By Yogesh Nama
Challenges faced by the meter tester on-site and technological advances have led to new designs, functionalities and features in the latest series of portable meter testing equipment. Today these devices do not merely serve as accuracy testing equipment, but also have highly advanced value added features. Among these are:
Instantaneous parameter display, which helps the tester to diagnose the condition easily and quickly on-site
- Vectorial display, which helps to identify the correctness of the connection of the meter and instrument transformer. The combination of instantaneous parameter and vectorial displays has become a useful tool to identify cases of energy theft
- Harmonic analysis: Harmonics cause excess losses, overheating of transformers, cables, etc., which lead to additional recording errors and frequent damage to transformers. This feature provides detailed harmonic analysis, including the amplitude of individual harmonics and phase angle with respect to fundamental. The high end equipment is capable of measuring and displaying the harmonic power (active, reactive and apparent power) and direction of the harmonic, which helps to identify the particular harmonics present in the network or generated by the consumer
- Burden measurement: The accuracy of an instrument transformer is dependent on the burden connected to its secondary side. In most cases the meter installation teams do not pay attention to the length of secondary cables or to the total burden of connected instruments at the CT/VT. Through this feature, the secondary burden of the instrument transformer can be measured to ensure that it is within the given range of the instrument transformer
- Accuracy testing of instrument transformers, including the ratio and phase angle error of the instrument transformer.
- Checking of the instrument transformer’s ratio
- Accuracy testing of various pulse outputs of complex and modern meters
- Accuracy testing of transducers.
Testing methods differ from country to country. These methods are mainly influenced by:
Access to the meter site: For example in Asian countries access to the meter and removal of the meter connections are difficult and restricted to limited authorities to prevent energy theft
- Testing regulations and practices: Some countries legally accept on-site testing when the meter is tested under similar conditions to laboratory testing, i.e. using a phantom load.
On-line testing at consumer load
In this method the tester connects the reference meter to the meter under test, i.e. voltage in parallel and current in series using a clamp on the CT. This is a quick and economic method but the tester has to rely on the running consumer load. The objective of such testing is to check the connection and any existing tamper condition, and to get a fair idea of the accuracy of the installation.
Off-line testing using phantom load
In this method the operator disconnects the meter under test and connects it with an electronic phantom load. There are two kinds of phantom loads available to cater for different needs and practices:
- Current phantom load, in which the test voltage is fed from the incoming mains supply. The magnitude and phase angle of the test current can be varied as per the test points and synchronised with the incoming mains voltage supply
- Voltage and current phantom load, in which the electronic phantom load generates the required test voltage, current, power factor and frequency. These test parameters are independent of the mains supply voltages.
Automated and uniform testing of all meters is possible using these modern phantom loads along with a reference standard and software. The results achieved in such uniform testing can be further systematically utilised in the following way:
- Statistical evaluation of meter type, brand, and performance of meters after given years of operation, which further helps deciding on test interval
- Qualitative feedback for future procurements of meters and instrument transformers
- Screening of a particular area or zone so that the surveillance interval/frequency of the particular zone can be increased or decreased.
This method, which requires the additional cost of a phantom load compared to the on-line testing method, but it is closer to laboratory testing, is currently used in European countries that have developed their standards and regulations accordingly.
The latest design of reference standards like the MT310, MT320 and MT3000 series can be used with high voltage and current sensors, and can perform on-line measurement up to 40 kV and 2,000 A. This feature is extremely useful to establish the overall accuracy as well the connection of a complete metering site that includes energy meters, CTs and VTs.In addition, using a high end reference standard like the MT3000 series it is possible to carry out on-line testing of CTs and VTs without removing them from site.
Further portability of an integrated system, i.e. a phantom load combined with the reference standard, has been achieved in the recent series of test systems without compromising metrological or electrical performance.