Section 7 of IEEE Std C57.12.00-2000 specified the general short circuit requirements for liquid-immerse distribution, power, and regulating transformers. For Category I and II transformers, symmetrical short circuit current shall be computed using transformer impedance only. For Category III and IV, system impedance should be included together with transformer impedance to calculate the symmetrical short circuit current. In our example our base MVA is 100 MVA, therefore it is a category IV transformer and should be calculated together with the system impedance. But for now we are neglecting the contribution of system impedance in our symmetrical short circuit calculation. By neglecting the system impedance, you are adding additional safety factor in your transformer short circuit withstand capability specification. In reality there is always the system impedance that will lower the calculated available short circuit current. But it is difficult to get the real time transmission line impedance value because the network is dynamic due to power flow changes, unplanned outages due to storm and planned outages for scheduled apparatus maintenance.

Link to Desmos: Simulation of Unbalanced AC Three Phase Systems

Is the zero sequence voltage, V₀ the same as the unbalanced voltage in a three phase transmission line ? In a WYE - WYE transmission transformer , where is the transmission line neutral to carry the unbalance current driven by unbalance voltage?

Credit to: Desmos Graphing by Sam Ortega 3/7/2020

Phasor Vector Rotation starts from from 0 ^° which is vector B-Phase (the reference vector), then rotates in CCW direction towards A-Phase, and finally C-Phase.
Frequently refer to as Phase Rotation is BAC
Remember the phasor rotation starts from 0 ^° moving counter clockwise until it reaches 360 ^° it can be displayed as 0 or 360 value. Therefore 0 ^° and 360 ^° is describing the same vector direction.

BAC phasor rotation seems to be associated with transmission line layout. For example from left to right (B-phase line-left, A-phase line, C-phase line-right) and from top to bottom ( B-phase line-top, A-phase line , C-phase line-bottom)

Do a simulation by adding a transmission line impedance to transformer short circuit impedance. What happen to the available short circuit current ?

In using this calculator to make decision if the transformer is operated within its factory % Z limit on short circuit withstand capability, you should remember to deduct some values of short circuit current dissipated by the transmission impedance in series with your transformer.

Next, very important to remember that digital fault recorder(DFR), fault current values are based on line current because the bushing current transformer (BCT) is its source of current signal which is measuring the line current. If the transformer WINDING is connected in WYE , then the line current of DFR and the transformer coil winding current are the same. But if the transformer WINDING is connected in DELTA, you need to divide the DFR fault current by square root of 3 , which is 1.732.

Then you make your comparison between actual DFR fault current vs transformer winding short circuit specified withstand capability based on its % Z from factory test.

Few excerpt of transformer categories from IEEE C57.12.00-2000

Category	Single Phase kVA	Three Phase kVA
III	1668 to 10000	5001 to 30000
IV	Above 10000	Above 30000

Using the ANSI method the maximum available fault current at the transformer bushing stud terminal will be:

This is valid for WYE primary winding configuration because the transmission line current will flow straight to transformer primary winding if it is connected in Wye. We say transmission line current is equal to the transformer winding current.

Refer to IEEE C57.12.00-2000 section 7.1.5.1 Symmetrical current (two-winding transformers) short circuit calculations. -- ANSI METHOD.

Now assuming the system impedance Z_sys = 5 % is in series connection with transformer impedance. Therefore the total impedance, Z_total = Z_tr 9.63 % + Z_sys 5 % = 14.63 %. The base short circuit current will be going down to 1715.851 Amp.

Refer to IEEE C57.12.00-2000 section 7.1.5.2 Asymmetrical current (two-winding transformers) short circuit calculations.

Transformer must also withstand the peak asymmetrical short circuit current during the first cycle equivalent to 16.67 millisecond.

ANSI method for calculating the peak asymmetrical short circuit current = K * I_sc.
If the x/r ratio of the system is unknown. ANSI method indicates the x/r ratio of the transformer should be used. Assuming the the x/r ratio of the given 100/150/180 MVA power transformer is 25 based on factory test result data. Calculate the peak assymetrical short circuit current the given transformer must be able to withstand during the first cycle. From IEEE C57.12.00-2000 Table 7B-1 for x/r = 25
use K =
Therefore the peak asymmetrical line short circuit current x = Amp. Keep in mind, this value was based on x/r = 25. You must change your K-factor value depending on the transformer x/r value that you are analyzing.

For delta connected primary winding , the transformer winding fault current = transformer terminal stud transmission line fault current divided by square root of three or 1.732.

This is valid for DELTA primary winding configuration because you need to divide the line current by 1.732 to get the winding current in a transformer that is delta connected.

Therefore the base MVA peak asymmetrical winding short circuit current for delta winding is
x k factor = Amp. Keep in mind, this value was based on x/r = 25. You must change your K-factor value depending on the transformer x/r value that you are analyzing.

HOW TO USE: Enter new number on any white input boxes shown below. Answer is automatic.
For scenario analysis: change the % Z value and the K - Factor value

When you change %Z or K-factor or both you should click all the white input boxes below to do new calculation base on new changes that you have made.

How do we interpret the two (2) second short circuit withstand capability of the transformer? This will be my next topic for research.

Explain infinite bus in relation to estimating transformer maximum available fault current.

LinkedIn Post discussion by Doug Millner about transformer vector group

LinkedIn Post discussion by Doug Millner about transformer Sequence Network and Thevenins Network

LinkedIn Post discussion by Georg Schett about Transient and sub-transient short circuits at synchronous generators

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