Centralized Substation Protection, Automation & Control Systems (CSPACS)

by Alexander Apostolov, USA

We have gladly accepted this elevated level of functional integration without worrying much that we are putting all our eggs in one basket. And it is because the individual functions are not isolated but integrated in a sophisticated system that can survive the challenges of its environment.
Accepting the high levels of functional integration in our everyday lives now is transitioning into our professional lives as protection, automation and control specialists. We are already using for a couple of decades multifunctional intelligent electronic devices. This experience will make it easier to get to the next level – Centralized Substation, Protection, Automation and Control Systems (CSPACS).

What are we doing?
IEC 61850 is the standard that we can consider as the glue that allows us to combine the advanced computer and communication technologies into sophisticated protection, automation and control systems. Digital substations are becoming a reality in many countries around the world and are accepted as the standard for future substations for many major utilities.
The availability of powerful industrial computers and real time operating systems are creating an environment supporting the next step in the evolution of digital substations - Centralized Substation, Protection, Automation and Control Systems (CSPACS).
This idea is definitely not new – it was first introduced half a century ago by George Rockefeller in his paper "Fault Protection with a Digital Computer" published in 1969 in the IEEE Transactions on Power Apparatus and Systems. The concept was implemented by Westinghouse which developed the first digital relay with the Prodar 70 between 1969 and 1971. It was commissioned in service on a 230 kV transmission line at Pacific Gas & Electric's Tesla substation in February 1971 and was in service for six years. With the development of microprocessor technology, the protection and control systems became distributed, based on multifunctional Intelligent Electronic Devices (IED)s. However, at the same time Ontario Hydro in Canada developed an Integrated Protection and Control System (IPACS). Their analysis convinced them that when each device independently acquires and processes the power system signals from current and voltage instrument transformers, circuit breakers and disconnectors, tap changers, etc. in order to perform its assigned function it limits the efficiency of the system.

The first disadvantage is the cost associated with purchasing and installing each device acquiring power system signals independently. Wiring the same current, voltage and binary signals to multiple IEDs is costly and time consuming, something that was not acceptable to them. On top of that was the need for exchange of signals between the individual devices for the implementation of different protection schemes – something else that was typically done using hardwiring. That is why they decided to take advantage of modern microcomputers, digital signal processors, analog-to-digital converters, optical sensors and fiber optic communication technologies to acquire and process electrical power system signals in a new and more efficient way – in a centralized protection and control system.
The concept was proven when the first integrated protection and control system was placed in service in July of 1994. All the system functions were enabled, but the protection functions were operating in a monitoring mode for a six-month period to verify that the system would not initiate incorrect trip signals. After the six-month monitoring period the different protection functions were gradually armed, thus proving that fully integrated substation protection, control, monitoring and recording is technically feasible and practical, providing a more efficient and flexible solution.

The IPACS can be considered as a pure centralized protection and control system, because it has direct hardwired – copper or fiber – interfaces to the substation equipment and conventional instrument transformers or optical sensors, while all the analog to digital conversion and digital signal processing is performed by the IPACS. Also, all the protection, control, monitoring and recoding functions are implemented in the central unit. (Figure 1).
About the same time a centralized integrated protection and control system for a small distribution substation was implemented by me in a universal, user programmable protection IED and presented at the Georgia Tech Protective Relay conference in 1995.

The higher level of functional integration achieved by such centralized systems definitely improves the efficiency of the protection and control system but does not eliminate all the problems that we experience in traditional hard-wired systems. Even that we save a lot by eliminating the hardwiring between all the different measuring, protection and control devices by integrating them in a centralized system, we still have the extensive hardwiring between the control house and the substation equipment, as well as issues such as current transformers saturation and open CT circuits.
With the development of IEC 61850, resulting in communications-based analog interfaces using streaming sampled values, as well as GOOSE based switchgear monitoring and control interfaces, it is clear that we are shifting towards a new generation of centralized systems that bring together the best of both worlds – the benefits of the digital substation and the efficiency of the centralized system.

However, from a pure definition point of view the IEC 61850 based CSPACS is what we will call a hybrid system, because at least the digitalization of the analog and binary signals from the substation equipment is performed by the different process interface devices, while protection, automation, control and many other functions are running on the central substation computer.

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