51G protection
51G protection
(OP)
Hello,
I am working in a facility trying to refurbish a fan/motor control center. I noticed that regardless of the load(50A,150A,225A,400A) the ct selected has been 200:5A for the 51G protection for each feeder. I was told this is standard practice.
What are the pit-falls in doing so (if any)? My concern is the ct primary being smaller than the FLC for the application.
Regards,
TULUM
I am working in a facility trying to refurbish a fan/motor control center. I noticed that regardless of the load(50A,150A,225A,400A) the ct selected has been 200:5A for the 51G protection for each feeder. I was told this is standard practice.
What are the pit-falls in doing so (if any)? My concern is the ct primary being smaller than the FLC for the application.
Regards,
TULUM






RE: 51G protection
The load current is not really an issue, since the CT will only see the imbalanced current resulting from a line-to-ground fault.
The CT ratio depends on several factors including the type of system grounding (solid or resistance) and the type of relay being used.
In most cases, there is nothing downstream of this relay to coordinate with, so the ground relay can be quite sensitive. In fact, many people use a 50G relay - instantaneous trip with 50/5 zero sequence CT.
One concern can be saturation of a low ratio CT on solidly-grounded system. If you are trying to selectively coordinate with something downstream this could be an issue. When an instantaneous relay is used, the thinking is that the relay will still operate even if the CT saturates.
RE: 51G protection
RE: 51G protection
Impedance grounded.. and end of line (no downstream protection to coordinate with)
I think I need to "edumacate" myself a bit more on the subject.
As always, insightful and appreciated.
TULUM
RE: 51G protection
Good stuff on CTs and motors.
RE: 51G protection
With low impedance grounding, the 200:5 CT should be no problem - the only problem might making sure you have enough ground fault current to detect and relay reliably.
If it's high impedance grounding, the fault current will probably be too low for this setup to work reliably.
RE: 51G protection
Thanks again. The NGR is a 10A, 347V resistor (on the secondary of a 1500kva 4160v-600v dy transformer)
Regards,
TULUM
RE: 51G protection
One paper on the subject is: John P. Nelson, Pankaj K. Sen, High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999 at www.neiengineering.com/papers/paper1JN.pdf
RE: 51G protection
Is the resistor rated for 10A continuously?
RE: 51G protection
The resistor is continuosly rated. The protective device is a startco SE-105 (on feeders) and an se-325 on the NGR itself. I wish I could add more... I installed and worked on many of these, but I am not to familiar with the engineering behind them.
I understand the basics behind the zero sequence current ct (this is what the feeder ct's are), but what is a zero sequence voltage detection scheme?
Regards,
TULUM
RE: 51G protection
A limitation of zero-sequence overcurrent in low-voltage, high-resistance-grounded systems is that it can be hard to coordinate with various levels of overcurrent protection—given the comparatively low currents during faults. Negative-sequence voltage, instead of a CT in the grounding-resistor lead, typcially uses a ø-ø rated VT in parallel with the resistor.
The VT serves a low-pickup, high-continuous-withstand voltage relay, often restrained for third-harmonic voltage. However sensitive, it is more of a non-directional “all-or-none” method of fault detection or indication.
RE: 51G protection
Negative-sequence overvoltage relays are typically referred to as ‘59G’ or ‘59N’ devices. Single-function examples with inverse-time characteristics are Baser BE1-59N [www.basler.com/html/pcs59-87.htm#BE1-59N] or General Electric IAV-51D [www.geindustrial.com/products/brochures/iav.pdf]
RE: 51G protection
RE: 51G protection
Thank you, David. I goofed.
RE: 51G protection
The zero-sequence voltage detection is just three PTs - grounded wye on the primary and broken delta on the secondary. You put a voltage relay (in parallel with a resistor to prevent ferro-resonance) across one corner of the secondary delta. The voltage that shows up there is 3Vo, so it is good indicator of a ground fault. There can be some harmonic voltages present, so the 59G relay often is equipped with some filtering.
Blackburn's Protective Relaying or the old Westinghouse Applied Protective Relaying books have better explanations.
RE: 51G protection
Regards,
TULUM