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PN2-600-630A-U3-KEAZ Inom = 597A Shutdown current 630

When operational (technological) overloads and emergency modes occur as a result of circuit malfunctions, currents flow through the electrical circuits of the emergency circuit in excess of the rated values ​​for which the electrical equipment is designed.

As a result of exposure to emergency currents and overheating of conductors, electrical insulation is damaged, contact surfaces of connecting busbars and electrical devices burn and melt. Electrodynamic shocks cause damage to busbars, insulators and reactor windings.

To limit the amplitude of emergency currents and the duration of their flow, special devices and systems for protecting electrical equipment are used. Protection devices must turn off the emergency circuit before its individual elements fail.

In case of large overloads or short circuits, the protection devices must immediately turn off the entire electrical installation or part of it with maximum speed to ensure further operation or, if the accident is a consequence of the failure of one of the circuit elements, prevent the failure of other electrical equipment.

In the event of small overloads that are not dangerous to the equipment for a certain time, the protection system can act on a warning alarm for the information of operating personnel or on the system automatic regulation to reduce the current.

Since the main factor leading to the failure of electrical equipment is the thermal effect of emergency current, according to the principle of construction, protective devices are divided into current and thermal.

Current protective devices monitor the values ​​or ratios of currents flowing through equipment.

Thermal protection devices directly measure the temperature of electrical equipment.

Semiconductor devices have low overload capacity compared to other power equipment, and increased demands are placed on the protection devices of semiconductor rectifiers and other converters. Protective devices in installations with semiconductor rectifiers are selected based on the permissible overload characteristics of power diodes or thyristors, taking into account the fact that other equipment located in the emergency circuit will also be protected, since it has a greater overload capacity.

The use of certain means of protection is determined by the parameters of the power circuit of the converter and the overload capacity of semiconductor devices.

Regardless of the installation parameters and the type of protective devices and systems used, the following general protection requirements are distinguished.

1. Speed ​​– ensuring the minimum possible protection response time, not exceeding the permissible one.

2. Selectivity. Emergency shutdown should be performed only in the circuit where the cause of the accident occurred. And other sections of the power circuit must remain in operation.

3. Electrodynamic resistance. The maximum current limited by protective devices must not exceed the permissible electrodynamic resistance value for a given electrical installation.

4. Overvoltage level. Disabling the emergency current should not cause overvoltages that are dangerous for semiconductor devices.

5. Reliability. Protection devices should not fail when emergency currents are switched off.

6. Noise immunity. When interference occurs in the auxiliary network and in the control circuits, the protection devices should not trigger falsely.

7. Sensitivity. The protection must operate in case of all damage and currents dangerous to semiconductor devices, regardless of the location and nature of the accident.

Selection of fuses.

Fuses are selected according to the following conditions:

1) according to rated network voltage:

Unom.prev. >= Unom.s.,

where Unom.prev. – rated voltage of the fuse;

Unom.s. – rated network voltage;

2) according to the long-term design current of the line;

Inom.in. >= Idol. ;

where Inom.in. – rated current of the fuse link;

Idlit – long-term design current of the circuit.

In addition, when using instantaneous fuses, the fuse link should not burn out from short-term current impulses, for example, from the starting currents of electric motors. Therefore, when choosing fuses for such electrical receivers, it is also necessary to fulfill another condition:

Inom.in. >= Istart / 3.1,

where Istart is the starting current of the motor.

There is often a need for protection main line, through which a group of electric motors is powered, and some or all of them can be started simultaneously. In this case, fuses are selected according to the following ratio:

Inom.in. >= Icr / 3.1 (under light starting conditions)

Inom.in. >= Icr / (1.5 – 2) (under severe starting conditions),

where Icr = I’start + I’dur – maximum short-term line current;

I’start – the starting current of an electric motor or a group of simultaneously switched on motors, when starting which the short-term line current reaches highest value;

I’last – long-term calculated current of the line until the moment of starting the electric motor (or group of electric motors), determined without taking into account the operating current of the started electric motor (or group of motors).

For three-phase electrical receivers alternating current;

where Rnom is the rated power of the electrical receiver (or group of electrical receivers), kW; U – rated voltage (for AC power receivers – linear network voltage), kV;

- Power factor; – Electric motor efficiency.

Selection of circuit breakers.

The selection of circuit breakers is made based on rated voltage and current, subject to the following conditions:

Unom.a. >= Unom.s.; Inom.a. >= Iduration;

where Unom.a. – rated voltage of the circuit breaker;

Unom.s. – rated network voltage; where Inom.a. – rated current of the circuit breaker; Idlit – long-term design current of the circuit.

In addition, the following must be correctly selected: rated current of the releases Inom.rast.; installation current of the electromagnetic release element of the combined release Iset.el.magn.; rated current of the thermal release setting or thermal element combined release – Inom.set.heat.

The rated currents of the electromagnetic, thermal or combined release must be no less than the rated current of the motor:

Inom.rast. >= Inom.motor

Installation current of an electromagnetic release (cut-off) or an electromagnetic element of a combined release, taking into account the inaccuracy of the release and deviations of the actual

starting current from catalog data is selected from the condition

Iset el.magn. >= 1.25 Istart. = 1.25 3.1 7 = 27 A Ip = 7 Ip

where Istart. – motor starting current.

Rated current of installation of thermal release or thermal element of combined release:

Inom.set heat. >= Inom.motor

Installations of circuit breaker releases are also selected to protect the circuits of other electrical receivers of the power supply system, for example, control circuits. measuring instruments etc. (if the need arises, since in most cases to protect devices and other similar electrical receivers low power For reasons of sensitivity it becomes necessary to use fuses). It should be taken into account that if a circuit breaker with an electromagnetic release is installed in the circuits of electrical receivers, when switched on, inrush current surges do not occur, then there is no need for detuning from the latter and the installation current of the electromagnetic release in this case should be selected as low as possible.

