GB2149928A - Arrangement for determining the value of AC power or AC consumption energy - Google Patents

Abstract

A multiplication product consisting of voltage and consumption current and produced by means of a first multiplier (1) normally includes, as an alternating error component, a term of angular frequency 2 omega . That term causes an error which cannot be disregarded on a percentage basis when making short- term measurements. In order to eliminate that error and thus permit accurate measurement values to be rapidly determined, the voltage and the consumption current are also additionally phase-shifted by means of respective phase shifters (4,5) through angles (90 DEG - alpha ) and (90 DEG + alpha ) respectively, and are then multiplied together by means of a second multiplier (2) to form a second multiplication product. The two multiplication products are finally added together by means of an adder (3) to eliminate the alternating error component. Fig. 3 shows a modification wherein error components are removed from a three-phase power meter. <IMAGE>

Description

SPECIFICATION Method and circuit arrangement for rapidly determining tlie value of ac power or ac consumption energy The onvention relates to a method and circuit arrangement for rapidly determining the value of AC power or AC consumption energy.

The difference between a power or watt meter and an electricity (a.c. consumption) meter is that the electricity meter integrates the instantaneous power with respect to time. In terms of the conventional Ferraris meter, that means that the speed of rotation of the rotor disc represents a measurement in respect of instantaneous power, and and the number of revolutions as recorded by the counting mechanism represents a measurement in respect of the energy consumed.

Therefore, this type of electricity meter is at the same time also a power meter.

Electricity meters and power meters have an input for the voltage u(t) and the current consumed i(t), which are multiplied together within the electricity meter or power meter by means of a multiplier, to form the multiplication product p(t) = u(t). i(t). Such an arrangement is disclosed in German specification DE-A-3 011 060. The multiplier is for example a combination of a known mark-space amplitude modulator operable to generate a pulse train whose amplitude is proportional to the consumed current i(t) and whose pulse/pulse gap (or mark/space) ratio is proportional to the voltage u(t), with an integrater which is connected to the output thereof and which is operable to determine the area content of the pulse train and thus to determine the value of the product p(t) = u(t).i(t).

The mode of operation of a power meter will be described hereinafter with reference to mathematical equations, for the situation involving measuring active power. The same considerations apply in a coresponding manner in regard to measuring apparent power or reactive power.

Definitions of the symbols used are as follows: u(t) = U.cos xt, i(t) = I.cos (t + ), p(t) = U.cos cot.l.cos(cut + (p) = (1 /2)U.l.[cosg + cos(2wt + )] = P0 + P,.cos(2w.t + cup), P0 = (1 /2)U.l.cos , P, = (1 /2)U.l and = = 27of, with f = 50 hz, if 50 Hz is the mains frequency.

In that respect, U and I are the amplitudes of the ac voltage and the alternating current consumed respectively, X is the angular frequency and (p is the voltage/current phase difference between consumed alternating current and ac voltage.

Therefore, the multiplication product p(t) comprises two terms and is equal to the sum of half the active power being sought, U.l.cosç and the alternating error component P, cos (2cot + (p) which has an angular frequency of 2w. In electricity meters, the above-mentioned alternating error component of the angular frequency 26 can normally be disregarded as integration thereof in respect of time is at most equal to the area of half a period and thus, in the course of a long integration period, is negligible on a percentage basis, in comparison with the integration value of P0 = 1/2 U.l.cos , which rises increasingly in the course of time.

In test equipment, on the other hand, the presence of the alternating error component of the angular frequency is a serious disadvantage as any measurement must be concluded as quickly as possible, and thus at the end of the measuring operation the above-mentioned error component may not be negligible, on a percentage basis, in comparison with the integration value of PO. In order, for example, to measure to a degree of accuracy of 0.01%, the measuring time would have to be 104. 10 ms = 100 seconds, which is much too long for automatic adjustment on a test station. In that connection, the time 10 ms represents the value of half the period of the mains voltage.

