Circuit breaker DC voltage. Electric machines

Circuit breakers direct current are used to disconnect the circuit under load. At traction substations, circuit breakers are used to disconnect 600 V supply lines during overloads and currents short circuit and to cut off the reverse current of the rectifier units in case of reverse ignition or breakdown of the valves (i.e., internal short circuits during parallel operation of the units).

The electric arc is extinguished by automatic switches in the air on the arcing horns. Arc elongation can be done by magnetic blowing or in chambers with narrow slots.

In all cases of disconnection of the circuit and the formation of an electric arc, a natural upward movement of the arc occurs along with the movement of the air heated by it, i.e., thermal blast.

Mainly used high speed circuit breakers.

Rice. 1. Oscillograms of current and voltage when the short-circuit current is turned off: a - by a non-quick-acting switch, b - by a quick-acting circuit breaker

The total time T for breaking the short circuit current or overload by the circuit breaker consists of three main parts (Fig. 1):

T \u003d t o + t 1 + t 2

where t0 is the rise time of the current in the disconnected circuit to the value of the set current, i.e., to the value at which the tripping device of the circuit breaker operates; t1 is the circuit breaker opening time, i.e., the time from the moment the set current is reached to the moment the circuit breaker contacts begin to diverge; t2 - arc burning time.

The rise time of the current in the circuit t0 depends on the parameters of the circuit and the setting of the circuit breaker.

The intrinsic opening time t1 depends on the type of circuit breaker: for slow-acting circuit breakers, the intrinsic opening time is in the range of 0.1-0.2 sec, for high-speed circuit breakers it is 0.0015-0.005 sec.

The arc burning time t2 depends on the magnitude of the interrupted current and the characteristics of the arc extinguishing devices of the circuit breaker.

The total shutdown time of a non-quick-acting switch is in the range of 0.15-0.3 sec, of a fast-acting one - 0.01-0.03 sec.

Thanks to its short tripping time, the high-speed circuit breaker limits the maximum value of the short-circuit current in the protected circuit.

At traction substations, high-speed automatic DC switches are used: VAB-2, AB-2/4, VAT-43, VAB-20, VAB-20M, VAB-28, VAB-36 and others.

Switch VAB-2 is polarized, i.e., responsive to current in only one direction - forward or reverse, depending on the setting of the switch.

On fig. 2 shows the electromagnetic mechanism of a DC switch.

Rice. Fig. 2. The electromagnetic mechanism of the VAB-2 circuit breaker: a - a section of the circuit breaker, b - the limits of the limiting wear of the contacts of the VAB-2 circuit breaker, (A - the minimum thickness of the fixed contact is 6 mm, B - the minimum thickness of the movable contact is 16 mm); 1 - holding coil, 2 - magnetic circuit, 3 - switching coil, 4 - magnetic armature, 5 - upper steel bar, 6 - anchor, 7 - main yoke coil, 8 - calibration coil, 9 - U-shaped magnetic circuit, 10 - current carrying output, 11 - adjusting screw, 12 - shunt plate, 13 - flexible connection, 14 - stop, 15 - armature lever, 16 - armature lever axis, 17 - fixed contact, 18 - movable contact, 19 - contact lever, 20 - axis contact lever, 21 - axle with roller, 22 - stop lever, 23 - opening springs, 24 - rod, 25 - adjusting screws, 26 - bracket, 27 - holding coil core

The armature lever 15 (Fig. 2, a) rotates around the axis 16, passed through the upper steel beam 5. In the lower part of the lever 15, consisting of two silumin cheeks, a steel anchor 6 is clamped, and in the upper part there is a spacer sleeve with an axis 20, around which the contact lever 19, made of a set of duralumin plates, rotates.

A movable contact 18 is fixed in the upper part of the contact lever, and a copper shoe with a flexible connection 13 is fixed at the bottom, with the help of which the movable contact is connected to the main current coil 7 and through it to the output 10. Stops 14 are attached to the lower part of the contact lever on both sides, and on the right side there is a steel axle with a roller 21, to which two opening springs 23 are attached on one side. On the other hand, the opening springs are fixed with the help of adjusting screws 25 in the bracket 26, which is fixedly mounted on the steel beam 5.

