CH632108A5 - Electrical circuit for simulating the thermal behaviour of a three-phase electrical apparatus or machine - Google Patents

Description

632108

PATENT CLAIMS

1.Electrical circuit for simulating the thermal behavior of a three-phase electrical device or machine, the heating of which is simulated in the circuit by charging at least one capacitor controlled by the current consumption of the device or machine, characterized by at least one current feed circuit for generating a capacitor charging current whose rise is both a function of the opposing component and a function of the co-operating component of the current consumption.

2. Electrical circuit according to claim 1, characterized in that the current feed circuit has a first part to form a first signal voltage, which is substantially proportional to the absolute value of the largest amplitude difference occurring between two phases of the current consumption of the device or machine, by approximately one to obtain a size proportional to the opposite component of the absorption current.

3. Electrical circuit according to claim 2, characterized by a connected to the output of the first part of the second part to form rectangular pulses, the width of which is proportional to the ratio of the first signal voltage to a substantially proportional to the moving component of the current consumption second signal voltage.

4. Electrical circuit according to claim 3, characterized by a connectable at the input of the device or machine transducer and a circuit connected to the output of the transducer to form the second signal voltage.

5. Electrical circuit according to claim 3, characterized by a pulse-amplitude modulation circuit controlled by the output signals of the first and second parts, which generates an output signal which is essentially proportional to the square of the opposing component and inversely proportional to the idling component.

6. Electrical circuit according to claim I, characterized in that the capacitor charging current is a function of the second degree of both components.

7. Electrical circuit according to claim 6, characterized in that the function has no linear elements in the components.

8. Electrical circuit according to claim 5, characterized in that the pulse amplitude modulation circuit has a field effect transistor.

9. Electrical circuit according to claim 1, characterized in that the capacitor charging current consists of the superposition of several currents, one of which is substantially proportional to the square of the opposite component of the current consumption.

10. Electrical circuit according to claim 2, characterized in that the first part is a diode decoupling network constructed from the same sub-circuits for each phase.

There are circuits known, mostly referred to as thermal images, which are used above all as control devices for protective devices of three-phase electrical devices or machines (which will be referred to in the following as “operating means”), such as in particular motors and also transformers or cables. The image shows capacitors that are charged with charging currents, the strength of which is a function of the loss of operating fluid, and discharged via resistors that simulate the corresponding thermal resistance of the operating fluid. The technically most important and often only provided charging current is generally proportional to the square of the operating current. When using two RC elements (e.g. to protect rotating machines or transformers), the capacity of the first simulating the copper heat capacity and that of the second simulating the iron heat capacity of the equipment, its thermal behavior can be accurate for many operating conditions enough to be simulated to reliably control the triggering of a downstream protection relay. Such a circuit is described in particular in Swiss Patent No. 623,431 dated February 3, 1978.

However, there are cases in which - for example as a result of asymmetrical loading of the network or even complete failure of a phase with three-phase current - the individual operating currents become unequal and their mutual phase shifts differ in size. This has e.g. B. in a three-phase machine that no circular rotating field, but an elliptical rotating field is generated. This corresponds to two rotating fields rotating in opposite directions, each of which can be assigned a symmetrical three-phase current, a procedure which is referred to as the decomposition of an asymmetrical three-phase system into its symmetrical components. These are referred to as co-rotating or counter-rotating components, the latter obviously generating additional losses and torques, which leads to increased heating of the machine. It is an object of the invention to propose a supply circuit for the thermal image which, with a reasonable circuit outlay, delivers a capacitor charging current which also takes into account the heat development therein caused by asymmetry in the current consumption of the equipment

For this purpose, the electrical circuit according to the invention for simulating the thermal behavior of a three-phase electrical device or machine, the heating of which in the circuit is simulated by charging at least one capacitor controlled by the current consumption of the device or machine, is characterized by at least one current feed circuit for generation a capacitor charging current, the increase of which is both a function of the opposing component and a function of the co-operating component of the current consumption.

