Detection of capacitor bank fuse/switch failure
A method for detecting the operating condition of each phase of a three-phase capacitor bank in a power distribution system, including the steps of, if the RMS neutral current is above the bank status closed setting, comparing the phase angle of neutral current (IN) with respect to voltage on one of the phases (VA, VB or VC) and sending an alarm signal when the comparison of the phase angle of neutral current with respect to the phase voltage differs appreciably from the expected phase angles.
1. A method for detection of the operating condition of each phase of a three-phase capacitor bank in a power distribution system, comprising the steps of:
measuring one of the phase voltages (VA, VB or VC) and the neutral current in the capacitor bank;
comparing a RMS neutral current with all open (bank status open) and all closed (bank status closed) settings to determine if all the three phases of the capacitor bank are open or closed;
if the RMS neutral current is above the bank status closed setting, comparing the phase angle of neutral current (IN) with respect to voltage on phase A (VA), or phase B (VB) or phase C (VC);
sending an alarm signal when the comparison of the phase angle of neutral current (IN) with respect to the phase voltage (if VA is used add 0° , if VB is used add 120° or for VC add) 240° differs appreciably from the phase angles indicated in the following chart:
Phase angle of I N with respect
to VA (If VB is used add
OPEN/CLOSE state of each Phase
120° or for VC add 240° to
A
B
C
the angles shown below)
OPEN
OPEN
OPEN
N/A
CLOSE
OPEN
OPEN
90°
OPEN
CLOSE
OPEN
−30°
OPEN
OPEN
CLOSE
210°
CLOSE
CLOSE
OPEN
30°
OPEN
CLOSE
CLOSE
270°
CLOSE
OPEN
CLOSE
150°
CLOSE
CLOSE
CLOSE
N/A.
2. The method as set forth in claim 1 , wherein the appreciable difference compared between the phase angle of neutral (IN) with respect to the phase voltage comprises approximately the phase angle indicated in the chart ±10°.
3. A method as set forth in claim 1 , wherein measuring one of the three phase voltages in phases (VA), (VB), (VC) and calculating neutral current comprises as follows:
neutral current ( I NE ) =magnitude of ( I A _ cap * a +I B _ cap * b+I C _ cap * c )
Where
a=1 if phase A is closed else a=0
b=1 if phase B is closed else b=0
c=1 if phase C is closed else c=0
I A _ cap =(1000* Q CAP )/(3* V A )
I B _ Cap =(1000* Q CAP )/(3* V B )
I C _ cap =(1000* Q CAP )/(3* V C )
Q CAP =“3 Phase Capacitor Bank Size” in KVArs
V A , V B and V C are phase to ground voltage phasors and I A cap , I B cap and I C cap are calculated Capacitor Bank current phasors.
4. The method as in claim 3 , further including the step of detecting a high resistance contact of a switch wherein the neutral current is less than an undercurrent pickup setting.
5. The method as in claim 3 , further including the step of detecting capacitor bank short circuit wherein the neutral current is greater than a neutral overcurrent setting.
6. An apparatus for detection of the operating condition of each phase of a three-phase capacitor bank in a power distribution system, comprising in combination:
a voltage meter for measuring one of the phase voltages (VA, VB or VC) and a current meter for measuring the neutral current in the capacitor bank;
a controller for comparing a RMS neutral current with all open (bank status open) and all closed (bank status closed) settings to determine if all the three phases of the capacitor bank are open or closed and if the RMS neutral current is above the bank status closed setting, then opening or closing each phases when required while comparing the phase angle of neutral current (IN) with respect to voltage on phase A (VA), or phase B (VB) or phase C (VC) and when the comparison of the phase angle of neutral current (IN) with respect to the phase voltage (if VA is used add 0°, if VB is used add 120° or if VC is used add 240°) differs appreciably from the phase angles indicated in the following chart:
Phase angle of I N with respect
to VA (If VB is used add
OPEN/CLOSE state of each Phase
120° or for VC add 240° to
A
B
C
the angles shown below)
OPEN
OPEN
OPEN
N/A
CLOSE
OPEN
OPEN
90°
OPEN
CLOSE
OPEN
−30°
OPEN
OPEN
CLOSE
210°
CLOSE
CLOSE
OPEN
30°
OPEN
CLOSE
CLOSE
270°
CLOSE
OPEN
CLOSE
150°
CLOSE
CLOSE
CLOSE
N/A.