Failed Digital Motor Relays 2

[zak]

Hi,

We had about 30 of the above failed within a short time span on a single switchboard.We found out that some components on the auxilliary card of the relays burnt. We thought that is due to over-voltage on a particular phase only(in this case yellow phase). Is it possible? There was no problem on other digital relays on the incomers or the motors on other switchboards.

My question are where did the over-voltage(if it is) comes from? If from the utility, shouldn't the incomer relays detect it and trip the incomer breakers? What are the actions should be taken to prevent from recurring? What are the means to protect the relays? We are using 240V 50Hz supply for the relays. Do you think a DC supply for the relays is a possible solution? Is it possible for the transformer(if there is something wrong with it) to that particular switchboard be the source of over-voltage? Any ideas or suggestions?

Your prompt reply is really appreciated.

rgds,

Zak

 

[jbartos]

Suggestion/Comment:
1. It appears that digital motor relay category was affected only. Was there anything else?
2. It may be due to the relay product flaw.
3. High voltage spikes/surges may have occurred. This needs a treatment in terms of transient voltage suppressors, or power line conditioners.
4. Higher voltage harmonic content may have occurred. This needs harmonic filtering that is sometimes part of power line conditioners or may be implemented separately via harmonic filters (active, passive, or hybrid).
5. High ambient temperature for that product.
6. Electromagnetic interference. This needs to trace a source and either remove the source or mitigate it in terms of EMI shielding/filtering.
7. DC offset in the power supply due to even (second) harmonics. This needs to isolate/mitigate the dc offset, e.g. by a harmonic filter, isolation transformer or capacitor.
8. Frequency deviation from the rated frequency. This needs a power line conditioner or AC to AC converter via a DC section, i.e. AC to DC to AC.

 

[peterb]

Sounds as though the common factor here affecting relays on a number of circuits may be the power supply. This was the weak point of early generation solid state relays and a lot of progress has been made over the years in addressing the problem. Most utility grade digital relays today have power supplies that can accommodate a very wide variation in supply voltage and are tolerant of potentially damaging transients.
In assessing the suitability of the relays for use on your system, you should evaluate whether they have been qualified to the appropriate standards for surge withstand capability - IEEE C37.90 &/or IEC 255 series.
The source of the overvoltage could be switching of reactive loads (such as solenoid valves); if this occurs on the power supply circuit, the system relays would not respond - they will operate for steady state conditions, not transients - you would need transient suppression if this is the problem. You haven't indicated the relay type, but as mentioned above, "utility grade" relays have transient protection built in; if you are using some "industrial grade" (eg small rail mount type) relays, then you may have to consider external surge protection. Also, check the system and equipment grounding carefully.
DC supply would definitely be preferred for the relays. Assuming that there is a station battery for the switchboard breakers and other relays, this would be a cleaner supply - again, assuming that the motor relays are capable of taking the DC supply directly.
Have you discussed the problem with the relay manufacturer? They normally will be very anxious to assist in resolving this type of difficulty.