JP2007198844A - Electric power measuring instrument, and motor multi-relay - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power measuring instrument capable of measuring precisely three-phase electric power using one transformer for detecting a phase-to-phase voltage, and a motor multi-relay built in therewith. <P>SOLUTION: This electric power measuring instrument is provided with a current detecting circuit for detecting instantaneous currents in optional two phases, a voltage detecting circuit for detecting instantaneous voltages in the optional two phases, a reception circuit for receiving instantaneous voltages in two phases different from the optional two phases, and a computation circuit. Instantaneous electric power is calculated in the computation circuit, based on values of the instantaneous currents in the two phases detected by the current detecting circuit, values of the instantaneous voltages detected by the voltage detecting circuit, and values of the instantaneous voltages received by the reception circuit. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a power measuring device that measures the power of a three-phase power load, and a motor multi-relay that incorporates the power measuring device to protect and monitor a motor.

  Many low-pressure motors are used to operate pumps and valve opening / closing devices in various plants such as steelworks and petroleum refining chemistry. Power supply to these many low-voltage motors is concentrated in an open / close control device called a control center. In the control center, a load control device called a motor multi-relay is attached to each motor. The motor multi-relay protects the motor as a load by performing overload protection, open phase protection, ground fault protection, undercurrent protection, power overload protection by power measurement, power low load protection, and the like. As a result, the load device driven by the motor is also protected.

  FIG. 2 is a diagram for explaining the configuration of a conventional power measurement circuit employed in the motor multi-relay. The motor multi-relay 50 receives the R-phase and T-phase motor currents detected by the two current transformers 51 and 52 and the voltage between the R and T phases detected by the transformer 53. In order to measure three-phase power by the usual two-watt meter method, it is necessary to detect another inter-phase voltage, for example, a voltage between R and S phases, in addition to the R-T phase. However, to do so, it is necessary to increase the number of transformers to two. The power value is calculated by A / D converting the measured voltage and current into digital values and multiplying them. Therefore, if the input interphase voltage is increased, an A / D conversion circuit needs to be added.

  Increasing the number of detected interphase voltages leads to high costs. Additional installation space is also required. Therefore, in the power measurement circuit of FIG. 2, assuming that the three-phase voltage is balanced, the three-phase power is calculated from the R-T phase voltage and the R-phase and T-phase currents (for example, patents). Reference 1).

However, since the power measurement circuit of FIG. 2 performs the calculation assuming that the three-phase voltage is balanced, there is a problem that the measurement accuracy is lowered when the voltage is unbalanced.
JP 2001-359297 A

  The present invention has been made to solve these problems of the prior art, and the problem is that it is not necessary to install two transformers for phase-to-phase voltage detection, but only to install one transformer. An object of the present invention is to provide a power measuring device capable of measuring phase power with high accuracy and a motor multi-relay incorporating the power measuring device.

  The invention described in claim 1 for solving the above-described problem is a power measuring apparatus for a three-phase load, comprising a current detection circuit for detecting an instantaneous current of any two phases and an instantaneous voltage between any two phases. A voltage detecting circuit to detect, a receiving circuit for receiving an instantaneous voltage between two phases different from the arbitrary two phases from the outside, a value of a two-phase instantaneous current detected by the current detecting circuit, and the voltage detecting circuit An electric power measuring apparatus comprising: an arithmetic circuit that calculates instantaneous electric power from a detected instantaneous voltage value and an instantaneous voltage value received by the receiving circuit.

  According to the power measuring device having such a configuration, it is possible to use only one transformer for detecting the interphase voltage necessary for instantaneous power measurement. Also, even if one transformer is used, the instantaneous power is calculated using the instantaneous current value of two phases and the instantaneous value of two phase voltages, so even if the three-phase voltage is unbalanced, the instantaneous power is accurate. The power can be measured.

The invention according to claim 2 is the power measuring device according to claim 1, further comprising a transmission circuit for transmitting the instantaneous voltage detected by the voltage detection circuit to the outside.
According to the power measuring apparatus having such a configuration, it is possible to provide the instantaneous value of the interphase voltage detected by itself to a power measuring apparatus that requires the instantaneous voltage value between different phases.

