What is primary current injection testing, and what are its applications? What kind of test equipment is needed for primary injection testing, and what features should users expect to find in the latest test sets? The answers to these questions and many more are supplied by Damon Mount of Megger
Primary current injection testing is most usually associated with high current and high voltage power distribution systems of the type found in an electricity substation, or in a large industrial installation. The principle it is actually very straightforward: a test current is injected into the primary side of a system – which is often but not always some form of protection scheme – to determine how the system behaves at particular levels of current.
The system under test might, for example, comprise a circuit breaker with an over-current trip relay that operates via a current transformer (CT). By injecting a predetermined current into the circuit breaker, it is possible to determine whether the relay will trip at this current and, if so, how long the current needs to flow before the trip is initiated.
Something similar, of course, could be achieved by injecting a test current directly into the trip relay – that is, on the secondary side of the CT. This is secondary current injection testing and it is widely used, not least because much lower currents are needed than are typically required for primary injection testing.
Secondary injection testing is undoubtedly valuable, but it does not check all of the components in the system. In the scenario discussed, it would not, for example, reveal a defective CT. Neither does it truly mimic the operating conditions – the heating effect of the primary current will not be present and, in some types of test, this can significantly affect the results obtained.
For these reasons, there are many situations where primary injection testing is considered useful if not essential. Because it tends to be somewhat disruptive – the plant under test must be taken out of service and de-energised and then arrangements must be made for the high current connections needed for the test – primary injection is most usually performed as part of the commissioning procedure for new plant or after major modifications have been carried out. In some instances, however, it can also be an invaluable aid to faultfinding.
Test sets used for primary injection are invariably built specifically for this purpose. Their primary function is to supply a lot of current – tests typically involve injecting currents from 100 A or so up to 20,000 A. Equipment capable of delivering these sorts of currents is never going to be physically small or lightweight, but remarkable strides have been made over recent years in making primary injection test sets more manageable.
One way this has been achieved is by using modular current sources, so that for lower test currents only one or two sources are needed, but for higher test currents additional current sources can be added. Test sets that adopt this approach are often assembled on wheeled trolleys that can accommodate the control unit plus up to three or four current source modules. This arrangement makes the test sets much easier to handle.
Test equipment manufacturers have also noted that only a few applications of primary injection testing involve the highest currents – many requirements can be satisfied with test sets rated at no more than 5000 A, which paves the way for smaller mid-range units. In addition, the highest currents are usually only required for a comparatively short time, to test, for example an instantaneous overcurrent relay, so the test sets do not need to be continuously rated for their maximum current output. Once again, this allows size and weight to be reduced.
Weight and size are not, however, the only areas where progress has been made. Another useful development is the introduction of test sets where the control unit can be connected to the current generator by a comparatively long control cable. This allows the current generator to be placed very close to the equipment under test, thereby minimising the length of the high current test leads needed, which makes testing easier and more practical.
To ensure versatility, primary injection test sets need to be able to offer options to cope with a wide range of burdens since, if they do not, there is the possibility that they will not be able to deliver the required test current into the impedance presented by the equipment under test plus the test cables. In the best test sets, this issue is addressed by allowing the output voltage of the current generators to be raised at the expense of output current, so that the total power the test set is required to deliver is not increased unduly. This option is particularly valuable when testing CTs, circuit breakers and busbar joints.
Another option of great value is an integral timer that can be set to inject the test current for an accurately controlled time, preset by the user. This makes it easy to perform complete circuit breaker tripping time tests that encompass both the relay and the CTs, by injecting the actual fault currents. Auxiliary voltage and current measuring inputs facilitate the testing of CTs and good test sets can provide a wide range of data, including impedance, resistance, virtual power, active power, reactive power, and power factor, together, of course, with CT ratio and polarity.
A fast acting hold feature for the measuring functions, which is provided in conjunction with a “stop” input further enhances usefulness, as it allows readings to be frozen by applying a signal to the stop input. This makes it possible, for example, to record data relating to the exact moment that a protection relay operates during a test. Some instruments, when used for circuit breaker testing, can even be configured to automatically freeze the measurements at the instant the breaker trips without the need to use the stop input.
A feature that is just starting to become available on the latest primary injection test sets is zero-crossover synchronisation. This ensures that the test current is turned on only at a zero crossing point, which eliminates DC offset effects and also ensures the best possible repeatability for test results.
One issue that has been perennially troublesome in carrying out primary injection tests has been heating of the equipment under test while setting up and adjusting the test current. This effect has even been known to trip a breaker under test during set up before the test proper has commenced. Work-arounds are available – test engineers can perform the set up very quickly to minimise heating, or they can prevent tripping at least by isolating the trip circuits. Neither of these options is particularly convenient, however.
Fortunately, there is a better solution, in that test sets are now available with a so-called I/30 function. This, as its name suggests, reduces the programmed current output of the test set by a factor of 30. Since this means that the heating effect is reduced by a factor of 900, test engineers using this function can take as much care and time as they need in setting up the test with absolutely no risk of significant heating. And, when they are ready to start testing, the output current can be returned to normal at the push of a button.
Some of the principal applications of primary injection testing, including the testing of circuit breakers and CTs, have already been mentioned in this article. Some test sets can, however, also be programmed for more complex functions, such as testing automatic reclosers and sectionalizers.
Finally, it is worth bearing in mind that the high-capacity current source at the heart of every primary injection test set is also useful in its own right as convenient way of providing current to carry out heat runs on busbars and other types of switchgear assembly, and for testing ground grid installations where the test set is used to inject current between a reference ground and the ground to be tested. Measuring the voltage drop and the percentage of current flowing through the ground grid then enables an accurate assessment to be made of the ground grid’s performance.
Primary current injection tests are among the most valuable tests that can be carried out on power systems as they take into account the performance of every component and are, therefore, the most reliable way of assessing the performance of the system under real world operating conditions. In the past, however, primary injection testing has been fraught with inconvenience, not least because of the size and weight of the equipment involved, and because of its limited capabilities.
Fortunately, things have changed and, as we have seen, the latest primary injection test equipment is much more user friendly – and far less back breaking! For all those involved in the commissioning and maintenance of power distribution systems this could, therefore, be a very good time to take a closer look at how primary injection test sets have changed in recent years, and to look again at the benefits that this form of testing undoubtedly offers.
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