T-305E User Manual Version 3.5 | 18
6.2
Impulse flashover method
This mode is used to locate the range of faults with low resistance and high DC leakage
current. The corresponding testing method is called the Impulse Flashover method. This
works by initially charging the capacitor bank and, when the voltage reaches a certain value,
the contactor operates connecting the capacitor to the faulty cable to break-down the fault.
The resulting current flow will trigger the Kehui T-906 connected to the internal current
coupler via the TRIGGER/SIGNAL port.
The Impulse Flashover method is suitable for most flashover faults. As with the DC method,
the waveform obtained by the DC Flashover method is relatively simple and easy to
interpret.
Operational procedure:
i.
Turn the
MODE
knob to the
PULSE
position.
ii.
Power on the T-305E, the green
STOP
button lights up.
iii.
Turn the voltage
ADJUST
knob to the zero position, which enables the
START
button.
iv.
Press the
START
button, which then lights-up and the green
STOP
light turns off. Inside
the instrument, the noise of the HV protective device can be heard.
v.
Turn the
ADJUST
knob slowly until the voltage rises to the chosen value, press the
PULSE
button to ascertain whether the voltage is sufficient to break-down the fault. If the fault
does not break-down, raise the voltage further and repeat the process until break-down
occurs.
vii.
The swing of the pointer of the kV meter will indicate the fault status. A small swing
indicates the fault has not broken down; if it is large, the operation is successful.
viii.
When the break-down voltage value has been found, check whether the T-906 has
triggered. If it has but the waveform is not satisfactory, the discharge voltage should be
increased and the process repeated until a suitable waveform is obtained.
ix.
Once the process is completed, discharge the cable and surge generator manually using
the discharge rod (see section 5.1 Using the discharge rod).
6.3
Impulse Current Method (ICM) Testing
In this case other suitable makes of TDR unit can be used, connected using a suitable current
sensor attached to the earth wire on the HV output. If the Kehui T-906 is used, an external
current sensor is not required, as it can be directly connected to the TRIGGER/SIGNAL port
using the Lemo cable supplied.
Operational procedure:
i.
Turn the
MODE
knob to the
PULSE
position.
ii.
Turn the
TEST METHOD
to
ICM
.
iii.
Select a suitable capacitor bank setting.
iv.
Power on the T-305E, the green
STOP
button lights up.
v.
Turn the voltage
ADJUST
knob to the zero position, which enables the
START
button.
vi.
Press the
START
button, which then lights-up and the green
STOP
light turns off. Inside
the instrument, the noise of the HV protective device can be heard.
