Transformer-Arrester Connection on Pole-Mount Transformers
Transformer-Arrester Connection on Pole-Mount Transformers
(OP)
I work for a electrical distribution utility. This past season we have had over a dozen transformers that were struck by lightning, resulting in the transformer spliting from the top of the transforer close to the primary bushing and down to the tap changer (we use dual voltage 7.2kV/14.4kV transformers). Our current method of connection is from the overhead line, through the cutout and then to the primary bushing. The arrester is connected by connecting a jumper from the bushing wire to the top of the arrester. The arrester lead is grounded to the tank ground.
It seems to me that this connection will incourage a ligtning surge to go through the bushing, resulting in our tank-splitting problem. Does this seem logical?
It seems to me that this connection will incourage a ligtning surge to go through the bushing, resulting in our tank-splitting problem. Does this seem logical?






RE: Transformer-Arrester Connection on Pole-Mount Transformers
The arrester have to be between the incoming line and the equipment to be protected.
Lmax = {{v [ BIL / (1 + Mp)] – Ures } / 2 x S} - (H arrester + H structure)
Where:
Lmax : max allowable distance between arrester and equipment(m)
H arrester : Height of arrester (m)
H structure : Height of structure(m)
V : velocity of wave propagation (m/ms) » 300 m/ms for overhead lines
150 m/ms for cables
BIL: Basic insulation level of equipment to be protected (kV)
Mp : protective margin required : Minimum IEC recommendation = 20%
Ures : Residual voltage of arrester (kV)
S : Steepness of incoming wave (kV/ms) » 2000kV/ms
Protective distances
kV max distance
11 on trsf tank
22 on trsf tank
33 3m
44 5m
66 6m
88 6m
132 7m
275 18m
400 24m
Regards
Ralph
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Basic overvoltage-protection concepts are partially addressed in IEEE Std C62.22.1-1996 …Connection of Surge Arresters to Protect Insulated, Shielded Electric Power Cable Systems
A significant improvement can be achieved by reducing the “open air” cross-sectional area enclosed by the arrester, jumpers and transformer bushings/can.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Our practice is to mount the arrestor beside the transformer's 'cutout' disconnect switch, with the 'hot' lead connected in parallel with the source side of the cutout, and then the arrestor is connected to ground below the transformer's ground connection, and then sharing a common ground wire down the pole to earth.
The cutout is usually ~1m from the transformer bushing.
It was felt that this maximizes the arrestor's ability to 'shunt' the lightning surge around the transformer.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
If the local connection to earth is poor, the near-zero impedance of a MOV arrestor will still not be a very good shunt, since it is in series with the ground wire and earth grounding.
We have found that simply using a single ground rod is insufficient if the local ground resistivity is high (soil conditions are rocky, etc.).
RE: Transformer-Arrester Connection on Pole-Mount Transformers
One of the reasons I was told we connect from the cutout to the arrester (in series) is that the cutout will blow and indicate that the arrester needs to be changed out. Otherwise, the arrester may be bad and no one ever knows it. Sometimes the arresters that have blown look normal from the outside (from what I was told).
With respect to the arrester ground lead connection, we currently connect it to the tank ground. I also thought it would make more since to connect it directly to ground, but I read some previous posts that said connecting to the tank would reduce the induced L di/dt voltage between the windings and the tank inside the transfomrer.
Any thoughts?
Thanks for all of your responses.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
RE: Transformer-Arrester Connection on Pole-Mount Transformers
RE: Transformer-Arrester Connection on Pole-Mount Transformers
What is meant by transfomer splitting? is the case splitting? or the winding is blwon to pieces or both?
Sounds like the arrester never worked, all energy is passing through the transfomer case or winding.
tinfoil's point equally valid. You better have good earth connection to dissipate the energy. Current has to compelete the circuit and it will take path of least impedance.
Although you want to connect to the 'can' but not depend on the 'can' to carry the surge current, rather have a solid condcutor all the way down to good earth electrode system. (using a pad or lug on the can as junction).
RE: Transformer-Arrester Connection on Pole-Mount Transformers
USDA-RUS bulletin 1724e300
4.3.5 Surge Arresters Transformers, regulators, and other substation equipment are particularly sensitive to transient overvoltages. For the highest degree of equipment protection, surge arresters should be installed as close as practical to the equipment being protected. In most instances, power transformers can be furnished with surge arrester mounting brackets to facilitate installation. Separate arrester stands can also be used, or the arresters can be installed on adjacent switching structures. For voltage regulator applications, the surge arresters are normally installed directly on the regulator tanks.