  In the power measuring device according to claim 1, the arithmetic circuit further calculates the average power by averaging the calculated instantaneous power over a predetermined period of three-phase power. An electric power measuring apparatus characterized by calculating.

  According to the power measuring device having such a configuration, it is possible to use only one transformer for detecting the interphase voltage necessary for measuring the average power. Also, even if one transformer is used, the average power is calculated using the instantaneous current value of the two phases and the instantaneous value of the voltage between the two phases. The power can be measured.

  According to a fourth aspect of the present invention, in the power measuring device according to the first or third aspect, the arithmetic circuit calculates a current value detected by the current detection circuit and a voltage value detected by the voltage detection circuit. A storage circuit that temporarily stores the value of the instantaneous voltage received by the receiving circuit, and a predetermined time as a communication required time from when the instantaneous voltage is detected until the receiving circuit receives the value An electric power measuring apparatus that calculates instantaneous electric power based on a current value and a voltage value temporarily stored in the storage circuit.

  The instantaneous value arrives at the receiving circuit with a delay of the time required for communication. In this configuration, as the voltage and current values detected by itself, values previously detected for the required communication time are used, and the instantaneous power is calculated based on these and the received voltage values. Therefore, the influence of delay time due to communication can be reduced, and the power measurement accuracy can be further increased.

According to a fifth aspect of the present invention, there is provided a motor multi-relay for motor protection and monitoring, characterized by incorporating the power measuring device according to any one of the first to fourth aspects.
The motor multi-relay having such a configuration can acquire highly accurate power information regarding the instantaneous power, average power, power consumption, etc. of the three-phase load even when the three-phase voltage is unbalanced. Therefore, the three-phase load can be protected and monitored with high accuracy.

  Hereinafter, an embodiment of a power measuring device and a motor multi-relay incorporating the power measuring device according to the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a motor multi-relay 1 including the power measuring device 2 and their connection relationship.

  Three-phase AC power, which is a load, is supplied from a three-phase AC power source 6 composed of R, S, and T phases via a circuit breaker 8 and a magnetic contactor 9. Between the circuit breaker 8 for wiring and the magnetic contactor 9, a transformer 10 for detecting the R-T phase voltage is provided. Between the magnetic contactor 9 and the motor 4, an R phase change for detecting the R phase current is provided. A current transformer 11, a T-phase current transformer 12 for detecting a T-phase current, and a zero-phase current transformer 13 are connected.

  The motor multi-relay 1 includes a power measuring device 2, a control circuit 16, a display device 17, and an input device 18. The control circuit 16 is a core circuit that performs control to protect the motor 4 from overload, phase loss, ground fault, shortage current, power overload, power low load, and the like. Although not shown in detail, the control circuit 16 is a microcomputer, a zero-phase current detection circuit that captures the output current value of the zero-phase current transformer 13, and an input interface circuit that captures the operating states of the circuit breaker 8 and the magnetic contactor 9. , An output interface circuit for operating the circuit breaker 8 and the magnetic contactor 9, an input interface circuit for taking in power information from the power measuring device 2, an input / output interface circuit for operating the display device 17 and the input device 18, and the like. A communication circuit for communicating with the motor multi-relay 1a is provided.

  The input device 18 is for inputting control conditions for controlling the motor 4. The display device 17 is for displaying a control state, an abnormality content, an alarm, and the like. The motor multi-relay 1 controls and protects the motor 4 based on control conditions input from the input device 18, power information acquired from the power measuring device 2, voltage information, current information, and the like. At the same time, the control device, the details of the abnormality, an alarm, etc. are displayed on the display device 17.

  The power measuring device 2 is for measuring the power supplied to the motor 4. The power measuring apparatus 2 includes an arithmetic circuit 21, A / D converters 22, 23, a transmission circuit 24, a reception circuit 25, a power supply circuit 27, a transformer 10, an R-phase current transformer 11, a T-phase current transformer 12, and a resistance. R1 and R2 are provided.

  The primary side of the transformer 10 is connected to the R phase and S phase of the three-phase AC power source 6. A voltage obtained by stepping down the R-S phase voltage at a constant ratio is output to the secondary side. The power supply circuit 27 receives power supply from the secondary side of the transformer 10 and supplies power to each circuit in the power measurement device 2 and the motor multi-relay 1. The analog voltage output to the secondary side of the transformer 10 is converted into a digital value in a short cycle by the A / D converter 22 and input to the arithmetic circuit 21.

