Reducing Misoperations by Boosting Protection System Design Quality
By Tim Farrar, TRC
The protection system design for a typical substation involves many interrelated drawings, calculations, studies and development of specific protective relay settings. Their accuracy has long been recognized as critical to the successful, safe and secure construction, commissioning and operation of a utility substation. This article outlines steps grid owners and operators should take to improve the quality of protection system design and protective relay settings to reduce the potential for protection system misoperations.
The North American Electric Reliability Corp. (NERC) is the regulatory authority whose mission is to ensure the reliable operation of North America’s bulk electric system (BES). Under NERC’s mandatory standard PRC-004-5(i), utilities are obligated to review all protection system operations to identify, report and correct those operations that qualify as misoperations. NERC lists six specific misoperations categories.
A misoperation generally entails:
“- Any failure of a protection system element to operate within the specified time when a fault or abnormal condition occurs within a zone of protection.
“- Any operation for a fault not within a zone of protection (other than operation as backup protection for a fault in an adjacent zone that is not cleared within a specified time for the protection for that zone).
“- Any unintentional protection system operation when no fault or other abnormal condition has occurred unrelated to on-site maintenance and testing activity.
NERC Requirements and Measures
Each generation and transmission owner, as well as any distribution provider that owns BES equipment must meet the following objectives:
“- Review all protection system operations on the BES to identify misoperations
“- Analyze misoperations to identify the cause(s)
“- Develop and implement corrective action plans to address the cause(s) of misoperations
“- Submit protection system misoperation data to NERC
NERC’s Protection System Misoperation Data Collection
The purpose of this data collection is to continue consistent reporting of misoperation data to NERC through a standardized template for performance analysis. NERC will analyze the data to:
“- Develop meaningful metrics to assess protection system performance
“- Identify trends in protection system performance that negatively impact reliability
“- Identify remediation techniques to reduce the rate of occurrence and severity of misoperations
“- Provide focused assistance to entities in need of guidance
“- Publicize lessons learned to the industry
To facilitate uniform information gathering in this area, in 2016 NERC moved to an online system called the Misoperations Information Data Analysis System (MIDAS).
Incorrect Relay Settings,
Logic and Design Errors
The most significant sources of misoperations are the result of incorrect relay settings, logic and design errors. Improvements in this area, therefore, will provide the most near-term benefit because they can be controlled and improved with process changes. Reduction of incorrect relay settings, logic and design error will move the industry toward reliable operation of the BES.
The typical electric utility workforce is aging and many are retiring. These experienced, knowledgeable, senior-level employees are commonly being replaced with more junior staff. This makes a solid and up-to-date quality control (QC) plan more important than ever to achieve effective knowledge transfer, maintain a high level of excellence and avoid past mistakes.
Engineering consultants are being used to not only develop the protection system design and relay settings but also to act as the owner’s engineer and to carry out the utility’s QC plan, among other important engineering roles. This again demonstrates the necessity of having a solid and up-to-date QC plan for use within the company and by contract technical experts.
Technology changes in utility substations, including the new IEC-61850 communication standard, also require utilities to refocus on quality. These automated substation architectures have changed the protection system physical designs considerably. Some utilities now rely on extensive development and use of logic diagrams for these platforms. Expanding the QC plan to the software and other supporting documentation for these platforms is also crucial.
Having detailed and well-thought-out design standards is an important element in the QC plan. Design standards are typically developed for common substation equipment like line protection, transformer protection and bus protection. In addition, design standards should include a set of standard drawings, such as schematic diagrams, wiring diagrams, layout drawings and materials lists, as well as relay setting documentation, including settings, basis documents and philosophy documents. These standards can be used as a template to develop the site-specific drawings, documents and settings that are modified to accommodate individual applications.
Design standards are part of the QC process. The checking team uses them as a tool to verify that a new design meets the utility’s requirements. Quality improvement is a big benefit of design standards, but other upsides include consistency of designs among multiple design teams and/or consultants, improved design efficiency and the elimination of differences strictly due to personal preferences. Design standards also can lead to operational and maintenance improvements by reducing the number of unique designs that operators and maintenance personnel must work with and understand.
Peer Reviews and Point-to-Point Checks
Peer reviews and point-to-point wiring checks are the most important elements of a good QC plan. The peer review process for both protection system designs and relay settings involve one or more competent and qualified people (engineering-related) to review and evaluate the design and identify any errors or omissions.
A common practice is to perform a peer review of protection system schematic diagrams for the design’s functional accuracy. This can be performed before the point-to-point wiring and interconnection diagrams are developed, thus capturing any errors early in the design process. When possible, it is best to have the protection engineer perform this peer review to provide overlap between the protection system design and relay settings.
Point-to-point checking is the process of verifying that all wiring points on schematic diagrams are developed accurately on the wiring diagrams. Typically this is done by tracing and highlighting each circuit on each drawing. This process must be included on all design components to ensure the entire design’s accuracy.
Checklists and Sign-off Lists
As a final step, it’s important to use checklists and sign-off lists to ensure that everything related to protection system design and relay settings are complete and accurate, meet the client’s scope of work and design standards and have been peer-reviewed with all point-to-point wiring checks.
A common practice is to use a checklist of very specific tasks that must be completed as part of the process. These tasks can be technical in nature (for example, use only one ground per current transformer and voltage transformer circuit) or non-technical (for example, make sure the proper border, revision block, text size, font, blocks/cells and client CADD standards have been followed).
It is also a common to require a signature or the initials of the person performing the QC task. This provides a record of who performed a specific task on the checklist and assigns personal accountability to the QC task.
The sign-off procedure is most effective when presented in a multi-step, closed-loop process described below:
Step 1: The designer signs off, indicating the document is complete and self-checked
Step 2: The checker signs off, indicating the peer review or checking is complete
Step 3: The designer again signs off, indicating all checking corrections and comments have been resolved or included in the document
Step 4: The checker again signs off, indicating all checking corrections and comments have been resolved or included in the document
The Institute of Electrical and Electronics Engineers (IEEE) Power System Relaying and Control (PSRC) Committee develops standards, guides and recommended practices to assist the industry in applying best practices. These technical documents are essential reference information to any protection system design and relay setting development. A list of the protection related standards and guides are available on the IEEE website.
NERC also provides useful industry technical information. One of NERC’s useful services is its lessons learned resource, which features published documents that clearly identify common problems that have been experienced and the corrective actions taken.
Other Quality Control Elements
The following list of additional QC elements also should be incorporated in a good QC plan:
“- The role of cost, scope and schedule
“- Communication with the entire design team, including other engineering disciplines
“- Communication with the client
“- Establishing qualifications for the roles in the design
and QC process
“- Document control
“- As-built documentation
“- Monitoring and measuring the effectiveness of the QC process to provide feedback and improvement
“- Ongoing training program
The paramount goals of a QC plan are to provide high-quality protection system design and relay settings that meet client scope and standards, without errors and omissions. The potential for successful implementation and achievement of reliability objectives will be greatly improved through this approach. Resources available from IEEE, NERC, including the checklists and guidance referenced in this article, will help utility professionals understand and apply all the elements necessary in a QC plan to achieve this goal.