A high precision system - for metrology institutes and test houses - Yogesh K Nama, ZERA GmbH, Germany
The meter testing requirements of manufacturers and utilities are different. Manufacturers' needs are based on the volume and fixed design of the meters, while utilities want to be able to test various types of meter in volume. Metrology institutes or test houses, however, which are responsible for meter design approval, require more sophisticated systems that can test various types of energy meter and meter testing equipment, as well as the ability to approve the design of newly developed energy meters. This paper discusses the requirements and characteristics of such test systems and proposes a unique solution for such institutes.
The test system for metrological institutes or test houses has the following requirements:
- Generation of voltage, current and phase angle through electronic amplifier
- Operation control through computer
- Ability to test various types of electromechanical and electronic meter
- Ability to test various meter testing equipment (called Sub-Standard Meter SSM or working standard) used by utilities and manufacturers
- A highly efficient and stable source
- High precision in measurement, very low standard deviation – e.g. 20ppm
- Very low distortion even at non linear load
- Sufficient output power to feed the voltage and current
The test system requires some special features for the testing of energy meters or meter testing equipment under special circumstances, or in order to perform the test as per IEC standards. Hence test systems should be capable of:
- Generating harmonics, which are required to test the meter as per IEC standards
- Generating special waveforms or chopped waveforms
- Creating the unbalance in magnitude and phase angle
- Reading electronic meters in order to check the various functions of the meter
- Testing SSM or working standards through pulse output
In 1996 ZERA began the development of a Universal Meter Testing System to meet these criteria, using the latest technology.
The basic principle of operation is shown in Figure 1. PC software gives a command to the waveform generator to generate the digital signal for the amplifier according to the desired setting values or test parameters. The waveform - generator is the central unit of the synthetic waveform generation. It generates the set points for the digital control of the voltage and current amplifier unit, and carries out the closed loop control of the test setting and control changeover operation during the test procedure. The resolution of output setting per phase is 3600 samples per period and 16 bit in amplitude.
The use of ASIC/LCA technology makes the system more versatile and useful. This unit is also responsible for the superim-position of harmonic components in each phase, generation of waveform, transmission of ripple control telecommands and phase control. The closed loop control is applied by the waveform generator to ensure the high stability and accuracy of the setting. The waveform generator has six channels (one for each phase) and a further two channels can be added.
The voltage and current amplifier is designed to provide sufficient power for full range testing (e.g. 400VA for each phase in the voltage circuit and 1000VA for each phase current circuit). The amplifier is designed for high efficiency, high stability, precise setting and low distortion even at nonlinear loads.
The high precision comparator (COM303-03) is controlled by the waveform generator and provides the actual value feedback for amplifier output correction. That means output value will be more precise.
The comparator is connected directly to the test circuit without any additional CT. This means that measurement accuracy will be the same as comparator accuracy.
The output pulses of each SSM under test or rotor mark or LED blinking of the energy meter sensed by the scanning head are compared at each measuring position by means of an error calculator. The error calculator indicates the percentage error of the device under test up to 0.001% resolution. The measuring interface device helps in configuring the error calculator located at each measuring position.
The picture in Figure 2 shows the test bench supplied to one of the national metrological institutes. It is suitable for testing five energy meters or three SSMs at a time. The system contains the high precision comparator, which has an accuracy of 50 PPM in voltage and current measurements, and 100 PPM for power/energy measurements. The comparator can be supplied duly calibrated by PTB, Braunschweig or DKD laboratories. It has a temperature coefficient of 1PPM/K and a standard deviation of 20 PPM. It can be checked by a DC standard cell, which is normally required by any metrological institute to ensure proper working of standards as per ISO/IEC guide 25.
The test bench contains five error calculators and three special devices to receive the frequency or pulse output of the SSM. The error calculators are connected with a parallel busbar and can be interchanged. The modularity concept makes the whole system very flexible, so that it can be upgraded on demand in the future.
The power source contains one voltage amplifier and three current amplifiers and one waveform generator, as described above.
The test bench is supplied with a PC and suitable software to control all the components of the system and conduct the testing of energy meters or SSMs. The software is also suitable for superimposing the harmonics and special waveforms required for design testing. Automatic tests can be performed as per desired load points and the resulting data will be stored in the PC.