High Voltage Power Supply
The high voltage power supply employed is a 260 kV, 83A variable frequency resonant test set (RTS) that complies with IEC standards 60840 and 62067 and operates within the frequency range 20-300 Hz. A schematic of the test setup is illustrated in Figure 2. As can be seen, a blocking impedance is placed between the power supply and the high voltage connection to the cable under test with two goals in mind: firstly, the blocking impedance protects the RTS in the unlikely event of cable failure; secondly, it effectively filters any high frequency noise originating from the RTS and improves the signal-to-noise ratio of the power supply when performing PD measurements.
A capacitive voltage divider provides for a voltage reference for the control unit of the power supply. The common point grounding of the entire test circuit is connected to station ground. A 6 inch (15 cm) wide copper foil provides a high frequency ground path while a stranded, insulated aluminium conductor positioned directly on top of the copper sheath constitutes the power frequency ground. Signal coupling is provided by attaching a High Frequency Current Transformer (HFCT) sensor around the ground link from the cable joint toward the link-box. High frequency currents induced as a result of any partial discharge activity in the joint or in the cable section will be coupled to the HFCT sensor and measured by a conventional partial discharge monitor.
The commercially available partial discharge monitor used has a 350 kHz to 800 MHz bandwidth and measures the amplitude (in mV) as well as phase angle of any signal detected. A pulse count rate for various categories of magnitudes and phase-angles is also generated. The phase angle reference is provided by a low-frequency winding embedded in the HFCT sensor.
While IEC 60840 and 62067 provide basic guidance on the waveform, frequency and prescribed voltage to be employed during the overvoltage test, there is still no standard prescribed procedure for PD measurements. As such, there is some variation in the measurement procedures employed by various service providers in this field.
The measurement protocol followed in the tests described here consists of the following:
1. Upon tuning the high voltage power supply to the appropriate resonant frequency, a relatively low voltage (on the order of 30 to 40 kV) is applied for two minutes during which various diagnostic parameters are checked to ensure that the system is functioning properly.
2. Voltage is increased to the nominal line-to-ground potential (U0) and held for a further two minutes while diagnostic parameters are confirmed as normal.
3. The voltage is raised to the prescribed level specified in the IEC standards for a period of one hour.
In order to enable performing PD measurements on all accessories during the limited 1-hour hi-pot test, PD detectors can be installed at each of the accessories in the circuit. Signals from these devices are then fed back to a remote test operator for display and analysis. This approach requires the availability of a communication path between the individual joints and the location of the test operator – something that is now increasingly possible as more and more installations of HV cable incorporate fiber communication links into the initial work. This approach, while expensive, has significant advantages in that each sensor point can be observed simultaneously in real-time.
Where such communication networks are not available, the PD must be recorded sequentially at each individual accessory. For significant cable lengths, the time required to carry out these measurements usually exceeds the one-hour hi-pot test duration. In these cases, the PD levels at the maximum possible number of joints are recorded during the one-hour hi-pot test, while the remaining accessories are PD tested at an applied voltage of between Uo and the specified one-hour hi-pot level. This level must be agreed to by the parties involved and has usually been in the range of 1.2U0.