Surge Protection on Open Ended MV Cable
Surge Protection on Open Ended MV Cable
(OP)
I would appreciate any comments on the use of surge protection on open ended cables. I am doing a MV (11 kV) substation design connected to ring feeders that will be split during normal operating conditions. Long lengths of cables (up to 1500m) might be energised but not connected to a busbar/load. I should add that series reactors are used in the network (main incoming sub) to limit the fault level.
I have talked to an experienced engineers that know of cable failures on open ended cables. He reason that these failures are due to reflection of switching surges causing doubling of the amplitude of the surge and insulation failures. This make sense but I have never seen a MV cable installation with surge arrestors installed inside the metal clad switchgear panels (only motor feeders). I have the following querries:
(a) Is the installation of surge arrestors on a cable distribution system common practise (esp with XLPE cables)?
(b) Is long or short cable lengths worst?
(c) Where should you install the surge arrestors?
(d) Should I use normal Metal Oxide Surge arrestors?
Any other comments regarding this will be appreciated.
Thanks.
KJvR
I have talked to an experienced engineers that know of cable failures on open ended cables. He reason that these failures are due to reflection of switching surges causing doubling of the amplitude of the surge and insulation failures. This make sense but I have never seen a MV cable installation with surge arrestors installed inside the metal clad switchgear panels (only motor feeders). I have the following querries:
(a) Is the installation of surge arrestors on a cable distribution system common practise (esp with XLPE cables)?
(b) Is long or short cable lengths worst?
(c) Where should you install the surge arrestors?
(d) Should I use normal Metal Oxide Surge arrestors?
Any other comments regarding this will be appreciated.
Thanks.
KJvR






RE: Surge Protection on Open Ended MV Cable
Long cable lengths are worse.
If installed, they should be at the terminations of the cable.
Normal metal-oxide surge arresters would be used.
RE: Surge Protection on Open Ended MV Cable
(b) Is long or short cable lengths worst? Long cable is worst
(c) Where should you install the surge arrestors? At the riser pole and end reflection point(s).
(d) Should I use normal Metal Oxide Surge arrestors? Yes
Any other comments regarding this will be appreciated. In recent years, new information has been revealed concerning lightning and its impact on underground power distribution systems. For instance, the IEEE Std 1299/C62.22.1-1996 states that the voltages developed on an unprotected line prior to flashover propagate along the line as traveling waves. At the surge impedance discontinuity of the overhead line to cable transition, part of the wave is reflected and not all of the surge propagates into the cable. A typical UG cable surge impedance is around 40 Ohms and only about 18% of the incoming surge voltage on the OH line would enter the cable because of the surge impedance discontinuity. However, because the voltage on the line prior to flashover could be 1000 kV or more, the voltage passing through to the cable can be excessive.
Therefore, improved protection is provided by the use of surge arresters at the terminal pole, mid-point of the cable run and at the open point.
RE: Surge Protection on Open Ended MV Cable
The 11kV system is an insulated cable (induvidualy screened cores) system only - no overhead lines that can be influenced by lightning. The 66 kV incomers are overhead lines feeding onto 66/11kV transformers. All lightning surges must past the 66kV surge arrestors and propogate through the transformers along a 1.5km cable. Is this possible?
jgchrist mentioned that switching surges will not be large enough to damage 11 kV cable with a BIL of 95kV at the open end. If that is true, is it still worth while installing surge arrestors on the open ended cables - or will it end up as dead capital hidden in the termination box of the switchgear?
KJvR
RE: Surge Protection on Open Ended MV Cable
The cost of those studies including the facility engineering time may be more than $100K.
Several options may be available to evaluate the risk:
a) Do nothing and accept the shortage in cable life or failure if happen.
b) Expend resources in study with open chance to recommend surge arrester.
c) Specify the arrester without detail studies.
NOTE: Cable insulation levels are 100%, 133% or occasionally 173%. BIL rating is typically used for equipment and power apparatus insulation level.
RE: Surge Protection on Open Ended MV Cable
RE: Surge Protection on Open Ended MV Cable
KJvR
RE: Surge Protection on Open Ended MV Cable
RE: Surge Protection on Open Ended MV Cable
Are you talking about direct strike on the 66kV incomer lines that will cause currents into the earth mat at the surge arrestors in the 66 kV substation OR indirect strikes close to the 11 kV substations causing voltage rise in the earth mat due to induction as normally happen on overhead lines?
One question: if the voltage spike is due to a direct lightning strike on the 66 kV lines, and increase the earth mat potential, will it not reduce the phase to screen (earth) voltage in the cable because the impulse also propogates through the transformer onto the the phases? I must admit I have never thought of what you just told me and run some ATP simulations.
Thanks
KJvR
RE: Surge Protection on Open Ended MV Cable
I would not expect much of the surge to propogate through the transformer because of the high impedance of the transformer to surge current. Also, the surge may not be on the 66 kV conductor.
On the other hand, when doing direct stroke protection design for an outdoor substation, you don't consider that if the shielding successfully intercepts a stroke, the ground grid potential rise may flashover the bus insulation. At least not if you design in accordance IEEE Std 998-1996, IEEE Guide for Direct Lightning Stroke Shielding of Substations. Is this a problem with IEEE Std 998?
RE: Surge Protection on Open Ended MV Cable