2. correct me if I wrong, but should the zero sequence MVA of motor (MVAsc0) be .....0?
in a figure before, motor will contribute to earth fault through the netral connected to earth?

My apologies! You are correct. I had it wrong. I had to go back to Moon Yuen's original paper on the MVA method. (He was my mentor when I started out. He would be quietly correcting me about now). We do include the motor in the MVA method zero sequence diagram. Why would the motor data include Xo=15.38% if that was not the case! So motor?s MVAo = 13.69 MVA, same as the positive and negative sequence. (Typical induction motor?s zero sequence impedance is about ? the positive. I am not certain about synchronous motors. Check your data.).

By the way...please refer to fig.(DIAGRAM A,B) (Attach), are there something wrong of calculation MVAsc of 3 winding trafo?

The 3-winding calculation looks correct, provided all the winding impedances are based on the 10 MVA rating. If the 7% Zpt and Zst for the secondary and tertiary windings are based on the 5 MVA or 3 MVA winding rating, you need to convert them to a common 10 MVA base.

By the way?Your sketch also shows motor Locked Rotor current of 453.5%. This translates to an Xd? = 1/4.535 = 0.22 or 22%.

I?ll stop posting before I make more errors. Good luck.

 

[iceworm]

... I had to go back to Moon Yuen's original paper on the MVA method. ...

rc -
Do you have a reference where one might get a copy of Yuen's paper? Mine is a pretty ratty pdf of a fourth copy. I'm guessing your's is clean and readable.

ice

 

[rcwilson]

The paper was first published in IEEE Transactions on Industry Applications, Vol. IA-10, No.2, March/April 1974, page 261-272. It is available from IEEE Explore.

 

[iceworm]

thanks

ice