Heroic developments in relay testing
Niclas Wetterstrand, business development
Progress is a wonderful thing but, particularly in the world of electrical technology, progress in one area can produce significant challenges in another. Recent developments in protection relays have called for almost heroic efforts from test equipment manufacturers to devise products that will test the latest relays easily, effectively and safely while retaining the versatility needed to work with older types.
There is without doubt a wide choice of protection relay test equipment on the market; indeed, many of the most popular models are made by Megger. So it would perhaps have been understandable if the members of the company’s design team had been content to rest a while on their laurels and devote their efforts to producing useful but relatively minor upgrades.
However, word reached their ears that the requirement of some users and potential users of relay test equipment were not being adequately met and that there was a pressing need for a new and innovative test set to fill a growing gap in the market.
Some users were reporting that they needed to test simple voltage and current relays but that the latest three-phase types were so intelligent that they couldn’t be tricked by injecting just one phase. Others were saying that they needed to perform tests on three-phase relays connected to the 230 V AC network, but that simpler test sets couldn’t produce a sufficiently high output voltage.
A complaint heard from many users was that they’d bought sophisticated – and therefore costly – test sets to gain access to just a few of the advanced options provided but, in their applications, up to 90% of the remaining functionality would never be used. And an almost universal comment concerned the need to combine versatility with simplicity and safety.
Many three-phase test sets can be configured to provide higher voltages and currents for single-phase testing but this typically involves setting up a tangle of cables and connectors. Not only is this time consuming, it’s all too easy to make mistakes that could endanger both the instrument and the user.
At first it seemed unlikely that it would be possible to produce a reasonably priced instrument that would address all of these issues, but the intrepid members of the protection relay test equipment design team are not easily daunted! They continued to chip away at the challenge until a practical solution started to take shape in the form of a multifunction testing “toolbox” for substation protection testing.
In developing this innovative solution, key factors the designers took into account were the need for modern distribution relay protection schemes to be tested by three-phase secondary injection and also the requirement for test sets to provide high-power, high-amplitude outputs for use when carrying out single-phase tests on current and voltage transformers, and primary injection into switchgear busbars. And, of course, they knew that changing between three-phase and single-phase mode had to be fast, easy and safe.
The “toolbox” concept was fine but, as is so often the case, translating it into reality was much more of a trial. In particular, finding a convenient way of switching between three-phase and single-phase operating modes without that inconvenient tangle of wires proved, at first, to be something to be something of a headache.
Then inspiration struck – don’t use flexible wires, use rigid links to make the necessary connections! At a stroke, the inconvenient cable tangle was eliminated and, after some very careful sketching and measuring, it even proved possible to position the sockets for links in such a way that it would be physically impossible to make the wrong connections. These simple yet novel ideas would allow convenience to be combined with safety and speed, exactly as users were requesting.
Equally innovative thinking was applied to other aspects of the instrument’s design. It was provided with three fully isolated current channels that can be connected in series or parallel and are capable of delivering 35 A DC or AC over the range 10 to 600 Hz. The maximum capacity is a generous 250 VA and the compliance voltage is 50 V at 5 A. For single-phase testing, the three outputs connected in parallel can deliver up to 105 A for 10 seconds intermittent or 60 A continuous.
The instrument also incorporates four fully isolated voltage generators that, once again, can be connected in series or parallel. Individually, these provide up to 300 V DC or AC from 10 to 600 Hz and have a maximum capacity of 125 VA. In single-phase applications, with three of the voltage generators connected in series, output voltages up to 900 V can be produced, leaving the fourth voltage generator available to power the relay. As might be expected, the frequency, amplitude and phase angle of all voltage and current sources can be independently controlled.
That’s a great start, but the concept for this instrument was that it should be a multifunction engineer’s “toolbox”, so a few more tools were needed. Those ultimately provided include an analogue ammeter and an analogue voltmeter, together with facilities for measuring a wide range of parameters such as phase angle; active, reactive and apparent power; power factor; resistance; reactance and frequency.
Further versatility was built in by providing a binary output and four independently programmable binary inputs, complemented by a timer that can, if required, be used independently of the current and voltage generators. A USB port for firmware upgrades and for uploading and downloading test files was also incorporated.
The design team could by now be content with the hardware specification for their new instrument, but the best hardware in the world is of little value if it’s difficult to use – and this has been an all-to-common problem in the past with protection relay test systems. The team was therefore determined to ensure that the new instrument’s user interface would be intuitive, simple and user friendly.
To achieve this, the interface centres around a touchscreen that provides access to a wide range of pre-configured virtual test instruments, allowing the required test function to be selected quickly and easily. The main virtual instrument provides timing tests as well as options for the manual determination of relay pick-up and drop-out points together with general facilities for generating, injecting and measuring currents and voltages.
Other virtual instruments include a CT magnetisation instrument, a prefault-fault instrument, a ramping instrument and a sequence instrument. An impedance instrument is also provided, which allows relays to be tested directly from the impedance plane with conversion from impedance to voltages and currents carried out automatically by the test set.
Full manual control and configuration is also supported and, in addition to the touchscreen, the instrument is provided with a large rotary knob that can be configured as required to control the current and voltage generators. Test configurations and test results are stored in internal non-volatile memory and can be readily transferred to and from a PC via the instrument’s USB port.
An advanced mode that allows harmonic generation is provided. When this mode is selected, each individual voltage or current generator can be set to produce a harmonic waveform, and a second or even third layer of harmonics can be superimposed over the fundamental frequency.
Will the endeavours of the Megger design team in developing the new instrument, which has been designated the SVERKER900, ultimately be successful in vanquishing the challenges of complexity, high cost and lack of versatility in protective relay testing? Only time and the market will tell for sure but initial indications are favourable.
The new instrument is already attracting attention for use in applications that include commissioning and maintenance of distribution substations and generator installations; testing of electromechanical, static and numerical protection relays; plotting current transformer excitation curves; burden measurement for CTs; polarity/direction measurement; impedance measurement; single- and three-phase injection testing in switchgear; checking SCADA annunciation and measurement values; and performing network measurements. A veritable engineer’s toolbox indeed!