WO2002014881A9

WO2002014881A9 - Method and equipment for power measurement in an alternating-current system

Info

Publication number: WO2002014881A9
Application number: PCT/FI2001/000722
Authority: WO
Inventors: Olavi Hirsimaeki
Original assignee: Enermet Oy; Olavi Hirsimaeki
Priority date: 2000-08-18
Filing date: 2001-08-15
Publication date: 2002-12-12
Also published as: FI20001831A0; DE10196481T1; FI110964B; AU2001282197A1; WO2002014881A1; DE10196481B4; SE521894C2; SE0300391L; SE0300391D0; FI20001831A

Abstract

Current of the measurement object is conducted through a first transformer based on the induction phenomenon into a measuring circuit, whereby such a current measurement signal is available in the measuring circuit, which is proportional to the time derivative of the consumption object current. Voltage of the measurement object is conducted directly or through a second transformer into the measuring circuit, whereby such a voltage measurement signal is available in the measuring circuit, which is proportional to the consumption object voltage. The integration is applied to the voltage measurement signal instead of the current measurement signal, and the transient power of the consumption object is calculated by multiplying at each moment of calculation the value of the current measurement signal by the value of the integrated voltage measurement signal.

Description

Method and equipment for power measurement in an alternating-current system

The method concerns a method according to the introductory part of claim 1 for power measurement in an alternating-current system.

The invention also concerns an equipment according to the introductory part of claim 5.

hi this application, the term consumption object means equipment connected to an electric mains, by which equipment electric power is either taken out from the mains or electric power is supplied to the mains.

In a kWh meter, wherein the method according to the invention may be used, power is normally measured in such a way that a signal proportional to the current of the consumption object and a signal proportional to the voltage of the consumption object are first formed. Then the transient active power is calculated by multiplying by each other at a certain moment the value of the signal proportional to the said current and the value of the signal proportional to the said voltage. The energy consumed in the consumption object is calculated by integrating the power over the desired time interval.

Measurement of the current in a consumption object is generally based on a resistance, which turns the current into voltage, on an iron or ferrite core current transformer, on the induction phenomenon, where the primary current induces the voltage of the secondary coil, on magnetic sensors, e.g. a Hall sensor or a magneto-resistive sensor) or in certain cases on an optical fibre sensing the magnetic field. In all others, except the transformer based on resistance, the measurement is based in one way or another on utilisation of the magnetic field caused by the primary current. hi the induction phenomenon, alternating current travelling in the primary circuit of the current transformer brings about in the current transformer a magnetic flux changing in relation to time. The said magnetic flux for its part induces a voltage in the secondary circuit of the current transformer. This voltage in the current transformer's secondary circuit is proportional to the time derivative of the alternating current in the current transformer's primary circuit. In order to find out the value of the alternating current travelling in the current transformer's primary circuit, the voltage in the current transformer's secondary circuit must be integrated in relation to time.

h state-of-the-art solutions, a current measurement signal obtained from the current transformer's secondary circuit is integrated, whereupon the said integrated current measurement signal is multiplied by the voltage measurement signal in order to find out the power. Such solutions have usually used active integrators, that is, such which are equipped with an amplifier. This is so because the output signal of an inductive current transformer is usually of a low level. When a passive RC filter even at the basic frequency further dampens the signal to approximately a hundredth part, it has not usually been possible to use a passive integrator in state-of-the-art solutions.

The applicant's FI Patent 98865 presents a method based on the induction phenomenon for measuring alternating current, a measuring sensor intended to measure alternating current and its use in a kWh meter. In the method, a gradi- ometer of at least the first order is fitted inside the electric conductor system or in its immediate neighbourhood, whereby the current travelling in the current system will induce a voltage in the gradiometer. The form of the current conductor system and the form of the gradiometer' s coil structure are adapted in such a way to each other that the output signal is essentially independent of any minor changes occurring in the relative position of the current conductor system and the gradiometer. The applicant's FI Patent Application 20001048 presents a current transformer based on the induction phenomenon for measuring alternating current. The current transformer includes a primary current conductor, wherein there are two current loops of essentially circular shape connected in parallel. The current loops are located concentrically in parallel planes located on top of each other and at a distance from each other. Between the current loops there is at least one gradiometer in a plane or planes parallel with the current loop planes. The primary current travelling in the current loops of the primary current conductor is connected by way of a magnetic field to the said at least one gradiometer and therein it brings about a voltage proportional to the primary current.

The main characteristic features of the method according to the invention are presented in the characterising part of claim 1.