The present invention provides a method of rapidly determining the value of ac power or ac consumption energy by evaluation of a first multiplication product formed from voltage and consumption current, wherein one of the two parameters voltage and consumption current is additionally phase-shifted through an angle (90' - cu) and the other parameter is additionally phase-shifted through an angle (90 + CL), CL representing a constant angular value, a second multiplication product is formed from the two phase-shifted parameters, and both multiplication products are added together to provide a determination of the value.

Such method provides for determining the value of ac power or ac consumption energy, as quickly and with little delay as possible, without the alternating error component of the angular frequency 2w causing any significant error and without large additonal equipment expenditure on the equipment which is in any case provided in a test station.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a block circuit diagram of an apparatus for measuring ac power, nullifying the alternating error component of the angular frequency 2w, in accordance with one embodiment of the invention; Figure 2 shows a vector diagram of the voltages present in the apparatus of Fig. 1; and Figure 3 shows a block circuit diagram of an apparatus with three multipliers for eliminating errors and with nullification of the alternating error component of the angular frequency 2w, in accordance with another embodiment of the invention.

The same reference numerals denote the same components in all the figures of drawings.

The apparatus illustrated in Fig. 1 comprises a first multiplier 1, a second multiplier 2, an adder 3, a first phase shifter 4 and a second phase shifter 5. A voltage u(t) directly feeds a first input of the first multiplier 1 and feeds a first input of the second multiplier 2 by way of the first phase shifter 4. A further voltage u,(t) which is proportional to the consumed current i(t) feeds a second input of the first multiplier 1 directly and a second input of the second multiplier 2 by way of the second phase shifter 5. The single output of the first multiplier 1 and the single output of the second multiplier 2 are connected to respective inputs of an adder 3 whose output forms the output of the apparatus.The output voltage of the first phase shifter 4 which produces a phase shift of (90' - CL) is denoted by u2(t) while that of the second phase shifter 5 which produces a phase shift of (90 + a) is denoted by u3(t). The values of the two phase shifts may also be interchanged.

Fig. 2 shows the vector diagram of the voltages u(t), u1(t), u2(t) and u(t), wherein U denotes the vector of u(t), U, denotes the vector of u,(t), U2 denotes the vector of u2(t) and U3 denotes the vector of u3(t). As the consumed current i(t) and thus also the proportional voltage u,(t) are shifted relative to the voltage u(t) through a phase angle , there is an identical angle q both between the vector Ú and the vector U1 and between the lines shown in broken line in Fig. 2, perpendicular to the two vectors U and U,. The vector U2 forms an angle of (90' - CL) to the vector U and is of the same length as the latter.The vector U3 forms an angle of (90 + a) to the vector U1 and is of the same length as the latter.

In Fig. 3, the voltages u(t), u,(t), u2(t) and u3(t) feed three multipliers 6, 7 and 8 by way of a four-pole change-over switch 9. The single output of each of the three multipliers 6, 7 and 8 is applied to a respective input of a three-input adder 10, the output of which forms the output of the apparatus. By means of the four-pole change-over switch 9, the switching arms of which are denoted by references 9a, 9b, 9c and 9d, there is only ever a combination of two of the three multipliers in operation at a time, more specifically the combination 6, 7 or the combination 7, 8 or the combination 6, 8. The three multipliers 6, 7 and 8 are for example the multipliers of respective single-phase power meters or they are the three multipliers of a three-phase power meter or electricity meter which, as is known, has one multiplier per phase.The change-over switch 9 has three positions in order in each case to render operable another of the three possible combinations 6, 7 or 7, 8 or 6, 8.

In the first position of the change-over switch 9, the first switching arm 9a applies the voltage u(t) to the first input of the multiplier 6, the second switching arm 9b applies the voltage u1(t) to the second input of the multiplier 6, the third switching arm 9c applies 8,he voltages u2(t) to the first input of the multiplier 7 and the fourth switching arm 9d applies the voltage u3(t) to the second input of the multiplier 7.

In the second position of the switch 9, the third and fourth switching arms 9c and 9d do not apply the voltages u2(t) and u3(t) to inputs of the multiplier 7, but to the corresponding inputs of the multiplier 8.

In the third position of the switch 9, unlike the connections in the second position, the first and second switching arms 9a and 9b do not apply the voltages u(t) and u1 to inputs of the multiplier 6, but to the corresponding inputs of the multiplier 7.

