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.
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Under voltage relay setting 1
- Thread starter ozziemick
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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.
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.
jbartos
Electrical
- Jan 15, 2000
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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.
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.
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