Standards Based Engineering of PAC Systems

Author: Alexander Apostolov, USA

The IEC 61850 international standard for substation and power system communications is not just defining a new protocol.

It is also introducing abstract models of primary and secondary substation equipment, communications systems and the relationship between all of them. It also defines an XML based format for the description of the above in a standard way that can be used at different stages of the engineering process based on an object oriented approach.

The “V” system engineering model
Many different engineering process models have been developed over the years to specify a series of steps that make up the systems engineering approach. Since it was first developed in the 1980s, the “V” model has been refined and applied in many different industries and with some modifications can be successfully used in the design of protection, automation and control schemes. The model defines different phases in the system engineering process covering the specification, implementation and verification of the system. Different “V” models use a varying level of detail in the number of steps required. The phases in the process related to the engineering of a PAC system can be briefly described as follows:

Concept of Operations
The concept of operations is based on the protection, automation, control, monitoring and recording philosophies. They are typically the result of many years of experience and development and cover a wide range of issues, such as the types of functions and schemes to be used for different applications depending on the criticality of the application and the type of primary system component being protected and controlled.

Requirements and Architecture
The requirements for a specific project depend on the place of the substation in the electric power system. They are the result of dynamic stability studies that can be used to define the critical fault clearing times for specific types of faults or other abnormal conditions. The results from the analysis are then used to specify the performance requirements for the different functions and schemes, as well as to determine the need for redundancy in order to meet the above requirements in case of a component failure. Based on the requirements the PAC system engineer can select the type of system architecture that can be used in the detailed design of the specific project. 

Detailed Design: The detailed design depends on the specific substation design, the concept of operation and the requirements and architecture, as well as on the specific primary and secondary equipment to be used in the project. The primary equipment will determine the interfaces between it and the PAC devices, while the selected IEDs will define both the interfaces with the process and between themselves for the implementation of different PAC schemes.
The detailed design is based on the use of IEDs that are in the list of approved devices after passing the rigorous acceptance testing process.

Implementation    : The implementation phase defines the complete configuration of the substation protection, automation and control system and also includes the panels’ layout, the HMI and the configuration of the individual IEDs. The communications architecture for the PAC system is defined during this phase as well.

Integration, Test and Verification: The integration testing is performed based on test plans designed during the architectural design phase and is intended to ensure that the individual devices can be integrated in protection and control schemes that meet the functional and performance requirements.
System Verification and Validation: The system testing is performed based on test plans that are designed during the system design phase based on the requirements specification. The system verification and validation includes the user acceptance testing which typically is performed as Factory Acceptance Testing (FAT) and Site Acceptance Testing (SAT) - (including end-to-end testing).

Operations and Maintenance: Once the substation PAC system passes the SAT, it can be put in trial-operation (optional phase) or normal operation. Maintenance is then performed preferably using condition or performance based maintenance, instead of time based maintenance.

Object-Oriented Standards Based Engineering of Protection, Automation and Contro Systems
Intelligent (microprocessor -based) Electronic Devices (IED) for data acquisition, protection, metering, and control have gained widespread acceptance and are recognized as essential to the efficient operation and management of substations. Their integration in hierarchical substation protection and controls systems over a substation local area network allows significant improvement in the functionality of the system without any increase in the cost. This integration process in substations using IEC 61850 as the communications protocol is based on object models that require the use of appropriate tools to represent the complex architecture of the substation, the communication system and the multiple functions in the IEDs themselves. A major part of the engineering of a substation automation system is related to the architecture and configuration of the secondary equipment in the substation. This requires the development of a formalized format that allows the description of all different elements and their relationships. IEC 61850 defines the object models of the different types of primary and secondary equipment, as well as their functionality in the substation.

The object-oriented approach to the engineering of the substation protection system is based on the system hierarchy and contains nested objects with different levels of complexity that can be defined as part of the standardization process. At the top of the hierarchy is the substation protection automation and control system (SPACS) that contains multiple instances of bay protection, automation and control schemes (BPACS), each defined as a complex object – SPACSO or BPACSO (see Figure 3).

Each BPACS contains multifunctional IEDs, defined in the object-oriented design process as protection, automation and control objects (PACO) with scheme specific functionality.
Each PACO contains multiple logical device objects (LDO) with specific functionality:  Protection; Automation; Control;    Measurements; Monitoring; Recording; Analysis; Others.

Each LDO can contain one to many sub-logical devices sLDO. The sLDO at the bottom of the protection system/scheme hierarchy contains the Function Elements (FE), the smallest functional objects that are represented by Logical Nodes in the IEC 61850 model.

A substation protection and automation system also includes different tools for visualization and control of the primary and secondary substation equipment - the substation HMI. The user can navigate through the multiple views of the substation one line or communications diagrams, or check the status or settings of a specific IED. The development of the HMI and the mapping of the multiple analog and binary signals from the IEDs is a very labor intensive process that also can be subject to errors at different stages of the engineering process.
The standardization process typically defines bay level objects, but more and more utilities are going in the direction of using standard substations, especially at the sub-transmission level of the electric power system.

It is important to understand that standardization, like everything else, has benefits and drawbacks. The analysis of both clearly shows that the benefits are much more than the drawbacks, especially if we consider the long-term benefits against the short-term drawbacks. Even though it will impose an initial cost and resource burden, in the long run it will lead to significant cost savings and improvements in the quality of the secondary systems. The benefits of such an approach can be further improved if the standardization applies, not only to the protection schemes' engineering, but also to IED configurations, settings, logic, etc.

Although some non-monetary benefits might be achieved in the short term, the standardized designs should be applied for a period of time in order to realize the full benefits anticipated, but this period should not be so long that the technology becomes obsolete or too far out of date compared with the latest available. The development of standard secondary schemes is based on:

  • Utility standards
  • Utility best practice
  • National standards
  • International standards:   IEC; IEEE
  • Industry best practice: CIGRE reports; IEEE Power System Relaying Committee reports

Detailed analysis of the standardization of protection and control schemes, definition of a standardization process, the benefits and challenges of this approach based on the experience and practices of many utilities from around the world is available in the brochure published by CIGRE working group B5.27 "Implications and Benefits of Standardized Protection and Control Schemes."

The contributions of this work, combined with the best practices from the established standardization process within a utility provide the foundation for the standardization strategy described in this article.

Power. Flexible. Easergy.
Protecting your electrical assets? today and tomorrow