Portable oscilloscope hints for the field technician - Part 2

Following my previous post on possible uses of a portable oscilloscope, I have run into another few situations where my Fluke 125B has come in really handy. So I decided to turn this into a 3-article series. For this article, I will dive a bit deeper into how you can perform a quick verification of a generator's synchronisation system. In the last and final article we'll do a deep-dive into data analysis based on an example of measurements with a portable knock sensor, mentioned in article no. 1.

Before diving into the use of the oscilloscope on this topic, it is worth clarifying 'synchronisation'. In short: when a (synchronous) generator is started up and ready to be connected to the electrical grid for production of electricity, it is important that the AC voltage produced by the generator is very 'similar' to that of the electrical grid. By this we mean that the voltage, the frequency and the phase angle all match within a certain tolerance. Only then is it safe to close the generator circuit breaker (GCB). Closing the GCB when the generator is not synchronised is a guaranteed recipe for disaster! A reasonably detailed explanation can be found on Wikipedia - synchronization.

Modern generators all have one or more components in their control system that will ensure automatic synchronisation prior sending a close command to the generator's circuit breaker. Sometimes there is a synchroscope, which allows the operator to verify the process of synchronisation graphically - which is useful as sometimes it takes a bit of time.

But if you're equipment is new, or was upgraded - how do you verify that the synchronising equipment is correctly performing its function? There is of course specialist equipment used by most electrical contractors that can be used to verify the function. But when in need, a trusted portable oscilloscope with just 2 channels can give you a very good indication too.

Before you proceed: safety first! Ensure always to comply with site safety regulations and as a very minimum ensure all lines and busbars are isolated and tested dead before connecting test gear.

Setup:

• Channel A: connect this input to the GCB closing signal and use it as a trigger for refreshing the screen
• Channel B: connect this input to a differential voltage probe (eg. MicSig DP)
• Differential probe: connect to a single phase on either side of the GCB

Test:

• Lock the GCB out or disable it from closing
• Run the generator up and enable the synchronisation process
• Every time the synchroniser gives a command to close the GCB, you should see the differential voltage between the grid and the generator reduce to practically zero. See image below.

This approach has of course limitations but it does give a quick and easy graphical indication that the GCB close command comes at a reasonable point in time when synchronising is happening.

Intrigued or confused? Get hold of someone at Stark Engineering Consulting - we'd be happy to pass on some more details to get you going.

#automation #jenbacher #starkengineering #TipTuesday