RU2093839C1 - Three-phase mains active-current measurement technique - Google Patents

Abstract

FIELD: electric measurement technology. SUBSTANCE: all phase currents are sequentially time-integrated within half-cycle of modulating voltage whose frequency is three-fold that of current being measured, that is, within space T/6, where T is period of current under measurement. Measurement time and yield of results make up one sixth of period and all phase currents are measured by means of one channel. EFFECT: improved speed and facilitated procedure. 2 dwg

Description

 The invention relates to electrical engineering and is intended for use in symmetric three-phase three-wire electrical networks when measuring active current.

 There is a method of measuring the active current of a three-phase network, including the measurement of phase currents and voltages, the formation of a modulating sequence of pulses from the measured phase voltages, modulating the phase currents with these pulses, integrating the modulated signals for a fixed time interval and storing the integration result (A. S. USSR N 679889, class GO1 R 19/22, 1979. Abramovich B. N. Krugly A. A. Excitation, regulation and stability of synchronous motors (L. Energoizdat, 1983, p. 107 111).

 When implementing this method, a signal proportional to the active component of the current of each phase is formed as a result of averaging or linear voltage of any two phases shifted by a quarter of the period. Thus, the period of measuring the active current of one phase is half the period of the measured current (10 ms for industrial networks with a frequency of 50 Hz). The measurement of active currents of all phases involves the use of three independent measuring signals for each of the phases, which complicates the technical implementation.

 Thus, the disadvantage of the known methods of measuring the active current of a three-phase network is low speed and the complexity of the technical implementation.

 Of the known technical solutions, the closest to the result achieved is the method for measuring the active current of a three-phase network, including measuring phase currents and voltages, generating a modulating pulse sequence from the measured phase voltages, modulating the phase currents with these pulses, integrating the modulated signals for a fixed time interval and storing an integration result that is proportional to the active current (A.S. USSR N 1048418, class GO1 R 19/22, 1983).

 With this method, a signal proportional to the active component of the current of each phase is formed as a result of averaging over the half period of the phase voltage of the other two phases shifted by a quarter of the period. Consequently, the period of measurement and formation of the result is half the period of the measured current (for general industrial networks 10 ms). To measure the active currents of all phases, the use of three independent measuring channels for each of the phases, i.e. the technical implementation of the method is complicated.

 Thus, the disadvantages of the known method are that the measurement of the active component of the three-phase current is carried out with low speed, as well as the complexity of its implementation.

 The purpose of the invention is to increase speed and simplify the technical implementation of the method of measuring the active current of a three-phase network.

 This goal is achieved by the fact that with the known method of measuring the active current of a three-phase network, which includes measuring phase currents and voltages, generating from the measured phase voltages a modulating sequence of pulses, modulating the phase currents with these pulses, integrating the modulated signals for a fixed time interval, and storing the integration result, which proportional to the active current, additionally form a modulating sequence in the form of rectangular pulses, the following x with a frequency equal to the trebled frequency network and synchronized with one of the phase voltages and phase currents integration modulated operate alternately during each half cycle of the modulating signal.

где U_M амплитудное значение напряжения;
ω частота.The essence of the proposed method for measuring the active current of a three-phase network is as follows. The phase voltages of a three-phase symmetric network are equal to:

where U _{M is the} amplitude value of the voltage;
ω frequency.

Φ фазовый сдвиг.Network phase currents

Φ phase shift.

где U_e напряжение, соответствующее уровню логической единицы.The voltage with a frequency equal to three times the frequency of the network, and synchronized with one of the phase voltages, for example U _A (t), is
u ₀ (t) = U _M0 sin3ωt.
The modulating pulse sequence is described by the expression

where U _{e is the} voltage corresponding to the level of a logical unit.

где k_M коэффициенты передачи модулятора.The phase currents modulated by the signal u _M (t) are equal, respectively:

where k _{M are} the transmission coefficients of the modulator.

где

коэффициент пропорциональности.The integrals from the modulated phase currents, taken sequentially for phases B, A and C, alternately for the time interval T / 6, respectively, are equal to:

Where

coefficient of proportionality.

 Therefore, all the integrals of the modulated phase currents taken over the corresponding time interval are proportional to the active circuit current.

 Thus, if phase currents and phase voltages are measured in a three-phase symmetric network, a modulating sequence of rectangular pulses is formed from the measured phase voltages, followed by a frequency equal to three times the frequency of the network and synchronized with one of the phase voltages, modulate all phase currents with this signal and sequentially integrate If they are alternately during each half-cycle of the modulating signal, the results will be proportional to the active current of the network. In this case, the measurement time of the presentation of the results is one sixth of the period of the measured current, i.e. increases the speed of the measurement process. In addition, to implement the method, only one channel can be used that serves all three phases sequentially in time, which simplifies the technical implementation of the method.

 An example of a technical implementation of the proposed method is shown in FIG. 1, which shows a functional diagram of a device for measuring the reactive current of a three-phase network. The device includes a power supply network 1, the first 2 and the second 3 measuring current transformers, an algebraic adder 4, a measuring transformer 5, a switch 6, a frequency multiplier 7, a modulator 8, a threshold element 9, an integrator with a reset 10, a sample-storage device 11.

