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3 Phase Feeder Capacitive Charging Current
2

3 Phase Feeder Capacitive Charging Current

3 Phase Feeder Capacitive Charging Current

(OP)
The normal line to ground capacitive charging current for one phase is the line to neutral voltage divided by the capacitive reactance in ohms. Let's assume that each phase has a current of one amp when the system is unfaulted ( Ia +Ib +Ic = 1). When one of the phases is grounded, the total system charging current is 3 times normal phase to ground current.

During a fault on one of the phases in a feeder, and the voltage on the other phases stepped up 1.73 times line to neutral, will a zero sequence current transformer/detector on the feeder see 1 amp or 3 amps charging current?

RE: 3 Phase Feeder Capacitive Charging Current

I would say 1.73A if each phase normally has a 1A charging current.

I don't follow the Ia+Ib+Ic=1 during normal operation, I would think Ia+Ib+Ic=0 because it is a balanced three phase capacitive load.  

Then when you get a fault your charging current on the high phases increases to 1.73 times the normal charging current.  (1.73X more voltage same capacitance.)  But this is unbalanced to ground, so I would think I would see 1.73A on the ground to balance the charging currents.

I had a false trip on a resistively grounded system with sensitive ground line differential protection.  The ground differential was set lower than this unbalanced charging current, so I bumped it up.  This was my best guess, but perhaps there is a large flaw in my reasoning, please feel free to point it out.

RE: 3 Phase Feeder Capacitive Charging Current

I think mrg probably meant Ia=Ib=Ic=1 for the unfaulted case.  
Are you considering here the currents in an unloaded feeder during a fault on another feeder connected to the same busbar?

RE: 3 Phase Feeder Capacitive Charging Current


One amp [for 480V] but all the experts say it needs to be empirically verified.  Just don’t pop any of those Fluke 1kV fuses—they are pricey.  Check for absence of neutral-shift voltage, then do the current measurement.

See: www.neiengineering.com/papers/paper1JN.pdf

§VI. DESIGN CONSIDERATIONS    …the resistor and equivalent capacitance are in parallel. Since the resistor is placed in the neutral of the transformer, the zero-sequence current will flow through it. As a result, that current makes the apparent resistance of the grounding resistor appear to be three times as large.

General references..
www.geindustrial.com/products/manuals/GEI-72116.pdf
www.sea.siemens.com/pde/product/Swbd_pdfs_3_29/LV_High_Resistance_Grounding.pdf
Chapter 6 of Beeman’s 1955 Industrial Power Systems Handbook
   

RE: 3 Phase Feeder Capacitive Charging Current

Comment on the previous posting paper. The medium voltage high resistance system grounding usually uses a single phase distribution transformer with the resistor and relaying on its secondary to avoid the medium voltage to propagate to the relaying circuits.

RE: 3 Phase Feeder Capacitive Charging Current

Suggestion: Draw a circuit diagram that would include for example:
1. Delta - Wye transformer
2. Downstream circuit breaker
3. Transmission line
4. Transmission line capacitances to ground
5. High-resistance distribution transformer in the transformer wye grounded neutral
6. Ground fault, e.g. in phase A downstream of the circuit breaker, or another circuit breaker, e.g. branch circuit circuit breaker.
Then, IFault=IR-jIC
Where
IC=IACN+IBCN+ICCN
IACN=0
IR will actually flow through the neutral and distribution transformer primary.

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