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§ 19 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 will briefly review microprocessor-based electricity meters to not bore you much. The main difference between such counters from conventional, induction meters is that they are a small "on-board computer". In such counters, there are practically no moving parts that measure the consumed electric current. The counter usually consists of measuring current and voltage sensors (transformers with improved characteristics), measurement circuits (ADC - analog-to-digital converters), a microcontroller that processes digital signals, memory for storing the counter data. All counter information is displayed on a liquid crystal display. Counters are usually fed from voltage circuits connected to them. In addition, it is good practice to install backup power on 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 current consumption is determined using current transformers. In the future, there is a multiplication of current and voltage signals through the ADC on a high-performance microprocessor with a RISC-set of instructions. All received information is recorded in the counter memory and displayed in parallel on the liquid crystal display. The simplest circuit of the counter is shown in Figure 1.

Simple measurement scheme of MP counter

In addition, additional information outputs (current loop, number-pulse relay, in which the pulse frequency is proportional to the consumed electric power, RS-485 output, etc.) can be introduced in various models of the meter. Practically all counters have a memory for storing the counter program, measured values ​​(i.e. active and reactive energy) as well as a list of various significant events (the number of entries in the meter, power outages, daylight saving time, etc. )

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

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

Obviously, it is most profitable to work at a night tariff (the price for 1 kWh is 4 times less). In old induction meters you couldn’t take into account transitions to different time tariffs (for this you would have to put a person at the meter who was monitoring when, for example, the peak tariff came and recorded the readings). As a way out, it would be possible to use a computer-based information collection system in real time, followed by manual partitioning into tariff intervals. 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 company (Moscow, Russia) can operate as a VAF (volt-ampere phase meter), show the actual value of current and voltage, harmonics, etc. (see figure 2).

Alpha +

Counter of the company ABB VEI Metronika

Power consumption graph

An example of power consumption graphs.

Counter program window Alpha +

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

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

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

Schlemburger counter

Counter Programming Screen

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

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