Trip circuit supervision relay for 5MW motor breaker 4

[hottip]

In our plant has been decided (because of bad design) to replace fuse-vacuum contactor protection scheme for a motor with breaker protection. The motor is 5MW, 11kV. For this purpose an entire switchboard will be changed.
The existing motor protection relays will be re-used. One of the relays is a multifunction protection relay (protecting the motor for: locked rotor, motor overload, too long start, etc.) and the other relay is a trip-lockout relay.

My question is: do we need as well with the new breaker protection sheme of the motor, to have a trip circuit supervision relay ?

 

[applekor]

hottip:

A little clarification please. By "trip circuit supervision relay", do you mean a relay that tells you whether or not you have power to your trip circuit?

Mike

 

[stevenal]

Many multifunction protective relays can supervise trip circuit continuity if wired and programmed for the task.

 

[ScottyUK]

Hi Applekor,

In UK parlance, a TCS relay allows a low current to bleed through the tripping coil, thus proving that the tripping coil and any contacts or connectors other than the actual trip relay contact itself are intact and that the tripping battery source is available. The current is far below the actual operating current of the coil, and has no practical effect on breaker operation.

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

I don't suffer from insanity. I enjoy it...

 

[applekor]

Scotty:

Ah...gotcha.

hottip:

In that case...what stevenal said.

Mike

 

[dpc]

To answer the original question, it isn't necessary to have a trip circuit monitor, but it's a good idea. It's also standard practice (in US, at least), to wire the red light (breaker closed) in series with the trip coil to verify power and continuity.

 

[applekor]

It's also common (though not necessarily required) in the US to install an undervoltage device on a circuit breaker used for such applications (i.e. direct motor starting).

Generally a solenoid is used. The basic idea is that the coil for the undervoltage device is connected in parallel with the entire trip circuit, thus sealing it in when potential is present. If potential is lost, the coil deenergizes. A spring then causes the solenoid plunger to extend and operate a cam or plate connected to the circuit breaker's tripping mechanism.

This prevents a situation in which your protective device attempts to clear a fault in the motor circuit, but cannot because of loss of power to the trip circuit. The downside is that disruption of power to the trip circuit for any reason (including accidental loss due to maintenance of nearby systems, etc.) WILL cause your motor breaker to trip open.

Generally speaking, anytime that I've seen a circuit breaker directly controlling a motor (rather than a starter), there has been some form of failsafe tripping protection installed (in your case a TCS relay).

Hope this helps,

Mike

 

[ScottyUK]

The TCS isn't really fail-safe, it just provides an alarm when there is a problem. For true fail-safe you'd need redundant tripping coils and contacts, but they aren't commonplace until you reach very large HV breakers. The use of an undervoltage coil or no-volts release for an MV breaker is an interesting idea - it's not common in the UK, or at least not common on modern gear. My experience on the older bulk oil gear is limited, so perhaps the UVR coil is something we have forgotten when we started building things down to a price instead of up to a standard.

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

I don't suffer from insanity. I enjoy it...

 

[hottip]

Guys,
Thank you very much for your answers.

@applekor: the undervoltage contact was in the old scheme (fuse-contactor) and will be present as well in the breaker new design.

I found this with google:

'A TCS relay is used to monitor the integrity of a circuit breaker's trip coil and associated wiring. Such a relay may be connected to monitor the trip circuit with the breaker in both the open and closed positions, or just for the closed position.
A TCS relay issues an alarm if it detects trip circuit supply failure or circuit breaker tripping mechanism failure.'

My question is: is it normal for a TCS relay to TRIP the breaker if the TCS trips? I mean in case of TCS detecting 'no supply voltage present' (for example) and will give an alarm, in the same time will trip the motor breaker?

 

[ScottyUK]

A TCS relay only lets you know that the breaker can't trip because there is a problem with the tripping circuit. The only way it would be able to initiate a trip would be either through a redundant tripping circuit, or with a UVR coil, neither of which are common on MV gear over here in the UK.


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

I don't suffer from insanity. I enjoy it...

 

[dpc]

If you are used to dealing with contactors and starters, you need to re-read and absorb ScottyUK's post. It's very important to understand that this circuit breaker requires control power to trip. If the trip circuit fails, such as burnout of the trip coil, the breaker will NOT trip, regardless of how hard you push the STOP button. All the trip monitor system can do is give you an alarm.

If the breaker has a true undervoltage release, it can trip without external control power - in fact you can't stop it from tripping - that's the downside of the UV release. It's always there when you need it, but it's also always there when you don't need it.

