Aging protection systems and limited financial resources demand improved resource allocation methodologies. Resource priority must be assigned to the assets that pose the greatest risk. Hydro One Networks Inc. (Toronto, Ontario, Canada) is aggressively optimizing expenditures on asset sustainment programs to address this situation. In 1999, the company initiated a comprehensive campaign to determine the condition of every asset it owns. The ensuing survey encompassed 63 different asset classes. Using the information collected, a health index was formulated for each asset class to aid in managing the sustainment program.

Protection systems present a unique situation. In contrast to the majority of power-system assets, during normal system conditions, protection systems remain dormant. In addition, protection systems store supervisory intelligence that cannot be physically observed. When compared to power-system assets, such as transformers and circuit breakers, the correlation between the risk of failure and observable deterioration is weak. To determine the remaining useful life of a protection system, the evaluation must be more extensive than just a physical assessment. A mixture of statistical evidence and qualitative evaluations is required to generate a meaningful health index for protection systems.

The protection-system health index identifies systems that pose significant risk to Hydro One. Using a database that monitors several characteristics regarding the condition of every protection system in service, a standardized evaluation is applied to each component. The characteristics monitored address both the operational risk of a protection-system failure and the financial risk of increasing operation and maintenance costs associated with aged systems. The result is an integrated condition rating that assists with the identification of protection systems approaching the end of their useful lives, allowing asset managers to optimize the protection-system replacement strategy.


The objective of the protection-system health index is to effectively evaluate every Hydro One transmission protection system. This is accomplished by using a standardization process to quantify several qualitative factors that comprise the risk presented by a protection system. Once the level of risk has been established in the health index, a straightforward comparison can be made across a diverse population of assets. This information allows the asset managers to identify the protection systems that pose the greatest risk and hence should be deemed to have reached their end of life. Replacement programs can then be developed to eliminate high-risk protection systems from the asset population.

In addition to providing information about the current condition of a protection system, the health index also can be used to predict a protection system's future condition. In many instances, Hydro One has applied a standard protection-system design to numerous applications over an extended period of time. For these standard designs, the older protection systems provide an indication of the rate at which the condition of the more recently installed systems can be expected to deteriorate. By applying an approximate rate of asset degeneration, the asset managers are able to forecast future protection-system health indexes. Sustainment programs can then be designed to mitigate both the immediate and impending risk.


An asset condition review assembles the information required for the foundation of a health index. The Hydro One protection-system asset condition review was applied as a field survey. A detailed method of quantifying each protection-system characteristic assessed was provided to field technical staff. Adhering to the guidelines, the staff collected quantified information describing the condition of every protection system in service. Since the protection systems are dispersed over a large geographic area, several individuals were required to complete the survey. Providing rigorously standardized guidelines ensured that human interpretation was limited and increased the validity of the data collected.

The first step involved in creating an asset condition review is to clearly identify the asset of interest. For the protection-system asset condition review, a protection system was defined as any group of relays designed to protect a single power-system element (bus, line, transformer, breaker, feeder or capacitor). The assessment survey was partitioned according to the principal components of a protection system. Hydro One identified three principle components: the primary measuring relay, the auxiliary relays, and the hardware and connecting equipment. The assessment survey was designed to collect information about the component characteristics that are known to impact the end-of-life decision. A condition rating was selected to describe each protection-system component characteristic assessed, using a scale of declining condition from CR1 to CR5 (Table 1).

For protection systems, the majority of the information contained in the asset condition review is used to quantify the condition of the primary measuring relay. The auxiliary relays were only assigned a condition rating for the reliability of the relays and their degree of silver migration (a term used to define the transfer of silver from the contacts of an auxiliary relay onto the terminals of its mounting case which, if the silver bridges two terminals, can cause relay failure). To complete the assessment of the protection system, the wiring, terminations and panels were each assigned a condition rating based on an evaluation of the degree of deterioration and the presence of hazardous materials.

Specific characteristics of primary measuring relays were identified as requirements to properly quantify a relay's condition. Detailed condition rating benchmarks were developed for each characteristic. In some instances, advances in relay technology (electromechanical, solid-state and digital) made the component characteristics surveyed inapplicable to modern relays. For these characteristics, a condition rating of CR1 was assigned to modern relays. The asset condition review survey collected ratings for the following:

  • Physical condition

    During a visual inspection, worn contacts, leaking capacitors, frayed insulation and reduced mechanical strengths were identified. The visual inspection was highly relevant for electromechanical relays, but provided limited information about the condition of solid-state relays and digital relays.

  • Calibration drift

    In electromechanical relays, this indicated electrical or mechanical failure. In solid-state relays, calibration drift indicated failed electronic components. The degree of drift from the original settings is a powerful indication of a relay's risk of failure.