In the off position, the lever system (armature lever and contact lever) is rotated by disconnecting springs 23 around axis 16 until the armature 6 stops against the left rod of the U-shaped magnetic circuit.

The making 3 and holding 1 coils of the circuit breaker are powered by own needs direct current.

To turn on the circuit breaker, you must first close the circuit of the holding coil 1, then the circuit of the closing coil 3. The direction of the current in both coils must be such that the magnetic fluxes created by them are added to the right core of the core of the magnetic circuit 9, which serves as the core of the closing coil; then the armature 6 will be attracted to the core of the switching coil, i.e., it will be in the “On” position. In this case, the axis 20, together with the contact lever 19, will turn to the left, the opening springs 23 will stretch and will tend to rotate the contact lever 19 around the axis 20.

When the switch is turned off, the magnetic armature 4 lies on the end of the core of the closing coil and, when the switch is turned on, remains attracted to the end of the core by the total magnetic flux of the closing and holding coils. The magnetic armature 4 is connected by means of a rod 24 to the locking lever 22, which prevents the contact lever from turning until the movable contact stops against the fixed one. Therefore, a gap remains between the main contacts, which can be adjusted by changing the length of the rod 24 and should be equal to 1.5-4 mm.

If the voltage is removed from the switching coil, then the electromagnetic forces holding the armature 4 in the attracted position will decrease and the springs 23, using the locking lever 22 and the rod 24, will tear the armature from the end of the core of the switching coil and turn the contact lever until the main contacts close. Therefore, the main contacts will close only after the closing coil circuit has been opened.

Thus, the principle of free tripping is implemented for VAB-2 circuit breakers. The gap between the magnetic armature 4 (otherwise called the armature of free tripping) and the end of the core of the closing coil in the on position of the switch must be within 1.5-4 mm.

The control circuit provides for the supply of a short-term current pulse to the closing coil, the duration of which is only sufficient to have time to transfer the armature to the “On” position. The closing coil circuit is then automatically opened.

The presence of a free trip can be checked in the following way. A piece of paper is placed between the main contacts and the contactor is closed. The switch is turned on, but while the contactor contact is closed, the main contacts should not close and the piece of paper can be freely removed from the gap between the contacts. As soon as the contactor contact is open, the magnetic armature will be torn off from the end of the core of the closing coil and the main contacts will close. In this case, the paper will be sandwiched between the contacts and it will not be possible to remove it.

When the switch is turned on, a characteristic double blow is heard: the first - from the collision of the armature and the core of the switching coil, the second - from the collision of the closed main contacts.

The polarization of the circuit breaker consists in choosing the direction of the current in the holding coil, depending on the direction of the current in the main current coil.

In order for the circuit breaker to open the circuit when the direction of the current in it changes, the direction of the current in the holding coil is chosen so that the magnetic fluxes created by the holding coil and the main current coil coincide in direction in the core of the closing coil. Therefore, with current flowing in the forward direction, the main circuit current will assist in keeping the breaker in the closed position.

In emergency mode, when the direction of the main current is reversed, the direction of the magnetic flux created by the main current coil in the core of the switching coil will change, i.e. the magnetic flux of the main current coil will be directed against the magnetic flux of the holding coil and at a certain value of the main current, the core of the switching coil will demagnetize and trip springs will trip the breaker. The speed is determined to a greater extent by the fact that while the magnetic flux decreases in the core of the switching coil, the magnetic flux increases in the core of the main current coil.

In order for the circuit breaker to open the circuit when the current increases above the setting current in the forward direction, the direction of the current in the holding coil is chosen so that the magnetic flux of the holding coil in the core of the closing coil is directed against the magnetic flux of the main current coil when the forward current flows through it. In this case, with an increase in the main current, the demagnetization of the core of the closing coil increases, and at a certain value of the main current, equal to or greater than the setting current, the circuit breaker turns off.

The setting current in both cases is controlled by changing the value of the holding coil current and changing the gap δ1.

The current value of the holding coil is regulated by changing the value of the additional resistance included in series with the coil.

