More On Wye Wye Connected Transformers 1

JWLunden · Mar 30, 2023

thread248-481670

In the United States, in underground mining applications it is legally required that the source secondary transformer have a neutral point, and the neutral must be grounded through a resistor. The resistor value is chosen to limit the maximum phase to ground fault current to 15 amps or less. For a 3 phase 600 volt system, the resistor value will be 23 ohms or more. In all cases, the source transformer’s primary is delta connected. All loads on the transformer secondary are connected phase to phase. Ground fault detection relays are utilized for instantaneous tripping of the branch circuits if a phase to ground fault condition occurs.

After reading the preceding discussion, it appears that a primary wye connection whose neutral point is solidly grounded will permit secondary zero sequence current to flow. Furthermore, it appears that the magnitude of secondary zero sequence current will in essence only be impeded by the resistor.
Agree or disagree?

Thank You, Jim

FacEngrPE · Mar 31, 2023

OP said:
All loads on the transformer secondary are connected phase to phase.

So the only time current can flow into the neutral point via the NGR is during a fault. Therefore there can't be any third harmonic passing through the NGR, except when the system experiences a ground fault.

che12345 · Mar 31, 2023

Mr JWLunden (Electrical)(OP)30 Mar 23 23:23
".... #1. In all cases, the source transformer’s primary is delta connected. All loads on the transformer secondary are connected phase to phase. Ground fault detection relays are utilized for instantaneous tripping of the branch circuits if a phase to ground fault condition occurs.
#2. After reading the preceding discussion, it appears that a primary wye connection whose neutral point is solidly grounded will permit secondary zero sequence current to flow. Furthermore, it appears that the magnitude of secondary zero sequence current will in essence only be impeded by the resistor..."
1. If the trafo secondary in Y and the Neutral is grounded through NGR, why "Ground fault detection relays are utilized for instantaneous tripping of the branch circuits if a phase to ground fault condition occurs" ? A small time delay say < 1s would be desirable.
2. If the trafo primary is in D, the secondary is Y, with Neutral grounded directly or through NGR; any ground fault on the secondary will be within the secondary circuit only.
2.1. If the trafo primary is in Y with Neutral grounded, the secondary is Y with Neutral grounded directly or through NGR; any ground fault on the secondary will be limited in the secondary circuit only.
Che Kuan Yau (Singapore)

dpc · Mar 31, 2023

Ignoring the resistor for a moment, for a grounded wye - grounded wye transformer there will generally be LESS line-to-ground fault current on the secondary side compared to a delta-wye transformer, due to inclusion of the primary side zero sequence impedance in the circuit. In a delta-wye transformer, the wye-side zero sequence current is sourced by the transformer itself since Io can flow in the delta limited mainly by the transformer impedance. The fault current is not much influenced by how the loads are connected. Adding a neutral grounding resistor will limit the fault current in similar manner for either a grounded wye or a delta primary.

LionelHutz · Apr 3, 2023

Well, with Y-Y connected transformer the transformer impedance and primary zero sequence impedance will be in "series" with the grounding resistor. But, in practice the grounding resistor is such a high impedance compared to the others that it is the only impedance that needs to be considered when calculating the secondary ground fault current.

JWLunden · Apr 3, 2023

I want to thank everyone for their helpful posts. In response to che12345, in mining applications the ground fault trip for each branch circuit must be instantaneous. A ground fault condition is considered an emergency, and the power must be removed to prevent shock and/or electrocution. The loads are fed by portable trailing cables that are very prone to damage. Men operating the machines are in immediate danger during a ground fault.

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