Todayâ€™s reference meters offer multiple functions
Test requirements are becoming more and more complex as meter manu-facturers develop more complex measuring instruments. In addition, the liberalisation of energy supply in many European countries has caused supply companies to review the systems and processes they have in place as they go about the business of selling electric energy.
The Hamburgische Electricitäts-Werke (HEW) has taken up these new and – for many energy suppliers – unusual challenges. For some years now the utility has devoted a great deal of attention to its customers; for example, 99% of all complaints are worked out on site with the end user. This made the use of mobile measuring technology essential, and HEW has gained considerable experience in this field, which has been of value in establishing the features they require of their mobile test equipment.
MOBILE MEASURING TECHNOLOGY TODAY
We have said that the features required of devices used in the testing of meters on site – and indeed of the installation as a whole – increase as meters become more complex. Another factor to take into account is how often the measurements have to be carried out. Because testing personnel often have to operate under difficult conditions and time constraints, testing devices should conform to certain important criteria.
First, the devices should be both light and small, because they sometimes have to be carried from the vehicle to the testing site in difficult conditions, such as in high or low outdoor temperatures or along winding staircases. Then installation and operation has to be straighforward, to prevent the possibility of errors. The test results have to be accessible without manual calculations, and should be presented clearly and directly on the device. The results should also be available for possible later analysis.
The measuring accuracy and functionality should provide for the testing of all meters with the same device. The current and voltage ranges should therefore be wide, so that re-clamping or switching with different measuring points is not necessary.
The testing devices should also provide additional measuring values, to allow for an advanced analysis of the measuring point. Circuit errors and abnormal conditions at the testing site, such as unpredicted changes of the load at certain times, asymmetries or high reactive loads can thus be detected. These values can be analysed later by the network service engineers.
Another desirable feature of a modern reference meter is to measure the burden of the voltage and current transformers. The burden of a modern electronic meter is quite different to the burden of the classic Ferraris meter. The high accuracy class of the meter, eg 0.5S, is just a wrong impression, if the transformer generates an additional error of 1% due to wrong burdening.
LATEST GENERATION REFERENCE METER
HEW’s division of energy measuring technology has tested a reference meter against these requirements. Manufactured and marketed by MTE Meter Test Equipment AG in co-operation with EMH Energie-Messtechnik GmbH, the PRS 1.3 is a portable reference standard with an accuracy of 0.05%, weighing about 5.5 kg. (See Figure 1). It is handy and robust, with the ability to connect two scanning heads and communicate with a PC or Notebook, making it an ideal meter service tool. The device is believed to be appropriate equipment for the meter service inspector, who is generally responsible for all voltage ranges from low to high voltage networks. When closed it is able to resist monsoon rains or sandstorms.
This device is capable of measuring all relevant values (U, I, P, Q, S, E, angle, harmonic spectrum etc) in 0.2 seconds at an accuracy of 0.05%, and of rep-resenting them on its graphic display. Two pulse inputs allow for the simultaneous measurement of independent pulse frequencies. For example, by using two scanning heads, the LEDs for active and reactive energy of a four quadrant meter can be scanned and the measuring errors can be determined at the same time. Alternatively the measuring errors can be determined simultaneously with the LED and a transmitting contact. In the case of a conventional measuring set consisting of an active and reactive energy meter, both meters can be tested at the same time.
Another important function for on-site testing of the installation and the analysis of the mains condition is the graphic representation of the vector diagram and the harmonic analysis. (See Figures 2 and 3). All the values can be stored in the device’s memory and transmitted to a PC via a serial interface for further processing, or alternatively a printer can be attached to the interface, allowing on-site printing.
In addition to the classic measurement methods of a reference meter, the new device can be used to determine the actual burden of voltage and current instrument transformers. After entering the required values and connecting to the secondary circuit of the transformer, the display shows the impedance and the actual burden. These values can be stored in memory or printed out on the spot (see figure 4).
One new function of the device allows for the testing of current transformers, and under certain conditions even voltage transformers. To do this the primary circuit of the transformer is connected to the plugs of phase 1 and the secondary circuit to those of phase 2. The device computes the ratio error and the phase displacement angle of the transformer from the measured values. An additional instrument transformer has to be connected in the case of a voltage transformer test if the primary voltage exceeds the maximum input phase voltage of 480 V.
When testing a current transformer, the primary circuit is directly connected to the current clamps of phase 1 and the secondary circuit to the clamps of phase 2. To be able to work with wires of higher diameter as well as with lower currents using normal lab wires with banana plugs, there are two of each current connection socket. Transformer clamps can also be used — for example error compensated transformer clamps for currents up to 100A result in measuring deviation of only 0.2%.
If higher currents have to be measured, other transformer clamps can be connected directly to the current input. Measuring accuracy is ensured by determining the characteristic error of the transformer clamps, and storing the actual ratio error of the clamp and the phase displacement angle separately in the device for each phase. They are automatically compensated for when measuring in the primary circuit or testing a current transformer with the clamps.
Today a reference meter should do more than compute the sum power and energy and the measuring error of a meter – many other functions are required. Today’s reference meters are multifunctional measuring, evaluating and registering devices; perhaps we should think of another name for this new technology?