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On-line Partial Discharge (OLPD) Condition Monitoring of Outdoor HV Switchyards to Support Risk-Based

Asset Management Policies 1M Seltzer-Grant*, 1R Giussani and 2T Chitifa

1 HVPD Ltd, Manchester, UK 2 Scottish and Southern Energy Power Distribution, Portsmouth, UK

*Corresponding author: m.seltzer-grant@hvpd.co.uk

Abstract:

Aging infrastructure in the UK electricity network is encouraging utilities to investigate methods for diagnostic testing and life extension. There are also safety concerns with outdoor HV substations where insulation failure can have drastic consequences such as substantial damage to other equipment and risk of personal injury. The installation and commissioning of a remotely accessible condition monitoring system, focussed on partial discharge detection on outdoor cable sealing ends is presented. The system utilises non-intrusive sensors such that it can be retrofitted to in-service cable systems. The data capture methodology for identification of PD signals and noise reduction methods are discussed. A remotely accessible graphical user interface (GUI) and SCADA integration allows the operator to gain insight into the condition of the cable sealing ends and alerts should the condition worsen. Initial field results and case studies are presented. 1. Introduction Partial discharge (PD) testing has long been established as a means for detection of localised points of degradation in high voltage insulation. As a diagnostic technique PD testing is advantageous in that it is possible to perform on-line partial discharge (OLPD) measurements with equipment in service. The technique is now being looked upon by transmission operators as a way to assess the condition of high voltage plant which can in turn be used to help assess the risk it poses. This paper gives an overview of a project currently under way to evaluate OLPD monitoring as a means to reduce the criticality scores of assets on a transmission network, particularly those in substations located in or close to public places. Recent work to install OLPD monitoring systems and some initial results are reported. 2. Partial Discharge for Condition Assessment of HV Equipment Partial discharges can occur at various points in the insulation system, such as voids, defects or between insulation layers and can lead to the growth of electrical trees and tracking, an example of this is at a 110 kV cable termination shown in Figure 1; in this case there had been an incorrect installation of the stress cone. OLPD testing can be done as part of routine maintenance as a spot test, with temporary monitoring equipment or monitored continuously with permanently installed equipment. Spot testing and temporary monitoring are advantageous in that the several plant items can be assessed with one piece of test equipment or be carried out on a service basis.

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However it should be noted in some situations PD levels and occurrences can vary over time [1] and so continuous monitoring can give a truer picture of the insulation condition and also give alerts to operators instantaneously when a worsening condition is detected. This is of particular interest in cases where a fault could develop in the time between spot PD measurements, or use of temporary monitoring equipment.

Figure 1 Example of PD damage to 110kV XLPE cable terminations

3. Use of PD Monitoring for Reduced Risk Transmission owners in the UK have a common methodology for prioritising asset replacement. The methodology requires a risk based approach from the combination of an assessment of asset health and assessment of asset criticality. Criticality is defined as having three elements, safety criticality, environmental criticality and system criticality. Overall criticality is taken as the highest of the three elements. If a substation is located in a public place, its plant will end up with a high safety criticality score because a disruptive failure is likely to send shards of porcelain outside of the substation which could result in serious harm or even fatality. The current criteria which define safety criticality are only the location and impact of failure of an asset, neither of which can be changed without relocation. The result is that plant in such locations always appears high up on the list of asset replacement priorities, regardless of the condition of the plant. Operators are therefore seeking ways to reduce the criticality score of assets. Various options exist including:

• Construction of solid barriers; which have planning consent issues and block views into substations with increased likelihood of theft and vandalism.

• Replacement of plant and equipment with different failure mechanisms such as dry type transformers or composite bushings. This is an expensive option, especially where equipment is in otherwise good condition.

• Reduction of the likelihood of disruptive failure by continuously monitoring the condition of the assets and performing the associated proactive interventions.

Scottish and Southern Energy Power Distribution (SSEPD) are presently undertaking a project to evaluate the last option as a means of improving asset criticality indices by assessing the condition of plant with continuous PD monitoring that integrates with internal Supervisory Control and Data Acquisition (SCADA) systems. Complementary technologies using radiometric methods and electromagnetic PD sensors are being utilised, this paper focuses on the later system.

