Guest Paper: Using Online Condition Monitoring In Substations To Achieve Business Benefits

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A. Kingsmill, S. Jones and P. McIntyre of TransGrid, Australia

Abstract

In recent years a range of monitoring devices have become available that provide continuous, real-time condition monitoring of substation equipment. Utilities have implemented these devices with varying degrees of success as interest in the use of online monitoring has grown.

This paper examines the use of online monitoring and presents a methodology that has the aim of deriving business benefits. It reviews the merits of different types of monitoring devices, analyses considerations that can be taken into account in developing a strategy for online monitoring, and discusses practical considerations for fitting condition monitoring devices across a network.

The paper also reviews the experiences of TransGrid in implementing an online monitoring strategy and the impact of this technology on asset management.

1. introduction

The introduction of electricity markets worldwiide has imposed operational demands on electricity networks in an environment of cost reduction and increased risks and liabilities. Some of the main asset management drivers in this environment are:

• financial drivers of revenue and cost;
• financial drivers of revenue and cost;
• safety and the environment [1]

A modern approach to condition monitoring can benefit utilities in each of these areas by facilitating changes to asset management practices. Within such an approach, the effective use of online monitoring has potential to provide significant benefits in the areas of condition assessment and asset management.

The key benefits of online monitoring can be summarised as follows:

• improved maintenance practices - resulting in reduced maintenance time and cost, fewer scheduled outages and outages of shorter duration;
• improved defect response - in terms of shorter response time and better ability for appropriate response;
• greater effectiveness of real-time decision making - for example in short-time transformer overload situations and asset utilisation reviews;
• early detection and possible prevention of equipment failure, especially catastrophic failure - resulting in reduced technical and non-technical costs associated with such failure;
• long-term data acquisition and understanding about equipment performance; and
• better equipment condition history than can sometimes be achieved using traditional diagnostics.

While condition monitoring in general is a powerful information resource, a pro-active approach on the part of utilities is required to realise the benefits of modern online monitoring devices.

TransGrid is a government-owned corporation responsible for the operation and management of the high voltage electricity transmission network in New South Wales, Australia. It is a network comprising 75 substations and power station switchyards, and over 12,000 kilometres of transmission lines operating at up to 500kV. The network forms a major part of one of the most extensive interconnected power systems in the world, with interconnections to the neighbouring states of Victoria, South Australia and Queensland.

The network is well suited to the benefits of online condition monitoring, with a number of remote substations across the state and equipment with a range of both age and condition. TransGrid has installed extensive monitoring facilities at a number of key sites and on key equipment.

As the use of online monitoring has grown, and experience in this area has been gained, TransGrid has used the ensuing knowledge to develop a strategy on the use of online monitoring across its network.

2. An Integrated Strategy

Online monitoring is potentially a powerful tool for utilities in responding to asset management drivers and streamlining asset management practices. The ability to have some level of information on asset condition available in real-time has extensive implications for decisions facing asset managers every day

To date, the approach to online condition monitoring in worldwide electricity markets has been either gradual and not well coordinated, or reactive and not well planned. Some utilities have rightly approached online monitoring devices with caution, and have installed them mainly on a trial basis for evaluation purposes. Others have employed online monitoring as a means of reducing maintenance requirements or even as an "insurance policy" in response to otherwise reduced maintenance requirements. Others have decided that the general cost of online monitoring at this stage is preclusive to the use of this technology.

Online monitoring technologies are now at the stage where the industry generally has acquired an understanding of their purpose, effectiveness and limitations and where the benefits of online monitoring can be effectively realised. What is needed is an integrated strategy that considers substation assets, available devices and their effectiveness, and an overall system and processes to bring the devices together and deal with their information at a network-wide level.

TransGrid has developed an integrated strategy addressing both the application of monitoring devices, and systems and processes required to manage their information. It includes:

• an extensive review of online monitoring devices available on the Australian market including factors such as cost, reliability and usefulness;
• a review of past equipment failures and how these may have been detected by online monitoring;
• development of policies and responsibilities for analysis and review of online monitoring data;
• consideration of a system to manage online monitoring data and information;
• a review of TransGrid's existing online monitoring installations, their status and any outstanding issues;
• consideration of existing equipment that would benefit from online monitoring;
• consideration of online monitoring that would be advantageous on new equipment;
• implications of online monitoring for maintenance practices; and
• consideration of additional issues such as staff training.

