Cable testing and diagnostics on medium-voltage cables with VLF sine voltage sources

 

Author: Martin Jenny, Product Manager at BAUR Prüf- und Messtechnik GmbH, Sulz (Austria) 

 

Evaluating the condition of cable systems helps medium-voltage network operators with efficient network planning and maintenance. Cable diagnostics can be made cost-efficient and easy to integrate into day-to-day operations by deploying measurement technology that uses a VLF sine voltage source (Very Low Frequency). This source can be used for cable testing and diagnostics. It weighs less and delivers reliable results.

 

Diagnostic measurements are necessary for utilising the condition evaluation of cable systems for better investment and maintenance planning. For example, such measurements provide information on the aging behaviour or on hidden faults. Mains operators that would like to carry out diagnostics in addition to power-on tests (cable testing) are therefore concerned about how to achieve significant results while keeping time and cost outlays as low as possible. Among other things, the significance of the measurement results depends on the voltage source of the testing and measuring device. Various sources are available on the market, such as 50 Hz resonance systems, VLF sine 0.1 Hz (VLF = Very Low Frequency), DAC (DAC = Damped AC) and VLF Cos-Rect (Cosine Rectangle or also called 50 Hz slope). This article describes why a test with the VLF sine 0.1 Hz presents the best solution for daily tasks.

 
An essential aspect is that a voltage source with VLF sine 0.1 Hz is not only suitable for cable testing, but also delivers good results during diagnostic measurements. The tan delta measurement (also called dissipation factor measurement) and the partial discharge measurement (called PD test below) are relevant here, see Text Box. An overview of the properties of various voltage sources clarifies this.

 
Characteristics of various voltage sources

 
Essential requirements for the voltage source include:

• Suitable for cable tests / withstand voltage tests
• High measurement accuracy during dissipation factor measurement
• Significant results during the PD test (inception and extinction voltage, PD level and phase-resolved PD pattern), and good localisation of the PD
• High reproducibility of results to guarantee the comparability of staggered measurements and various cable routes in the network
• Possibility of applying different methods in parallel, i.e. combine and save time
• Light in weight, easy to handle, easy connection, easy operation, short test duration

 

Table 1 shows a comparison of the voltage sources in relation to the various requirements. With reference to the withstand voltage, almost all standard voltage sources in the market have been found suitable in theory and in practice. However, the VLF 0.1 Hz sine voltage is the only voltage that is suitable both for measuring partial discharges and for the dissipation factor measurement (tan delta). Note that it depends on the voltage shape: To achieve reliable results independent of the cable route, an ideal sinusoid (with constant frequency) is an advantage. For the measurements carried out with an ideal sine, users can compare the measurement results of various cable routes or joint types.

 

Table 1: Comparison of various voltage shapes with regard to different practical requirements

Requirement	VLF Sine	VLF Cos-Rect	50 Hz resonance systems	DAC
Withstand voltage test in compliance with IEC, VDE (CENELEC), IEEE	Yes	Yes	Yes	Yes, IEEE standard in preparation
Load-dependent test signal	Yes	Swing phase varies in the region of 30-250 Hz acc. to IEEE400.2 [7], reload phase varies depending on load	Test frequency depends on cable length	Test frequency depends on cable length
Tan delta measurement accuracy	High (1*10E-4)	Unsuitable for tan delta	High	Medium
Tan delta sensitivity / comparability	High	Unsuitable for tan delta	Medium, sensitivity less than with VLF	Medium, load-dependent
PD localisation possible	Yes	Yes	Yes	Yes
PD level and PD pattern comparable with measurement at 50 Hz	Yes	Not yet studied in sufficient detail	Yes	Yes
PD inception voltage comparable with measurement at 50 Hz	Yes	Not yet studied in sufficient detail	Yes	Yes
Compact voltage source	Yes	Yes	No	Yes

 

With regard to the measurement of the dissipation factor, it is apparent that a VLF sine measurement has an edge even over the 50 Hz measurement owing to the high precision and sensitivity. At a low frequency of 0.1 Hz, the tan delta values for PE-insulated cables are higher, which enables better detection of an increase in the tan delta. Due to its positive properties, the 0.1 Hz sine has already been considered in some standards (IEEE 400.2-2013) where both test level and limit values for different regions are available.

 

The suitability of various voltage sources for the partial discharge measurement has already been discussed in different scientific publications. Most publications discussed the comparability of the measurement results with those tested at operating frequency (50 and 60 Hz). To sum up, the following result can be derived from publications [1] to [6]:

A VLF Cos-Rect voltage source produced about 5.5 times the PD level (approx. 5,500 pC) during comparison of measurements with 2 x U0 on six worn joints as opposed to the measurement at 50 Hz and VLF sine [6]. The measurement with 50 Hz and 0.1 Hz sine showed almost identical levels. The higher measured values with the VLF Cos-Rect voltage source present a higher load for worn joints. In addition, on comparing a sine and rectangle voltage source [6], it was determined that the waveform of the test voltage has a greater impact than the increase in the level from 2 x U0 to 3 x U0.