 In this device, the outputs of two current transformers 2 and 3 are connected respectively to the first and third inputs of switch 6, the second input of which is connected to the output of the algebraic adder 4, the first and second inputs of which are connected to the outputs of the first 2 and second 3 current transformers, the output of a three-phase measuring voltage transformer 5 through series-connected frequency multiplier 7 and threshold element 9 are connected to the combined control inputs of series-connected switch 6, the modulator 8, an integrator 10 with reset and sample and hold device 11, whose output serves as output of the apparatus.

Timing diagrams explaining the principle of operation of the device are shown in FIG. 2, where the output signals of the elements of the device are indicated by the symbol uc and the index corresponding to the number of the element in the functional diagram (Fig. 1). Two measuring current transformers 2 and 3 and a three-phase voltage transformer 5 are connected to the power supply network 1. At the outputs of the first 2 and second 3 current transformers, signals are proportional to the currents of phases A and C. These signals are fed to the inputs of the algebraic adder 4, at the output of which a signal is generated proportional to the current of phase B:
u ₄ (i _A + i _C ) = k _T i _B ,
where 4 k _{T is} the proportionality coefficient of current transformers 2 and 3.

The signal u ₇ from the voltage measuring transformer 5 is fed to the input of the frequency multiplier 7, the output of which is voltage
u ₇ = k ₅ k ₇ u _M sin3ωt,
where k ₅ and k ₇ are transmission coefficients of a voltage transformer 5 and a frequency multiplier 7, respectively.

Выходной сигнал u₉ порогового элемента 9 имеет вид прямоугольных импульсов, следующих с частотой, равной утроенной частоте питающей сети 1. Эти импульсы управляют коммутатором 6, модулятором 8, интегратором со сбросом 10 и устройством выборки-хранения 11. Коммутатор 6 осуществляет поочередное подключение к информационному входу модулятора 8 выходов алгебраического сумматора 4, первого 2 и второго 3 трансформаторов тока. Следовательно, выходной сигнал коммутатора 6 имеет вид:

Напряжение u₆ поступает на информационный вход модулятора 8, коэффициент передачи k₈ которого равен k₈ 1 при u₉ 0 и k₈ -1 при u₉ u_e. В результате напряжение на выходе модулятора 8 равно:

Сигнал u₈ поступает на информационный вход интегратора 10, который устанавливается в начале каждого такта измерения передними и задними фронтами импульсов u₉ в состояние u₁₀ 0. Напряжение на выходе интегратора 10 в конце каждого такта измерения

где

коэффициент пропорциональности;
k₈ и k₁₀ коэффициенты передачи модулятора 8 и интегратора 10;
m 0, 1, 5.The signal u ₇ from the output of the frequency multiplier is fed to the input of the threshold element 9, the voltage at the output of which

The output signal u _{9 of the} threshold element 9 has the form of rectangular pulses following with a frequency equal to the triple frequency of the supply network 1. These pulses control the switch 6, the modulator 8, the integrator with reset 10 and the sampling-storage device 11. Switch 6 carries out an alternate connection to the information the input of the modulator 8 outputs of the algebraic adder 4, the first 2 and second 3 current transformers. Therefore, the output signal of the switch 6 has the form:

The voltage u _{6 is} supplied to the information input of the modulator 8, the transmission coefficient k _{8 of} which is equal to k ₈ 1 for u ₉ 0 and k ₈ -1 for u ₉ u _e . As a result, the voltage at the output of modulator 8 is equal to:

The signal u _{8 is} supplied to the information input of the integrator 10, which is set at the beginning of each measurement cycle by the leading and trailing edges of the pulses u ₉ to the state u ₁₀ 0. The voltage at the output of the integrator 10 at the end of each measurement cycle

Where

coefficient of proportionality;
k ₈ and k ₁₀ transmission coefficients of modulator 8 and integrator 10;
m 0, 1, 5.

Therefore, at times t mT / 6, m 0.1.5, a signal is generated at the output of the integrator 10, which is proportional to the active current of the three-phase electric network 1. At these times, corresponding to the leading and trailing edges of the pulses u ₉ , the contents of the integrator 10 are recorded to the sampling-storage device 11. The value u ₁₀ (mT / 6) recorded in the sampling-storage device 11 at the end of the measurement clock acts on the output for the next clock, the duration of which is equal to one sixth of the period.

 Thus, using the known method for measuring the active current of a three-phase network of a modulating sequence in the form of rectangular pulses following with a frequency equal to three times the frequency of the network, and in-phase with one of the phase voltages, and sequentially integrating all phase currents of the modulating signal during each half-cycle, improve performance and simplify the technical implementation of the method.

 Using the proposed method for measuring the active current of a three-phase network in various monitoring and control systems for electrical devices and equipment will improve their technical characteristics.

Claims (1)

 A method for measuring the active current of a three-phase network, including measuring phase currents and voltages, generating a modulating sequence of pulses from the measured phase voltages, modulating the phase currents with these pulses, integrating the modulated signals for a fixed time interval and storing the integration result, which is proportional to the active current, characterized in that the modulating sequence is formed in the form of rectangular pulses following with a frequency equal to the triple frequency of the network, and synchronized with one of the phase voltages, and the integration of the modulated phase currents is performed alternately during each half-period of the modulating signal.

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