Shifts in power generation call for reliable relay testing
by Damon Mount
Technology innovations tackle new electrical supply challenges
—Sponsored article by Megger
Distributed generation, also known as embedded generation, is a hot topic at the moment. With more and more generation being connected to the network, those concerned with its stability have to deal with increasing electrical power generation and distribution challenges.
Electricity supply networks were originally designed to supply electricity in a one-way system. Electricity was generated by a number of large power stations and delivered to the transmission grid for transportation over long distances to the distribution network, where the electricity was supplied to consumers locally.
While this is still the most common way of delivering electrical power, the increased use of distributed generation has changed the face of the network.
With encouragement from government policies, feed-in tariffs and the increase in renewable generation systems such as wind and solar, the electrical distribution network is increasingly becoming both a generation and distribution system.
While financial and environmental benefits can be gained by generating power locally, the approach can cause problems for the network. With multiple generation plants connected to the network, power is being fed back into it, reversing the usual power flows.
This can cause issues with the components of the system, resulting in contributions to the electrical fault level. Thermal ratings of equipment can be exceeded as can the capacity of the system.
Issues relating to power quality can also occur. Generation can contribute to the harmonic content of the supply, cause voltage unbalance and fluctuation, resulting in load problems and flicker. Then there’s the impact of the connection and synchronization with the frequency of the network.
If you plan to connect anything but the smallest source of power to the supply network, you have to put suitable protection in place to ensure in the event of a fault, your connection won’t compromise the security of supplies to other consumers on the network.
Protection relay test sets are readily available to do some or all of these tests but they typically split into two categories: very simple instruments capable of performing limited tests with only single-phase testing; and very sophisticated three-phase instruments, with provision for automatic testing of almost any type of relay.
Some single-phase test sets are extremely versatile, having the capability to test protection relay operation using variable voltage, current, and frequency outputs to determine the relay tripping times and levels. These single-phase sets tend to be relatively easy to use and have high power capabilities, allowing them to test the older high-burden over-current relays. They can’t, however, test three phases simultaneously.
The sophisticated three-phase test sets, by contrast, are easily capable of testing the latest relays but they’re necessarily expensive and many users don’t need the extensive range of options they offer.
Because of their sophistication, these high-end test sets have a relatively steep learning curve. Most G59 protection schemes do, however, require a test set with three-phase capability.
It’s clear there’s a need for an affordable three-phase relay test combining safety and versatility with ease of use. The best hardware in the world is of little value if it’s difficult to use, and this has been an all-too-common problem in the past with protection relay test systems.
To address this need, a relay test set conceived as an engineer’s “multifunction testbox” for protection testing has been developed. A key feature is an intuitive user interface that makes it unnecessary to use a PC to operate the instrument.
This new and innovative test set—the Megger SVERKER 900 centres around a touch screen, providing access to a wide range of pre-configured virtual test instruments.
In developing this innovative solution, designers took into account 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 on high burden relays or when carrying out single-phase tests on current and voltage transformers.
Full manual control and configuration are 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.
To make the instrument into a truly versatile testbox, additional tools are provided. These 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.
There’s little doubt protection relay testing in the distribution network is increasingly becoming a mainstream activity for electrical engineers. Fortunately, the latest test sets are making relay testing more accessible both technically and financially, with a variety of specification and testing capabilities.