Overcurrent relay at generators: True RMS / DC ?
Overcurrent relay at generators: True RMS / DC ?
(OP)
Hi guys,
what current is usually measured by the overcurrent protective relays installed at synchronous generators below 10 MVA?
Do they measure the true rms value (that is, all harmonics) or are some harmonics filtered out?
How about the DC offset during a fault or transformer inrush? Is it included in the rms value?
Is there a general practice or does it depend on the manufacturer and his definitions?
Thanks.
Regards
what current is usually measured by the overcurrent protective relays installed at synchronous generators below 10 MVA?
Do they measure the true rms value (that is, all harmonics) or are some harmonics filtered out?
How about the DC offset during a fault or transformer inrush? Is it included in the rms value?
Is there a general practice or does it depend on the manufacturer and his definitions?
Thanks.
Regards






RE: Overcurrent relay at generators: True RMS / DC ?
Basically any relay has a low pass analog filter.
Then simple over current relays are usually calculating the RMS value simply integrating in a discrete way (samples) the sinusoidal current wave. In that case the DC component and harmonics not cleared by the analog filter are part of the RMS calculation.
More complex relays (like most of the recent generator protections) are performing a DFT calculation. In that case the DFT works as a digital filter removing up to the harmonic number (nsamples/2). Greater harmonics should be removed by the analog filter. The DC component could be removed in a digital way subtracting to all samples the value equal to the difference of two samples at 180°.
Suggestion: read the relay manual looking for any clue about what the relay is doing
RE: Overcurrent relay at generators: True RMS / DC ?
Old electro-mechanical relays (instantaneous trip units) do respond nicely to dc offset.
Regarding fault current data - it is generally given in symmetrical current - this would not include any dc offset. Asymmetrical rms current would include the dc offset.
David Castor
www.cvoes.com
RE: Overcurrent relay at generators: True RMS / DC ?
Another question: are instantaneous pickup currents defined in Apeak or Arms?
If an instantaneous current relay should operate with a 0,04 sec time delay, I suppose the corresponding curent pickup value is defined in Arms, since the relay has 2 whole periods (50 Hz) to measure the rms value.
Thanks!
Regards
RE: Overcurrent relay at generators: True RMS / DC ?
Some relays give you the peak as an option if you want, it gives a faster time, but DC is then included
RE: Overcurrent relay at generators: True RMS / DC ?
I am still new to protection system, so don't judge too hard.
A 3-phase overcurrent relay at a generator normally monitors each phase and will pick up and subsequently trip the generator even if the current at ONE phase only exceeds the threshold, right? For example due to too high current at one phase when a transformer is being energized.
Thanks in advance
Regards
stason
RE: Overcurrent relay at generators: True RMS / DC ?
RE: Overcurrent relay at generators: True RMS / DC ?
a 51 is an inverse time OC element. What about the definite time OC element and instantaneous one?
RE: Overcurrent relay at generators: True RMS / DC ?
What's the question? They will trip on any one phase, two phases or three phases. Just like the 51 element.
RE: Overcurrent relay at generators: True RMS / DC ?
According to wikipedia:
# 50 - Instantaneous Overcurrent Relay
# 51 - AC Inverse Time Overcurrent Relay
so I asked if the definite time OC relay would also trip due to overcurrent at one phase only.
BTW, does the Def.time OC relay have an ANSI number as well or does it share 51 with the Inverse Time OC relay?
RE: Overcurrent relay at generators: True RMS / DC ?
If you want to learn about relays, look up manuals of a few relays of reputed manufacturers and not Wikipedia.
Rafiq Bulsara
http://www.srengineersct.com
RE: Overcurrent relay at generators: True RMS / DC ?
3.1.51 Device number 51—ac inverse time overcurrent relay
A device that functions when the ac input current exceeds a predetermined value, and in which the input
current and operating time are inversely related through a substantial portion of the performance range.
Note that EI, VI, STI, etc. are all types of inverse. Definite time is not.
RE: Overcurrent relay at generators: True RMS / DC ?
RE: Overcurrent relay at generators: True RMS / DC ?