  Resistors R1 and R2 are connected to the R-phase current transformer 11 and the T-phase current transformer 12, respectively. Voltages proportional to the R-phase current and the T-phase current are generated at both ends of the resistors R1 and R2. The voltage is converted into a digital value by a 2-channel input A / D converter 23 in a short cycle and input to the arithmetic circuit 21.

  The arithmetic circuit 21 is configured using a microcomputer, and is provided with a known CPU, a RAM 28 as a storage device, a ROM, an input / output interface circuit, a bus connecting them, and the like. The arithmetic circuit 21 is connected to the control circuit 16.

  A transmission circuit 24 and a reception circuit 25 are further connected to the arithmetic circuit 21. The transmission circuit 24 is for transmitting the R-S phase voltage detected by itself to a power measurement device (not shown) in the other motor multi-relay 1a. The receiving circuit 25 is for receiving the interphase voltage detected by the power measuring device from the power measuring device in the other motor multi-relay 1a. Note that a communication line between the control circuit 16 and another control circuit may be used as a communication line between the power measurement device 2 and another power measurement device.

Next, a power measurement method of the power measurement device 2 having such a configuration will be described. The instantaneous values of the R-phase, S-phase, and T-phase voltages of the three-phase voltage supplied to the motor 4 are Vr, Vs, Vt, the instantaneous values of the phase currents are supplied to Ir, Is, It, and the motor 4. Let P be the instantaneous value of the active power. The electric power P is calculated by the following formula.
P = Vr · Ir + Vs · Is + Vt · It (1) Further, since the zero-phase current is zero, the following equation is established.
Ir + Is + It = 0 (2) Formula

Using the formula (2), erase Ir in the formula (1). The power P is calculated as follows.
P = Vr · (−Is−It) + Vs · Is + Vt · It
= (-Vr + Vs) .Is-(-Vt + Vr) .It
(−Vr + Vs) is a voltage between R and S phases based on the R phase, and (−Vt + Vr) is a voltage between T and R phases based on the T phase. When these values are represented by Vsr and Vrt, the electric power P is represented as follows.
P = Vsr · Is−Vrt · It (3) Formula

Similarly, when Is in the equation (1) is deleted, the power P is calculated by the following equation.
P = Vts · It−Vsr · Ir (4) where Vts is a voltage between the TS phases with the S phase as a reference.
Similarly, when It in the equation (1) is deleted, the power P is calculated by the following equation.
P = Vrt · Ir−Vts · Is (5)

  The power measuring device 2 of the present embodiment detects R-phase and T-phase current instantaneous values Ir and It. The undetected S-phase current instantaneous value Is can be calculated using the relationship of equation (2). That is, the arithmetic circuit 21 can know any of the three-phase instantaneous current values Ir, Is, It.

  Regarding the interphase voltage, the power measuring device 2 of the present embodiment detects the voltage Vsr between the RS phases. Therefore, for example, if the voltage Vrt between the R and T phases not detected is known, the arithmetic circuit 21 can calculate the value of the instantaneous power P using the equation (3). When the voltage Vts between TS phases is known, the value of the instantaneous power P can be calculated using the equation (4).

  Therefore, in the power measurement device 2 of the present embodiment, an instantaneous voltage value between the R and T phases or between the T and S phases is acquired from another power measurement device. Communication is used for the acquisition. And the instantaneous electric power P is calculated by (3) Formula or (4) Formula using the acquired value. The average power is obtained by integrating the value of the instantaneous power P over a predetermined period and calculating the average value during that period. The amount of electric power is obtained by continuously integrating the value of the instantaneous electric power P.

  As described above, the power measuring device 2 detects any two-phase current of the three-phase current. If the currents of the two phases are found, the currents of the remaining phases are obtained by calculation, and eventually the phase currents of all three phases are found. In order to calculate the instantaneous power P, it is necessary to know two inter-phase voltages in addition to these current values. Therefore, one interphase voltage is detected by itself, and the remaining one interphase voltage is acquired by communication from the outside. Then, the instantaneous power P is calculated by selecting one of the expressions (3), (4), and (5) according to the two interphase voltages found. The power measuring device 2 measures the instantaneous power P based on such an idea. The instantaneous value of the interphase voltage detected by the power measurement device 2 itself is output from the transmission circuit 24 for use by other power measurement devices.

