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§ 19 The principle of operation of electronic meters.



The article was taken here: http://www.railway.te.ua/pc_meter.htm

Microprocessor electricity meters.

Hello, dear reader. On this page, we briefly, in order not to bore you too much, consider microprocessor-based electricity meters. The main difference between such meters and conventional ones is that they represent a small on-board computer. In such meters there are practically no moving parts that measure the consumed electric current. The meter usually consists of current and voltage measuring sensors (transformers with improved characteristics), measurement circuits (ADC - analog-to-digital converters), a microcontroller processing digital signals, memory for storing meter data. All information of the counter is displayed on the liquid crystal display. Meters are usually powered from voltage circuits connected to them. In addition, it is good practice to install backup power to the meter (in the form of various batteries). They are designed to maintain the integrity of important information when the meter is disconnected from the power supply. The values ​​of current consumption is determined by means of current transformers. Subsequently, the current and voltage signals are multiplied via an ADC on a high-performance microprocessor with a RISC command set. All received information is recorded in the memory of the meter and is displayed in parallel on the LCD display. The simplest scheme of the counter is shown in Figure 1.

Simple measurement circuit for MP counter

In addition, additional information outputs may be introduced in various meter models (current loop, number-pulse relay, in which the frequency of the pulses is proportional to the electricity consumed, RS-485 output, etc.). Virtually all meters have memory for storing the meter's work program, measured values ​​(i.e., active and reactive energy) as well as a list of various significant events (number of meter inputs, power loss, switching to winter and summer time, etc.) ).

A distinctive feature of such meters is the ability to meter electricity at a tariff. This means that you can pay for the consumed electricity in different ways. For example: there are 3 tariffs for calculating electricity - peak (usually morning and evening hours), semi-peak (this is almost all day except morning and evening) and night. The price for consumed electricity varies depending on the tariff. With a one-rate tariff of 1 kW, it costs UAH 0.1271, but if you decide to pay by tariffs, then you will need to multiply this price by the following factors:

- peak rate - 1.8
- half-rate - 1.02
- night tariff - 0.2

Obviously, it is most advantageous to work at the night rate (the price per 1 kWh is 4 times less). In the old induction meters you could not take into account transitions to different time tariffs (for this, you would have to place a person at the meter who watched for example when a peak tariff occurred and recorded the readings). As a way out, it would be possible to use a computerized system for collecting information in real time with subsequent manual division into intervals according to tariffs. However, it is much easier to install a microprocessor counter, whose internal program itself would monitor the transition from one tariff zone to another and record the power consumption separately for each interval.

In addition, such a counter can be a good diagnostic tool. For example, the "Alpha +" counter of ABB VEI Metronika (Moscow, Russia) can work as a VAF (volt-ampere-phase meter), show the effective value of current and voltage, harmonics, etc. (see figure 2).

Alpha +

ABB VEI Metronika Company Counter

Electricity consumption graph

An example of power consumption graphs.

The program window counter Alpha +

Figure 2. A copy of the program screen counter "Alpha +".

In addition, given the high accuracy class of such a meter (0.2 - 0.5) and the absence of a self-propelled motion (i.e., a spontaneous disk movement, as in an induction counter), we can say that today such meters are the most convenient option for electricity metering .

In order for the meter to read and display data, it is necessary to enter a program that will “indicate” to the meter what to do: how to measure, what to measure, where and in what form to record. To do this, you must have a personal computer with a special program supplied with the meters, the meter itself, a special transmitter for transferring data from the computer to the meter. Moreover, by and large, all the counters are programmed according to the same rules: the user does not need to write programs in some programming language - all that is required of him is only to answer the questions of the program and mark the necessary work options for the slider with the mouse. An example of the program window for the "Quantum D300" counter by Schlemburgier (Figure 3).

Counter firm Schlemburge

Counter Programming Screen

Another of the useful properties of these meters is the possibility of creating on their base an information system for collecting and processing data on energy consumption. Those. Based on such meters, you can create an automated information collection system. Suppose you have several objects (substations or small factories for the production of products) and you want to control the power consumption of objects. There is nothing simpler - you put microprocessor counters on metering points, connect a modem (pulse relay, RS485 output) to them and collect all the information on one PC in your office. Even if you need to reprogram the counter - you can do it remotely, via a modem. And then the question immediately arises: And if I am a "bad person" and I want to get in and change the data in the meter? On this account, the counter has its “answer to Chamberlain”. Usually counters have several levels of access to their information (for reading, for modification, for reprogramming). In addition, the memory of the counter contains information about all attempts to read and enter it. So you can determine when the data in the meter changed.

Here, dear reader, I finish my story so that you can take a short break. Next time we will talk with you and various automated systems that can be built on the basis of microprocessor counters. Sergey Kibitkin.