Henning Oetjen - Product manager cable products
Partial discharge (PD) measurements are increasingly used as a reliable and non-destructive diagnostic method for detecting weak spots in the insulation of underground cables. Partial discharge measurements are also routinely used in laboratories for testing cable reels prior to commissioning and in the field to verify installation quality.
Typically, many factory-testing standards require the use of 50/60 Hz high-voltage power supply when performing laboratory tests. However, the use of 50/60 Hz supplies has proven to be impractical when it comes to field-testing, due to high energy generation requirements.
The most important factor to consider when choosing an alternative test frequency is that the partial discharge characteristics at the new frequency must be similar to those at 50/60 Hz, otherwise the results cannot be reliably interpreted. This is especially true when measuring partial discharge inception voltage (PDIV), the voltage at which partial discharge first occurs.
Partial discharge inception voltage is one of the most important parameters used to characterise partial discharge. If the PDIV measurement at the new frequency is higher than at 50/60 Hz, it may create false negatives, making problems appear non-critical when they could in fact be critical at the operating voltage.
Many research papers have addressed the comparability of partial discharge characteristics at various test frequencies and with various wave shapes. This article provides a quick overview of the most commonly used test wave shapes.
0.1 Hz sinusoidal
The very low frequency (VLF) sinusoidal wave shape was introduced for partial discharge testing in the 1990s. In a scholarly paper, “Applied Voltage Frequency Dependence of Partial Discharges in Electrical Trees”, researchers reported that PD is frequency dependent and diminishes at low frequencies. It is, therefore, challenging to measure partial discharge at low frequencies such as 0.1 Hz.
A Megger research paper entitled, “Influence of the Test Voltage Wave Shape on the PD Characteristics of Typical Defects in Medium-Voltage Cable Accessories” showed a greater than 300% difference when interfacial discharge was measured at 50 Hz compared to 0.1 Hz. Additionally, the authors of the paper conducted extensive literature research on previous publications comparing PDIV measurements at 50 and 0.1 Hz. Seven papers reported a difference between the two values that ranged from 10 to 250 percent.
This huge discrepancy is due to the characteristics of interfacial discharge. Most interfacial discharges in cable systems occur at the terminations and in splices, and are very dependent on the voltage gradient. A change in voltage gradient could make the discharge 500 times smaller at 0.1 Hz compared to 50 Hz, which is a critical factor to consider when making measurements with a VLF sinusoidal test voltage.
Damped alternating current
Over the past 10 years, the damped alternating current (DAC) technique has been established as a very effective method for partial discharge testing. This method is one of the voltage shapes listed for PD testing in IEEE 400.3: “Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field Environment”.
Electric utilities have collected numerous examples of successful field test data that show a very strong correlation between 50/60 Hz and DAC results. This correlation prompted a broad comparative study of commercially available medium-voltage cable diagnostic systems by Centro Elettrotecnico Sperimentale Italiano Giacinto Motta (CESI), an Italian company that provides testing and certification services, energy consultancy, engineering and technology consulting for the power sector globally.
Table 1 shows the different voltage shapes compared in the study. Testing was performed on five cables. Three parameters – the partial discharge inception voltages, the location of partial discharge spots and the PD pulse amplitudes – were selected as the comparison criteria. Figure 1 shows an excerpt from the results. Overall, the damped alternating current method proved very similar and the most comparable to 50-Hz testing while 0.1 Hz sinusoidal showed the largest deviation.
Figure 1: Comparison of PDIV at 50 Hz with each test system. Overall, the DAC method (green) was most closely comparable with 50/60 Hz testing and the 0.1 Hz sinusoidal (red) showed the largest deviation.
Table 1: Test voltage wave shapes used in a study carried out by engineering company CESI
0.1 Hz cosine rectangular
The first very low frequency systems used a cosine rectangular (CR) wave, which proved very effective, and is still widely used because the time interval of its polarity change replicates that of a 50/60 Hz wave. Figure 2 shows the characteristic shape of the 0.1 Hz CR wave compared to the damped alternating current and the 0.1 Hz sine wave.
In 2003, German author and scientist Daniel Pepper performed in-depth research to evaluate merits of using a triangle voltage shape and a very low frequency cosine rectangular voltage shape as voltage sources for partial discharge testing on solid dielectric power cables. Both wave shapes performed well for this purpose; however, the cosine rectangular showed higher partial discharge levels, especially for sliding discharges.
Figure 2: Comparison of DAC, 0.1 Hz sine wave and 0.1 Hz CR wave shapes. The time taken for the polarity reversal with the VLF CR wave shape closely matches that of the DAC but its peak voltage is maintained for five seconds until the next cycle in the VLF CR system.
