RU1814138C - Protected self-contained three-phase network - Google Patents

Abstract

The invention relates to electrical engineering, in particular to relay protection, and can be used in a three-phase autonomous network with isolated neutral with a frequency-controlled electric motor for protection against short circuit. A three-phase autonomous network contains a three-phase current sensor, a three-phase voltage sensor, an electromagnetic power measuring unit, a current polarity determination unit, comparators, a logic unit, a pulse duration discriminator, a single vibrator, an actuator. Summary of the invention: short circuit detection is carried out by comparing the duration of the negative and positive instantaneous values of the signals from the output of the current polarity determination unit and the electromagnetic power measurement unit for a period. This eliminates false triggering of the actuator unit and allows the use of frequency converters with a reverse diode bridge to reset the energy of the electric motor to the DC link of the inverter. 2 ill. Yo

Description

The invention relates to electrical engineering, in particular to relay protection, and can be used in a three-phase autonomous network with isolated neutral with a frequency-controlled asynchronous motor for protection against short circuit.

The purpose of the invention is to expand the scope of application on a network with frequency converters having reverse diode bridges for discharge of energy into a DC link.

Figure 1 shows a functional diagram of a three-phase autonomous network with protection; Fig. 2 shows voltage plots illustrating the operation of the device. .

A three-phase autonomous network with protection contains a three-phase current sensor 1, a three-phase voltage sensor 2, an electromagnetic power measurement unit (BEM) 3, a current polarity determination unit 4, comparators 5, 6, an element BAN 7, a discriminator 8 of pulse duration, a one-shot 9, an executive unit 10, a frequency converter 11, consisting of a series-connected controlled rectifier 12, a DC link 13 and an inverter 14 with a reverse bridge (not shown in Fig. 1), an electric motor 15. connected to the cable 16.

The outputs of the current sensors 1 and voltage 2 are connected to the inputs of block 3,

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with

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the output of which through the first 5 comparator is connected to the first input of the BAN 7 element, the second input of which through the second comparator b is connected to the current polarity sensor 4. The output of the element 1 is FORBID connected through series-connected discriminator 8 and one-shot 9 to the Executive unit 10.

A three-phase autonomous network with protection operates in the following modes: normal motor operating mode with variable frequency f and voltage U at U / f const; electric motor starting mode; FC-HELL generator braking mode; short circuit mode in the cable network (emergency mode). Network operation modes are presented in FIG. 2.

During normal long-term and starting operation of the electric drive (Fig. 2, a), the output of the electromagnetic power measuring unit 3 contains rectangular pulses that take positive and negative signs for the period, and the positive half-wave duration is longer than the negative duration (plot Uz, FIG. 2a). A positive sign of the power signal indicates that energy is supplied through the inverter to the motor, while a negative sign indicates that energy is supplied by the motor through the inverter to the filter. By applying these pulses to the input of the comparator 5, at its output we obtain rectangular pulses of positive polarity, the duration of which corresponds to the time during which the energy is given by the engine to the filter (diagram Us, Fig. 2, a). This signal is fed to the first input of the BAN element.

During normal operation of the electric drive, the current in the DC link 13 of the inverter without a return bridge, where the current polarity determination unit 4 is turned on, flows from the rectifier 12 to the inverter 14 during the period of operation of the motor 15 and the output of the latter should have a level 1 signal. But since the frequency converter has a reverse diode bridge of energy discharge to the DC link, at the output of block 4, the signal for the period will have positive and negative signs corresponding to the direction of energy transfer for the period, and from NOSTA positive half wave greater than the length of the negative (1M diagram, Figure 2, a). A conditionally positive half-wave for the period will correspond to the discharge mode of the filter capacitance in the DC link (current flows from the filter to the inverter - the current direction in Fig. 1 is shown by a solid line), conditionally negative half-wave will correspond to the charge of the filter through the reverse diode bridge (current flows from the inverter to the rectifier, the current direction in Fig. 1 is shown by a dashed line). This signal is fed to the input of comparator 6, at its output we obtain rectangular pulses of positive polarity, the duration of which corresponds to time, during

whose filter capacity is charged through the reverse diode bridge and a current in the DC link 13 flows from the inverter 14 to the rectifier 12 (diagram Ue, Fig. 2, a). These pulses are fed to the second input of element 5 and PROHIBITED 7. At the output of element 7, we obtain positive pulses, the leading edge of which coincides with the trailing edge of the pulses from the output of the comparator D and the trailing edge - with the trailing edge of the pulses from the output of the comparator 5 (plot U, Fig. 2, and). In other words, the duration of the pulses from the output of the PROHIBITION 7 element corresponds to the time of the mismatch of the signals from the output of the electromagnetic power measuring unit 3 and the unit 4 for determining the polarity of the current in the DC link of the inverter. From the output of element 7, these pulses are fed to the input of discriminator 8, where their duration is compared with a given

0 In normal motor operation of the electric drive, as well as in the start-up mode, the time of mismatch of the pulses at the outputs of blocks 3 and 4 is less than the set value and at the output of discriminator 8, the signal is logic 5 O (plot Us, Fig. 2, a). The one-shot 9 does not start, and the shutdown signal of the actuation unit 10 is not received.

In regenerative braking mode

0 of the IF-AD system (Fig. 2, b), the inverter 14 goes into a rectifier mode, as a result of which the direction of energy transfer changes: it is transferred from the motor 15 to the DC link 13 of the ПЧ-11.

