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the impact of pulse-width modulation on the error of induction electricity meters and losses in an asynchronous motor

Effect of pulse width modulation on error induction electricity meters and losses in asynchronous engine

A. P. Popov, A. O. Chugulev, A. A. Gorshenkov, S. M. Klevansky

Siberian State Automobile and Highway Academy (SibADI)

The results of the study of the error of induction meters of electrical energy, as well as electrical losses in an asynchronous motor under high harmonics in the current and voltage curves using the frequency converter of the Mitsubishi company ( E 500 FR - E 540-5,5 K - EC ) in as a source of non-sinusoidal voltage. It is shown that the error of induction electricity meters and electrical losses in an asynchronous motor with non-sinusoidal modes in PWM circuits increase by several tens of percent.

As is known, in power supply systems due to an increase in electricity consumers operating in a pulsed mode, as well as systems with pulse-width modulation (PWM), frequency converters in electric drive systems with asynchronous motors, non-linear loads, thyristor converters, etc. there is a high level of higher harmonics.

In this regard, the issue of measuring electrical energy in these conditions remains relevant, despite the fact that a significant number of works, for example, [1: 6] , deal with issues of measuring electrical energy, both under sinusoidal modes and under non-sinusoidal electromagnetic processes.

To measure electrical energy in power supply systems, both induction and electronic electricity meters are currently used. Moreover, the latter are most often based on analog-to-digital converters using microprocessor calculators, that is, in the process of calculating electricity, sampling of time measurements and quantization of input signals proportional to current values ​​of current and voltage on the load is used, which inevitably generates an error in the calculation of energy.

This paper presents the results of a study of the error in induction meters of electrical energy, as well as power losses in an asynchronous motor under conditions of a high level of harmonics in the current and voltage curves. In this case, a special electronic meter of electrical energy was used, which allows to obtain reliable information in non-sinusoidal conditions caused by PWM.

As such an electronic meter, an electronic meter specially developed for this purpose was used, providing with a fairly high accuracy the calculation of the current value of electricity compared with an induction counter, in accordance with the expression:

, (1)

Where - instantaneous voltage on the load;

* - instantaneous load current;

- current measurement time.

In the block diagram of such a counter as a multiplier of instantaneous values and a pulsed multiplying device, a pulse integrator and a digital pulse counter are used, which makes it possible to ensure the total measurement error of the current value of electric power of the order of several tenths of a percent (0.1–0.2%) in conditions of high harmonics at frequencies of multiples of 50 Hz, up to frequencies of several tens of kilohertz, and use it as an exemplary means of measuring electrical energy.

In this paper, the goal is not to describe the complete structural and principal electrical circuits of such a meter (such information may be provided to interested organizations and institutions). One of the tasks is to determine the possible level of error in an induction counter in non-sinusoidal modes with a high level of distortion of the current and voltage curves on the load.

The studies were conducted using a Mitsubishi E 500 FR - E 540-5.5 K - EC frequency converter with a rated power of 5.5 kW. As a load, heating elements and an asynchronous motor were used. The block diagram of the installation with heating elements and time diagrams of currents and voltages are shown in Fig. 1 and fig. 2

Fig. 1. Block diagram of the installation: Wh 1, Wh 3 - induction electricity meters CO 505; Wh 2, Wh 4 - electronic electricity meters; TT - current transformer; DN - voltage sensor; PE - frequency converter; R n - load resistance.

Before conducting the experiment under non-sinusoidal conditions, a check was carried out for identifying the readings of electronic and induction meters when working on the same load in a close to sinusoidal mode. The device connection diagram is shown in fig. 3. The timing diagram of the voltage curve at the load is shown in Fig. 2a

but)

b)

at)

Fig. 2. Time diagrams of phase voltages (a and c) and phase currents (b and c)

input and output PE for the case of linear active load

Fig. 3. Diagram of testing induction and electronic meters on

identification of indications at the mode close to sinusoidal

During the experiment, the following operating mode of the frequency converter was used:

- frequency of the main harmonic of the voltage at the output of the emergency state f = 50 Hz;

- PWM frequency of the voltage at the output of the emergency state - 1 kHz;

- load resistance of the frequency converter R H = 38 ohms (mode close to nominal)

Several experiments were carried out with a fairly accurate measurement of the operating time of electricity meters and the recording of their readings.

According to the readings of electronic electricity meters, the average value of the efficiency of the frequency converter at the specified load was determined:

Where - the average power at the output of the emergency;

- average value of power consumption;

(The standard deviation of the readings from the average value was 0.05%)

As a result of the measurements carried out according to the scheme of fig. 1 relative values ​​were established * the difference between the readings of electronic and induction electricity meters as a percentage of the input and output of an emergency, which, taking into account the statistical processing, were the following values:

, .

From the obtained results, it follows that with the same load values ​​under non-sinusoidal conditions in PWM circuits, the main error of induction electricity meters is several dozen times greater than their basic error in the sinusoidal mode.

The results of the study were obtained, as already mentioned, for linear active load. Due to the fact that PEs are mainly used to power asynchronous motors (BP) in order to regulate the speed, an experiment was conducted to determine the power losses in the BP when it is powered by the Mitsubishi PE 500 FR - E 540-5.5 K - EC . For experimental studies, an asynchronous motor AIR100 L 2 Y 3 (nominal power 5.5 kW, 3000 rpm) was used. A dc generator loaded on a heater with mixed excitation was applied as a load of arterial pressure. The measurement of the power consumed by the arterial pressure and the load under the sinusoidal mode was previously carried out. After processing the experimental data, it was found that when the BP is powered from the emergency state, all other conditions being equal, the power loss in the BP increases by 30% compared with the sinusoidal mode. This leads to a change in the thermal mode of blood pressure and the need to reduce its load. The reasons for the increase in blood pressure losses in non-sinusoidal regimes are known and are not discussed in this work. The main goal was to establish the level of these losses.

Findings:

one. For the first time, the level of the basic error (tens of percent) of induction electricity meters under non-sinusoidal conditions created by PWM was experimentally established.

2    Losses in regulated asynchronous motors powered by a state of emergency also increase by several tens of percent compared to the standard power mode, which leads to overheating of the arterial pressure and the need to reduce the power of the load.