Delta-Wye HRG Transformer with REF 1

[DM61850]

Can you set a REF protection on a transformer that has high impedance resistive ground? I am looking in the manual of the SEL-787 and it looks like it says to not use it if you have a high impedance ground but later it mentioned it can be used on transformers with grounding resistors. Can you use REF but it is just not recommended on high resistive or inductive ground or just not inductive grounding?

This is what the SEL-787 manual says:

"Restricted earth-fault (REF) protection comes from a zero-sequence
directional element that provides sensitive detection of ground faults near the
neutral of a grounded wye-connected transformer winding. The REF element
is intended for resistance and solidly grounded transformer applications. This
element should not be applied to protect high-impedance grounded
transformers."

 

[DM61850]

What is standard protection on a transformer with high resistive ground?

 

[dpc]

At medium voltage, resistance grounding can be low-resistance (200-1000 A) or high-resistance (5 amps or less). Generally, low-resistance schemes can use some type of ground current detection, while high-resistance grounding fault detection is generally done by a voltage element across the resistor (with harmonic filtering). So it probably depends on the type of resistance grounding.

 

[DM61850]

This is a 13.8kv/4.16kv transformer, and medium voltage by IEEE is above 1kV. By your description, if the ground fault current is limited to 400 A, this is really a low resistance system and not high resistance grounded. I guess people are calling it high resistive ground but it really isn't? I saw that the IEEE 142 definition is as follows:

IEEE Standard 142 defines a high resistance grounded system as one with a purposely inserted resistance that limits ground-fault current to levels such that the fault current (usually thought of as less than 10 A) can flow for an extended time without exacerbating damage.

So, 400A too high for that definition?

 

[dpc]

400 A is, without question, considered low-resistance grounding and you must trip as soon as practical. Damage can still be significant.

 

[rcw retired EE]

On high resistance grounded system the 5-10A fault current is too low for secure protection with the phase current transformer ratios on the low voltage winding.

If your transformer is 20 MVA, rated current on the 4.16 kV side is 2775 Amps and a typical CT ratio would be 4,000/5 (144% of rating). The 10 Amp fault current is only 0.25% of the CT. CT errors in adding the phase currents and comparing to the neutral current can cause false trips.

With low resistance grounding and a fault current of 400A, the current signal is 10% of the CT rating, giving a better margin for CT errors.