Surge arrester on neutral terminal of transformer 2

[lume7006]

Hello guys,

In a project we are participating, there is a transformer wye-wye grounded on both sides, the HV side is grounded through a surge arrester, while the secondary (LV) side is through a resistor.

My question is:
Has anybody dealt with something similar (H0 terminal earthed through a surge arrester)?

I will thank if somebody had some clues or references to understand the application and some possible effects on the protections scheme.

Regards,

 

[m3ntosan]

@lume7006,

the ones I've oprerated were similar sizes and voltage levels, the surge arresters were rated for 72 kV, for Y windings at 110 kV. It's the same path to earth, by operating an isolator and/or earth switch you're getting the surge arrester in/out the circuit.

May you grow up to be righteous, may you grow up to be true...

 

[electricpete]

Please refer to EPRI Copper book and EPRI fault level management, grounded through a surge arrester is grounded,low or high resistance grounded depending on surge arrester characteristics.

I don't think many would agree with that. Surge arrester is basically an open circuit until a certain overvoltage appears accross it. Are you talking about the EPRI copper book... which chapter?


=====================================
(2B)+(2B)' ?

 

[slavag]

Hi, its standard practic in lot of systems.
Reason, reduce a level of ground fault.
Scotty is right, in smaller system with lot of generation for example

 

[veritas]

I agree with electricpete, when a neutral bushing has a surge arrester it is ungrounded and protected by the surge arrestor. Conduction is only possible with excessive overvoltages to minimise insulation damage.

 

[odlanor]

Lume7006(1)
...because the surge arrester does not allow an effective connection to earth at 60Hz frequency.
Example
If neutral is solidly grounded: BIL reduced 450kV at phase terminal winding and graduated up to 60kV at the neutral.
Maximum overvoltage between good phase during a ground fault= 0.80 *112 =89.6 kV
Ohio Brass OZn surge arrester selected for phase terminals:
Minimum continuous overvoltage = 88kV
Duty cycle = 108kV
Discharge voltage for 10kA = 244kV

If neutral transformer is ungrounded: BIL shall be 550kV
Maximum overvoltage between good phase during a ground fault= 1.05 *112 =117.6 kV
Ohio Brass OZn surge arrester selected for phase and neutral terminals:
Minimum continuous overvoltage = 115kV
Duty cycle = 144kV
Discharge voltage for 10kA = 332kV

lume7006(2)
1- If phase terminals surge arrester were 144kV then refrated wave of 332kV will be refleted to phase winding with double value(664kV) and if 72kV surge arrester fails, 60kV insulation of winding will break !
2- Insurance company of transformer will not pay for this accident because transformer was not sized in accord to standards!

m3ntosan
My example refers to 380kV of NPAG book and 345kV Korean system.
There are some utilities in Brazil(few) that only requires the generating producer using transformer with isolated neutral to connect to system for avoiding increase the fault-to-ground 138kV system.

 

[cuky2000]

Grounding transformer neutral via surge arrester create a hybrid connection. Under normal operating conditions grounded or ungrounded the transformer performance are very similar. However; during asymmetric switching or ground fault scenarios, the system very fast could be connected to ground (earth) reducing the overvoltage in the unfaulted phase(s).

Wye-wye transformer connection in HV systems has limited applications. Grounding Y-Y transformers via surge arrester is not a common practice in the ANSI marketplace. The risk exposure vs. benefits is difficult to justify for high reliable installation.

 

[odlanor]

lume7006(2)
ERRATA
1- If phase terminals surge arrester were 144kV then refrated wave of 332kV will be refleted to phase winding with double value(664*60/450 kV) and if 72kV surge arrester fails, 60kV insulation of winding will break !

cuky2000
if you sized Grounding transformer neutral for
X0/X1 < 3 - system effectively grounded
3 < X0/X1 < 10 - system middle grounded (1-phase earth fault current less then 3 phase fault)
X0/X1 > 10 - system ungrounded.

 

[cuky2000]

Hi edlanor,

X0/X1= infinite (no path for the zero sequence) Under normal operating conditions - ungrounded
X0/X1< 3 during phase to ground fault if the arrester operate.

 

[jghrist]

cuky,
What would be the advantage of this hybrid connection vs solid grounding of H0? How would you size the arrester? It would have to have a low breakdown voltage and be able to withstand ground fault current.

 

[cuky2000]

Hi jghrist,

More than overcurrent, the concern is mainly overvoltage issues during ground fault, switching operation or transient conditions.

Theoretically, the advantage of a hybrid grounding system is to stabilize the system voltage in the event of transient conditions and allows specify Y-Y transformer with lower than full BIL to reduce initial costs.

There is not large world wide experience grounding transformers via surge arresters and there is no standard or guidelines to address the surge arrester sizing for this application. One sizing method is modeling the system using a transient program such as EMTP but, this also costly and requires challengers such as determining accurate parameters such as saturation curves, capacitance, etc for the transformer model.

As I mentioned earlier, the benefit in term of performance and cost reduction may be difficult to justify the risk of grounding the transformer via surge arrester.

 

[odlanor]

cuky2000
X0/X1 > 10 meand high impedance grounded with BIL and surge arrester equivalent to ungrounded.

gentlemen,
All HV or EHV system are solidly grounded.

If you have only HV/EHV source stepup transformer solidly grounded with all stepdown transformer of substation system ungrounded you have either a system solidly grounded.

BUT with less shortcircuit to ground then

If you have all HV/EHV transformer solidly grounded.