The strategy was prepared taking into account plant performance information over a number of years, experience and case studies from within and outside TransGrid, benchmarking against practices and approaches of other transmission utilities worldwide, and advice from device and equipment manufacturers. The main focus of the strategy is on power transformers, circuit breakers and current transformers as some of the most significant substation assets, with some consideration given to other assets and available monitoring devices.

The key outcomes of this strategy for TransGrid have been:

• consolidation of existing monitoring systems;
• clearly defined responsibilities for review of online monitoring data;
• procedures for specification of device settings, alarm levels and functionality;
• establishment of commissioning procedures for online monitoring;
• strategies for further installation of online monitoring in the medium- and long-term;
• strategies for online monitoring to be fitted to new equipment;
• implementation of a centralised information system to manage and analyse online monitoring data; and
• development of a supporting IT network.

The following sections detail some of the key aspects of these outcomes.

3. Online Monitoring Devices

For the purposes of this paper, the terms "devices" and "monitors" refer to the discrete devices that perform online monitoring functions, and the term "systems" refers to a broader infrastructure used to consolidate information from a number of devices.

With the current focus on online monitoring in the electricity industry, the market has been flooded with a plethora of online monitoring devices. These range from single-function monitors such as dissolved gas in oil and moisture in oil monitors, to more complex devices that monitor a range of parameters from plant such as tapchangers and circuit breakers. The number of devices available is continually growing, as new ones are regularly developed and marketed.

TransGrid has completed a review of online monitoring devices available in Australia. The review has included factors such as cost, reliability, ability to change maintenance practices, ability to detect incipient or impending failure, usefulness of information for real-time decision making, and ease of fitting to equipment. A summary of the primary benefits of each type of device is shown in Table 1.

Type of device	Primary benefits to TransGrid
Transformer dissolved gas in oil	Detecting incipient or impending faults that generate key gases measured by the monitors, especially where these occur between routine oil sampling. Dissolved gas also has an effect on the ability of transformers to handle overload [2].
Transformer moisture in oil	Providing accurate moisture in oil information when transformer load and temperature is close to equilibrium, which can be used to calculate moisture in paper and has an effect on the ability of transformers to handle overload [3,4].
Transformer partial discharge	Detecting partial discharge faults early. However, these are also detected by online dissolved gas monitoring for hydrogen and acetylene.
Tapchanger monitors	Tapchanger monitors with extensive monitoring can facilitate condition-based maintenance and will also pick up various failure modes [5].
Temperature monitors	Real-time information for loading studies, determination of overload capability and loss of life calculations.
Oil level monitors	Detecting oil leaks and monitoring them over time, also watching oil level in conservator in situations such as overload.
Bushing and CT DDF monitors	Detecting bushing or CT failure that creates a change in DDF, also can remove the need for routine DDF testing if sufficiently reliable.
Insulator pollution monitors	Determining when insulator cleaning needs to be done in environments with significant pollution, avoiding external flashovers.
Circuit breaker monitors	Improved defect response and changes to maintenance practices which can reduce outage time and number of outages.
Battery monitors	Monitors that perform regular condition tests in service, such as high current tests, can reduce battery maintenance and detect faults. These are of particular benefit in substations with only one battery.

TransGrid has in-service experience with many of these monitors over a number of years, and has developed a set of criteria for evaluating their performance and suitability. The criteria include costs, performance, suitability and other factors, as follows.

Costs:

• the device itself, including installation
• routine maintenance on the device
• repair of defects with the device
• response to nuisance alarms

Performance:

• accuracy of readings
• consistency of readings
• defects with the device
• occurrence of nuisance alarms

Suitability:

• life expectancy
• ability to interface into broader online monitoring information system
• ability to handle the conditions in which it is installed, including climate and weather s

Other factors:

• ease of installation and use of device
• usefulness of data provided for the key benefits identified in Section 1 of this paper
• monitoring method including parameters measured, calculations performed, and conclusions inferred
• monitoring of leading, concurrent or lagging indicators of equipment failure

Considerations such as these will ensure careful selection of devices, which is essential for effective condition monitoring. However, it is equally necessary to consider each case in which devices can be installed to ensure the attainment of business benefits.