 

With reference to the PD tests with VLF sine, all mentioned publications state that the PD inception voltage can be compared with the voltage of the 50 Hz measurement when tests were conducted on field objects (i.e. not artificially prepared objects/laboratory structures). In artificially created faults, the inception voltage during the VLF test and the 50 Hz test sometimes differed from each other. Therefore, artificial faults and test bodies created in the laboratory are not suitable for selecting the optimum voltage source for field use [4].

 

With regard to the PD level and the PD pattern (distribution of measured values), the listed publications similarly showed that results with VLF sine 0.1 Hz are comparable with the results of 50 Hz measurements. This applies for worn joints in plug-in or heat shrinking technology. There were no relevant differences in the location of partial discharges.

 

A comparison of methods on four cable routes with a total of 42 different fault positions between VLF sine 0.1 Hz, a 50 Hz resonance system, a 20-400 Hz resonance system and DAC showed that no single technology seems distinctly better than the other during the tests on various medium-voltage cables [2]. We were unable to prove a clear correlation between the PD magnitude and the inception voltage and the voltage source during the described study. While choosing from the examined voltage sources, the user should preferably consider practical criteria such as accomplishment of tasks, weight, versatility and easy handling.

 

Implications for practical use

 

For practical use, other aspects besides measurement accuracy and reliability must be taken into consideration. The following aspects are important during use in the field:

 

• Easy transport and easy connection of measurement technology
• Staff expenses, training
• Time required for the connection
• Time required for the measurement
• Cost-benefit ratio
• Relevance of measurement results for future maintenance

 

Under these aspects, the VLF voltage sources can register low weight and its compactness as plus points compared to a 50 Hz voltage source. Furthermore, as the VLF sine source is considered an option for cable testing and diagnostic measurements (tan delta and PD), network engineers can perform all relevant measurements on new and old cables with only one voltage source. In comparison to conducting different measurements/tests with different voltage sources, using a single VLF sine voltage source definitely saves time, as less time is spent establishing connections. By working with one universal voltage source, it is also possible to apply test and measurement methods in parallel, e.g. during the Monitored Withstand Test (MWT). By Monitored Withstand Test (MWT), one means the partly simultaneous implementation of cable testing and cable diagnostics with tan delta methods. As the test engineer needs to connect only one device for the MWT and then start a related workflow, it takes little additional time to perform the usual test after installing a new cable or repairing a cable route and simultaneously determine the cable condition.

 

A combination of testing and diagnostic measurement - the MWT - offers the following benefits:

 

• Easy test setup, easy sequence (no additional connections and no introduction to MWT required)  Reduction of test duration (e.g. from 60 min to 15 min) if the cable is in good condition
• No cable overload
• Result evaluation in real time
• Easy interpretation of cable condition with smiley symbol on display
• Precise results on cable condition

 

Collecting additional information

 

Information from both the integral dissipation factor measurement and the local partial discharge measurement complement and/or confirm one another and give network and maintenance engineers more/better criteria for evaluating their assets.

 

The following example shows why it is important to keep the option of a dissipation factor measurement and a partial discharge measurement open. Typically, defective joints, e.g. defective fittings or those with electrically conducting faults, can be detected with the PD test (see Table 2). However, this is not the case with moist joints. In this example (see Figure 1 and 2 as well as Table 3), during tests in a network in Hong Kong, the tan delta measurement delivered pertinent information. The tan delta standard deviation on conductor 2, identified in Fig. 1, suggests a moist joint, as the PD test cannot detect any PD activity (too much moisture). The MWT, i.e. the combination of cable testing and tan delta measurement over 15 minutes, caused the joint to dry out and the tan delta to drop significantly (Fig. 2). This reinforced the suspicion of moisture being present.

Standard deviation	Result	Measurements required	Measures required	Comment
< 0.010	Cable in good condition Water trees Only low PD	• tan delta • PD	None, as in good condition	Standard deviation tan delta low No PD, no high PD
0.010 to 0.080	Water trees and PD Only PD	• Tan delta • PD	Moderate ageing with regard to water trees PD concentration must be analysed	Moderate water trees – no immediate measures Replace joints if there is PD concentration
0.080 to 0.500	Water penetration in joints	Tan delta PD may show no high values	Only the tan delta shows the effect PD values suppressed due to water penetration, PD cannot be used as criterion	Sheath fault location can indicate the location of moist joints because leakage currents occur there Joints with indication of low PD must be investigated (in spite of low PD values)
> 0.500	High	Unsuitable for tan delta	Medium, sensitivity less than with VLF	Medium, load-dependent
Del	Del	Del	Del	Del
Del	Del	Del	Del	Del
Del	Del	Del	Del	Del
Del	Del	Del	Del	Del

Table 2: Example guideline for interpreting the standard deviation of tan delta

STDTD	0.5*U0 (kV)//3.5	U0 (kV)//6.5	1.5*U0 (kV)//10
L1	0.068	0.036	0.060
L2	4.453	2.313	9.343
L3	0.063	0.004	0.050

Table 3: Measured values for Figure 1

 

Voltage source for daily tasks

 
About ten years ago, Elektrizitätswerk Mittelbaden Netzbetriebsgesellschaft mbH (or E-Werk Mittelbaden for short) compared a VLF 0.1 Hz sine and a 50 Hz method for the PD test based on over 40 cable routes. As the 50 Hz method produced too many highly varying evaluations and in particular more negative prognoses at the time, which even today have not been considered failures, the company decided to use the VLF 0.1 Hz sine method. In the meantime, the VLF measurement with sine voltage has proved itself in hundreds of tests. This was evident in the diagnostic tests performed on 240 kilometres with 500 partial routes in the 20 kV network of E-Werk Mittelbaden on paper pulp insulated and mixed cable routes.

