Developments in protection relay testing

Recent developments have transformed the field of protection relay testing, with the latest multi-phase test sets in particular offering features and benefits far beyond those provided by even their recent predecessors. But exactly what can users expect from these new test sets? Let’s take a look.

Flexibility

Test sets with fixed configurations deny their users future proofing. In contrast, those that offer flexible configurations mean that future changes in requirements can be accommodated easily, conveniently and at modest cost. Flexible multi-phase test sets are now available that allow users to choose up to four voltage/current generators (VIGENs) to provide four voltages and four currents, or to combine the four VIGENs with one double-current generator to provide four voltages and six currents simultaneously, or even to combine three VIGENs with a single-channel voltage source (VGEN) to provide four voltages and three currents.

Other options available with some of the most versatile test sets include using five VIGENs with both of their channels configured as current outputs to provide a total of 10 current channels. This allows the latest multi-phase differential protection relays to be tested conveniently.

In addition, the ability to interconnect multiple units together, along with the convertible voltage/current channels, can provide up to 18 currents for testing multi-phase bus differential protection schemes.

Flexibility not only means that users can configure their test sets so that they match their exact requirements, but also that they can initially purchase a unit with a modest specification safe in the knowledge that they can upgrade it to a full four- or even five-channel unit in the future, should they need to do so.

Constant power output

The latest high-power voltage-current amplifiers deliver maximum compliance voltage to the load constantly during the test, and range changing is done automatically under load. This ensures better test results and saves time, as the outputs don’t have to be turned off to change ranges, as was typically the case in older instruments.

In many cases, constant power output eliminates the need to parallel- or series-connect channels when testing high-burden relays. Outputs are regulated and, therefore, they are unaffected by changes in load impedance which, for example, occur with saturating loads like inverse and extremely inverse time characteristic electromechanical overcurrent relays.

Higher output current

Best-in-class current amplifiers now provide 30 A at 200 VArms per phase continuous and up to 60 A for 1.5 seconds. Six of these amplifiers can be paralleled to provide 180 A at 1,200 VA or up to 360 A at 1,800 VA for 1.5 seconds.

Low range with higher accuracy

The availability of a 1 A range provides enhanced accuracy at low test currents. Low current ranges typically also have reduced switching noise, which improves regulation and accuracy with test currents as low as a few tens of milliamps.

Constant power output voltage amplifiers

The new voltage amplifiers provide higher current output at low test voltages, from 30 V to 150 V. This is important for users who want to test a complete panel of relays as a single operation, as this is often impossible with older test equipment that has voltage amplifiers with a limited VA rating. The new amplifiers also solve the problem of testing old-style electromechanical distance relays that can only be tested using a high-power voltage source.

Compact construction and light weight

The best of modern test sets are very much smaller and lighter than their predecessors. Specifiers and intending purchasers should, however, exercise a little caution in this respect. There are compact, lightweight units available that have achieved their diminutive dimensions and weight by sacrificing performance, particularly in terms of output power. These units are best avoided as, for most users, their limitations outweigh their benefits.

Large, high-resolution colour display

An important innovation is the introduction of large, high-resolution touch-screen colour user interface systems that provide intuitive automatic, semi-automatic and manual control of the test set. In leading instruments, the screens are complemented by a context-aware control knob that allows easy and convenient adjustments to be made to key parameters related to the particular test being performed.

Automatic ramp, pulse ramp and pulse ramp binary search capabilities

The best test sets have all of these capabilities and they also allow users to use the control knob to ramp outputs up and down manually. When using the automatic ramp functions, users can easily adjust the ramp increment to suit their requirements.

Click-on-fault impedance relay test screen

Another key feature found in the latest instruments is a click-on-fault test screen facility for easy and convenient testing of impedance relays. Typically, all the user needs to do is to select from a menu the generic operating characteristics of the relay to be tested, or to select its exact operating characteristics from the instrument’s relay library.

Click-on-fault impedance relay library

An integral relay library means that users simply pick from a list of predefined operating characteristics for popular relays from manufacturers such as ABB, AREVA, General Electric and SEL. They then enter the relay settings and select the type of fault desired – line-to-earth, line-to-line or three-phase. The test will then progress down a line as a ramp, pulse ramp of pulse ramp binary search, looking for the relay to operate. With the shot test method, users can also select a test point within and outside the characteristic to quickly check the relay under test.

Real-time click-on-fault impedance test screen

This feature allows users to see, on the screen, the operating characteristic of the relay under test with the test pointer moving in the impedance plane to indicate exactly where the test impedance is, in real time. When the relay operates, a coloured test point is displayed showing where the relay operated – the test point is shown in red if the relay failed the test, green if it passed. The user can also see the test vectors changing in real time, or see positive-, negative- and zero-sequence vector values. When the test is complete, the test results are displayed for each defined test point with pass/fail indication.

Binary inputs and outputs

User-configurable binary inputs to monitor contacts and high-current binary outputs are invaluable, and in leading instruments they can even be allocated user-defined names, such as Trip1 or Recl1, to make them even easier to work with.

Overcurrent testing with IEC/IEEE curve algorithms and relay-specific curves

The test set evaluates timing tests by automatically comparing actual and theoretical operating times. The user doesn’t need to refer to the manufacturer’s time curve to determine error, which means that this feature is a big time saver. The test set displays the time curve for the relay under test, plots the test results automatically with the time curve, and determines pass/fail.

Dynamic sequencing test capability

This feature provides multi-state dynamic testing to include trip and reclose timings tests for, typically, up to 15 states, including lockout. Each binary input and output can be set with each state along with voltage, current amplitudes and phase angles. With each state, the user may input values of voltage, current, phase angle, frequency and phase angle, as well as setting the binary input sensing. With some instruments, three-pole tripping as well as single-pole can be simulated.

Simplified testing of three phase transformer differential relays

In order to correctly test new three phase transformer differential relays, three compensations must be taken into consideration by the relay; current magnitude compensation, phase angle compensation and the zero sequence current compensation. By performing transformer differential tests for each sequence component verifies that the differential protection will be stable for all symmetrical and non-symmetrical external faults, and through-load conditions. In addition, tests will also confirm that the differential relay will trip for any internal fault. The most modern test systems can not only provide the six currents required for testing two winding transformer differential relays, but they also should include both ANSI and IEC transformer models to accurately calculate the required test currents and phase angles relationships that get applied to the relay. From the transformer model, the test system should be able to take into consideration the transformer vector groups, be able to eliminate zero sequence when testing single phase to earth faults. Using a touch screen to select test lines, it should also be able to perform dual slope characteristic tests using one of the seven different Biasing equations, and one of the four different slope characteristics that the modern microprocessor based relays use today.

Manually defined harmonic waveforms

A recently introduced feature on top-of-the-range test sets, this allows users to seasily create harmonic waveforms consisting of a fundamental plus superimposed second, third and fourth harmonic waveforms. Individual harmonic amplitudes or phase angles can be easily ramped as required. This feature provides harmonic restraint testing with a single current channel and is also useful for checking the performance of power quality meters.

With the latest relay test sets offering so many new and useful features, now is a very good time for those involved with protection system testing to take a long hard look at the test equipment they are currently using. Undeniably, a new test set represents a significant investment, but if the new test equipment prevents just one significant power outage it will, for most users, have paid for itself many times over.