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4. Monitoring System Overview The system consists of OLPD sensors permanently installed to the plants under test and connected to nearby monitoring nodes. The monitoring node inputs are multiplexed which allows multiple assets to be monitored from a single unit. Sensor attachment points vary by the equipment monitored, but mainly these are High Frequency Current Transformers (HFCT) sensors on power cables and cable earthing, Transient Earth Voltage (TEV) sensors on cable terminations and metal-clad plant housing and adapters on the transformers bushing tap. The sensors used are all non-intrusive; however due to access restrictions at site it was necessary to make the installation during a scheduled outage. The bushing tap adapters had to be made custom to the transformer in some cases. Examples of sensor connections are shown in Figure 2 here HFCT and TEV installed on the sealing end of a 132 kV cable are visible and in Figure 3 where a bushing tap installed on the 132 kV side of the transformer is visible.

Figure 2 Example of HFCT and TEV installation on 132 kV cable sealing end. A) overview of the outdoor HV Switchyards; B) details of one of the 132 kV sealing end; C) details of outdoor HFCT

installation; D) outdoor TEV installation.

Figure 3 Bushing tap installation on 132kV transformer bushing taps

A

B C D

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The monitoring nodes connect to a server which is either located in the control room or in the same cabinet as the monitoring nodes. The server handles data collation, signal analysis and graphical user interface (GUI). The data points and alarms on each monitor can also be connected to SCADA networks at the site using industry standard protocols, for example Modbus and DNP3. It is also possible for data to be interrogated externally by HVPD to provide assistance in the data analysis and reporting. The architecture of a typical system is shown in Figure 4.

Figure 4 PD monitoring system schematic

5. Data Analysis and Noise Reduction As is common in PD measurements, data is acquired synchronously to the power cycle of the high voltage. This allows for phase resolved PD patterns (PRPD) to be observed as means to identify different types of PD and also can aid in identification of noise interferences. The system acquires data at a high sampling rate of 100MS/s which allows the PD waveforms to be digitised and the data to be clustered based on pulse shape parameters to aid separation of different PD sources and segregating noises. An example is given in Figure 5

Figure 5 Example of pulses clustering on the base of different parameters. A) PRPD plot where

pulses are displayed on the base of each pulse amplitude and phase; B) pulses displayed accordingly to pulse peak and pulse width; C) pulses displayed accordingly to pulse area and

pulse rise time.

A B C

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6. Case Study – Monitoring 275kV Transformer Bushings With the monitoring system installed on a 275/20 kV transformer, high PD signals were observed on one of the bushings with a TEV sensor attached onto the transformer tank below the bushing. It was subsequently decided to replace the bushing. Initial investigations indicated a potential issue with the grounding of the bushing tap point. An example of the de-noised PD trend before and after replacement is shown in Figure 6. The PRPD pattern before replacement of the bushing is shown in Figure 7.

Figure 6 Peak PD level before and after bushing replacement detected with TEV sensor

Figure 7 PD phase pattern before bushing replacement detected with TEV sensor

7. Conclusions Continuous PD monitoring systems have been installed and will remain under observation for the next 2 years to evaluate potential further use of the technology in business as usual (BAU). Some initial results have shown its use in identifying and confirming potential issues such that remedial action can be taken or the criticality score adjusted accordingly. The project also aims to start addressing the limitations of routine interspersed PD inspections of equipment and to have the data integrated with the SCADA system. If successful, the method can then be adopted in monitoring assets with high safety criticality to enable earlier intervention. Although this intervention may not be necessary in every case, it is a precaution that is justified for safety reasons and still more cost-effective than alternatives. References

[1] M Seltzer-Grant, D Denissov, L Renforth, R Mackinlay, H Schlapp, F Petzold, “On-line PD Spot Testing and Continuous Monitoring for in Service Power Cables – Techniques and Field Experiences”, JiCable, Versaillles, France, 2011