5 But the signal at the output of block 4 will again take negative and positive signs for the period, but with a negative half-wave duration longer than the positive one (plot U4,

0 figure 2, b). This signal is fed to the input of comparator 6, the output of which will be rectangular pulses of only positive polarity with a duration corresponding to the time during which 5 turns in the DC link 13 flow from inverter 14 to rectifier 12 (the filter capacitance is charged through the reverse diode bridge , (plot Ue, Fig.2, b). This signal is fed to the second input of the element BAN 7.

In the case of generator braking, the operating mode of the electric motor 15 also changes. The engine starts to transfer energy to the filter, and at the output of the electromagnetic power measuring unit 3, rectangular pulses are generated for a duration of a negative half-wave longer than the positive one (plot Us. Fig. 2 , b), which are fed to the input of the comparator 5. At the output of the latter, we have a positive rectangular signal, the duration of which corresponds to the burden during which the engine gives energy to the filter converter l (plot Us. Fig.2, b). This signal is fed to the first input of the element BAN 7. However, despite the fact that compared with the motor and starting operation, the time of the generator mode (the time during which the engine gives energy to the filter) and the time of charging the capacitance through the return bridge (t because there was a change in the direction of the current in the DC link of the inverter), the ratio between the duration of the signals from the outputs of blocks 3 and 4 did not change, and the duration of the signal at the output of element 7 (plot U, Fig. 2, b) turns out to be less than a predetermined threshold drive duration riminator 8. At the output of the latter there will be a signal of the zero level (plot Us, fig. 2, b), the one-shot 9 does not start and the signal to turn off the executive unit 10 is not received.

In the event of a short circuit in the cable 16 between the electric motor 15 and the frequency converter 11 (emergency mode, Fig. 2, c), the motor enters the state of energizing the short circuit point and begins to transfer energy to the short circuit location.

Accordingly, the ratio increases for the period between the duration of the negative and positive half-waves of the signal at the output of block 3 in the direction of increasing the duration of the negative half-wave (plot Ua, Fig. 2, c). Then, at the output of the comparator 5, we have a positive rectangular signal, the duration of which corresponds to the time the engine feeds the short circuit point (diagram Us, Fig. 2, c), which is fed to the first input of the BAN 7 element.

The signal from the output of block 4 for determining the polarity of current in the short circuit mode will not change and will correspond to the normal mode of operation of the IF-HELL system, since energy is supplied from the network through. IF to the short-circuit point and does not reach the engine (plot U4, FIG. 2, c). Then at the output of the comparator 6 at short circuit a positive rectangular signal will be present (plot U6, Fig. 2, c). In this case, the time of mismatch of the signals from the output of block 3 and block 4 increases sharply and the duration of the 5-signal at the output of the BAN element (plot U, Fig. 2, c) becomes longer than the set discriminator 8. At the output of discriminator 8, a logical signal appears 1 (plot Us, Fig. 2, c), which starts a single-shot 9. At the output of the latter, a positive pulse of a given duration appears, which, upon entering the actuating unit 10, causes the motor 15 to disconnect from the converter 11

5 frequencies.

In the proposed device, in comparison with the prototype, due to the newly introduced blocks (comparators, the element BAN, pulse discriminator, single vibrator) and their corresponding connection between each other and with the polarity detection sensor and the electromagnetic power measuring unit, the signs of the instantaneous current in the constant link are controlled

5 current and instantaneous electromagnetic power of the electric motor during the period of change of the output frequency of the frequency converter, which allows you to reliably determine the direction of transmission of electrical energy

0 for the period in the frequency converter having a reverse diode bridge of the inverter, and the direction of energy transfer at the connection point of the electric motor to the cable network, and after matching the processed

5 signals in the element BAN and discriminator distinguish short-circuit mode and normal network mode. The use of an inverter with a reverse bridge in the prototype, periodically transferring the electric motor energy to a constant current filter in both normal motor and generator modes, leads to the appearance of single and zero signals at the output of the sensor for a period of frequency change by

5 output of the frequency converter, which are fed to the input of logic circuit I. The unit and zero signals will also be sent to another input of this circuit from the electromagnetic power block (as in the prototype). The ratio of the duration of these signals over a period depends on the load mode of the electric drive and can lead to the appearance of a unit at the output of the AND circuit and false triggering of the fifth unit.

In the proposed device, false positives protection against short circuit are absent, since the control of the direction of energy transfer in the inverter and the electric motor is carried out by compiling the duration of the positive and negative instantaneous values of current and electromagnetic power for the period. This makes it possible to use frequency converters with a reverse diode bridge with reliable protection against short-circuit, i.e. the scope of the network is expanding.

Claims (1)

SUMMARY OF THE INVENTION A three-phase autonomous network with protection, comprising a thyristor frequency converter, made in the form of a series-connected controlled rectifier, a DC link and an inverter, as well as a current polarity detection unit included in the DC link, the output of which is the first output thyristor frequency converter, the second output, which is the output of the inverter, is connected to the first output of the Executive unit, the second output of which is connected by a cable to an asynchronous electric motor a body to the stator windings of which the inputs of the current and voltage sensors are connected, the outputs of which are connected to the inputs of the electromagnetic power measuring unit, which is related to the fact that, with in order to expand the scope of application on a network with frequency converters having reverse diode bridges for dumping energy into a DC link, two comparators are additionally introduced into it, and series-connected element BAN, a pulse duration discriminator and a one-shot, and the output of the electromagnetic power measuring unit is connected to the input of the first comparator, and the first output of the thyristor frequency converter is connected to the input of the second comparator, the outputs of the first and second comparators are connected respectively to the first and second inputs of the BAN, the output of the one-shot is connected to the disconnecting input of the Executive unit. .Fig {