 

[stevenal]

Is tripping the only way to get someone's attention? Otherwise there would be no reason to trip for an open trip circuit even if you had a second trip coil available. The purpose of a TCS alarm is to alert that repairs need to be made.

 

[hottip]

Well in this case that's bad news. The problem is that our plant is an island from power point of view. We have turbines and there is a possibility that the under voltage condition to occure. And we have 5 motors in the same condition that I described.
There is a small possibility that the breaker of one of the motors not to open when is required if undervoltage shall occur. In this case I believe that the very expensive 5MV motor can be damaged (I mean the bars from the rotor can be affected by the powerful momentul generated by the voltage drop and by the re-apparition of a normal voltage)

 

[stevenal]

So the multifunction relay is programmed to trip for undervoltage, but you suspect the undervoltage condition will cause insufficient tripping power? Sounds like you are tripping on AC? The best method is to trip off a battery backed up DC supply. Sometimes a capacitor trip employed, or a constant voltage transformer can be used. Both the relay and breaker are built to standards that will allow some voltage drop in the tripping voltage supply and still function. Maybe you can set your relay to trip prior to reaching this lower level.

 

[applekor]

dpc:

I would like to add a note to your comments. Depending on the manufacturer and model of the circuit breaker there is still likely a manual, mechanical trip on the circuit breaker frame. Obviously this doesn't help you much in a fault situation on the motor leads or windings, but you WOULD still (technically) have motor control.

As for the undervoltage trip, most modern multifunction relays (should) allow you to specify an undervoltage function disable that you can logically implement into a startup scheme.

stevenal:

I agree. DC is the best to use in these situations. If this is not an option, hottip, then you might consider adding an auxilliary (gasoline/diesel) generator or battery + inverter setup into your trip circuit supply for use during black starts or outages.

Regards,

Mike

 

[waross]

If the breaker has a true undervoltage release, it can trip without external control power - in fact you can't stop it from tripping - that's the downside of the UV release. It's always there when you need it, but it's also always there when you don't need it.

For one under voltage release solenoid, you may be able to use a UPS on the undervoltage release solenoid to let your relay scheme decide when to inhibit under-voltage tripping.
On the other hand, it was pointed out to me in a previous thread that on a large breaker the trip solenoinds may be so large that power consumption becomes an issue if the solenoids are continously energised. Hence battery banks and shunt trips on large installations.
respectfully

 

[dpc]

Breakers with UV release have a separate UV release coil that is distinct from the trip coil.

 

[hottip]

There is 110 Vdc tripping system + batteries .
My question is related with the faults that can happen inside the motor and the breaker will not be tripped because of the fault with the tripping circuit itself.
There is nobody in HV room, so such an alarm from a TCS relay can go unnoticed.
The other scenario can be like this:
Breaker on, motor working => suddenly the tripping circuit is faulty, TCS alarm which can go unnoticed => suddenly under voltage situation in the plant power system => the undervoltage relay gives command to the breaker to open => breaker cannot open (trip circuit fault) => the undervoltage condition of the plant finished (so again will be normal voltage of the power system) => normal voltage re-applied for an electrical motor which was not working at the nominal rpm (because of undervoltage) => high torque applied for the rotor shaft => motor damaged ????
Sorry for this long story but I'm really interested of your experience wich such a system. I do not have an experience with motor breakers.

 

[ScottyUK]

There is no absolutely no purpose in fitting a TCS relay if it only alarms locally in an unmanned substation. You must take the alarm signal to a location where it will be noticed and investigated. Once the alarm is received and the fault investigated, a decision may then be taken based upon the nature of the fault to (e.g.) fix the problem, trip the breaker manually, live with the problem for the short term, open the upstream breaker feeding the board, etc.

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

I don't suffer from insanity. I enjoy it...

 

[nightfox1925]

I also recommend, in addition to the trip circuit supervision on the protection relays and trip coil circuit, to provide circuit supervision relays on the closing circuit and on the lockout relays. Bear in mind that the lockout relay coil is as vital as the breaker trip coil. Further, the control circuits of both closing and tripping be supervised. The trip coil and lockout relay coils should be monitored both in trip or closed condition (since series contacts is wired along the coils). Alstom model MVAX31 can montior both coil and the overall control circuit. Normally, each lockout and trip coil will have their individual supervision relays. All these relay NC contacts maybe wired in series and interfaced to a local annunciator or a remote manned facility as an urgent alarm.

If the motor is a very vital process equipment, you may want to consider also providing circuit breakers with dual trip coils with a breaker failure relay.