• Saravanan Balamurugan

A Protection Engineer's Diary: Simulation of a Symphony

You are a protection testing engineer and you walk into your site (a thermal power plant in this case). You are surrounded by huge rotating machines, buzzing to their own tune, gigantic transformers which convert the raw energy from the generators for the EHV transmission networks (400kV in this case), humming, as usual, and motors whirring with the raw sound and steam hissing to make sure you are paying attention to it all. It is a symphony for all the Power Systems Engineers out there. You can feel the power in the air (pun intended), along with the countless people making sure that this orchestra does not play out of tune, or worse, forget the song.

You enter with your Secondary Injection Test Kit, wearing your safety gear (Safety First !!!). Everyone looking at you with mixed feelings of tension, wonder and questions. You are here to here to make sure that the band of high potential 'musicians' are safe, without disturbing the music or disrupting the players. You are here to work ONLINE!!! Only the 140km 400kV Quad Conductor Line has been shut down just for you.

You settle down near the Main Line Protection Relay. You isolate the device under test, you connect your kit, make the wiring as per the schematics for current & voltage injection and test feedback from the relay, you see inquisitive people from various teams surround you to see your work, you create the test plan, you note down the relay details & settings, you open the software, perform simple wiring checks, and ready to inject your first fault. You look at everybody around you. Time slows down. You hold your breath when you click the 'Inject' button, waiting for the smooth click of the Master Trip Relay and... Injection Stops... Your ears are open for any missing 'tunes' or any additional 'booms' from the circuit breakers. You make sure the system is healthy and the orchestra continues its symphony of buzz and hums. You let out a breath of relief, gaining your confidence to continue testing till completion.

It does not matter how many times you have verified the configuration of the relay or checked the schematics of the panel under test, you are still going to miss out on a few, rather important details regarding the tripping / inter-tripping circuit. I won't hesitate to admit that I have tripped the remote end of a 100km transmission line (via inter-tripping), without even realizing it. So, what do you do?

You use a mysterious setting called the "Test Mode". You either create your own over-complicated Boolean logic as interlocks to provide physical isolation and/or use the Test Mode option from the Relay Menu Switch on Test Mode and do whatever you want, nothing happens to the live part of the system (Disclaimer: Not completely true. Read more about this based on the relay manufacturer). But what about the voltages and the currents? What about the Digital Inputs / Outputs? More importantly, what if there were no wires for any of them? What if all you could see was an ethernet/fibre optic cable?

I am talking about digital substations. Welcome to a Digital World, where the entire information, including measurements, statuses, tripping signals etc. flows through a communication network. What happens to you and your entire experience?

I recently attended a webinar by Omicron, titled "Protection Testing in Digital Substations", where a concept of signal simulation was brought up by the presenter. Digging deeper, I found a paper [1], presented by engineers from SEL, where they discuss two most important IEC61850 Edition 2 test features: Mode Control (widely referred to as “Test Mode”) and Simulation.

Testing a hardwired system varies significantly from a virtual setup such as any digital substation. The equivalent of live system isolation in such a system is a digital blocking, to make sure that there are no unwanted operations because of your work.

IEC 61850 Edition 2 provides an improved, yet still overly complex, explanation of the preferred testing terms and methods, and it specifies how IEDs should operate based on both the Mode (Mod) and the Behavior (Beh). Based on the flags within the GOOSE messages, the SV (Sampled Values) and the GOOSE messages are processed. With this clarity from the standard, protection engineers can test their system without complicated logic or time-consuming relay operations.

For instance, testing an in-service transmission line having a line differential protection function, the IEDs at both ends of the transmission line are set to Mod.stVal/Beh.stVal = Test/Blocked. All the remaining IEDs in the scheme (21L/67L, 50BF, and the bay controller) remain unchanged (Mod.stVal/Beh.stVal = On). With both 87L relays’ Mode/Behavior = Test/Blocked, no output contacts will be closed by these relays and the protection testing routines can be started. The published trip signals by the 87L relays will not be processed by other IEDs in the network; these trip signals will be discarded due to the (.q) field value mismatch. This physical and digital isolation will prevent 87L relays and other IEDs from tripping the breaker. Hardwired inter-trips do not operate as the output contacts do not close.

The GOOSE control block contains a Simulation attribute which is set to TRUE for a simulated message. Simulation is the state where the IED is configured to process simulated GOOSE messages (LPHD.Sim.stVal = TRUE). With Sim.stVal = TRUE, once the IED receives a subscribed GOOSE message with the Sim bit set, the IED will stop processing the normal GOOSE message in favour of the simulated message. A Simulated device will even process normal data if it does not receive any message with the Sim bit set.  Thus, a device in Simulation mode subscribing to multiple GOOSE or SV messages may process both simulated streams and actual streams simultaneously depending on the Sim flag of the published messages.

There are still a few caveats and pitfalls which still needs to be navigated in the standards & in its clarity, and implementation of such features from the manufacturers play a vital role in maximizing such an approach. You can use products from different vendors such as Omicron, who provide you with the option of playing around with the SV and GOOSE Messages. This approach reduces the pressure of a protection engineer (to a certain level...)

You walk into a substation (a digital one, in this case), connect your kit to an ethernet switch, simulate the currents/voltages as sampled values, use the Simulation mode, play to your heart's content, and resonate with the symphony of a perfectly tuned orchestra.

References:

[1] Edson Hernández, Tovah Whitesell, and Karen Leggett Wyszczelski, "A Practical Guide to Substation Testing Using IEC 61850 Mode and Behavior", Power and Energy Automation Conference.

[2] Baradi, Dinesh. (2019). Implementation of test mode in IEC 61850 Edition 1 and Edition 2.

DISCLAIMER: This article represents the opinion of the author and is not an advertisement of any kind.

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