Generator neutral grounding - Reactance type 3

[RRaghunath]

I am aware of high impedance grounding as most popular and current practice for generator systems in power plants. I only read about reactance grounding and resonance grounding in text books.
Today I came across a 23MW (GE-Frame V), 11kV generator that has its neutral grounded through a reactor limiting the earth fault current to a max. of 400A. The generator is connected to a 11kV SWB and directly feeds loads.

Have any one come across such practice?

Thanks in anticipation.

 

[xabproject]

Dear RRN,
All the 3 nos 30MW Generators (generating at 11kV) of CPP, Khammam Chemical Refineries is the one installation that comes to my mind immediately. E/F Current limited to 20A(!?)
Best regards

 

[RRaghunath]

Xabproject,

Thanks for your post. Could you please tell me what is the earth fault protection provided - method, type and make of relay, setting etc.

 

[busbar]

The purpose of this grounding scheme is to provide an extremely sensitive means of detecting phase-to-ground faults on the generator voltage system and to limit the fault current to a very low value so that iron burning associated with generator insulation faults to ground is minimized. Excerpt from IEEE C62.92.2-1989—Grounding of Synchronous Generator Systems.

 

[jwerthman]

Reactance grounding is commonly used where the generator must supply unbalanced line-to-neutral load. Resistance grounding cannot be used in this application because the resistor losses would be prohibitive and the resistor would run quite hot due to the continuous neutral current.

 

[jbartos]

Suggestion: A type of system grounding is selected according to results of grounding calculations. If the system grounding calculations reveal the ground fault charging current 5A to 10A flowing through the ground back to the neutral of generator or transformer, the high-resistance system grounding is applicable. If the ground fault charging current is higher than 10A (approximately), then the medium-resistance system grounding is applied. Also, low-resistance system grounding may be applied. Reactance system grounding is also possible. See, IEEE Std 142-1991 (Green Book), section 1.4.4 Reactance Grounding. The term reactance system grounding describes the case in which a reactor is connected between the system neutral and ground. Since the ground-fault that may flow in a reactance-grounded system is a function of the neutral reactance, the magnitude of the ground-fault current is often used as a criterion for describing the degree of grounding. In a reactance-grounded system, the available ground-fault current should be at least 25% and preferably 60% of the three-phase fault current to prevent serious transient overvoltages (X0<=10X1). This is considerably higher than the level of fault current desirable in resistance-grounded system, and therefore reactance grounding is usually not considered an alternative to resistance grounding. In most generators, solid grounding, that is, grounding without external impedance, may permit the maximum ground-fault current from the generator to exceed the maximum three-phase fault current that the generator can deliver and for which its windings are braced. Consequently, neutral grounded generators should be grounded through a low-value reactor that will limit the ground-fault current to a value no greater than the generator three-phase fault current. In the case of three-phase four-wire systems, the limitation of ground-fault current to 100% of the three-phase fault current is usually practical without interfering with normal four-wire operation. In practice, reactance grounding is generally used only in this case and to ground substation transformers with similar characteristics.