4. Effective Monitoring

It is clearly not cost effective to install condition monitoring on every asset, indeed in many cases utilities have found that the costs of some devices are preclusive to their widespread use.

A strategy for online monitoring with the aim of achieving the key benefits identified in this paper will necessarily require attention given to the justifications for installing each device. These justifications will include both technical and non-technical factors, which can be quantified and expressed via cost/benefit analysis.

Factors influencing technical justification include reduction in routine maintenance, improvement of defect response, early detection of equipment failure, increased equipment availability and improved decision making. In general these can be quantified relatively easily. Commercial effects of loss of supply due to equipment failure can also be taken into account.

Non-technical justifications include factors such as staff safety, environmental considerations and social effects of loss of supply. They are generally more difficult to quantify in terms of cost, but can be evaluated in terms of point scores and should at the least be identified when considering a strategy for these devices.

There are a number of variables that influence the above factors. These are:

• remoteness of substation under consideration;
• criticality of substation or items of plant in terms of network performance and availability;

Figure 1 shows an outline of the sheet TransGrid has used for the cost/benefit analyses:

Benefits (per year in $) Routine maintenance	…………
Defect response	…………
Failure detection – technical cost	…………
Failure detection – non-technical cost (e.g. LOS)	…………
Equipment availability	…………
Real-time decision making	…………
Long-term data acquisition	…………
BENEFIT PER YEAR	…………
Costs (per year in $) Cost of capital	…………
Annual depreciation	…………
Routine maintenance of CM device	…………
Defects with CM device	…………
Responses to unreliable readings COST PER YEAR	…………
Non-technical factors to consider (in points) Staff safety	…………
Environmental considerations	…………
Social effects of loss of supply	…………

TransGrid’s experience has been that certain types of online monitoring can be justified for 330kV power transformers, largely for technical reasons. Dissolved gas in oil and tapchanger monitors are being installed, initially targeting transformers that are critical for supply reliability and those with possible conditions warranting continuous monitoring. Moisture monitors are being targeted at transformers with known high moisture content. New transformers are purchased with dissolved gas, tapchanger and bushing monitors, as those which detect the most common modes of transformer failure [6]. Also, transformer maintenance has been reduced where online monitors are fitted, as identified in Table 1.

Circuit breaker monitoring can be justified on the basis of increased system availability. TransGrid is purchasing online monitoring on all 330kV circuit breakers, with particular emphasis on those installed on critical feeders, and has reduced routine maintenance requirements for circuit breakers with online monitoring.

For other plant such as current transformers, batteries and smaller power transformers the main reasons for installing online monitoring are non-technical reasons, often where there are particular concerns for aspects such as staff safety. Several years ago TransGrid became aware of type faults on some current transformers, and part of the response was to install CT monitoring systems at some sites until the replacement program was completed. At one substation a CT monitor detected and prevented explosive failure of a current transformer.

For other plant such as current transformers, batteries and smaller power transformers the main reasons for installing online monitoring are non-technical reasons, often where there are particular concerns for aspects such as staff safety. Several years ago TransGrid became aware of type faults on some current transformers, and part of the response was to install CT monitoring systems at some sites until the replacement program was completed. At one substation a CT monitor detected and prevented explosive failure of a current transformer.

Although there will be many factors in common, each utility will have different requirements and constraints to consider when developing an online monitoring strategy. With proper consideration, utilities can develop a condition monitoring strategy that is effective, reasonably comprehensive, and supports the broader asset management strategy to achieve business benefits.

5. Supporting Systems

An aspect of online monitoring that has received little significant attention to date is effective management of the information provided by these devices.

In a network with many and varied monitoring devices, numerous software packages are required for their interrogation and configuration. To interrogate the devices staff are required to be on site, connected via dial-up modem or at best connected via a LAN interface to each device. To minimise the effort to obtain online monitoring data, it is advantageous to have a centralised system that automatically reads and manages data from online devices without the need for proprietary software.

There are some recent products available that gather information from a range of devices on an item of plant or even across a substation. However, a large number of these systems are needed to extend across a high voltage network. TransGrid has taken the approach of developing a centralised system that gathers and manages information from condition monitoring devices in multiple substations across an entire network.