  As described above, the power measurement apparatus 2 according to the present embodiment acquires one of the interphase voltages not detected by itself from the other power measurement apparatus. It is used for power calculation. Therefore, it prepares only one transformer for detecting one interphase voltage. In this way, the required transformer can be reduced from two to one. Since the number of interphase voltages to be detected is reduced, the number of A / D converters that convert the voltages into digital values can also be reduced. Since the power calculation is performed in the same manner as the two-watt meter method, accurate power measurement can be performed even when the three-phase voltage is unbalanced. These advantages more than make up for the disadvantage of requiring an extra transmitter circuit 24 and receiver circuit 25.

Note that the current detection circuit described in the claims is configured by the R-phase current transformer 11, the T-phase current transformer 12, the resistors R <b> 1, R <b> 2, and the A / D converter 23 in the above embodiment. The voltage detection circuit includes a transformer 10 and an A / D converter 22.
Moreover, although the motor 4 was taken up as a load in the said embodiment, loads other than a motor, for example, loads, such as a heater, may be sufficient.

(Modified embodiment)
The power measurement device 2 in the above embodiment may be modified as follows. In the power measuring device 2, the instantaneous value of the other interphase voltage that is not detected by the power measuring device 2 is acquired from the other power measuring device by communication. In this case, the arithmetic circuit 21 receives the instantaneous value with a delay of the required communication time. If the required communication time is so large that it cannot be ignored, an error occurs in the instantaneous power P that is calculated due to a phase difference between the voltage and current detected by itself and the voltage acquired through communication.

  In order to reduce this error, the voltage, current, etc. detected by itself may be delayed by the required communication time. In order to do so, the voltage and current values detected by the A / D converters 22 and 23 are stored in the RAM 28 in the arithmetic circuit 21 in time series. Then, when calculating the instantaneous power P, the instantaneous values of the previous voltage and current corresponding to the required communication time are read from the RAM 28, and the instantaneous power P is obtained using these values and other interphase voltages received by communication. Is calculated.

  In this way, since the phase difference caused by the communication required time can be reduced, the measurement accuracy of the instantaneous power P can be increased.

It is a block diagram which shows the structural example of an electric power measurement apparatus and a motor multi relay including the same. FIG. 2 is a view corresponding to FIG.

Explanation of symbols

  In the drawings, 1 is a motor multi-relay, 1a is another motor multi-relay, 2 is a power measuring device, 4 is a motor, 10 is a transformer, 11 is an R-phase current transformer, 12 is a T-phase current transformer, 16 is A control circuit, 21 is an arithmetic circuit, 24 is a transmission circuit, 25 is a reception circuit, and 28 is a RAM (memory circuit).

Claims (5)

A power measuring device for a three-phase load,
A current detection circuit for detecting an instantaneous current of any two phases;
A voltage detection circuit for detecting an instantaneous voltage between any two phases;
A receiving circuit for receiving from the outside an instantaneous voltage between two phases different from the arbitrary two phases;
An arithmetic circuit for calculating instantaneous power from the value of the two-phase instantaneous current detected by the current detection circuit, the value of the instantaneous voltage detected by the voltage detection circuit, and the value of the instantaneous voltage received by the reception circuit; A power measuring device comprising:   The power measuring apparatus according to claim 1, further comprising a transmission circuit that transmits the instantaneous voltage detected by the voltage detection circuit to the outside. The power measuring device according to claim 1,
The arithmetic circuit further calculates an average power by averaging the calculated instantaneous power over a predetermined period of three-phase power. In the electric power measurement apparatus according to claim 1 or 3,
The arithmetic circuit includes a storage circuit that temporarily stores a current value detected by the current detection circuit and a voltage value detected by the voltage detection circuit, and the instantaneous voltage value received by the reception circuit and the instantaneous voltage are The instantaneous power is calculated based on the current value and the voltage value temporarily stored in the storage circuit previously for a predetermined time as a communication required time from when the reception circuit receives the value until it is detected. A power measuring device.   A motor multi-relay for motor protection and monitoring, comprising the power measuring device according to any one of claims 1 to 4.

Images