# Arrester Application bringing it all together!

## Presentation on theme: "Arrester Application bringing it all together!"— Presentation transcript:

Arrester Application bringing it all together!
Selection (see EM Section 14 SI 1.00 and IEEE C Overvoltage Energy Margins Coordinating across the transformer

Arrester Selection – Step 1
a. Select an MCOV rating that is just greater than the maximum continuous operating voltage of the system b. Check the system maximum temporary 60hz over-voltage (magnitude and duration) with the arrester temporary over-voltage capability from the manufacturer’s curve c. Choose the higher rating from a or b above

Arrester Selection – Step 2
a. Select the arrester class based on the available fault duty. The available fault duty should be less than the arrester pressure relief rating b. Check the energy dissipation capability of the arrester. This is given in kilo-joules per kv of rating (either MCOV or duty cycle rating). Usually this is determined from switching studies of the transmission system and is not a concern on the distribution system.

Arrester Selection – Step 3
Calculate the protective margins and check they are adequate: PM1> 20% Front-of-Wave PM2> 15% Lightning PM3> 15% Switching

Arrester Selection – example: Let’s pick arresters for the 230kv system. Step one is to select the MCOV rating. 1a. Maximum voltage for the 230kv system is 242kv from ANSI C84.1 – 1995 so the MCOV must be at least = 242kv = kv Standard ratings available above 139.7kv are: IEEE C

Arrester Selection – example cont:
1b. The next step is to check the maximum possible system 60hz over-voltage First let’s check the maximum voltage rise on the un-faulted phase for a phase to ground fault. System analysis shows a worst case maximum voltage rise of 197kv for a phase to ground fault on the system (COG of 81.4%). Assume a clearing time of 5 cycles. Next check for the possibility of back-feed during clearing of a 230kv fault. Consider a kv delta-wye transformer. A high side fault is also back-fed from the 34.5kv system:

Neutral shift And Back-feed:
The 230kv breakers clear in 3-5 cycles Neutral shift And Back-feed: Phase to ground fault 197kv 242kv The 230kv line fault is back-fed from the 34.5kv system until the reverse power relaying clears (up to 1 sec). Arrester voltage kv 140kv Neutral shifts during clearing of a phase to ground fault The xfmr high side arresters must withstand the increased phase to ground voltage on the unfaulted phases until the fault clears

Arrester Selection – example cont:
1c. The highest stress for the back-feed condition could be a maximum of 242kv for 1 second. The minimum MCOV = 242/1.46 = 165.7kv 1.46 Pick a 228kv duty cycle:

Arrester Selection – example cont:
2a. Next select the arrester class: The worst case phase to ground fault duty on the 230kv system is at Doubs and is around 50ka. Need to select a station class arrester since the fault duty is > 16ka. 2b. Check transmission line discharge duty. Transmission studies show the maximum switching duty on the 230kv system is within the 7KJ/kv rating for standard station class metal oxide arresters.

Arrester Selection – example cont:
3a. Calculate the protective margins and check they are adequate: PM1> 20% Front-of-Wave PM2> 20% Lightning PM3> 15% Switching Normally margins are calculated for the transformers since they usually have reduced insulation (less than full BIL)

Protective Ratios for Liquid Filled Transformers
CWW = CWW FOW PR1 60 HZ BIL = BIL LPL PR2 BSL 20KA Voltage = BSL SPL PR3 FOW 10KA 5KA SPL Discharge voltage at Appropriate system Impulse current 1.0 10 100 1000 10000 .1 Time in u-seconds

Oil Filled Equipment characteristics

Transformer BILs:

Arrester characteristics

Arrester characteristics from the manufacturer:

Protective Margins PM1= (PR1-1) x 100% > 20% PR1 = CWW/FOW
PR2 = BIL/LPL PR3 = BSL/SPL PM1= (PR1-1) x 100% > 20% PM2= (PR2-1) x 100% > 20% PM3= (PR3-1) x 100% > 15% Example: kv transformer high side: PR1 = CWW/FOW=(825x1.1)/(559) = 1.62 PM1= (PR1-1) x 100%=(1.62-1)x100%= 62% PR2= BIL/LPL= 825/(525) =1.57 PM2= (PR2-1) x 100% =(1.57-1)x100%= 57% PR3 = BSL/SPL=(825x.83)/(451)= 1.52 PM3= (PR3-1) x 100%=(1.52-1)= 52%

Why are the low side margins so high?
Protective Margins Example: kv transformer low side: PR1 = CWW/FOW=(200x1.1)/(95.8) = 2.30 PM1= (PR1-1) x 100%=(2.30-1)x100%= 130% PR2= BIL/LPL= 200/(84) =2.38 PM2= (PR2-1) x 100% =(2.38-1)x100%= 238% PR3 = BSL/SPL=(200x.83)/(66)= 2.52 PM3= (PR3-1) x 100%=(2.52-1)= 252% Why are the low side margins so high?

Coordination across the transformer:
Should you worry about Mixing Silicon Carbide and metal oxide arresters? Consider a temporary 60hz over-voltage on the 230kv side of 200kv ph-gnd: SiC Duty cycle high side = 228kv Will the high side arrester conduct?? V low side = 200 x (34.5/230) V low side = 30.0kv V low side = 30.0 kv Arrester Duty cycle = 228 kv MOV low side arresters would be stressed since MCOV= 24.4 kv Use MOVs on BOTH sides of the transformer

OMU:

The OMU (optical metering unit)
What is the BIL? BSL? 60 hertz withstand? How do you know? What impulse testing has been done? Is there a design test report? Should it be protected by an arrester? Is the insulation self-restoring? Will it flashover on the outside or the inside? What happens when it loses SF6? What is the withstand at reduced gas pressure? Can we add gas if its energized? Should we trip it for low gas? What is the minimum pressure that is acceptable? When its switched out could the switching surge cause a flashover? (could the porcelain explode?)