RELAY vs. BAY CONTROL UNIT vs. DIGITAL TRIP UNIT vs. IED 1

[tszpun]

Hello Experts,

Could somebody explain me what are the differences between : RELAY vs. BAY CONTROL UNIT vs. DIGITAL TRIP UNIT vs. IED ?

What i think is :
RELAY = very generic naming, if it is used for CONTROL, we can call "control relays" as BAY CONTROL UNITs.
DIGITAL TRIP UNITS are mostly associated with small LV circuit breakers. It is like miniature relay, without any accessories like shunt trip release, UV release, communication etc. It only trips based for overcurrent fault.
IED = Intelligent Electronic Device. Basically all "protection" & "control" control electronic relays can be considered as IED.

Could somebody confirm my understanding ?

 

[cranky108]

The term Relay had a specific meaning at one point in time. Now with microprocessor technology it is somewhat generic.
Control was also specific to a control function like open and close. But control was also rase and lower, so it has always been a little generic.
But protective relay still provides a protective function, and in the regulated world has some specific requirements around it. The term control is not regulated, and there is a tendency to try to push protective functions into it to either avoid regulations, or avoid cost.
There is a difference in perceived quality in industrial and utility grade devices so the naming can be convoluted some.
A bay control is to control one or more breakers used to make up a bay. This may include some functions that seem protective in nature like sync-check, or breaker failure.
A protective relay is more protection oriented and would include over currents, or impedance, or differential, or some combination.

 

[tszpun]

Thank you for the reply.
Could you more ellaborate on "bay control unit"? I saw this terminology mostly associated with GIS. So, its really confusing, sometimes engineers refer to RELAY, sometimes to IED, sometimes to BAY CONTROL UNIT. Are they same ?

 

[thermionic1]

On GIS, all the breaker & line disconnects are electrically operated as well as the ground switches. Some form of electrical interlock is required to prevent opening a disconnect under load. Some form of electrical interlock is required to prevent the operation of a ground switch, unless the breaker is open and disconnected. There are plenty of GIS installations where the electrical interlocking is performed via the breaker / switch status contact and auxiliary relays. Control of traditional GIS is via conventional trip/close switches, Local/Remote switches, etc. A Bay Control unit, at a minimal level will have the control and interlocking functions performed via digital logic in the BCU. It's fewer wires, status contacts and auxiliary relays required. Some GIS manufacturers make a BCU just for this purpose. A BCU is an IED.

Modern Protection relays are also IED's. Some protection relays have enough I/O and internal logic available to perform all of the functions that a BCU can do. Additionally, as mentioned, breaker related protection and control functions such as sync check and breaker failure can be implemented in the same unit. A relay such as the SEL-451 can easily be a BCU and a protection relay, with HMI screens showing a mimic display of the various switch and breaker statuses. A SEL-421 / 411L can do the same, adding line protection functions as well.

I installed a 6 breaker GIS about a year ago. Our utility practices are to have traditional control switches for trip/close, etc, so all the interlocking was status contacts and aux relays. We have SEL-451 relays installed for 25/50BF/79, but choose to keep that as a separate device. I'm sure ABB tried to sell us a BCU, along with their relays, but this is what we choose. The upside of contacts and aux relays is that it is easy to understand and troubleshoot. In 30 years it still will be. Will a BCU?

Some Utilities send personnel on site to perform switching, even if most of the switching is remotely performed by SCADA via the system operator. I could imagine if there are users that don't send someone to supervise the switching, a BCU programmed from the factory with full FAT, may be an attractive option. Controlled switching may be another reason to have a dedicated BCU.

 

[tszpun]

Thank you @thermionic for detailed answer.
Indeed, the terminology "bay cobtrol unit" comes a lot for GIS. In some manufacturer documents there were a lot notion about "bay cobtrol unit", "IED", "relay", "local control panel". The fact is that all the just mentioned terminologies were used interchangebly. Thats why i was wondering the difference between them.

To summarize:
1. IED is a generic name. It can refer to numerical relay, or to bay control unit.
2. Local control cabinet is a panel that houses the BCU or IED.

Since we are already discussing about GIS, let me ask you one thing that is still not clear to me. Do you know, where does the trip coil (itself) for the circuit breaker of GIS is located? Are they located in stand-alone cabinet (for example Local Control Cabinet), or are they located in the circuit breaker gas insulated compartment of GIS? I had a request to energize trip coil of circuit breaker. But i do not know where the trip coil is actually placed.

Thank you.

 

[thermionic1]

The trip coil is located at the breaker mechanism, which is external to the gas compartment. Access has to be easy for maintenance, inspection and testing In some cases, the "coil" is more of a solenoid, where the solenoid is energized and the plunger then hits a trip release latch. I would not put my fingers near the trip coil/solenoid while the breaker is charged and the trip circuit is energized. Certain manufactures also use hydraulic systems to assist in the charging of the spring, so be weary of high pressure hydraulic hoses that could fail - they don't fail often, but you DO NOT want hydraulic oil in your bloodstream.

An IED (Intelligent Electronic Device) can be a number of things. A digital (numerical) relay, a digital meter, a SCADA device, Load Tap Changer Control, etc. I have a little box that converts 4-20mA signals to DNP over ethernet, so I guess that could be an IED as well. In my opinion the term is way too generic to have any real meaning, especially in the 21st century where nearly all devices used for the monitoring, protection and control of the power system are digital. I personally don't hear the term IED used often as it also has other meanings used outside of the protection and control world.

A local control cabinet can be the place where all of the relays, meters, control switches are housed. The ones I've worked on have two doors and internally also serve as the interface point (terminal blocks) for the external cabling from the GIS. The LCC's are generally installed in a different room (or floor) from the GIS. It's really no different from a relay panel, just bigger to have room for all of the control switches, indicating lights, operations counters, etc. I suppose if a BCU was used, the LCC could be very compact. There are multiple motor operated switches, so the motor overload / contactors can be mounted in the LCC. I suppose the LCC also allows the GIS to be "localized" better. The GIS is generally built to IEC standards, but that's not really helpful for personnel used to ANSI standards and nomenclature. So fill the LCC with ANSI devices, schematics & wiring diagrams and have it interface with IEC switchgear and it's probably a little easier for everyone. I've seen mini control panels mounted near the GIS, but I'd assume that's not so popular. In the event of some catastrophic failure of the GIS where the gas blow out / rupture ports operate, I'd rather be some distance away in order to breathe.

 

[tszpun]

The trip coil is located at the breaker mechanism, which is external to the gas compartment. Access has to be easy for maintenance, inspection and testing In some cases, the "coil" is more of a solenoid, where the solenoid is energized and the plunger then hits a trip release latch. I would not put my fingers near the trip coil/solenoid while the breaker is charged and the trip circuit is energized. Certain manufactures also use hydraulic systems to assist in the charging of the spring, so be weary of high pressure hydraulic hoses that could fail - they don't fail often, but you DO NOT want hydraulic oil in your bloodstream.

Mhm.... I was having doubt whether the trip coils are placed in LCC or in GIS itself. This part really helped me to clear my doubt.

Thank you very much sir for sharing your expertise.
It really helped me a lot.

Regards,