For many years electricity utility companies have realised the importance of performing measurements and tests on-site at the metering installation. The meter test equipment industry continually supplies and develops new and improved products that reduce and simplify on-site efforts. The latest meter test equipment with enhanced functionality and high measurement accuracy determines meter accuracy and provides additional information relating to the conditions at the mains points.
This paper describes how MTE Meter Test Equipment AG proposes to simplify these tasks by means of an innovative modular portable test system.
THREE MODULES ALLOW MANY COMBINATIONS
In the third quarter of 2003 MTE Meter Test Equipment will introduce a new, accurate, portable test system PTS400.3, which comprises several interchangeable modules. The system consists of a reference standard PRS400.3 of class 0.02, a programmable power source PPS400.3 available in two versions of up to 12 A or 120 A, and a control module PCS400.3. The control module controls the other modules, either separately or as a combination of reference standard and source.
All modules can be assembled and controlled easily. (See Figure 1). The control module auto-matically recognises the modules it is connected to, so a reference meter PRS400.3 may be upgraded with a source PPS400.3, producing a one-position portable test system. Operation of the system begins immediately after connecting both modules.
THE REFERENCE STANDARD PRS400.3
The system’s reference standard is based on well-known digital measurement value retrieval, fast analogue-digital conversion and calculation of the values using fast signal processors. (See Figure 2). Reference standards are used as standards for meter testing at a stationary installation, but in the field are more often used for the measurement of all the mains parameters. The PRS400.3 offers the following main functions:
- Simultaneous testing of up to three meters or registers of a multi-functional meter.
- Internal memory for measurement results and customer data.
- Vector diagram, harmonics spectrum, wave form and rotary field display for analysis of the mains conditions.
- Mains supply via the normal mains voltage.
- Active, reactive and apparent power measurement in three-phase or four-phase circuits with integrated error measurement and pulse output for energy.
- Voltage measurement.
- Current measurement, direct and with current transformer clamps up to 3000 A or hot sticks.
- Active, reactive and apparent power measurement per phase and sum of all phases.
- Phase angle, power factor and frequency measurement.
- Burden measurement of PTs and CTs.
- Accuracy test of PTs and CTs.
- Measuring of current, voltage and power transducers.
LOAD CURVE MEASUREMENT ON SITE
It is often necessary to carry out on-site testing of meters, especially precision meters of class 0.5S and 0.2S, to ensure that the mains losses are minimised when billing larger energy amounts. In such cases, it is important to determine the entire load curve of the meter in the shortest possible time, to compare it with the load curve of the previous measurement and to discover possible deviations early.
Deviations in the range of a few tenths per cent may lead to significant differences in the billing statement for transformer meters at medium or high voltage measurements. Take the example of a meter of class 0.2S installed via current and voltage transformers in a 110 kV transformer station. The applied transformers have a transformation ratio of 110 000 V : 100 V and 500 A : 1 A. One kWh measured by the meter is thus in fact equivalent to 550 000 kWh.
It follows that, if meter accuracy deviates by 0.1 % with a certain load, 550 kWh more (or less) are measured than the meter has registered. If over time the error values of the individual meters change, it may lead to deviations in the power balance sheet and to controversy between supplier and consumer, because the load values measured differ.
There is therefore a need to control the metrological functions of precision meters at regular intervals. The removal and replacement with spare meters has disadvantages, not least in terms of storage capacity. Moreover, every meter has its own load curve, which complicates comparing the measurement results. The more cost-effective alternative is testing the meter at the measurement site.
To do this a precision standard and a load source are required. The test equipment should be easy to use, have a high level of automation, guarantee high stability and repeatability of measurement values and withstand transport from one measurement site to another without loss of the metrological properties. The reference standard described above, when extended with a power source to the portable test system PTS400.3, satisfies these requirements. (See Figure 3).
THE POWER SOURCE
The PPS400.3 portable power source may be used separately or as an enhancement of the reference standard PRS400.3. It is produced in two versions, for the supply of transformer meters with a maximum current of up to 12 A and a wider range source of up to 120 A. The source is designed to generate any network independent of its supply voltage, e.g. three-phase four-wire, two-phase three-wire, single-phase two-wire or T-net. Generation of harmonics in both voltage and current circuits as well as ripple control signals is optionally available.
The source module can easily be connected to the reference meter, and the control software automatically recognises the model, starting the automatic measurement of a meter’s load curve. Controlling the source is carried out by using the control module.
The PPS400.3 source is developed to be fully operational without the reference meter. It can either be controlled manually using the control module or via the serial interface. The PPS400.3 source can therefore also be used in a stationary, PC-controlled test system, without the control module.
THE CONTROL MODULE PCS400.3