  • Failures

    The number of protection-system failures attributed to each primary measuring relay was collected for use in generating an in-service mean time between failures (MTBF) index. Once the MTBF was calculated for every relay model, a condition rating can be assigned to each primary measuring relay based on the MTBF for the relay model it employs.

  • Nondiscretionary obsolescence

    The impact of the primary measuring relay on Hydro One personnel was addressed. Adherence to company standards designed to enable safe testing and maintenance was evaluated. Availability of spare parts, manufacturer support and the ability to accommodate changing system conditions were also considered.

  • Discretionary obsolescence

    The ability of the primary measuring relay to provide modern capabilities (diagnostics, increased sensitivity/speed) was evaluated with respect to its impact on the customer.

  • Age

    Known or estimated installation dates were collected for use in determining the residual life of each primary measuring relay. The expected useful life of a relay was estimated to vary from 20 to 50 years according to the technology employed in its construction. Again, a condition rating was applied to each relay once its residual life had been calculated.

  • In-service performance

    The ability of the relay to operate with only scheduled maintenance was assessed.

The culmination of the Hydro One protection-system asset condition assessment was a database that contains quantified information regarding the condition of each of the 10,500 Hydro One protection systems that are installed throughout the province of Ontario. The results provide a standardized measure with which to make comparative evaluations across a diverse population of assets. To ensure the continued effectiveness of this database, field technical staff updates the condition ratings of a protection system each time scheduled or emergency maintenance is performed. This database establishes the basis of information from which the Hydro One protection-system health index is derived.


Once a condition rating has been assigned to each of the component characteristics surveyed, a formula must be developed to integrate the condition ratings. The formula will determine the health index score for each protection system. To generate a suitable evaluation of the information the component characteristics assessed should be weighted according to the relative risk, both technical and financial, that each present to the protection system. The weighting of the Hydro One protection-system health index was determined using a Delphi process. It is displayed in Table 2 (the weightings for the primary relays are depicted in the pie chart). To determine the health index score for a protection system, the estimated weights were multiplied by the health factor corresponding to each of the recorded condition ratings and summed.


The results of the protection-system health index identify individual protection systems that pose an immediate risk to Hydro One. An overview of the asset population's health, as well as a basis from which to develop protection-system sustainment programs, can also be derived from the health index. The introduction of a corporate protection-system fault tree database, to monitor failure modes, has been proposed to aid in determining the frequency of protection system failures that can be attributed to specific components and improve failure mode data. Including new component characteristics, to reflect the advancement of relay technology, will also enhance the capability of the protection-system health index as a tool for sustainment planning. Overall, the protection-system health index has proven to be an effective tool for Hydro One asset managers.

Aaron Cooperberg is the protection and control sustainment manager in Asset Management for Hydro One. He joined the company (then Ontario Hydro) in 1977, working in the protection design group. There he designed protection systems for generation, transmission and distribution for 21 years. In 1998, he transferred to the newly created asset management organization at Hydro One in the position of senior network management engineer. He is registered with professional engineers Ontario with a license in protective relaying systems design.

John Ciufo is the protection and control strategies and standards manager for Hydro One. He has extensive background in protection, control and telecommunications systems for application in Hydro One Networks. Ciufo is a member of the Northeast Power Coordinating Council Task Force on System Protection, NERC System Protection and Control Task Force and the IESO Security Working Group. He sponsors many standards and development projects with organizations such as CEA Technologies, EPRI and Kinectrics. He is a registered professional engineer in Ontario.

Cole Tavener, an assistant network management engineer, joined Hydro One in 2005 after completing bachelor degrees in electrical engineering and economics at the University of Western Ontario. He has experience in the design, commissioning and management of protection and control assets. He is currently completing a masters of engineering degree in electric power engineering at the University of Waterloo.

Ian Bradley manages all programs for the sustainment of telecommunication, protection and control, and metering assets for Hydro One. He has 30 years experience in power-system operations, control, telecommunications, and the design and operation of large-scale transmission and distribution control centers. Bradley is a member of the Northeast Power Coordinating Council Task Force on Infrastructure Security and Technology and is a registered professional engineer in Ontario.

Table 1. Condition Rating Scale
Condition rating Health factor (%)
CR1 100
CR1 75
CR3 50
CR4 25
CR5 0
Table 2. Health Index Formula
Protection-system component Weight Characteristic Weight
Primary measuring relays 5 Visual inspection 5
85 Calibration drift 5
Residual life 20
Non-descretionary obsolescence 10
Descretionary obsolescence 5
In-service performance 5
Auxiliary relays 10 MTBF 70
Silver migration 30
Harware and connecting equipment 5 Panel 40
Wiring 40
Terminations 20