BIL rating and surge protection 1

[cvirgil]

1. Do the BIL levels for switchgear and transformers need to be coordinated. For example, if a switchgear is rated for 90kv BIL and the transformer is rated for 30kv BIL is the transfomer under rated?

2.Is it typical to provide surge protection on transfomers that are served on the primary from vacuum CB from switching surges?

 

[dpc]

I would put surge arresters on the primary of any transformer over 1000V. Vacuum breakers can create voltage transients that exceed old air or oil breakers.

What really needs to be coordinated is the BIL of your transformer winding and the rating of your surge arresters. If you get a voltage surge that exceeds the transformer BIL, it doesn't really matter what the switchgear BIL is.

It is much easier to build high BIL ratings in switchgear than in a transformer winding, so transformers will always be a weak link. That's why the surge arresters should be as close the transformer primary terminals as possible.

 

[cvirgil]

Is it typical to also install surge capacitors in addition to the surge arresters. It's my understanding that the arresters are ineffective at controlling a surge's rate of rise.

Could the sure arresters be installed in the breaker cubicle instead of at the transformer.

 

[dpc]

Surge capacitors are used on rotating equipment such as generators and large motors, but not on transformer arresters.

Surge arresters at the breaker would be better than nothing, but the transformer is much, much, much better protected when you put the arrester as close to the transformer primary terminals as physically possible. Grounding connections to the arresters should be straight, short and big to limit the impedance presented to high frequency impulses.

http://www.usace.army.mil/usace-docs/armytm/tm5-811-1/chap9.pdf

 

[cuky2000]

1. Do the BIL levels for switchgear and transformers need to be coordinated. For example, if switchgear is rated for 90kv BIL and the transformer is rated for 30kv BIL is the transformer under rated? Most people opt to investing specifying equipment with dielectric insulation with proven good performance rather than invest in expensive insulation coordination study to lower the equipment BIL. Since your case dielectric rating are unusual, something must be done to prevent a catastrophic failure.
[sub]Please provide additional details: System voltage, type of transformer (dry/liquid), indoor or outdoor installation, shielding protection, underground or overhead lines, lightning activity (keraunic or flash density level) and any other relevant information.[/sub]

2. Is it typical to provide surge protection on transformers that are served on the primary from vacuum CB from switching surges?. Yes, it is typical to provide surge protection for distribution and power transformer. For standard small transformer unit, surge protection sometime is not economically justifiable.

3. Is it typical to also install surge capacitors in addition to the surge arresters? It's my understanding that the arresters are ineffective at controlling a surge's rate of rise. Not, it is not typical to install surge capacitors in transformers. For standard dry type transformer with lower BIL than the liquid immersed units, surge-protective capacitors are specified in rare situations to correct problem applications particularly where cable length is inadequate to achieve transient voltage rate-of-rise control.
[sub]The advances in surge arrester combined with better performance of VCB switching eliminate in most cases the need for surge capacitor. [/sub]

4. - Could the surge arresters be installed in the breaker cubicle instead of at the transformer. This is not the best location for transformer surge protection. In addition to SA in the transformer, some switchgear designer add extra SA for UG cable protection and overvoltage control.

 

[cvirgil]

The details are:

Voltage: 13.2kV, grounded
Type of transformer: VPI dry type, 2000kVA, indoor

The transformer is fed from a 13.2kV VCB in a sub distribution switchgear line up. This is not the incoming utility service. The incoming utility service is approximatley 1000' feet away which terminates at in a 13.2kV main service switch (VCB). The 1000'feet of 15kV cable to the sub 13.2kv gear is installed underground.

 

[cuky2000]

For full protection, surge arrester may be required on the dry type transformer.

To confirm that, the electric characteristics and ratings of the supply transformer and surge arrester back in the distribution substation is required to be considered in the enclose figure.

 

[tommom]

30 KV Bil sounds like a low-voltage Bil rating. Make sure the high side isn't at least 60 kV.

 

[cvirgil]

If the transformer is close coupled to the medium voltage gear, would this setup be a canidate to install surge capacitors since the length of cable would be small to the primary side?

 

[cuky2000]

Not. Protection with surge arrester is sufficient if the transformer is rated with the standard BIL.

 

[romanskat]

cuky2000 stated that "expensive insulation coordination study to lower the equipment BIL." . Is it the protective relay coordination study? Or actually is some other kind of study?

Interested to know more on this.

Thanks.

 

[davidbeach]

Definitely a different study; makes the protective relay coordination study look like a piece of cake. Insulation coordination studies look at the voltages achieved during transient events, transients that happen in the 10s of kHz or higher frequency and require much more detailed models of the system under study. These studies in the realm of EMTP programs rather than the programs used for relay coordination.

 

[cvirgil]

Thanks. The tranformer has a lower BIL rating than the switchgear. Would that make a difference?

 

[cvirgil]

How does one typically size the surge arrestor and surge capacitors for a system? I;m intretse din fidng out what kV surge arrestor is need with a particulat BIL. Is protcetion of the BIL level affected by a higher surge arrrestor rating and grounding scheme?

 

[dpc]

You need to size the arrester to protect the transformer.

For modern arrestors, here's a simplified approach for determining the ratings:

For an ungrounded system (or system that could become ungrounded)or resistance grounded system - select an arrester with a Maximum Continuous Operating Voltage (MCOV) that is at least 105% of the normal LINE TO LINE voltage.

For grounded wye system that will always be reliably solidly grounded, select an arrester with an MCOV that is at least 105% of normal LINE TO GROUND voltage.

There are a lot of others things you could look at, such as temporary overvoltage curves, etc. But the above approach should always get you pretty close to the right arrester. Normally, this will provide adequate protection for the transformer windings. For large substation transformers, go with station class arresters.

 

[cvirgil]

In looking at this more closely it looks like there are three levels for Margin of Protection. Does anyone know how to find vaules for:

1. Chopped Wave Withstand value
2. Front of Wave Value.
3. Equivalent switching protection level value.

Also, is it typcial to use normal duty arrestier foir an indoor appliaction at medium voltage level with the majority of the utility line underground?