SU1157490A1 - Method of checking electromechanical wattmeters - Google Patents

Abstract

A METHOD FOR TESTING ELECTROMECHANICAL WATTMETERS by separately supplying voltage and current circuits, characterized in that, in order to increase the accuracy of the verification process, periodic signals of rectangular shape of a square wave are used as calibration signals, changing the indicator of the calibrated instrument by changing amplitude and temporal parameters of the generated rectangular signals determining the calibrated power value, and according to it and according to the indication of the instrument being scanned the error is after it. /

Description

ate

four

;about

Yg of fig. 1 The invention relates to electrical measuring technique and can be used when calibrating wattmeters of an electromechanical system. The known method of checking wattmeters with separate power supply to the current and voltage, which means that the nominal voltage and the test instrument are supplied with a nominal voltage and from the voltage source, the phase angle is set to the required phase angle and current windings of both devices with a scale mark on which calibration is performed. Then the readings of the instrument are compared and the accuracy of the instrument being checked is calculated. The same voltage is applied to the reference and tested instruments, one current flows through both instruments, one current and the phase shift angles between voltage and current in both devices are C}. The disadvantage of this method is a significant decrease in the accuracy of calibration with a decrease in CosV, due to the fact that at Cos H 0.5 the errors of all exemplary devices increase sharply and do not provide the necessary guaranteed spare parts between the test and the sample device. The closest to the invention is a method of calibrating wattmeters by separately supplying current and voltage circuits, and comparing the readings of the reference and calibrated instruments, in which the voltage vector in the reference instrument shifts relative to the voltage vector in the test instrument by a given angle, the phase of the current flowing through both devices to obtain a zero phase angle between the voltage and current in the sample device, after which the readings of the devices 2 are compared. The main disadvantage of this method is The accuracy of calibration is low due to the error due to the technical impossibility of setting the exact power factor value due to significant errors in the phase-shifting circuits of sinusoidal signals, especially in the frequency range 1a, also due to the error due to voltage and current vectors IHOCTH 4902 exemplary device, which is caused by the error in determining the moments of coincidence of the phases of voltage and current in the Exemplary device by a phase zero indicator. The purpose of the invention is to improve the accuracy of the verification process. This goal is achieved by the fact that, according to the method of calibrating electromechanical wattmeters, by separately supplying voltage and current circuits, periodic sequences of square-shaped square-wave pulses are used as verification signals, changing the readings of the calibrated instrument by changing the amplitude and time parameters of rectangular signals determined by the calibrated power value, and according to it and according to the indication of the instrument being scanned, the error of the latter is found. Figure 1 shows the equivalent circuit of the wattmeter circuit; Fig.2 block diagram of the device that implements this method. The essence of the method is as follows. Serial and parallel measuring circuit wattmeter You can. present a set of active and reactive elements (figure 1). The following abbreviations are added to the abbreviations of the following entries: dp of the series circuit (Fig. 1a) for the parallel circuit (Fig. 16) .cu ,,., E ,, U) Using these notation, we write the expressions for the impedance complexes of the measuring circuits:. i /, -d, de cv j W / u) o. Let the voltages U, and U, supplied when calibrating to a series and parallel wattmeter circuit from an exemplary generator, have the shape of a meander. Then the change in the amplitudes of their harmonics is described by the expression

C5)

iKI) l ° where (2K + 1) is the harmonic component number; Ug - the maximum value of the signal. Let also the voltage U, be delayed with respect to the voltage 1) 2 by the time f and differ in amplitude by K times (K can be equal to unity). Assuming the amplitude of the voltage L) is equal to Uo, we have

-jWot

e

where v

u) (, is the operating frequency of the wattmeter. The wattmeter readings are determined by the average over the period from the production of currents

I

t7 |

Hldt

or equivalent expression

x; -

35

U1

P (lk H-2U +)

where is the conjugate complex.

Writing the complexes of currents with consideration of impulses (3), (4) and substituting them into formula (8), after transformations we get

.KZKH) V

45

RU) (9)

 (2K + -1)

After the decomposition of the function R (q) into poles and a series of transformations, the expression for the wattmeter readings takes the form

ki

(IS)

ot-l- I,.,

where r ji ti.

This expression can be used as a calibration equation for the scale, with the value of the power factor CosV

during calibration, associated with

| ratio

Co5C / 1-A. (C. |

(AND)

those. the values of the functions in the right and left parts of the expression coincide when W × 0, ± I, ± 1. Under the accepted condition (13), the second term in the brackets of the expression (121 represents the error, the error in setting the power factor due to the reactive elements of the measuring circuit.