The invention will be explained in more detail below with the aid of preferred exemplary embodiments and the drawing. It shows:

1 is a circuit for detecting the current consumption of a motor,

2 shows a first embodiment of the asymmetry detection circuit according to the invention,

3 shows a representation of an asymmetrical rotating field, FIG. 4 f (Ig / Im) for different values of the parameter cp. FIG. 5 shows a block diagram of a second embodiment of the asymmetry detection circuit according to the invention,

6 b shows a detail from a possible variant of the circuit diagram of FIG. 6 a,

7 shows a time diagram of the signals in part W of FIG. 6a, FIG. 8 shows the relationship between asymmetry of the current consumption and pulse width T at the output of W.

Let Ii, L, h be the phase currents of the asymmetrical three-phase system, and IM, Ig. Io denotes its symmetrical components, where IM denotes the co-system, IG the opposite system, I0 the zero system. Then the relationship applies:

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632108

M

1 3

1 a a '

1 a a

111

device of Figure 2 performed, which delivers a signal characterizing the asymmetry at its output P5.

A preferred embodiment of the invention, described below, is based exclusively on the information which can be obtained from the 5 amplitudes of the current consumption at the input of the equipment in order to generate a signal which characterizes the degree of asymmetry.

(1)

With: a = eJ 2 7t / 3:

to with: a = ei 2 ti / 3

To simplify the following considerations, it should be assumed that there is no current-carrying neutral conductor, i.e. that:

Ii + I2 + I3 = 0+

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- »-

.

111

S1

M 1

2nd

eel

•

^ 6

2,

eel

I.

0

■ »«

(2)

M

+ a

In principle, the amount of IG required to take the asymmetry into account can be calculated from the values of the phase shifts and the amplitudes, and can therefore also be simulated on the basis of these variables using an analog circuit. Because I0 = 0, the following follows:

the.

A very economical solution with for the given I ^ =

Adequate accuracy is obtained if -

Determination of the size of the opposing system either only the 30 angular deviations or only the amplitude deviations are used.

When using the angular deviation, a reference signal with three times the operating frequency can be generated by means of a PLL, which indicates the desired position of the zero crossings. By evaluating the pulses at the phase comparator output, a signal proportional to the opposite system with sufficient accuracy for the given application can be obtained.

Fig. 1 shows a form of decrease in the measuring currents controlling the simulating circuit at the input of a three-phase motor and, if one considers the angle between the co-system and the gate. These currents are referred to at terminals 1, 2, 3 of the switching counter system with <p (Fig. 3, right):

* 2 + 3

I3 = a ïM + a '

(3)

(4)

(5)

^ i ^ M - + (vm + 2 (v1 *) cos ^

(6)

V1 *! =

i1 + (v1 *) 2 + 2 (V1 «) cos tf + i2 °> '

(7)

VXM =

\ jl + (IG / IM) 2 + 2 (VIM) 003 CM20) '

(8th)

Using the standardized current values (Ii / Im) »(WIm). (VIm), it is possible to set up a measure of the degree of asymmetry which is dependent only on the current amplitudes and which, in particular with small values, runs largely linearly with the asymmetry factor (Ig / Im). This measure is defined as follows:

f -

max die) -min (IK)

medium

K = 1,2,3

65

where the value of Imittei from the filtered signal is one

3-phase rectifier is obtained on the assumption that it is approximately equal to IM for small asymmetries. Figure 4 illustrates the relationship between f and the effective asymmetry for different values of <p. The linearity between f and Ig / Im is obviously very satisfactory.

