Sel SEL-300 Series, Application Manual

16

SEL Application Guide 2020-04

Date Code 20200326

C

ONSIDERATIONS

FOR

 P

ILOT

 P

ROTECTION

 B

ETWEEN

 D

ISSIMILAR

 R

ELAYS

Different relays with different sensitivities used in pilot schemes can cause issues with coordina-
tion. See 

Figure 16

 for an example system. The relay installed on Breaker 3 is an SEL-400 series 

relay and the relay installed on Breaker 4 is an SEL-300 series relay. When you are using two dis-
similar relays, there may be coordination problems because the relays may have different process-
ing speeds. Also, the SEL-421-3, SEL-421-5, and SEL-421-7 have high-speed distance elements 
to reduce operating time. These features make SEL-400 series relays faster than SEL-300 series 
relays, and this must be accounted for in the coordination time delays.

If a fault occurs on the line between Breakers 5 and 6 in a POTT scheme, Breaker 3 sends a PT sig-
nal to the relay at Breaker 4 (if the relay at Breaker 3 is set sensitively enough to detect the fault). 
If the protection at Breaker 4 is not sensitive enough to detect this reverse fault and Breaker 4 uses 
echo logic, as explained in [1], Breaker 4 echoes the received PT signal. The result is that 
Breaker 3 trips for an out-of-section fault. This misoperation can be avoided with more sensitive 
reverse-reaching protection elements at Breaker 4.

If the blocking relay is slow compared to the tripping relay, miscoordination can occur. To solve 
this problem, make sure Zone 3 always overreaches the remote Zone 2 by using 

Equation 1

 to set 

Zone 3 of the local relay. Only apply this equation for two-terminal lines. 

Equation 1

Now Zone 3 is set so that it overreaches Zone 2 of the remote relay with a margin of 25 percent. 

Equation 1

 also compensates for any differences in the CT and PT ratios at the local and remote 

terminals. Perform this exercise at least twice at each end of the line to make sure that Zone 3 is 
defined as explained previously. Doing this prevents tripping for an out-of-section fault.

For a reliable POTT scheme, the relays at each line terminal should have similar sensitivities. If the 
sensitivities are not matched, a misoperation or failure to operate could occur. When coordinating 
SEL-400 series relays with SEL-300 series relays, consult your fault study to ensure that each 
relay has adequate operating quantities for all internal and external faults.

For a more detailed discussion of ground-relaying sensitivities, refer to [6] [7]. These papers iden-
tify relay sensitivity limits and fault resistance coverage for various fixed and settable directional 
element limits. They also provide some useful tools for verifying relay settings in practice.

Figure 16

Example System Single-Line Diagram Showing Sensitivity Miscoordination

Zone 2

Zone 3

Zone 3

Zone 2

1

2

4

3

6

5

4

Bus A

Bus B

Z3

Local

Z2

Remote

PTR

CTR

------------









Remote



















CTR

PTR

------------









Local









Z1MAG

Local

–

•









1.25

•

=