Substation Telecommunications DC Bus Protection 2

[Tooch]

Can harmful voltage spikes, due to a substation apparatus operation (i.e. breaker trip/close), result on the load side bus of a dc-dc converter which is supplied by the substation backup batteries?

At our utility, wherever we have a telecommunications tower site within a substation, our current practice is to backup our 48V Telecom bus using our substation backup batteries through a dc-dc converter . The Western Electricity Coordinating Council(WECC) doesn't recommend this practice [Ref: WECC Guidelines for the Design of Critical Communications Circuits; Section 4.3.1]. We are trying to determine if we really need to change our dc backup arrangement.

For any of you who have a similar telecom battery backup arrangement, have you experienced any harmful transient voltages on the telecom side of the converter?

 

[alehman]

You would have to look at the specifications or consult the manufacturer of the converter. When circuit breakers operate there will be momentary dips on the substation DC bus voltage. If the input to the conveter falls below what it can tolereate, it may malfunction.

A DC-DC converter alone (without batteries on the load side) may not be considered sufficiently reliable for substation communications.

I would also be concerned about transients from lightning strike on the tower getting back in to your substation.

 

[busbar]

One consideration is the usual DC-station power serving protective relaying is typically ungrounded with ground detectors, where telecomm 48V is often solidly grounded on the positive terminal at the power supply {DC-DC converter in your case.}

Ground-potential rise during faults could conceivably induce noise into the comms gear, in particular where there may inappropriately more than one (non-obvious) bond between station ground mat and comms 48V gear.

Does WECC suggest an isolated battery set dedicated to comms only?

 

[stevenal]

http://www.wecc.biz/committees/OC/TOS/TELCOMWG/documents/design_of_critical_comm_circuits-Jan-02.pdf

So "critical" refers to protective relaying and remedial action, and the advice applies to non-substation hardened equipment. Who would select non-hardened equipment for this application? This may also have been written before IEEE took up the issue with standards like 1613.

 

[Tooch]

Thanks alehman for your response. I will contact my converter manufacturer to see if there are filtering specifications. In fact, I found out about a different manufacturer that sells EMI filter modules (input and output) for it's converter so voltage tranients seem to be an issue.

As far as lightning strikes on our towers go, we have them well protected with lightning arrestors and solid grounding.

Busbar: the WECC does indeed recommend a separate DC power system for non sub-hardened communication equipment, however our ISO doesn't address this issue so we're left to our best judgement. We've recently (last 5 years) started the practice of supplying comm equipment from the substation DC. That happened because our substation relays have changed from electromechanical to electronics; therefore, we've had to install substation chargers with so little output ripple that they were also adequate for the communications DC. My job is to determine if our use of a dc-dc converter is good engineering practice. If so, we won't install separate DC supplies. Thanks.

Stevenal: Because of our history of practice, we're trying to decide if we need to retrofit all our affected substations. Thanks for reference to IEEE 1613, I wasn't familiar with it.