The two respectively operated multipliers 6, 7 or 6, 8 or 7, 8, together with the three-input adder 10 shown in Fig. 3, perform the same function as the two multiplsers 1 and 2 with the adder3, in Fig. 1.

Description of operation The following equations apply: u(t) = U cos w t, i(t) = I cos (wt + (p) u1(t) = k i(t) = k I cos (cut + (p) = U1 cos (wt + ), with U1 = k.l, u2(t)= U2 cos(ot + 90' - CL) with U2 = U and u3(t) = U3 cos(wt + (p + 90 + a) with U3 = U1, wherein k denotes a constant, w denotes the mains angular frequency, t denotes time, a denotes any constant angular value and U, I, U1, U2 and U2 respectively denote the amplitudes of u(t), i(t), u1(t), u2(t) and u3(t).

The forst multiplier 1 shown in Fig. 1 forms a first multiplication product as follows: p(t) = u(t).u1(t) = U cos w t.(k.l) cos (wt + (p) = (k/2) U.l [cos (p + cos(2wt + g The second multiplier 2 in Fig. 1 forms a second multiplication product as follows: p1(t) = u2(t).u3(t) = U cos(wt + 90' - a).(k.l) cos (t + ç + 90 + a) = (k/2)U.I. [cos( + 2a) + cos(2cot + 180 + )] = (k/2) U.l.[cos ((p + 2a) - cos (2xt + )].

The adder 3 in Fig. 1 forms the following sum: p(t) + p1(t) = u(t).u1(t) + u2(t).u3(t) = (k/2).U.l.[cosç + cos(2azt + (p) + cos(# + 2a) cos(2cot + (p)] = (k/2).U.I. [cos(p + cos( + 2a)].

Two values of a are of particular interest: a = 0, that is to say, u2(t) is phase-shifted through 90 with respect to u(t), and u3(t) is shifted through 90 with respect to u1(t) and thus also with respect to i(t).

In that case: p(t) + p1(t) =(k/2).U.l (qos ç + cos (p) = k.U.l.cos (p = 2.k PO, wherein P0 = (1 /2)U.l cos g denotes the active power to be measured.

q7 + 2a = 90 , that is to say, the phase shifted voltage u2(t) is phase-shifted through 90 with respect to the voltage u3(t) which is also phase-shifted, as shown in Fig. 2. In that case: p(t) + p,(t) = (k/2).U.l(cos# + cos 90 ) = (k/2).U.I cos g = k.Po.

In both cases therefore the sum [p(t) + p,(t)] is proportional to the active power U.l. cos to be measured, and in particular is not falsified, as the alternating error component of the angular frequency 2z is no longer present. That means that either the precise value of the active power was instantaneously continuously present or that, in the case of the mark/space amplitude modulation process, it is independent of the duration of the integration time and thus it is possible to have short integration times with correspondingly short measuring times, without the degree of measuring imprecision increasing in that situation.

The circuit shown in Fig. 1 inviolves additional components and would therefore appear to give rise to additional apparatus expenditure. However, two or more multipliers are in any case generally present in test equipment so as to permit simultaneous testing of at least two singlephase or at least one three-phase power meter or electricity meter, while in the last-mentioned case an adder is also already present. These multipliers and adder can therefore be used as the multipliers 1, 2 and adder 3 or 10.

In the situation where a three-phase power meter or electricity meter is being adjusted, the circuit shown in Fig. 3 can be used, which then additionally serves to determine any errors that occur. In that case, the multipliers 6, 7 and 8 are the multipliers which are present in any case in the power meters or the electricity meters. The three multipliers may each have an unknown error X, Y or Z.

Three measurements are taken in succession, to form three equations having the three unknowns X, Y and Z, in which respect, when making each measurement, the four-pole changeover switch 9 is operated respectively to provide different combinations of two of the three single-phase power meters or two of the three phases of a three-phase power meter or electricity meter.