The main characteristic features of the equipment according to the mvention are presented in the characterising part of claim 5.

The solution according to the invention is based on the realization that when one wishes to measure the electric power of a consumption object, information about the real value of the consumption object's current is not necessarily needed. If the current transformer gives such a current measurement signal from its secondary circuit, which is proportional to the time derivative of the consumption object's current, integration may be applied to the voltage measurement signal of the voltage transformer and not to the current measurement signal of the current transformer's secondary circuit. The electric power is found out by multiplying the current measurement signal proportional to the current's time derivative by the integrated voltage measurement signal.

A signal can be integrated in relation to time in many different ways. The methods may be analogue either based only on passive RC or LR circuits or based on active circuits, that is, those equipped with an amplifier. In digital kWh meters, integration may be carried out by numeric methods by various algorithms of the time or frequency level. The integration works in a satisfactory manner when it approximates the transfer function Hι(jω) = Go)^"1 of the ideal integrator well enough within the frequency range concerned in each case. For example, in kWh measurement, the important frequency range is usually about 10 Hz - 1 kHz. Nor must integration produce any interfering signals in the other frequency ranges. Integration is an operation, which enhances low-frequency signals, and with direct current the amplification is even infinite.

Practical integrators should be implemented in such a way that their output signal mainly has the integral of the useful part of the input signal. Thus, the aim here is for the integrator to approximate the ideal integrator with sufficient precision within the desired frequency range and to dampen any components outside this frequency range. Nor should it generate any new signal in the output signal, such as e.g. DC components or any low-frequency noise. The integrator must be sufficiently linear, so that it will not bring about too many harmonious components.

Irrespective of the method by which integration is performed it is advantageous if the integrating signal is of a suitable magnitude or it can easily be made so, it contains no direct current components and it is a narrow-band signal. It is in fact more advantageous to apply integration to the voltage signal of the consumption object and not to the current derivative of the consumption object because:

- there is usually no need to amplify the voltage signal (e.g. 230 V) of the consumption object, whereby any harmful DC components caused by the amplifier's offset voltage are avoided the voltage signal of the consumption object usually contains no harmful DC component, or if it does, it is very small

- the voltage signal of the consumption object is of a considerably narrower band than the current signal of the consumption object, whereby the integration task is simplified.

Thus, in the method according to the invention, the real current of the consump- tion object is not found out, but it is still possible to calculate the electric power and energy consumed by the consumption object.

hi the following, the solution according to the invention will be described with reference to the circuits shown in the figures of the appended drawings, but the purpose is not to limit the invention only to these.

Figure 1 is a schematic view of the measuring circuit of a single-phase kWh meter known as such.

Figure 2 shows an active integrator known as such, that is, one equipped with an amplifier, which may be used to implement the invention.

Figure 3 shows a passive integrator known as such, which may be used to implement the invention.

Figure 1 is a schematic view of the measuring circuit of a single-phase kWh meter, which is known as such. The measuring circuit of the voltage arm includes a protection circuit 11, the purpose of which is to protect the kWh meter against over-voltage peaks arriving from the electric mains. The protection circuit is followed by a voltage circuit 12, which is connected to the voltage of the consumption object to be measured in between a phase wire and a neutral wire. In the voltage circuit such a signal level is formed of the mains voltage, which is suitable for a multiplier 15. The measuring circuit of the current arm includes a current transformer 13, which is followed by a pre-amplifier 14, by which the voltage signal obtained from the secondary circuit of current transformer 13 is amplified to a level suitable for multiplier 15. i multiplier 15, signals propor- tional to the voltage of the consumption object to be measured and proportional to the current are multiplied by each other, whereby the electric energy consumed by the consumption object is obtained. A mechanical drum counter or a digital LCD display may be used as the counter 16 in the measuring circuit, hi addition, the measuring circuit includes pulse outputs 17 according to the standard, whose pulse number is proportional to the consumption of electric energy, pulses/kWh.

In state-of-the-art solutions, the signal to be supplied to the multiplier 15 of the current arm is integrated before the value of the current measurement signal is multiplied by the value of the voltage measurement signal to find out the power. Thus, what is at issue is integration of the voltage of the inductive current transformer's 13 secondary side, whereby the integrator is located in the current arm in connection with the input gate of multiplier 15.

In the solution according to the invention, the signal to be supplied to the multiplier of the voltage arm is integrated before the value of the voltage measurement signal is multiplied by the value of the current measurement signal to find out the power. On the other hand, the signal to be supplied to the multiplier 15 of the current arm is not integrated. The integrator is located in connection with the input gate of the multiplier 15 in the voltage arm.