 

At E-Werk Mittelbaden, such cable routes have been diagnosed with a VLF sine 0.1 Hz by means of PD and, for about seven years, with the tan delta measurement as well. According to Werner Brucker, Director of Network Operations, using both these diagnostic procedures produces a good overall view of the ageing and condition of the network. Any partial routes classified as hazardous are then promptly replaced. Isolating defective partial sections results in huge savings, as entire cable runs do not have to be replaced.

 

In practice, VLF measurement has proved to be suitable during the commissioning tests of new or modified cable systems, for locating faults precisely and for using a simultaneous PD test in the future for detecting faults in the fittings so that the work involved in troubleshooting, such as installation faults, or during maintenance, e.g. excavation work, can be kept to a minimum.

 
Brucker emphasises the weight and suitability for daily use as an essential advantage of the VLF sine voltage source. The 0.1 Hz technology can be transported and operated by any employee, which would certainly not be possible with a 50 Hz system.

 
The use of a cable test van with two persons is rarely required, as the portable measurement and testing device is sufficient for most cable lengths. A cable test van is therefore only necessary during every seventh measurement or so.

 
E-Werk Mittelbaden has a clear cost advantage by applying the VLF 0.1 Hz technology, as measurements are easily performed by a single employee in a short time. Due to the short connection time and test duration as well as the need for fewer people, several cable routes can easily be tested per year. Routes or partial sections identified as critical during the measurements are scheduled for repair or replacement. Maintenance budgets can thus be planned accordingly. With the knowledge of weak points in the network and the condition-based maintenance it is possible to run the medium-voltage network with a low failure rate and in a cost-optimised manner, in spite of the growing cable stock.

 
The E-Werk Mittelbaden maintenance plan costs approx. €4 million, of which €2.5 million relate to the distribution network. The costs for cable diagnostics are currently €90,000 p.a.

 
From the comparative tests conducted before procuring the VLF equipment, Brucker knows that there were distinct differences between the VLF 0.1 Hz sine and 50 Hz measurements.

 
In practice, a changeover no longer seems relevant, as E-Werk Mittelbaden has become very well acquainted with the VLF 0.1 Hz measurements and their interpretation and can gather measured values with high reliability for classifying cable routes. Owing to vast experience, it is even possible to predict with relative accuracy whether a cable route is likely to fail in the short or medium term, thus making it easier to prioritise maintenance measures accordingly.

 

Conclusion

 
The VLF sine voltage source makes it possible for a single person to perform cable testing and diagnostics of a cable route with portable equipment. The ideal, load-independent sine form proved to be of benefit as far as the reproducibility of results is concerned and when good comparability of results is desired. The time-saving diagnostics thus offers the following advantages:


• Specific allocation of maintenance budget
• Cost savings by isolating faulty partial sections
• Lower failure rate
• Positive effect on the cost structure in ratio to the network failure rate
• Quality of new cable routes (detection of installation faults before any actual failure)

 

References

 
[1] The Use of the 0.1 Hz Cable Testing Method as Substitution to 50 Hz Measurement and the Application for PD Measuring and Cable Fault Location; M. Muhr, C. Sumereder, R. Woschitz

 
[2] Jicable 11 – Investigation of the Technologies for Defect Localization and Characterization on Medium Voltage Underground Lines; G. Maiz (Iberdrola Distribución, Spain)

 
[3] New Studies on PD Measurements on MV Cable System at 50 Hz and Sinusoidal 0.1 Hz (VLF) Test Voltage; K. Rethmeier, P. Mohaupt, V. Bergmann, W. Kalkner, G. Voigt

 
[4] Partial Discharge Measurements on Service Aged Medium Voltage Cables at Different Frequencies; G. Voigt, P. Mohaupt

 

[5] VLF-TE Messungen an betriebsgealterten Mittelspannungskabel (Abschlussbericht) [VLF PD tests on worn medium-voltage cables (final report)]; G. Voigt

 

[6] Grundlagenuntersuchung zum Teilentladungsverhalten in kunststoffisolierten Mittelspannungskabeln bei Prüfspannungen mit variabler Frequenz und Kurvenform [Study of the basic principles of partial discharge behaviour in plastic-insulated medium-voltage cables at test voltages with variable frequency and waveform]; D. Pepper

 

[7] IEEE 400.2-2013 IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz)