Primary injection testing: challenges and solutions
This article is an edited extract from a paper presented by Robert Probst of Megger at the 2017 PowerTest Conference hosted by NETA.
Introduction
Field personnel face a range of difficult challenges when carrying primary injection tests. These include: input power requirements, test circuit impedance, high current requirements, high impedance test objects, setting target currents, current decay, DC offset, comparing measured trip times with time-current-curves, data storage and trending, report generation, and the size and weight of the test equipment. This article looks at each of these challenges and discusses how they are being addressed in modern test equipment.
Input power requirements
Unfortunately, primary current injection instruments are not magic! Even modern solid-state power converter designs still need a step down transformer of a certain size to generate high currents at extra low voltages on the secondary side. Therefore, output power equals input power minus losses.
Having the correct mains input voltage and current capabilities available is very important to achieve the maximum output of the test equipment to test high rated circuit breakers. Testing at lower trip settings may be necessary if the low-voltage circuit breaker exceeds the capabilities of the test equipment, but this does not necessarily verify its proper operation. Modern instruments accept a wide range input power supplies, giving the option of working on different supply voltages—typically 100 - 240 V—to obtain more power and more flexibility.
Test Circuit Impedance
In terms of fundamental circuit theory, primary injection test sets behave like a current source. No real-world current source is ideal, so whereas the theoretical model of an ideal current source would provide infinite power (constant current at infinite open-circuit voltage), real-world current sources have definite limitations. Any impedance in the test circuit causes a voltage drop across the output terminals of the current source. Therefore the impedance, and thus the voltage drop, burdens the instrument. If the burden becomes too high, the required current can no longer be supplied. Therefore, it is very important for field personnel to optimise the setup as poor setup can result in a drastic decrease of the output capabilities of a primary injection test set.
Ideally, the only impedance burdening the output of the instrument would be the contact resistance of the circuit breaker pole itself. However, this is not achievable in the real world. Additional impedances are inevitably introduced and these cause additional voltage drops. These impedances principally result from the resistance and inductance of the test leads, and the transition resistances at the connection point of the leads to the instrument and the circuit breaker under test.
How can these issues be addressed? To minimise transition resistances, bolts need to be tightened or torqued sufficiently and lugs need to be properly crimped. To minimize lead resistance, users can increase the cross-sectional area of the leads either by using heavier cables or by connecting multiple cables in parallel. The leads should also be kept as short as possible. The loop inductance of the test circuit is related to its geometry and is proportional to the area encircled by the test leads and the circuit breaker under test. It is imperative to keep this area as small as possible. Twisting the leads around each other is a proven way to reduce the loop inductance dramatically.
High current requirements and high impedance test objects
Continuous current ratings of low-voltage circuit breakers range from less than 10 A for miniature circuit breakers (MCBs) up to 8000 A on big arc-fault circuit breakers (AFCBs). In the average panel, however, the majority of circuit breakers will be moulded-case circuit breakers (MCCBs) rated at 200 A or less—especially in nuclear power plants—with branch and feeder circuit breakers that have higher ratings. There is no test set available that is capable of covering the full range of breakers.
Modern primary injection instruments address this problem with a modular approach. Multiple test sets can be synchronized in a cluster and act together in master-slave mode. The cluster may be configured in parallel for higher current or in series for higher compliance voltage. Modular sources increase the efficiency of testing for the user because several technicians can work independently on different panels, being able to test the majority of CBs in the panel with a smaller portable unit. It is only necessary to put these units together in a cluster when testing the higher rated branch and feeder breakers.
The option to configure a cluster in series also allows the testing of high-impedance test objects where a high compliance voltage is necessary to drive the required test current. Figure 1 shows a cluster with two modern primary injection instruments configured in parallel to produce enough current for a large circuit breaker.
Figure 1 – Cluster of two primary injection instruments in parallel
Setting target test currents
Dialling in a specific test current on a classic primary test set is a highly manual process and also somewhat dependent on the experience level of the user. Typically it includes as a minimum programming the instrument for pulsed mode and then manually adjusting the variable autotransformer in steps until the target current is reached. Whenever the test object changes, the impedance of the test circuit changes as well, and the user has to repeat the procedure.
