Transformer Riddle No.33 - Grounding resistance calculations
i want to know how we detrmine (ngr)value in power transformer protection,i have a cause in my substation the (ngr) of power transformer 100 MVA 220/33/11 kv y/y/d at 25% loading the load is only in 33kv side 11kv side is not yet used this (ngr) was burend after afault in 33kv feeder which  tripped in its time by earth fault relay,iwant to know what cause the (ngr) burend

#1
Wed, December 2nd, 2009 - 16:18
Resistance grounding solves the problem of transient over-voltages, thereby reducing equipment damage. Over voltages caused by intermittent (arcing) faults, can be held to phase-to-phase voltage by grounding the system neutral through a resistance which limits the ground current to a value equal to or greater than the capacitive charging current of the system. Thus the fault current can be limited, in order to prevent equipment damage.
In addition, limiting fault currents to predetermined maximum values permits the designer to selectively co-ordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault. The line-to-ground capacitance associated with system components determines the magnitude of zero-sequence charging current. The resistor must be sized to ensure that the ground fault current limit is greater than the system's total capacitance-to-ground charging current. If not, then transient over-voltages can occur. The charging current of a system can be calculated by summing the zero-sequence capacitance or determining capacitive reactance of all the cable and equipment connected to the system.

For prevention of resistor burning the Rating power of selected NGR shall be considered regarding the time and amount of NGR through earth fault currents.
IEEE-32 is the standard used for rating and testing neutral grounding resistors. The most important parameters to consider from IEEE-32 are: the allowable temperature rises of the element for different “on” times; the applied potential tests; the dielectric tests; and the resistance tolerance tests that are required.
10 second rating: is applied on NGR’s which are used with a protective relay to prevent damage to both the NGR and the protected equipment. The relay must clear the fault within 10 seconds.
1 minute rating: One NGR is often used to limit ground current on several outgoing feeders. This reduces equipment damage, limits voltage rise and improves voltage regulation.Since grounds could occur in rapid succession on different feeders, a 10-second rating is not satisfactory. The one-minute rating is applied.
10 minute rating: is used infrequently. Some engineers specify a 10 minute rating to provide an added margin of safety. There is, however, a corresponding increase in cost.

The standard practice for neutral grounding in industrial plants has been:
a) 600 volt and lower systems – solid grounding
b) 2.4 to 13.8 kV – low resistance grounding
c) above 13.8 kV – solid grounding

The following factors should be considered when rating neutral grounding resistors:
a) The capacitance-to-ground charging current of the circuit being protected. Rule of thumb is:
- On systems of 600 volts or lower, 0.5 amp per 1000 kVA of transformer capacity.
-On medium voltage systems, (above 1000 volts), 1.0 amp per 1000 kVA of transformer capacity.
b) The maximum ground fault current to be permitted on the system, after taking into consideration point a) above. This determines the amount of fault damage considered acceptable under ground fault conditions.
c) The importance of maintaining production in the presence of a single ground fault.
d) The type and characteristics of the sensing relays, fault clearing relays, and circuit isolating devices. Ground fault relays are generally selected to operate at less than 80% of the maximum current allowed by the grounding resistor. To provide maximum system protection with minimum system damage, the trend is to select lower current ratings.
e) Safety to operating personnel.

#2
Thu, December 3rd, 2009 - 13:28
thank you very much about the information, also i want to add somethnig ,the 33kv feeder which tripped is empty feeder onely charging current with along distance,also the value of the resistance was designed to 50 MVA Y/Y/D transformer which hasd been replaced by 100 MVA transformer after uprating the substation and they didnot change the NGR.

#3
Thu, December 3rd, 2009 - 15:21
Grounding resistor rating ampere calculation criteria shall be considered base on equipment insulation protection (Ig >3Ic0) for high resistance NGR selection and power system stability (Refer to Machine Riddle No.5) for low impedance NGR selection. But about your problem regarding burning of NGR due to earth fault current, obviously there is not any coordination between your relaying protection system and current time withstand of grounding devices.
It seems the specified NGR to previous power transformer was selected regarding power system stability consideration, so in this case, perhaps can be said in rough role: you can select a NGR with rated current two times ( 100MVA/50MVA) of previous NGR current. For time withstand selection of NGR (10sec., 1 min., 10 min.,), I said some selection criteria in above post.