[i]When power transformers are protected by fuses, it is recommended that transformer surge arresters be connected on the line side of the fuses, as close as practical to the power transformers. This will minimize the stress on the fuse and help avoid partial melting of the fuse link when the surge arrester responds to a transient overvoltage.{/i}
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Thanks for your help.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
No the idea is to prevent over stressing the transformer insulation. Adding lead length (inductance) to either the line or ground side will increase the voltage at the can. See Ralph's post.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Change the connectins such that current does not pass through the can's skin, but through the conductor. Although attach the conductor to the can so can is grounded too.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
The transformer "splitting" may be caused by high fault currents on a failed transformer rather than by the surge voltage directly. Backup current limiting may be used to prevent this. See
http://www.sandc.com/webzine/021003_1.asp
http://www.sandc.com/edocs_pdfs/EDOC_001654.pdf http://www.cooperpower.com/Library/TheLine/pdf/990...
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Equally important is the rating of surge arrestor. If we consider the 11kV system to be solidly grounded type, a 9kV rated surge arrestor should be appropriate and the type shall preferrably be metal-oxide, gap-less.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
I wouldn't be as concerned for the actual impedance to earth as the total impedance between the arrestor and the transformer. You are trying to minimize the transient voltage seen by the transformer, therefore the arrestor ground should be connected close to the transformer tank ground.
The arrestor voltage rating is important. Also make sure your fuses and arrestors have adequate interrupting ratings.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
RE: Transformer-Arrester Connection on Pole-Mount Transformers
tank and the local earth ground must be very good.
Otherwise there is a high potential between the primary
and secondary which is surely well grounded at the load.
So inadequate pole ground could contribute
RE: Transformer-Arrester Connection on Pole-Mount Transformers
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Notice that there is not shield wire to protect from direct or near lightning strike. Traveling wave could create severe overvoltage at the transformer primary side resulting in component damage.
Possible mitigation strategy is considering installing line arrester and/or OH shield wire.
The table below shows the line performance against the number of flashover for different configurations
Summary of results of the studies at
100 Ohms footing resistance
Case Configuration Total Flashovers Per 100 km/yr
I No Protection 20.41
II OHSW Alone 13.92
I Arresters Top 2 Phase
Alternate Structures
No OHSW 14.68
II Arresters Lower Phase
Alternate Structures –
Plus OHSW 9.21
III Arresters All Phases
Every Third Structure 18.16
III Arresters All Phases
Alternate Structures 15.26
III Arresters Top Phases
Every Structure 10.35
III Arresters All Phases
Every Structure 0
=====================================================================
NOTE: Arresters on the top phase of every structure results in a 26% improvement over shield wire alone. Therefore, for new construction the elimination of the shield wire and use of top phase arresters results in a better performing line.
Reference:
http://www.hubbellpowersystems.com/powertest/liter...
RE: Transformer-Arrester Connection on Pole-Mount Transformers
I also found out that the transformers are splitting from the tap changer up to the bushing (determined by the fact that some of our xfmrs only split from the tap changer but not all the way to the top). Too bad we cant attach pics to this.
Based on the responses I'm thinking of changing to Cooky's connection, but running the arrester ground directly to the pole ground. I just can't see putting a lightning surge through the tank. I can't see how the L di/dt will make much of a difference with such a small transformer. Increasing the ground wire size may be beneficial as well.
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Lightning protection is only as good as the ground. If your soil has very few rocks you can use a Borit or Borzit ( 2 competitive companies ) to sink a 1.25 inch copper of steel water pipe way deep into the ground. Figure your budget on the basis that you will have to leave the drill bit at the bottom of the hole.
This is a cheap hand held directional drilling machine to which you hook up a water hose and an air drill. Their directions say that you can use an electric drill of the type that electricians use to drill holes in woodwork but an air compressor is cheap insurance.
Do not forget to use Burndy Penetrox A or A13 or Ideal Noalox on galvanized pipe threads. For copper or bronze pipe use Penetrox E.
Mike Cole, mc5w@earthlink.net
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Often, xmfr arrestors are the ONLY ones installed in the system for miles around. The lightning surge would then have to find another path to earth, likely damaging one or more insulators (which can be a bitch to find in the middle of the night during an electrical storm!).
RE: Transformer-Arrester Connection on Pole-Mount Transformers
RE: Transformer-Arrester Connection on Pole-Mount Transformers
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Most utilities outside of the USA have not even heard of the NESC standarda! ;)
RE: Transformer-Arrester Connection on Pole-Mount Transformers
Happiness is a way of travel, not a destination.