Referring to Figures 2 and 3, the following is a description of two integrators known as such and suitable for the solution according to the invention.

Figure 2 shows an active integrator 20, that is, one equipped with an amplifier. The circuit includes an operation amplifier 21, a first resistance Ri connected to the input gate of operation amplifier 21, and a second resistance R₂ connected in parallel in between the input gate and the output gate of operation amplifier 21, and a capacitor C. The transfer function of such a circuit is: H(jω) = -^

R_x jωCR₂ + l

The circuit must be dimensioned so that term ω_mi_n "C^ is of a sufficient magnitude, preferably > 100. At high frequencies (ω > Omin) the transfer function H(jω) of the connection approaches the function:

1 7^'ωCR, which may be written as:

— — - HΛjω) CR, ^J which again is standard times the transfer function of the ideal integrator. At low frequencies, the transfer function approaches the value -Ra/ i, which is the DC amplification of the connection. The amplifier's own offset voltage is also summed into the integrated signal by coefficient — R2/R1.

In a situation where a phase voltage Ui_n = 230 V of a consumption object connected to a low- voltage mains is conducted directly into the integrator, the integrator may be dimensioned so that R₂ = Ri = 1 MQ and C = 300 nF. Hereby the integrator's phase error at a frequency of 50 Hz is already reasonably small, the signal is of a suitable magnitude in the output of the integrator for further processing, and the DC amplification is about 1.

In a situation where a small signal of a few volts is conducted into the integrator, the ratio R₂/Ri must be high, preferably > 100. Hereby the DC amplification increases at least to a value of 100, which may be harmful.

The active integrator shown in Figure 2 is better suitable for integration of a high signal, e.g. 230 V phase voltage, than for integration of a low signal proportional to the current derivative. Figure 3 shows a simple passive RC integrator 30. The transfer function of the circuit is:

H(jω) = , jωRC + 1

The circuit must be dimensioned so that the term α R-C is sufficiently high, preferably > 100. This circuit functions in the same way as the circuit shown in

Figure 2, if R₂ = Ri is chosen for the circuit shown in Figure 2. However, no amplifier is needed in this passive circuit, whereby any harmful offset voltage of an amplifier is also avoided.

The circuit may be dimensioned so that Ri = 1 MΩ and C = 300 nF. Hereby the circuit dampens the phase voltage of the consumption object to a voltage level of 230 V, which is a suitable value for further processing of the signal.

In the foregoing only some examples of application of the solution according to the invention were presented, and it is obvious to the professional in the art that numerous modifications can be made in these within the scope of the inventive idea presented in the appended claims.

Claims

1. Method for measuring power in an alternating-current system, wherein current of the consumption object is conducted through a first transformer based on the induction phenomenon to a measuring circuit, whereby such a current measurement signal is available in the measuring circuit, which is proportional to the time derivative of the current of the consumption object, and wherein the voltage of the consumption object is conducted directly or through a second transformer to the measuring circuit, whereby such a voltage measurement signal is available in the measuring circuit, which is proportional to the voltage of the consumption object, characterised in that the integration is applied to the voltage measurement signal instead of the current measurement signal and that the transient power of the consumption object is calculated by multiplying at each moment of calculation the value of the current measurement signal by the value of the integrated voltage measurement signal.

2. Method according to claim 1, characterised in that integration of the voltage measurement signal is performed by an active RC circuit based on an operation amplifier.

3. Method according to claim 1, characterised in that integration of the voltage measurement signal is performed by a passive RC circuit.

4. Method according to claim 1, characterised in that integration of the voltage measurement signal is performed by numeric methods by an algorithm of the time or frequency level.

5. Equipment for implementation of the method according to claim 1, which equipment includes a first transformer (13), which is based on the induction phenomenon and by which the current of the consumption object is transformed into a current measurement signal proportional to the time derivative of the consumption object's current, and a multiplier (15), wherein the voltage measurement signal proportional to the voltage of the consumption object is multiplied by the current measurement signal, characterised in that the equipment in addition includes an integrator (20, 30), by which the voltage measurement signal is integrated, whereupon the transient power of the consumption object is calculated by multiplying at each moment of calculation the value of the current measurement signal by the value of the integrated voltage measurement signal.

6. Equipment according to claim 5, characterised in that the integrator (20) is an active RC circuit equipped with an operation amplifier.

7. Equipment according to claim 5, characterised in that the integrator (30) is a passive RC circuit.

8. Equipment according to claim 5, characterised in that the integrator is a numeric method based on the algorithm of a time or frequency level.