Changing the gap δ1 changes the resistance to the magnetic flux of the main current coil. With a decrease in the gap δ1, the magnetic resistance decreases and, consequently, the value of the breaking current decreases. Changing the gap δ1 is carried out using the adjusting screw 11.

The gap δ2 between the stops 14 and the cheeks of the armature lever 15 in the on position of the switch characterizes the quality of the closure of the main contacts and should be within 2-5 mm. The plant produces switches with a gap δ2 equal to 4-5 mm. The gap value δ2 determines the angle of rotation of the contact lever 19 around the axis 20.

The absence of a gap δ2 (stops 14 are in contact with the cheeks of the armature lever 15) indicates poor contact or no contact between the main contacts. A gap δ2 less than 2 or more than 5 mm indicates that the main contacts are in contact only at the lower or upper edge. The gap δ2 can be small due to the high wear of the contacts, which in this case are replaced.

If the dimensions of the contacts are sufficient, then the regulation of the gap δ2 is carried out by moving the entire switching mechanism along the frame of the switch. To move the mechanism, two bolts are released, with which the mechanism is attached to the frame.

The distance between the main contacts in the open position should be 18-22 mm. The pressing of the main contacts for switches with a rated current up to 2000 A inclusive must be within 20-26 kg, and for switches with a rated current of 3000 A - within 26-30 kg.

On fig. 2, b shows the movable system of the circuit breaker with the designation of the boundary of the limiting wear of the contacts. The moving contact is considered worn when dimension B is less than 16 mm, and the fixed contact is considered worn when dimension A is less than 6 mm.

On fig. 3 shows a detailed control scheme for the VAB-2 circuit breaker. The circuit provides for the supply of a short-term pulse to the switching coil and does not allow multiple repeated switching on when the power button is pressed for a long time, i.e. it protects against “voicing”. The holding coil is energized all the time.

To turn on the switch, press the "On" button, thereby closing the circuit of the coils of the contactor K and the blocking RB. In this case, only the contactor is activated, which closes the circuit of the closing coil VK.

As soon as the armature takes the “On” position, the closing auxiliary contacts of the switch BA will close, and the opening contacts will open. One of the auxiliary contacts shunts the coil of the contactor K, which will break the circuit of the closing coil. In this case, the entire mains voltage will be applied to the coil of the blocking relay RB, which, having actuated, by its contacts once again shunts the contactor coil.

To re-close the circuit breaker, open the power button and close it again.

The discharge resistance CP, connected in parallel with the holding coil DC, serves to reduce the overvoltage when the coil circuit is opened. The adjustable resistance of the LED makes it possible to change the current of the holding coil.

The rated current of the holding coil at a voltage of 110 V is 0.5 A, and the rated current of the closing coil at the same voltage and parallel connection both sections - 80 a.

Rice. 3. Wiring diagram switch control VAB-2: Off. - opening button, DK - holding coil, SD - additional resistance, CP - discharge resistance, BA - auxiliary contacts of the switch, LK, LZ - red and green signal lamps, Incl. - power button, K - contactor and its contact, RB - blocking relay and its contact, VK - closing coil, AP - automatic switch

Voltage fluctuations of operational circuits from - 20% to + 10% of the rated voltage are permissible.

The total time of switching off the circuit by the VAB-2 switch is 0.02-0.04 sec.

Extinguishing the arc when the circuit breaker breaks under load occurs in the arc chute using magnetic blast.

The magnetic blast coil is usually connected in series with the main fixed contact of the switch and is a coil of the main current-carrying bus, inside of which there is a core made of steel tape. To concentrate the magnetic field in the zone of arc formation on the contacts, the core of the magnetic blowing coil of the switches has pole pieces.

The arc extinguishing chamber (Fig. 4) is a flat box made of asbestos cement, inside which two longitudinal partitions 4 are made. A horn 1 is installed in the chamber, inside which the axis of rotation of the chamber passes. This horn is electrically connected to a moving contact. The other horn 7 is fixed on a fixed contact. To ensure a quick transition of the arc from the movable contact to the horn 1, the distance of the horn from the contact should be no more than 2-3 mm.

