The capacitor discharge technique is performed with a surge generator, nicknamed “thumper.” This device converts line power into high voltage, unidirectional, impulses that are transmitted into a faulted power cable. A simplified circuit of a surge generator connected to a faulted cable is shown in Figure 1.
Capacitor C charges to the voltage of the power supply. When switch S1 closes, the capacitor C discharges into the cable under test in the form of a high voltage impulse. The red curve of Figure 2 depicts the way that time affects the voltage at which a cable fault, represented by a non-linear resistance and gap, will flash over.
The gap G in Figure 1 will behave in the manner depicted by the red curve of Figure 2. Applying incrementally higher voltages to the gap and plotting the time lag until it sparks over, develops the curve. The curve shows that:
- The higher the applied voltage, the shorter the time lags before flashover occurs.
- There is always a minimum time lag, as indicated by the ‘Minimum Breakdown Time,’ under which the gap will never flash over.
- There is a minimum voltage, shown by the ‘Minimum Breakdown Voltage,’ below which a gap will not flash over within a typical test period of several minutes.
The curve demonstrates that for breakdown to occur in any particular cable fault, the applied impulse must reach a particular voltage and must last for a definite period of time before a gap or fault will flash over. Figure 2 also shows three different fault-locating impulses applied to the gap. Note that the rise times are short and, after reaching a controlled peak, their amplitudes decay at varying rates.
Impulse (a) has sufficient voltage and duration to cross the bold curve at point A and cause flashover. Impulse (b) has sufficient peak voltage, but is too short in duration to intersect the bold curve and will not flash over; impulse (c) has the same peak amplitude as (b), but is long enough in duration to cross the bold curve at point C and achieve flashover. Although the shape of the flashover characteristic curve is typical of all such gaps, the actual curve will vary with each cable fault and will be unpredictable. To locate a fault with the capacitor discharge technique, the surge generator must be capable of generating an impulse of sufficient voltage and time duration to create flashover. Performance of a surge generator is based on its output voltage and energy capability. As mentioned earlier, when switch S1, of Figure 1, closes the capacitor C discharges into the cable under test in the form of a high voltage impulse. The larger the value of capacitor C, the greater the amount of available energy for discharge in to the fault and thus, a greater probability of sustaining the applied voltage and intersecting the characteristic flashover curve of Figure 2. A surge generator’s available energy can be calculated as follows:
Energy (Joules or Watt Seconds) = (C/2) V2
Where C is the capacitance in microfarads and V is the applied voltage in kilovolts.
The length of the cable will also affect the peak amplitude of the voltage that reaches the fault. Effects of cable length on an applied voltage can be calculated as follows:
VE = VA (CS / CS + CC)
VE = the effective voltage at the fault,
VA = the voltage applied to the cable,
CS = Capacitor value of the surge generator, and
CC = Capacitance of the cable under test.
The surge generator or thumper serves as a critical element in cable fault location. The device’s voltage and energy capability determine the efficiency at which faults can be broken down and located. When selecting a unit for purchase or rental, consideration must be given to the type of cable and overall length being tested. Protec Equipment Resources carries a variety of cable fault locating systems from top manufacturers like Megger, High Voltage Incorporated and Innovative Utility Products. Let our experienced staff assist in setting you up with the right tool for your next job.
Megger’s PFS series and IUP’s popular FaultWizard.
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Editor’s Note: This post was originally published April, 2012 and has been updated freshness, accuracy and comprehensiveness.