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Under voltage relay setting

Under voltage relay setting

Under voltage relay setting

Hi, i'm an electrician at a pilot hydrocarbon processing plant in Australia. I'm trying to find a rule of thumb setting for the site incoming power supply under-voltage relay time delay. The reason is that the supply to the site was interrupted which caused equipment to stop. The result of this could have very serious consequences due to the nature of the process. The relay is set to trip the incomer circuit breaker at 70% of normal voltage in a time of 0.01 secs. The time delay was set by the high voltage switch gear vendor, but i'm wondering if the time  was set too short, due to the effect a trip has on the safety of the plant? My understanding of under-voltage protection is to prevent motors etc from gettig to hot because of increased current. I can't see why, if the time delay was say, 0.5 or 1 sec, the equipment wouldn't still be protected from prolonged "brownout's", but site power could be maintained if a very quick dip in power occured. I'm not sure of the duration of the dip we had, but we have two sub's with identical u/v relay's and setting's, but only one tripped.

RE: Under voltage relay setting

The appropriate time delay can depend on several issues, but the main issue is what purpose the original designer was trying to serve with this relay.  Is some function or process within the facility going to be damaged if a longer duration is used.  Settings from the factory can not be used unless they are made specifically for your application (default settings can be outrageous sometimes).

The motors will usually be protected from overheating by their individual overcurrent/overload protection.  One reason to use such a short time delay is to protect motors against reclosing. If your facility is served from a utility feeder that also serves other customers, it is a normal practice to re-energize the line after a fault (reclose) to minimize the duration of an outage (80-85% of faults on a utility system are temporary, which is not usually the case in a plant).  Synchrounous motors and large induction motors can be damaged by this.

Just some thoughts which may or may not be applicable.

RE: Under voltage relay setting

Suggestion: Visit industry standard Reference:
1. ANSI/IEEE Std 242-1986 "American National Standard IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems" (Buff Book)
1a. Page 144 Sect. 4.5 "Undervoltage Relay - Device No. 27" Describes various applications; e.g. Bus Undervoltage Protection for momentary dips, etc.; Source Transfer Scheme; Permissive Functions; Backup Functions; Timing Applications, etc.
The solid state instantaneous relays are probably with the shortest setting, e.g. less than 1.5 cycle=0.025 seconds. They would respond to a reverse phase sequence condition in addition to other undervoltage conditions.
Figure 48 on page 146 shows typical time-voltage characteristics of an Undervoltage Relay. For 70% of tap setting, the time ranges from about 0.8 second to about 9 seconds, which is a reasonably wide range for a wide area of applications. The time dial goes from 0.5 to 10, i.e. there are 11 time-voltage inverse curves.
Section 11.4.7 on page 460 describes an application of 27 relay related
to generator operational applications.
Section 13.5.4 on page 522 describes an application of 27 relay related
to automatic transfer, which should be delayed to coordinate for faults.

RE: Under voltage relay setting

From my post to the other forum:

rhatcher (Electrical) Apr 27, 2001

There are several parameters to consider for undervoltage relay settings. There is no fixed standard value to use for either time delay or dropout voltage. The combination of the settings you choose must be based on your situation. Unfortunately, if you are using a simple relay having a fixed time delay the result will always be somewhat of a compromise.

As you correctly stated, a longer time delay would allow the plant to ride through a sag or dip in line voltage. Your assumption that your plant could run for a few seconds at 70% voltage are probably sound. However, keep in mind that the relay probably has a fixed time delay. In other words, if you select 2 seconds time delay with a 70% dropout voltage, the relay will hold for 2 seconds at 69% voltage and for any voltage less than 69%. The down side becomes the case of a sustained voltage drop to low voltages. For example, if the voltage dropped to 50% the relay would still hold for 2 seconds and you would probably suffer some negative effects on your equipment.  

The flip side of the coin is the dropout voltage adjustment. A lower dropout voltage with a short time delay may seem like a way to avoid dropping out for "acceptable" sags but would act almost immediately if the voltage becomes dangerously low. For example, you could choose a 55% dropout voltage with a 0.01s time delay. The down side of this is the case of a sustained voltage sag occurred in the range between 55% and say 75%. For example, if the voltage dropped to 60% the relay will hold indefinitely although at that voltage damage will occur within seconds or minutes. Really, I think your current settings are somewhat of a compromise as you would not detect a drop to 75% voltage although at this level electric motors will be damaged in short time.   

I have worked for a customer to solve the same problem for him. In his case the UV relay was activating a transfer switch and an emergency power system with load shedding. We had the utility monitor the voltages to get an idea of what the problem was and at the same time he kept a log of power incidents. With this data we were able to determine a setting that would protect him without nuisance tripping. I do not have more time now to explain what we found and how we determined the settings, but I will say that the selected settings were in the range of about 80% for 2 seconds (if I remember correctly). At that time the combination of utility and customer data allowed almost perfect confidence in the settings. That was over 3 years ago and no "nuisance" trips have occurred nor any damage.

If you are interested I can go into what we found in a later post. I hope this helps.   


RE: Under voltage relay setting

I think jnims comments are worthy of close attention.
EPRI tells us that power interruptions of duration 4-10 cycles are very dangerous to motors.  Motor drifts out of phase with supply only to be reenergized out-of-phase with can take a big motor and toss it across your plant!  Shorter durations means motor does not drift out of phase to cause a problem, longer durations means motor voltage has enough time to decay before reclose.

You've told us enough information to know that your plant has had a transient... although not sure whether it was a voltage dip (not a problem with respect to out-of-phase reenergization) or a complete interruption/restoration (big problem).  The utility might have some ideas on what types of transients you saw and what you can expect in the future.

What are the other reasons for using a 27 for motor protection?  Overload should cover overheating.

RE: Under voltage relay setting

Comment to the previous posting May 6, 2001:
1. It is true that the frequently rating 4-10 c/s interruptions are dangerous to the motors, especially to synchronous motors since the asynchronous motors are more rugged. However, what causes these? Are these really the major design basis for the application of 27 relay?
2. More often occurring problems in the power distribution are continuously lowered power supply voltages because of the overloaded power distribution system of regulated Utilities. Then, the 27 relay application with respect to NEMA Std MG-1 requirements shall be checked (90% in some cases).

RE: Under voltage relay setting

Isn’t application of negative-sequence voltage relaying (47 device) generally more effective in the prevention of stator insulation damage (overheating) than simple undervoltage?  Related to that, is it more appropriate to use a high-speed, definite-time or inverse-time tripping characteristic for negative-sequence voltage protection?

RE: Under voltage relay setting

jbartos - I didnt mean to suggest this (out-of-phase reenergiztion) was the primary or only purpose for a 27 relay. I was asking a question...what are the other reasons for using a 27 relay.

Thinking about it a little more, it strikes me that the most common purpose for 27 device is to ensure that the motor is properly tripped during loss of power so the power supply is not subject to simultaneous inrush of all motors when power is restored.  Sometimes this function is provided in a controller, sometimes with 27 device tripping the breaker.  It would not seem like the time delay on this device would be critical, unless high-speed reclose of the supply is expected.

busbar - I agree that 47 relay is more effective than overload relays for protecting against voltage imbalance (even if we have overloads on all three phases).  The reason is that a given current produces more heating of the rotor during imbalance condition that the same curren would during normal condition due to the negative sequence currents in the rotor which are at approx 2*line frequency and therefore see a much higher resistance due to skin effect.

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