We investigate this error. We write it in the form:

FOR acosv- | r. -,, oya

 n "i V0 V J Formula (12) is the wattmeter scale equation for a meander-type input signal. Let us now analyze the emerging methodological errors. Let us single out in the expression (121 a component that does not depend on reactive elements, and we denote it by

Here, the fractionally rational function R (q), which characterizes the reactive properties of the meter measuring circuits, is equal to

R (V1

i1-ei))

Where

55

WITH")

(1b)

,

-nc1nircj, J2

From the expression (l6) it can be seen that the functions УП (отлич) differ from 11 only in a small region near V O and H t 7G. The width of this transition region for each function is approximately equal .. Outside. of this transition region with V70) dcos f 4-54 2 1 Finally, expression (18) can be written as follows: Aco5Ч - | - (From the analysis of expression (18), it follows that, if the residues A and Aj have different signs, then the transition region of LCs are possible maxima and minima of the DSo5CH function, which are of greatest interest for estimating errors. Let us find the extremal values of the cos cos Ч function. The maximum position is found by solving the equation a (ω5). formula for estimating the magnitude of the error due to n The difference between the reactive elements in the measuring circuits of the watt meter is 05.1 ,,, 4 (the estimate is valid for any relationship between the parameters of the parallel and the next 6 watt circuits and convenient dp of practical use. The quantitative analysis of the known wattmeters that was carried out using it shows that it is tenths of a percent and can be reduced by the introduction of ballast resistance. The present method of verification can be implemented by a device (FIG. 2). The device for implementing the method comprises an exemplary generator 1, a tunable divider 2 (Frequencies, a pulse shaper 3, a voltage amplifier 4, a tunable delay unit 5, a current amplifier 6 and device 7, generator output 1 of exemplary frequency is connected via a tunable frequency divider 2 to the input of a pulse shaper 3. The output of the shaper 3 pulses is connected to a point connecting the inputs of voltage amplifier 4 and tunable block 5 The output of the tunable delay unit 5 is connected to the input of current amplifier 6. The outputs of current amplifier 6 and voltage amplifier 4 are respectively connected to the measuring circuit and the voltage of the device being tested. The verification process is carried out as follows. The pulses from the generator 1 of the exemplary frequency are fed to the input of the tunable frequency divider 2. The required separation factor K is determined by the form-where T is the period of the test signal; Tjj is the period of the following pulses from the generator of the reference frequency. From the output of the tunable divider 2 frequency, the pulse sequence of a given frequency arrives at the input of the imager of 3 pulses. They form a sequence of square-shaped pulses of the square wave type. The law of their change is O where a is the amplitude value of the test signal. The generated pulse sequence is fed to the inputs of the voltage amplifier 4 and the tunable lock unit 5. The delay unit 5 is necessary to set the desired angle of the voltage vector and current in the calibrated ribor. This requirement is fulfilled by means of a time delay on the value of the signal in such a measuring circuit of the instrument being tested. Its value can be found by the formula where H is the required phase shift between the voltage and current vectors in the instrument being tested. From the output of the tunable block 5 of the delay of the periodic sequence of the pulser is fed to the input of the amplifier 6 current. Amplifiers A and 6, respectively, of voltage and current, serve to maintain a constant amplitude value of voltage and current in the measuring circuits of the instrument 7 to which they are connected. In measuring voltage and current circuits of the device under test, 7 periodic waveforms of square-shaped square-wave pulses with a constant amplitude value of voltage and current are posted. Further, the readings of the instrument to be tested 7 are compared with the calculated power value and determine whether the instrument corresponds to a given accuracy class. The advantage of the proposed method of checking wattmeters is an increase in the accuracy of verification, which is ensured by an increase in the accuracy of setting the amplitude values of test signals, as well as an increase in the accuracy of setting the power factor due to the formation of test signals based on discrete equipment, i.e. setting the power factor is possible with an accuracy determined by the value and instability of the reference frequency of the reference oscillator. It can be seen from formula (24) that when using a master frequency of 1 MHz and generating a test signal of 50 Hz, the accuracy of setting the phase of your shift is about 3.1 10. Achieving such accuracy by analogue means is impossible. In addition, there are more opportunities to automate the verification process. The device that implements the proposed method is semi-automated, and its detailed development is quite simple due to the use of unified units and blocks. Automating the verification process improves its efficiency and reliability.

Claims (1)

METHOD FOR TESTING ELECTROMECHANICAL WATCH METERS by separately supplying voltage and current circuits, characterized in that, in order to increase the accuracy of the verification process, periodic sequences of square-wave pulses of the meander type are used as calibration signals, the readings of the instrument under test are changed by changing the amplitude and time parameters of the formed rectangular signals determining the calibrated power value, and according to it and according to the indication of the instrument being verified, st last. ^and h - * K o — CZJ GUUUA ₀ U1 FIG 1