On the assumption that, in all three measuring operations, the circuit is supplied with the same voltages u(t), u,(t), u2(t) and u3(t), and thus the same values of the multiplication products p(t) are also present, the three following equations apply: -for measuring operation 1: p(t) + p1(t) + X + Y = MW 1 (1), -for measuring operation 2: p(t) + X + Z = MW2 (2) and -for measuring operation 3: p(t) + p1(t) + Y + Z = MW3 (3), wherein MW1, MW2 and MW3 represent the three measurement values as measured at the output of the apparatus.

The sum [p(t) + p,(t)] is equal to 2.k.Po in the case where cu = O, or equal to k P, in the case where (p + 20c = 90 , wherein P0 is determined for example by means of a calibrated reference power meter or reference electricity meter. For the sake of simplicity, taken hereafter is the assumption that k = 1 for the situation where a = 0 and (k/2) = 1 for the situation where g + 2or = 90 . Then, in both cases: p(t) + pr(t) = P,.

The errors X, Y and Z are the three single unknowns in the system of equations (1), (2) and (3).

Those three equations can be easily converted by introducing the terms: lav= MW1-p(t)-p,(t)= MW1 -P0, A2 = MW2-p(t)-p,(t)= MW2-Po and A3 = MW3-p(t)-pt(t) = MW3-P In that case, the equation system (1), (2) and (3) becomes: X+Y=h (4), X + Z = A2 (5) and Y+Z=A3 (6) By reference to those three equations, it is then possible mathematically to determine the value of the three unknowns X, Y and Z and then to use them for adjusting the three multipliers.

The phase-shifted values U2 and U2 and the non-phase-shifted values U and U, or the proportional currents may be all or partly produced by means of signal generators.

Claims (10)

1. A method of rapidly determining the value of ac power or ac consumption energy by evaluation of a first multiplication product formed from voltage and consumption current, wherein one of the two parameters voltage and consumption current is additionally phase-shifted through an angle (90 - a) and the other parameter is additionally phase-shifted through an angle (90 + a), CL representing a constant angular value, a second multiplication product is formed from the two phase-shifted parameters, and both multiplication products are added together to provide a determination of the value.

2. A method according to claim 1, wherein the angular value CL is selected such that the two phase-shifted parameters are phase-shifted by 90 relative to each other.

3. A method according to claim 1, wherein the value a is equal to zero.

4. A method according to claim 1, claim 2 or claim 3, wherein the two multiplication products are formed by means of respective multipliers both present in an electricity meter.

5. A method according to claim 4, wherein, for the purposes of adjusting three single-phase power meters with a common adder connected thereto, which each have a respective multiplier, three measuring operations are performed for the purposes of forming three equations having three unknowns, and the values of the three unknowns are then mathematically determined by reference to the three equations, wherein the unknowns are respective errors of the three power meters, and wherein, in each measuring operation, a respectively different combination of two of the three power meters is used for forming the two multiplication products.

6. A method according to claim 5, wherein the three single-phase power meters are respective phases of a three-phase power meter or electricity meter.

7. A method according to any one of the preceding claims, wherein the phase-shifted and non-phase-shifted parameters are all or partly generated by means of signal generators.

8. A method of rapidly determining the value of ac power or ac consumption energy, the method being substantially as herein described with reference to and as illustrated in the accompanying drawings.

9. A circuit arrangement for carrying out the method according to claim 1, wherein the circuit arrangement comprises a first multiplier, a second multiplier, an adder, a first phaseshifter and a second phase-shifter, wherein the two parameters voltage and consumption current are applied directly to the inputs of the first multiplier and also by way of respective ones of the two phase shifters to the inputs of the second multiplier, and the outputs of the two multipliers are connected to respective inputs of the adder.

10. A circuit arrangement for carrying out the method according to claim 5, wherein the circuit arrangement comprises three multipliers, a four pole change-over switch and a three-input adder, wherein the circuit wiring of the change-over switch which has three positions and the three multipliers is such that there is only ever a combination of two of the three multipliers in operation at a time such that the non-phase-shifted parameters are at the inputs of the one multiplier and the phase-shifted parameters are at the inputs of the other multiplier, and the outputs of the three multipliers are connected to respective inputs of the three-input adder.