TransGrid set out to develop a system that is in keeping with the following principles:

• integrated, that is, combining information from multiple devices across a number of substations into a common interface;
• accessible, that is, available to a range of users in a variety of locations, without the need for manufacturers’ proprietary client software;
• flexible, in terms of queries that can be performed and the format and detail of information returned;
• secure, that is, not allowing changes to data or control of substation functionality;
• reliable;
• offers good performance, even with minimal infrastructure and regardless of the size of the system; and
• expandable, to potentially encompass an entire high voltage network [7].

The outcome has been a web-based online monitoring system that manages information from online monitoring devices in multiple substations across the New South Wales high voltage network. The current extent of this system is shown in Figure 2, and additional sites and equipment are progressively being incorporated.

Figure 2 - TransGrid’s current system

The system operates by continuously storing information from online monitoring devices into databases at each substation. Staff can select information they would like to view, including information from multiple substations, in one query. The system then retrieves and displays the information required in the appropriate formats. It also provides users with the ability to view or download raw data, and send graphs and raw data via e-mail.

Figures 3 and 4 show screen displays from the system during the query process. Figure 5 shows the results of a query that includes multiple substations. In the query, the user has selected circuit breaker gas densities from Armidale, Regentville and Sydney South substations.

Figure 3 - Grid map used in query

Figure 4 - Substation operating diagram used in query

Figure 5 - Results from multiple substations

The system is able to handle the following types of information:

• analog quantities, such as DGA and moisture;
• digital quantities, such as extended circuit breaker alarms;
• records from protection relays, circuit breakers, tapchangers and battery tests; and
• plain text information, such as flag information from modern protection relays.

Even in its early stages the system has been able to provide a range of information on a variety of equipment, which has served a number of purposes. Figure 6 shows data for a transformer at Regentville substation which can be used for real-time load monitoring, performance modelling and load simulations based on actual data. The system has also been used to detect recurring problems with transformer automatic voltage regulation reverting to manual operation.

The main advantages TransGrid has found with this system are the facility of easy and flexible access to online monitoring data, and the ability to perform automated analysis of this data as required.

Figure 6 - Transformer data monitored by the system

6. Data Analysis

When systems are in place to manage online monitoring data, analysis can be both easy and comprehensive and the need for human involvement can be reduced.

There are a number of types of analysis that can be done on online data. These include:

• transformer loss of life and overload in real-time;
• calculation of moisture in oil and moisture in paper at times when transformers are close to equilibrium of temperature and load [8];
• analysis of dissolved gas results and variations, even on monitors that detect a weighted sum of gases;
• comparison of online condition monitoring results with offline results obtained via traditional methods, via an interface to other corporate systems;
• transformer temperature rise above ambient; and
• compensation of oil level for oil temperature.

TransGrid has implemented the above types of analysis, and is now able to provide better information for real-time decision making and equipment condition indication than is possible without online monitoring.

7. Conclusion

For online monitoring to achieve business benefits, its use must be clearly and coherently aligned to asset management goals and objectives. This can be achieved through a well thought out strategy that addresses both the application of monitoring devices, and systems and processes required to manage their information.

Through the use of online monitoring, workplace practices can become more efficient and utilities can respond effectively to asset management drivers in deregulated electricity markets such as in Australia.

Acknowlegement

The authors would like to express their thanks to TransGrid’s online monitoring strategy working group for the strategy information presented in this paper.

References

[1] Jones S., Gardner K., Brennan G.: Switchgear Issues in Deregulated Electricity Industries in Australia and New Zealand, CIGRE Session 2000, paper 13-068.

[2] IEC 354: Loading guide for oil-immersed power transformers second edition, IEC, 1991.

[3] Myers S.D., Kelly J.J., Parrish R.H.: A Guide to Transformer Maintenance, S.D. Myers Inc., 1981.

[4] Leibfried T., et al: Online Monitoring of Power Transformers – Trends, New Developments and First Experiences, CIGRE Session 1998, paper 12-211.

[5] CIGRE AP12: Australian/New Zealand Transformer Reliability Survey, AP12 Report to Australian National Committee of CIGRE, 1996.

[6] Kingsmill A., Jones S.: Real-Time Condition Monitoring: Maximising the Benefits Through Innovation, Distribution 2001, paper 161.00.