The electric arc that occurred when switching off between contacts 2 and 6 under the influence of a strong magnetic field of the magnetic blast coil 5 is quickly blown onto horns 1 and 7, lengthens, is cooled by the oncoming air flow and the chamber walls in narrow slots between the partitions and quickly goes out. It is recommended to insert ceramic tiles into the walls of the chamber in the arc extinguishing zone.

The arc extinguishing chambers of circuit breakers for voltages of 1500 V and above (Fig. 5) differ from the chambers for voltages of 600 V in large overall dimensions and the presence of holes in the outer walls for the release of gases and an additional magnetic blast device.

Rice. 4. Arc extinguishing chamber of the VAB-2 circuit breaker for a voltage of 600 V: 1 and 7 - horns, 2 - moving contact, 3 - outer walls, 4 - longitudinal partitions, 5 - magnetic blowing coil, 6 - fixed contact

Rice. Fig. 5. Arc extinguishing chamber of the VAB-2 circuit breaker for a voltage of 1500 V: a - chamber arrangement, b - arc extinguishing circuit with additional magnetic blowing; 1 - movable contact, 2 - fixed contact, 3 - magnetic blowing coil, 4 and 8 - horns, 5 and 6 - auxiliary horns, 7 - auxiliary magnetic blowing coil, I, II, III, IV - arc position during extinguishing

The additional magnetic blowing device consists of two auxiliary horns 5 and 6, between which the coil 7 is connected. As the arc lengthens, it begins to close through the auxiliary horns and the coil, which, due to the current flowing through it, creates an additional magnetic blowing. All chambers have metal pole strips on the outside.

For fast and stable extinguishing of the arc, the divergence of the contacts must be at least 4-5 mm.

The body of the switch is made of non-magnetic material - silumin - and is connected to a moving contact, so during operation it is under full operating voltage.

Switch automatic high-speed DC VAT-42

Operation of DC circuit breakers

In operation, it is necessary to monitor the condition of the main contacts. The voltage drop between them at rated load should be within 30 mV.

The contacts are cleaned from oxide with a wire brush (brushing brush). When sagging appears, they are removed with a file, however, the contacts should not be filed to restore their original flat shape, as this leads to their rapid wear.

It is necessary to periodically clean the walls of the arc extinguishing chamber from copper and coal deposits.

During the revision of the DC circuit breaker, the insulation of the holding and switching coils is checked in relation to the body, as well as the insulation resistance of the walls of the arc extinguishing chamber. The insulation of the arc chamber is tested by applying voltage between the main moving and fixed contacts with the chamber closed.

Before commissioning the circuit breaker after repair or long-term storage its chamber must be dried for 10-12 hours at a temperature of 100-110 ° C.

After drying, the chamber is mounted on the switch and the insulation resistance is measured between two points of the chamber opposite the moving and fixed contacts when they are open. This resistance must be at least 20 mΩ.

Switch settings are calibrated in the laboratory with current received from a low-voltage generator with a rated voltage of 6-12 V.

At the substation, the switches are calibrated with the load current or with the help of a load rheostat at a rated voltage of 600 V. A method for calibrating DC circuit breakers can be recommended using a calibration coil of 300 turns of PEL wire with a diameter of 0.6 mm, mounted on the core of the main current coil. By passing a direct current through the coil, the current setting value is set by the number of ampere turns at the moment the switch is turned off. Switches of the first version, produced earlier, differ from switches of the second version by the presence of an oil damper.

For the first time in the assortment of MPO Elektromontazh appeared automatic switches for switching and protecting DC circuits from short circuits and overloads.

In principle, in case of emergency, a “variable” automaton can also be put in a “permanent” circuit, but carefully recalculate the current rating by 1.3–1.9 times more, since when a direct current flows, noticeably more heating occurs conductors, including inside the device. But why, if they now already have specially designed and designed ones.

These are Schneider Electric C32 H-DC models, two-pole, with a breaking capacity of 10 kA, designed for rated currents of 2, 3, 6 and 10 A (No. K8012-K8015 according to our price list). Trip characteristic - C (type of instantaneous release, or dependence of the load off time on the magnitude of the current in the main circuit of the machine), that is, the setting of the electromagnetic release - in the range from 5 to 10 values ​​​​of the rated current - In. Thermal setting - (1.13–1.45) In. Such parameters are typical for lighting networks and installations with moderate starting currents. These can be industrial process automation and control systems, transport, renewable energy.