Modern primary injection instruments avoid any form of inconsistent pulsing by using software-driven control schemes together with the regulated output of the solid-state power converter. Before a test is initiated, the software injects a small fraction of the requested test current and measures the impedance of the test circuit. Based on these measurements, built-in algorithms calculate exact settings for the driver circuits in the power converter. This will bring the actual output of the test current to 100% of the requested current immediately after turn-on.
Current Decay
During testing, all of the components in the test circuit, especially the test leads and the circuit breaker pole under test, will heat up as a result of the high currents applied. This means that with no compensation, the test current decays as shown in Figure 2.
Figure 2 – Current decay
A classic primary injection test set must be manually adjusted by the user to compensate for this – otherwise the test results obtained may not be valid. As shown in Figure 3, modern primary injection instruments incorporate a continuous feedback loop that automatically regulates the current and holds it steady at the requested value. No manual correction is required.
Figure 3 – Automatic current regulation
DC offset
The instantaneous performance of a circuit breaker is sensitive to the presence of asymmetry, which may cause the circuit breaker to trip at incorrect current amplitudes and therefore after incorrect times, resulting in incorrect test results. While minimising the DC offset is extremely involved and time-consuming on a classic primary injection test set, modern primary injection instruments do not require the user to do this.
The software addresses the problem automatically in the same way as it deals with current decay and the setting of the target test current (see above). Before the test is initiated, a small fraction of the requested current is injected to measure the impedance of the test circuit. The turn-on of the solid-state power converter is then timed with respect to the correct phase angle, so that any DC offset is eliminated.
Comparing trip times with time-current curves (TCCs)
Manufacturers tend to compress all of the information about a circuit breaker’s time-current characteristics into a single plot that is typically provided in the service manual, supplemented by various notes. Figure 4 shows an example.
Figure 4 – TCC for Eaton (formerly Cutler-Hammer) Amptector II
If a user with a classic test set wants to test an Amptector II, this specific curve has to be readily available either as paper copy or in electronic form as a PDF version. Then the measured trip times have to be manually compared with the curve to determine whether or not the circuit breaker has passed or failed the test.
Modern primary injection instruments have built-in expandable software libraries with thousands of digitised TCCs. As illustrated in Figure 5, digital curves allow for a real time plot of the test results, and therefore eliminate the need to manually compare trip times against a paper copy or PDF version. Determining whether the measured trip times are within pass/fail limits is done automatically.
Figure 5 – Curve Library and digital TCC with live test results and pass/fail indication
Digital TCCs also provide the possibility of defining the set points of the TCC according to the actual settings on the trip unit of the CB. The software will then adjust the curve precisely to the configuration that the user is actually testing. See Figures 6 and 7.
Figure 6 – Software-adjusted digital TCC according to the actual short time settings on the AFCB trip unit; band = 0.2 s, short time pick-up at 2.0x sensor rating, i²t in
Figure 7 – Software-adjusted digital TCC according to the actual short time settings on the AFCB trip unit; band = 0.3 s, short time pick-up at 2.5x sensor rating, i²t out
Report generation, data storage and trending
After having completed the testing and successfully compared the test results against the curve, compiling a test report is the most time-consuming step. Modern primary injection instruments have an automatic report generation feature that allows the user to save and print a complete test report in the field, immediately after the testing has been carried out. This report includes the CB’s TCC, all of the test results in tabulated form as well as graphically indicated against the curve, header information and, if recorded, additional test data such as contact resistance or insulation resistance. Figure 8 shows an example.
Figure 8 – Test report generated automatically immediately after the completion of testing.
The test data is stored in a native format and can be exported to a database for future trending of the same or similar CB models.
Conclusion
Most of the challenges discussed in this article are effectively addressed by the latest generation of primary injection test sets. Solid-state power converter technology makes it possible to produce equipment that is smaller and lighter, yet has higher output capabilities. The software-based graphics user interface, the regulated output and the integrated expandable circuit breaker curve library introduce new tools that make primary testing easier, quicker, more consistent and more repeatable without any variance between different personnel. The software also provides features for automated report generation and integration of the test data into asset management and maintenance database systems.
About NETA
NETA is an ANSI Accredited standards developing organization that creates and maintains standards on electrical testing for electrical power equipment and systems. NETA is an association of leading electrical testing companies comprised of visionaries committed to advancing the industry standards for power system installation and maintenance to ensure the highest level of reliability and safety.