FIG. 5 shows a block diagram and FIG. 6a shows the circuit diagram of a circuit based on the determination of f as a measure of the asymmetry. Uv and -Uv denote by way of the positive and negative supply voltage of the amplifier. On the input side, to determine the amplitude values of the individual phases, instead of special and

632108

elaborate peak value detectors, the low-pass filter TF used. Their capacities charge up to a potential that is proportional to the maximum value of the one-way rectified vibration of the respective phase. The extreme values of the amplitudes (maximum and minimum) sought, which are subtracted by the differential amplifier OP4, are determined with the downstream diodes of the circuit part Q. The sensitivity of the circuit is determined by the trim potentiometers Pi, P2, P3, the greatest possible resistance values of which are essentially determined by the current transformers on the input side. At the output of the differential amplifier OP4, a trimmer P4 for zero point adjustment and compensation of the forward voltages of the diodes of the upstream circuit part Q is provided, while the transistor T2 together with a zero current detector (not shown) has the task of securing Ug = Ur when the motor is switched off and to block the output in this operating state. The signal processing unit W connected downstream of the differential amplifier, with the amplifiers OP5, OP6 and OP7, forms a voltage-pulse width converter, at the output of which pulses with a width proportional to the asymmetry factor Ig / Im are obtained. The mode of operation of this converter can be seen from the time diagram of FIG. 7, while FIG. 8 illustrates the pulse width as a function of the degree of asymmetry of the current consumption by the motor.

In FIG. 7, the time course of the signals at points a, β, y, 8 and e of converter W of the circuit diagram of FIG. 6a is illustrated by curves of the same name. The formation of the signals will now be explained on the basis of this circuit diagram.

At the non-inverting input of the operational amplifier OP5 there is a DC voltage Um, supplied by a 3-phase converter connection, which is proportional to the total motor current, while the square-wave signal e occurring at the output of the converter W controls the feedback of the operational amplifier OP5. The reference voltage UR via a resistor at the inverting input of OP5 defines the basis of the sawtooth signal β at the output of OP5, while the increase in the sawtooth determined by the dimensioning of the feedback circuit is proportional to UM, and in FIG. 7 as KUm is designated. The sawtooth signal is fed to the non-inverting input of an operational amplifier OP6, at the inverting input of which a voltage UG, designated with a and proportional to the opposite component, is present. If signals a and ß overlap, a periodic square wave signal of height UH appears at the output of OP6, the width of which depends linearly on UG-UR (see FIG. 7). This will make the capacitor

Cw charged, one terminal of which is held at a voltage -Uv which is below the reference voltage UR by a fixed value, for example 15V.

This capacitor Cw is thus quickly charged by y during the rectangular pulse and discharged in the remaining time, so that the signal 5 of FIG. 7 occurs at its other terminal. This is fed to the inverting input of an operational amplifier OP7, which is connected as a comparator with hysteresis, and its non-inverting input is therefore connected to a voltage divider located between its output and the reference voltage UR. To simplify the illustration, assume that it shares 1: 1. Then, at the output of the converter W, the periodic square wave signal of FIG. 7, designated e, is obtained, which, as desired, is at a higher of its two voltages during a time proportional to UG / UM and with which the transistor S is controlled. In addition, as mentioned, the periodicity of this signal is used by feedback for clocking the entire converter.

FIG. 6b shows a variant of the circuit part comprising the amplifiers OP6 and OP7 suitable for fast charging of the capacitor Cw of FIG. 6a.

To simulate the heat sources most influenced by the asymmetry in the engine, the formation of a capacitor charging current I is preferably of the form

I = Kt IG2 + K2IM2 (9)

desired, where K] and K2 are selectable constants. In order to be able to use existing standard circuits to generate the current profile defined by formula (9), a signal proportional to Ki • IG2 / IM is first formed with the FET transistor S (FIG. 6a) acting as a pulse amplitude modulator , the amplifier OP8 acting as a voltage follower serving to suppress drift and ripple.

Finally, the block diagram of FIG. 5 indicates how the signal obtained at the output Z of FIG. 6a can be supplied to an input of a known thermal simulation circuit (such as that described in the aforementioned Swiss Patent No. 623 431) in order to use it to be added to a current proportional to IM, according to which a capacitor charging current of the form defined in equation (9) is generated from this sum - using the multiplier M consisting of a voltage-current converter and integrator on the input side in the simulation circuit. This current can, for example, be fed into the capacitor simulating the copper heating of the motor of the image L present in the simulating circuit.

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7 sheets of drawings