26.4 kV XFMR Basic Insulation Level and Bushing Inserts/Lightning Arresters

[strap89]

Hello! Questions underlined.

-Working on a transformer installation. Primary is 26.4 kV Delta and the secondary is 4.6 kV Wye.

-Client wants a primary 125 kV BIL. I know this is acceptable (what standard defines this?), but typically 150 kV BIL is what's used.

-Because the transformer primary is delta connected...I'm getting mixed information on calculating the lightning arrester rating. At this point I'm at 27kV (22kV MOCV) to accommodate the 26.4 kV system voltage. I'm being told that even though the the primary is delta, the voltage rating of the lightning arrester is line to neutral, not line to line...this doesn't sit right with me. Which is it? Is it ok to have a higher than needed rating for a lighting arrester? Say I only need 18 kV and I specify 27 kV with a larger MOCV?

-Because the I'm at 27 kV for the lighting arrester, my bushing inserts are rated 35 kV. Is there any issue having 35 kV bushings on a transformer with a 125kV BIL?

~Strap

 

[Ingenieur]

phase to ground is how they are rated (I stand corrected, that is for grounded systems)
basically flashover

usually it is sized on 1.05 pu

you don't want larger as that will allow more fault thru before conducting

I don't see an issue using up-rated bushings

excerpt from Hubbel/Ohio Brass
In grounded neutral circuits, a 233kV system usually has a maximum line-to-line continuous voltage of 245kV rms. This 245kV divided by √3 equates to a 141.5kV line-to-ground voltage. The appropriate arrester for this application is an arrester with a U
c of 144kV since it is the lowest standard Uc that will meet or exceed the line-to-ground voltage.
On ungrounded or impedance grounded systems, use the following selection guidelines to choose the best arrester:
• For applications where a ground fault is expected to be removed within 30 minutes, the minimum U
c is the maximum system line-to-line voltage divided by 1.25.
• For extended operation under ground fault conditions as long as 2,000 hours, the minimum U
c is the maximum system line-to-line voltage divided by 1.11.

 

[strap89]

phase to ground is how they are rated (I stand corrected, that is for grounded systems)
basically flashover

usually it is sized on 1.05 pu

you don't want larger as that will allow more fault thru before conducting

I don't see an issue using up-rated bushings

excerpt from Hubbel/Ohio Brass
In grounded neutral circuits, a 233kV system usually has a maximum line-to-line continuous voltage of 245kV rms. This 245kV divided by √3 equates to a 141.5kV line-to-ground voltage. The appropriate arrester for this application is an arrester with a U
c of 144kV since it is the lowest standard Uc that will meet or exceed the line-to-ground voltage.
On ungrounded or impedance grounded systems, use the following selection guidelines to choose the best arrester:
• For applications where a ground fault is expected to be removed within 30 minutes, the minimum U
c is the maximum system line-to-line voltage divided by 1.25.
• For extended operation under ground fault conditions as long as 2,000 hours, the minimum U
c is the maximum system line-to-line voltage divided by 1.11.

Thank you. I assume Uc=MCOV. I didn't realize the MCOV needed to match the the line to neutral voltage for grounded systems. I may have just assumed the voltage class was what needed to match.

 

[jtinge]

Hello! Questions underlined.

-Working on a transformer installation. Primary is 26.4 kV Delta and the secondary is 4.6 kV Wye.

-Client wants a primary 125 kV BIL. I know this is acceptable (what standard defines this?), but typically 150 kV BIL is what's used.

-Because the transformer primary is delta connected...I'm getting mixed information on calculating the lightning arrester rating. At this point I'm at 27kV (22kV MOCV) to accommodate the 26.4 kV system voltage. I'm being told that even though the the primary is delta, the voltage rating of the lightning arrester is line to neutral, not line to line...this doesn't sit right with me. Which is it? Is it ok to have a higher than needed rating for a lighting arrester? Say I only need 18 kV and I specify 27 kV with a larger MOCV?

