The measurement of electrical quantities in a power system is one of the central tasks of every plant operator. For this purpose, current and voltage transformers supply the primary current and voltage values to measuring transducers, electricity meters and other measuring equipment. These measuring devices then forward the measured values by equivalent DC signals or via communication protocols to the data acquisition system. The measurement results provide the basis for system operation, for measuring the energy consumption for accounting purposes, and also for more extensive analyses.
The requirements regarding the accuracy of the measured data depend on the specific purpose. Regular functional testing and calibration of the measuring devices are either prescribed by mandatory standards and regulations or form part of an established quality assurance concept. All types of measuring equipment, such as power quality measurement devices, electricity meters or transducers, can only be considered to provide valid measured values after prior calibration. This requires a comparison of the device against a precise and reliable signal source for current and voltage.
Functional and accuracy tests of measurement devices are not only necessary at the place of operation in an electrical utility or industrial plant. Precision tests are also carried out on measurement equipment at the manufacturer’s premises, ranging from product development through type testing, certification and production testing to commissioning and handover to the end-user on site. In the development and production process, specific requirements are often applied to the test equipment, which can include the interaction with other test devices or their integration into an automated test environment. In acceptance testing, decisive factors in the choice of this equipment include: reliability, efficiency, test automation, data transmission and documentation. It is therefore advantageous to be able to exploit the synergies offered by a test device that is already available for other applications, such as protection testing.
Meter testing on-site and in a laboratory
Depending on the specific purpose of the meter, the accuracy requirements are different; CMC 430 and CMC 256plus accuracy allow testing class 0.2 meters as they are approximately 4 times more accurate than the equipment under calibration. If accuracy requirements are less, or for a plausibility check, all CMC test sets can be used.
These regular functional tests and the calibration of energy meters may take place in the laboratory or on site. Using one of the test sets mentioned above, tests can be performed without reference meters because the voltage and current sources of the test set are so precise that the test device itself is used as the working reference. For on-site testing in particular, this solution is much more convenient due to the substantial simplification of this test setup. Also, for laboratory calibration, the CMC is quite convenient because it can be used on its own or in conjunction with a reference meter. Especially for sporadic testing of meters it is a big benefit that the CMC test sets can also be used for protection testing. Both test sets, the CMC 256plus (Figure 1) and CMC 430 (Figure 2), provide test signals via high-precision voltage and current outputs (Figure 3). Furthermore, their inputs for the meter pulses allow closed-loop testing. To this end, special scanning heads for virtually any type of meter capture the pulses emitted by the meters.
For manual testing at site the CMControl P is the easiest and quickest way to control the test set. CMControl P is available as an App for Windows PC or Android tablets or as a dedicated front panel control device for a CMC test set. CMControl P provides an intuitive and guided user interface, so setting up a test is very convenient and feasible without special training. The new edition of the CMControl P App also offers testing without scanning heads. In this case, the meter readings before and after the energy injection are entered, the software then calculates the meter error which comprises the error of the measurement elements and the meter display (Figure 4).
The Meter module of the Test Universe software in combination with one of the CMC test sets allows for manual or automated testing of single and multifunction energy meters.
- Load test: Accuracy of measurement unit (time power method)
- Mechanism test: Accuracy of entire meter including display
- Gated Mechanism test: Testing internal meter registers
- Injection test: Quick check (wiring, sense of rotation)
- No-load test: No start-up at zero load
- Creep test: Start-up at low loads
The results of an automatic test are clearly summed up in a tabular test report (one line per test point).
For planning of all meter calibration activities, documentation of results and certificates, the ADMO maintenance management software can be used. It also takes care of supervision of the calibration status of the used test equipment and matches perfectly with the CMC test sets and the Test Universe Software.
Calibrating measurement equipment
Function and accuracy tests of measurement devices in electrical power systems are either prescribed by mandatory standards and regulations or form part of an established quality assurance concept.
Periodic calibration of in-house measurement equipment such as multimeters, transducers, current clamps, etc., is also a standard procedure in many enterprises. If measurement equipment is calibrated in an external laboratory according to the organisation’s quality assurance specifications (usually based on ISO 9000), in addition to a considerable financial overhead, the equipment is often unavailable for long periods of times. Because of their high accuracy, the CMC test sets are a versatile and portable solution for these applications. Their functionality makes calibrating measurement equipment a quick and safe in-house job to establish an internal calibration service that is very cost-efficient and can be performed when no routine tests are taking place.
Testing of Power Quality Meters
Many important production plants and other consumers require a certain level of power quality. On the other hand, non-conventional energy supplies, modern power converters in the network and non-conventional consumers lead to increasing distortion of the voltage and current waveforms in the network. Therefore, monitoring of the power quality becomes more and more important, especially in case of contractual obligations with important consumers.
Accordingly, more and more PQ-meters are applied, e.g. at connection points for decentralised power producers or feeders connecting industrial plants. To ensure correct monitoring of the power quality test and calibration of power quality meters is essential.
The PQ Signal Generator (part of the TU software) turns a CMC test set – ideally a CMC 256plus or CMC 430 with its high-precision voltage and current outputs – into a calibration tool that generates relevant power quality phenomena according to IEC 62586 and IEC 61000-4-30:
- Power frequency
- Power supply voltage
- Dips & Swells
- Voltage interruption
- Transient voltages
- Voltage unbalance
- Rapid voltage changes.
Depending on the type of CMC test set used, it is possible to generate voltage and current signals with superimposed harmonics (up to the 60th harmonic at 50 Hz or 50th harmonic at 60 Hz) and interharmonics up to 3 kHz or to enter harmonic magnitudes either in absolute values or in percentages of the fundamental value. For advanced applications, even fluctuating harmonics can be generated. The PQ Signal Generator allows the creation of comprehensive test sequences, to group test steps and to run repeatedly in a user-definable number of loops. If the test object provides a binary output, it can be used for automatic assessment of the test results. If no alarm contact is available, it is possible to perform a manual assessment.
Sample test plans typically used for type testing of PQ meters are available. A subset of these tests can be used for testing at site or for an acceptance test to select the best suitable PQ-meters for a given application.
Again, using the same test set and the same software platform makes this task very quick, easy and inexpensive.
OMICRON’s CMC test sets are not only a solution for testing protective relays and distribution automation schemes, they enable the user to benefit from a broad range of synergies. They can also be used for testing energy meters, power quality measurement devices, measuring transducers, Phasor Measurement Units and even to calibrate measurement equipment. These additional application scenarios make OMICRON CMC test sets much more efficient from a business point of view.
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