Test wave generation
VLF Cosine Rectangular Voltage
The VLF cosine rectangular voltage is generated by a circuit as shown in Figure 3. One of the most significant advantages of the CR technology is its ability to store and recover 90 percent of the energy within the charged cable via the choke. The stored energy is used to charge the cable with the opposite polarity during the next half cycle, within the same millisecond interval of the 50/60 Hz operating frequency. This allows substantially higher test loads to be driven with fairly small input power compared to sinusoidal VLF systems. VLF cosine rectangular systems with up to 25 μF and 20 to 80 kVrms are commercially available.
Figure 3: Block diagram of a VLF CR unit
Damped AC Voltage
The circuit used for generating a DAC voltage is fundamentally similar to the one used for generating a VLF cosine rectangular voltage. The only difference is how switch “S” operates. In the VLF cosine rectangular system, the switch reverses its position to allow polarity reversal. In the DAC system, this switch closes after allowing the cable to be charged to the test voltage, creating a damped resonance (fixed) circuit. The resonant frequency of the circuit is a function of the inductance of the choke, the capacitance of the auxiliary capacitor and the capacitance of the cable to be tested.
The two main advantages of very low frequency cosine rectangular technology are its energy efficiency due to its resonant design, and the polarity reversal time interval on the VLF cosine rectangular very closely matching the one at 50/60 Hz, so that it mimics the electrical stress on the insulation under operating conditions. This close time matching makes the technology an excellent candidate for use as a power supply in offline partial discharge testing.
The same two characteristics make VLF cosine rectangular technology a very effective tool for withstand testing (with or without partial discharge monitoring), enabling the testing of very long cables or simultaneous testing of three phases at 0.1 Hz. In contrast, damped alternating current technology is not ideal for withstand testing because it requires a substantial number of test cycles to generate an equivalent amount of electrical stress for the same duration. This shorter exposure time to the electrical stress at power frequency is exactly what makes DAC perfect for truly non-destructive partial discharge diagnosis.
Given the advantages of the VLF cosine rectangular wave shape, its performance as a voltage source for offline partial discharge testing was evaluated. In this study, partial discharge inception voltage and partial discharge levels were measured at the operating voltage (U0) on service-aged cross-linked polyethylene (XLPE) and paper-insulated lead covered (PILC) mixed cables using both VLF cosine rectangular and DAC methods. The damped alternating current method was chosen for comparison instead of 50/60 Hz because DAC results have already been established by numerous studies as being highly correlated with 50/60 Hz results. Table 2 summarizes the test parameters for each of the three tests.
Table 2: Summary of test parameters
Figure 4 shows the on-site test set up for partial discharge measurements. As discussed previously, both the DAC and VLF cosine rectangular test voltages can be generated with the same circuit by controlling switching behaviour. The measurements were performed with conventional coupling and without any hardware or software noise filtering.
Figure 4: Set up for partial discharge test
In summary, both methods produced very similar partial discharge inception voltage (PDIV) and maximum partial discharge (PDmax) values (refer to Tables 3, 4, and 5) with acceptable statistical fluctuations. Both methods were able to identify the same weak spots in all three cables.
PDmax values were generally slightly higher with VLF cosine rectangular. The VLF cosine rectangular waveform consists of a millisecond polarity reversal, followed by a five-second plateau of the peak voltage before the next cycle. This plateau phase most likely causes an accumulation of charges at the layered interfaces of the cable, resulting in higher PDmax values. Furthermore, this phenomenon might explain why the VLF cosine rectangular allows weak spots on longer cables to be located more easily.
Table 3: Comparison of PDIV and PDmax for DAC and VLF CR (Test 1)
Table 4: Comparison of PDIV and PDmax for DAC and VLF CR (Test 2)
Table 5: Comparison of PDIV and PDmax for DAC and VLF CR (Test 3)
Partial discharge measurements using VLF cosine rectangular test waves were benchmarked against the well-established damped alternating current method. The results showed that PDIV, PDmax, and the location of weak spots obtained by the VLF cosine rectangular method were highly comparable to the DAC method. This proves that the VLF cosine rectangular wave shape is a comparable and convenient voltage source for partial discharge testing in the field.
The similarity in the design of the VLF cosine rectangular and DAC voltage generation circuit means that it is possible for VLF cosine rectangular units to also generate a DAC voltage. The integration of both technologies offers users the flexibility of a single unit that can perform withstand testing, partial discharge monitored withstand testing, and non-destructive PD diagnostics with damped alternating current.
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