-Because the I'm at 27 kV for the lighting arrester, my bushing inserts are rated 35 kV. Is there any issue having 35 kV bushings on a transformer with a 125kV BIL?

~Strap

Assuming you are using a pad-mounted type transformer, IEEE Std C57.12.34-2015 would be applicable. Table 3 provides minimum BIL based on primary voltage rating. For 24,940 Grd Y/14,400 (V) Min BIL is 125 kV, for 34,500 Grd Y/19,920, Min BIL is 150 kV. Other IEEE stds may be applicable for transformer types other than pad-mounted.

Also, would suggest review of IEEE C62.22-2009 IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems. It provides the guidance for proper selection of surge arresters based on MCOV, TOV and grounding configuration of the source.

 

[strap89]

Assuming you are using a pad-mounted type transformer, IEEE Std C57.12.34-2015 would be applicable. Table 3 provides minimum BIL based on primary voltage rating. For 24,940 Grd Y/14,400 (V) Min BIL is 125 kV, for 34,500 Grd Y/19,920, Min BIL is 150 kV. Other IEEE stds may be applicable for transformer types other than pad-mounted.

Also, would suggest review of IEEE C62.22-2009 IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems. It provides the guidance for proper selection of surge arresters based on MCOV, TOV and grounding configuration of the source.

Thanks for the info. I keep reading that the maximum voltage primary voltage is somewhere between 26 kV and 26.3 kV for 125 kV BIL. I'm assuming that if a 26.4 transformer is equipped with +-2.5% taps that this is acceptable. I can't find a solid reference that permits 125 kV BIL on a 26.4 kV system. I've read that arrester characteristic and lead lengths play a role in determining protection margin, but I can't find any literature on how to find this value.

 

[Ingenieur]

I have a PP class from a grad course (15 week course) I took last term on transients
this class session covered SIL/BIL/surge/lightening arresters

very good info on sizing and selection, standards, etc.
If I still have it I will post it (pretty sure it's public domain stuff)

the prof that taught the class is the VP HV circuit breakers (Mitsubishi)
he knew his stuff is an understatement
great, patient, practical instructor

 

[strap89]

I have a PP class from a grad course (15 week course) I took last term on transients
this class session covered SIL/BIL/surge/lightening arresters

very good info on sizing and selection, standards, etc.
If I still have it I will post it (pretty sure it's public domain stuff)

the prof that taught the class is the VP HV circuit breakers (Mitsubishi)
he knew his stuff is an understatement
great, patient, practical instructor

Please! That would be greatly appreciated.

 

[jtinge]

Excerpt from IEEE Std C57.12.00-2006 IEEE STANDARD FOR STANDARD GENERAL REQUIREMENTS FOR LIQUID-IMMERSED DISTRIBUTION, POWER, AND REGULATING TRANSFORMERS

5.10 Insulation levels
Transformers shall be designed to provide coordinated low-frequency and impulse insulation levels on
line terminals and low-frequency insulation levels on neutral terminals. The primary identity of a set of
coordinated levels shall be its basic lightning impulse insulation level (BIL).
The system voltage and the type of transformer may also influence insulation levels and test procedures.
Therefore, power transformers are separated into two different classes as follows:
a) Class I power transformers shall include power transformers with high-voltage windings of
69 kV and below.
b) Class II power transformers shall include power transformers with high-voltage windings from
115 kV through 765 kV.
Table 4 lists BIL levels in current use at various system voltages; however, any BIL choice requires
careful attention to proper insulation coordination and to accurate assessment of the coefficient of
grounding as outlined in 5.10.3.


