Grounded Wye-Delta Transformer Bank Backfeed Short-Circuit Currents W. H. Kersting & Wayne Carr Milsoft Utility Solutions

Introduction Wye-Delta Transformer Bank – Serving a three-phase load A three-phase transformer Or three single-phase transformers – Serving single-phase and three-phase loads One centered tapped lighting transformer Two power transformers Question: To ground or not to ground the Wye connection.

Wye-Delta Transformer Connection

Ungrounded Wye-Delta Transformer Bank

Grounded Wye-Delta Transformer Bank

So What? Ungrounded Y-D – No backfeed short circuit current – Lose one phase and bank becomes single-phase Grounded Y-D – Backfeed current possible – Lose one phase and have open Y-D bank – Need to compute all short circuit currents for A-G fault

Test System Used to Compute Backfeed SC Currents for Grounded Y-D Bank

Overhead Lines

Overhead Line Phase Impedances

Methods of Analysis Symmetrical Components – Old fashion way to demonstrate backfeed Basic circuit and transformer analysis – Analysis of total three-phase circuit Reduced 8x8 matrix analysis – Reduce total analysis to just what is needed

Symmetrical Component Method

Overhead Line Sequence Impedances

Sequence Networks Connection Phase a-g Fault at Node 12

The Minus 3 Error Doc. Crosno (NMSU) was a stickler for accurate calculations when I was a student When he found an error it was an automatic -3 error (red rubber stamp) out of 10 points on the problem When I was preparing for this presentation I found a -3 error on the symmetrical component analysis. I used transformer impedance referred to secondary rather than primary side. Please ignore the results in the paper

Correct Sequence Network Analysis Results

Now What? The symmetrical component method has demonstrated – There is a backfeed current of amps/phase – This method assumes a balanced three-phase system (ignored the off diagonal terms of the sequence impedance matrix) Need to evaluate the short circuit currents with a total unbalanced three-phase system

Unbalanced Three-Phase System

Observations to Note

A-G Fault at Node 12

A-G Fault at Node 12 Results

Reduced Three-Phase Circuit Analysis Primary concern – Currents in the fault – Currents from the source – Backfeed currents from the transformer bank Recognize the three backfeed currents are equal Apply basic algebra to reduce to 8 equations and 8 unknowns

Reduced Analysis 8x8 Matrix

8x8 Matrix for A-G Fault at Node 12

8x8 Matrix Results

Comparisons of Results

WindMil Results Using Loop Flow Method

Ungrounded Wye-Delta Connection Analysis methods – 7x7 matrix – Set Zg = Results

Short Circuit Current vs. Rated Current Transformer Rated Currents – 100 kVA: 13.8 amps – 50 kVA: 6.9 amps With Zg = 5 ohms backfeed = 91.7 amps – 100 kVA: 564% over rated current – 50 kVA: 1,229% over rated current With Zg = 50 ohms backfeed = 19.3 amps – 100 kVA: 39.9% over rated current – 50 kVA: 79.9 % over rated current

Short Circuit Protection

Substation Transformer Primary Fuses Primary responsibility is to open on transformer faults Backup responsibility to open if low side breaker fails to open for downstream fault Transformer fuse must lie between inrush current and maximum melt curves

Primary A-G Short Circuit Current

Substation Transformer Fuses

Primary Fuse Curves Referred to Secondary for LG Fault

Primary Curves Referred to Secondary For LG Fault

Recloser and Xfm Fuses

Total Protection

Three Downstream Transformers 8x8 equivalent matrix method of analysis doesn’t work Symmetrical component model developed and analysis run WindMil using the loop flow method was developed and analysis run Symmetrical components and loop flow methods match very closely

Three Downstream Transformers WindMil Solution

Summary Grounded Wye-Delta Short Circuit Analysis – Symmetrical Components – Exact Circuit Analysis – Reduced Circuit Analysis Ungrounded Wye-Delta Short Circuit Analysis Short Circuit Protection – Primary Fuses – Recloser – Distribution Transformer Fuses

Conclusions Grounded wye-delta transformer banks provide backfeed currents for upstream faults involving ground Ungrounded wye-delta does not provide backfeed currents Backfeed currents can blow transformer fuses for upstream ground faults