by Alex Apostolov, Editor-in-Chief
Do your homework!
IEC 61850 has created an environment for innovation based on the peer-to-peer communications, object-oriented models and freedom of functional allocation that allows a wide range of possible system architectures that can meet in the most efficient way the specific requirements for reliability, security and efficiency of any user.
This amazing flexibility gives tremendous power to the protection, automation and control specialists, but like everything else in life it comes with a price – you must do your homework. We have to ask a lot of questions and answer them during the system design process.
When the topic is IEC 61850 based centralized substation protection and control systems, the first question is “What do we mean by that?”
We can define a pure centralized substation protection, automation and control system (CSPACS) as a system that integrates the acquisition and processing of power system currents and voltages, as well as switchgear status and based on the extracted data performs all required functions, such as transmission line, transformer and distribution feeder protection, supervisory control, disturbance and sequence of events recording, etc. If any abnormal condition is detected, the CSPACS will operate the circuit breakers to clear a fault or restore the power.
All of this is achieved based on hardwiring between the secondary sides of the instrument transformers and auxiliary contacts of the switchgear with the CSPACS.
While this was the approach that looked like the right one thirty years ago, this is not what we should be doing today. The reason is that even though 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 sampled values based analog interfaces and GOOSE based switchgear monitoring and control, it is clear that we are shifting towards CSPACS that do not meet the pure definition introduced earlier. This is because today we can take advantage of fiber optic communications between the devices providing the interface to the substation primary equipment and the centralized components of the system.
From this perspective it becomes obvious that this is a step in the evolution of the digital substations. As soon as we say that, there are many possible CSPACS implementations depending on the allocation of functions between the process interface devices and the substation servers performing the protection, automation, control and all the remaining functions.
This is where the issue for doing our homework comes into play. We need to optimize the system architecture based on the specific requirements for reliability, security and efficiency. We must analyze if performing the distance protection function for a specific transmission line at the substation server will be fast enough, considering the time to receive and process the sampled values, make a decision if the fault is within the zone of protection and send a GOOSE message to the switchgear interface device to trip the breaker and clear the fault. If this is not fast enough, we must consider including local distance protection elements being integrated in the transmission line process interface device that will operate based on the sampled values from the local merging unit and directly trip the breaker if necessary.
The allocation of function elements that require very high sampling rates, such as travelling waves and partial discharge detection is also more efficient in the process interface devices.
It is very exciting that today we have the technology that allows us to build highly reliable substation protection, automation and control systems, but we need to do our homework and figure out which centralized system architecture will best fit our specific requirements.
“The real trick in highly reliable systems is somehow to achieve simultaneous centralization and decentralization.”Karl E. Weick