Notes to Table 4.
NOTE 1—BIL values in bold typeface are listed as standard in one or more of ANSI C57.12.10 [B1],
ANSI C57.12.20 [B3], ANSI C57.12.22 [B6], IEEE Std C57.12.23 [B16], ANSI C57.12.24 [B6], ANSI C57.12.25
[B7], and IEEE Std C57.12.26 [B17].
NOTE 2—Single-phase distribution and power transformers and regulating transformers for voltage ratings between
terminals of 8.7 kV and below are designed for both Y and Δ connection, and are insulated for the test voltages
corresponding to the Y connection so that a single line of transformers serves for the Y and Δ applications. The test
voltages for such transformers, when operated and connected, are therefore higher than needed for their voltage
rating.
NOTE 3—For series windings in transformers, such as regulating transformers, the test values to ground shall be
determined by the BIL of the series windings rather than by the rated voltage between terminals.
NOTE 4—Values listed as nominal system voltage in some cases (particularly voltages 34.5 kV and below) are
applicable to other lesser voltages of approximately the same value. For example, 15 kV encompasses nominal
system voltages of 14 440 V, 13 800 V, 13 200 V, 13 090 V, 12 600 V, 12 470 V, 12 000 V, 11 950 V, etc.

 

[Ingenieur]

dang

the files are too big
one on SIL/BIL
and one on arresters

 

[Sahib]

Excerpt from IEEE Std C57.12.00-2006 IEEE STANDARD FOR STANDARD GENERAL REQUIREMENTS FOR LIQUID-IMMERSED DISTRIBUTION, POWER, AND REGULATING TRANSFORMERS

5.10 Insulation levels
Transformers shall be designed to provide coordinated low-frequency and impulse insulation levels on
line terminals and low-frequency insulation levels on neutral terminals. The primary identity of a set of
coordinated levels shall be its basic lightning impulse insulation level (BIL).
The system voltage and the type of transformer may also influence insulation levels and test procedures.
Therefore, power transformers are separated into two different classes as follows:
a) Class I power transformers shall include power transformers with high-voltage windings of
69 kV and below.
b) Class II power transformers shall include power transformers with high-voltage windings from
115 kV through 765 kV.
Table 4 lists BIL levels in current use at various system voltages; however, any BIL choice requires
careful attention to proper insulation coordination and to accurate assessment of the coefficient of
grounding as outlined in 5.10.3.


Notes to Table 4.
NOTE 1—BIL values in bold typeface are listed as standard in one or more of ANSI C57.12.10 [B1],
ANSI C57.12.20 [B3], ANSI C57.12.22 [B6], IEEE Std C57.12.23 [B16], ANSI C57.12.24 [B6], ANSI C57.12.25
[B7], and IEEE Std C57.12.26 [B17].
NOTE 2—Single-phase distribution and power transformers and regulating transformers for voltage ratings between
terminals of 8.7 kV and below are designed for both Y and Δ connection, and are insulated for the test voltages
corresponding to the Y connection so that a single line of transformers serves for the Y and Δ applications. The test
voltages for such transformers, when operated and connected, are therefore higher than needed for their voltage
rating.
NOTE 3—For series windings in transformers, such as regulating transformers, the test values to ground shall be
determined by the BIL of the series windings rather than by the rated voltage between terminals.
NOTE 4—Values listed as nominal system voltage in some cases (particularly voltages 34.5 kV and below) are
applicable to other lesser voltages of approximately the same value. For example, 15 kV encompasses nominal
system voltages of 14 440 V, 13 800 V, 13 200 V, 13 090 V, 12 600 V, 12 470 V, 12 000 V, 11 950 V, etc.

How are you sure OP is talking about liquid cooled transformer? May be it is dry type transformer with lower BIL and so requiring lightning arrester.

 

[jtinge]

How are you sure OP is talking about liquid cooled transformer? May be it is dry type transformer with lower BIL and so requiring lightning arrester.

Not sure about anything. My first post assumed certain configurations which were the basis for the posts. No replies indicated the OP was using dry type or liquid type transformers.

 

[Sahib]

Not sure about anything. My first post assumed certain configurations which were the basis for the posts. No replies indicated the OP was using dry type or liquid type transformers.

Dry type transformer is more liable to damages by surge and so I thought OP's concern resulted from such usage.

 

[jtinge]

Dry type transformer is more liable to damages by surge and so I thought OP's concern resulted from such usage.

Could be but we won't know for sure unless OP confirms.

 

[strap89]

Could be but we won't know for sure unless OP confirms.

The transformer is liquid filled oil.