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

2. 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

3. 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.

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

5. 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

6. 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:

7. 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

8. 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:

9. 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.

10. 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)

11. 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

12. Oil Filled Equipment characteristics

13. Transformer BILs:

14. Arrester characteristics

15. Arrester characteristics from the manufacturer:

16. 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%

17. 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?

19. 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

21. OMU:

23. 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?)

24. Thanks for your attention…… Your substation support staff!
Luxor substation