For DC switches, there is an unshakable requirement: observe the polarity of the power connection, as indicated on the device.

Automatic machines for constant and alternating current they differ not only in the size of the contacts - in the first they are more powerful, and the design of the protection (number of turns and wire diameter) is somewhat different.

But the main thing is that the constant magnetic field in the coils of the protection device affects the core not only many times stronger than the alternating one, which changes the intensity with a frequency of 50 Hz. It creates, depending on the polarity, either a pulling or pushing mechanical moment. You can't go wrong.

Our new machines are mounted on a DIN rail, size - 2 modules (36 mm), dimensions 81x76 mm, weight 250 g.

Modular DC circuit breakers, or more simply automata, are used in electrical networks and electrical installations, telecommunications cabinets, automation panels. Why are they called modular? The thing is that they are available in standard compact housings and are single-pole modules, which can consist of single-pole, two-pole or three-pole devices. According to the existing standard, the width of one such pole is 17.5 mm.

The DC circuit breaker differs from the usual one in that it breaks the circuit in the event of a short circuit or overload automatically. The design of the device includes several main elements:

Many people know from a school physics course that current can be variable and constant. If we can still say something with certainty about the use of alternating current (all household electrical receivers are powered by alternating current), then we know practically nothing about direct current. But since there are direct current networks, then there are consumers, and, accordingly, protection for such networks is also needed. Where are direct current consumers found and what is the difference between protection devices for this kind of current, we will consider in this article.

None of the types electric current not "better" than the other - each is suitable for certain tasks: alternating current is ideal for generating, transmitting and distributing electricity over long distances, while direct current finds its application in special industrial facilities, installations solar energy, data centers, electrical substations, etc.

Distribution cabinet of direct operating current of an electrical substation

Understanding the differences between AC and DC provides a clear understanding of the challenges faced by DC circuit breakers. Alternating current of industrial frequency (50 Hz) changes its direction in the electrical circuit 50 times per second and “passes” through the zero value the same number of times. This "passage" of the current value through zero contributes to the rapid extinction of the electric arc. In DC circuits, the value of the voltage is constant - just like the direction of the current is constant in time. This fact significantly complicates the extinguishing of the DC arc, and therefore requires special design solutions.

Combined graphs of normal and transient modes when disconnected: a) alternating current; b) direct current

One such solution is the use of a permanent magnet (3). The movement of the arc in a magnetic field is one of the methods of extinguishing in devices up to 1 kV and is used in modular automatic switches. An electric arc, which is essentially a conductor, is affected by a magnetic field, and it is drawn into the arc chute, where it finally fades.

1 - moving contact
2 - fixed contact
3 - silver-containing contact soldering
4 - magnet
5 - arc chute
6 - bracket

Polarity must be observed

Another and, perhaps, the key difference between AC and DC circuit breakers is the presence of polarity in the latter.

Wiring diagrams for single-pole and double-pole DC circuit breaker

If you are protecting a single-phase AC network with a two-pole circuit breaker (with two protected poles), then it makes no difference in which of the poles to connect the phase or neutral conductor. When connecting circuit breakers to the DC network, the correct polarity must be observed. When connecting a single-pole DC switch, the supply voltage is supplied to terminal "1", and when connecting a two-pole DC switch, to terminals "1" and "4".

Why is it so important? See video. The author of the video conducts several tests with a 10-amp switch:

1. Turning on the switch in the network with respect to the polarity - nothing happens.
2. The switch is installed in the network with reverse polarity; network parameters U = 376 V, I = 7.5 A. As a result: strong smoke emission with subsequent ignition of the switch.
3. The switch is installed with respect to polarity, and the current in the circuit is 40 A, which is 4 times its rating. Thermal protection, as it should, opened the protected circuit after a few seconds.
4. The last and most severe test was carried out with the same 4-fold overcurrent and reverse polarity. The result was not long in coming - instant ignition.

Thus, DC circuit breakers are protection devices used for alternative energy facilities, automation and control systems for industrial processes, etc. Special versions of the protective characteristics Z, L, K allow you to protect high-tech equipment of industrial enterprises.