Selection of thermal relays for magnetic starters.

Thermal relays are selected according to the rated motor current (or continuous rated current):

Inom.t.r >= Inom.motor ;

When choosing a thermal relay, you must strive to ensure that the installation current is in the center of the regulation range.

Results of calculation and selection of protection devices.

1. Requirements for choosing protection equipment.

When choosing on-board protection devices electrical networks the following requirements apply:

1. Protection devices must reliably operate and disconnect electrical circuits during short circuits and unacceptable overloads and must not give false alarms in normal modes.

2. When triggered, the protection devices must act to shut down, and their action must be irreversible (there should be no automatic re-enablement after the overload or short circuit has been eliminated). Restarting must be done manually.

3. Protection devices must provide selective (selective) disconnection of the circuit section with a short circuit. In this case, undamaged sections of the power supply system should not be disconnected. When a short circuit occurs in the power supply system network, the protection devices must make only those shutdowns that are necessary to eliminate the short circuit.

4. The sensitivity of the protection devices must be sufficient to operate at the lowest short-circuit current in the protection zone and under dangerous overloads.

5. Protection devices in AC power supply systems must respond to all types of short circuits: single-phase, two-phase and three-phase.

6. AC lines feeding directly consumers, for which single-phase modes are not allowed, must be protected by three-phase circuit breakers.

7. Protection devices must have sufficient speed to ensure the shortest possible interruption of power supply to consumers, preventing the occurrence of a fire or damage to elements of the power supply system and disruption of the stability of its operation.

8. To protect AC and DC networks, protection devices must be used that are approved for use in newly developed and modified products.

Note. In general, circuit breakers with free tripping should be used. Circuit breakers without free tripping are allowed to be used in cases where there are no circuit breakers with free tripping with the required characteristics.

9. Protection devices must be selected:

– according to the rated voltage of the circuit;

– in terms of the magnitude and nature of the current load.

10. The selected protective devices must provide protection for the wires.

11. Selected protective devices must be checked:

– resistance to short-circuit currents (electrodynamic, thermal stability and switching capacity);

– on selectivity of operation during short circuit;

– sensitivity to short-circuit currents.

Note. Devices designed to protect the emergency power supply system when powered from emergency sources are not tested for resistance to short-circuit currents. This check is performed while the system is powered from the main sources .

2. Methodology for selecting protection equipment.

Protection devices in primary distribution networks must be selected taking into account the long-term maximum current strength of the line, the number of channels of the split line, taking into account the uneven distribution of currents in the wires of split lines.

The rated current of the protection device for one channel of a split line of the primary distribution network is determined by the formula

Where I n.a.– rated current of the split line protection device, A;

I l– line current strength, A;

a- current distribution unevenness coefficient for on-board networks is taken equal to 1.075;

n– number of channels of the split line;

k– number of backup channels.

Let's consider the methodology for selecting protection devices for the secondary distribution network, which, as is known, provides power to electricity consumers directly from the switchgear and central control buses.

Feeder protection devices for electricity consumers must be selected based on the condition of ensuring normal operation of consumers when the current strength in the circuit is equal to or less than its rated value, as well as during non-hazardous overloads (for example, when starting a motor) in different conditions environment (temperature, vacuum).

Note. Protection of consumers in technically justified cases must be provided for by the developer of these consumers.

To protect circuits, protective devices must be selected with a rated voltage equal to or greater than the rated voltage of the protected circuit.

Devices for protecting consumer feeders must be selected taking into account the nature of the consumers’ work.

Based on the nature of their work, electricity consumers are divided into two main groups:

– consumers that do not have high continuous starting power and overload current (lighting devices, heating devices, transformers, unit control circuits, contactors, relays, etc.);

– consumers of electricity, including electric motors (various electric mechanisms, fuel and oil pumps, electric machine converters, fans, etc.).

For consumer feeders that do not have a large starting current, the rated current of the protection devices must be equal to the rated current of the consumer or have a greater value closest to it:

I n.a.³ I n.sweat, (2)

Where I n.sweat– rated current of the consumer, A.

For consumer feeders including motors with continuous and short-term operating modes, protection devices must be selected in accordance with the conditions:

Where t start. max – time at which the root-mean-square starting current of the consumer has its maximum value, s;

- response time of the protection device according to the time-current (also called ampere-second) characteristic for the environmental conditions in which the protection device is located at a current strength equal to I rms.start. max, s;

I rms.start. max – maximum root-mean-square value of the starting current, A.

t start. max and I rms.start. max are determined by the curve of changes in the root-mean-square starting current of the consumer over time. The root mean square starting current for any moment in time is determined from the oscillogram of the consumer's starting current (Fig. 1)

according to the formula

Where n t– the number of equal intervals in section t of the current change curve at start-up;

I 1 ,…,I nt– average current values ​​in intervals on a section of the curve, A.

Note. In approximate calculations, the value I rms.start. max for AC motors with starting time< 1 сек может быть принято равным 0,9I start. (I start.– the value of the starting current of the motors specified in technical conditions on them), t start. max can be taken equal to 0.5 s.

All of the above is illustrated in Fig. 2a and 2b.

For consumers of the second group, it is recommended to use thermal protection circuit breakers. This is explained by the fact that there are significant shortcomings in protecting such consumers with fuses. Let's show it. In Fig. Figure 3 shows the ampere-second characteristics of the circuit breaker and fuse with the same rated current, selected according to condition (3). The figure shows that for the circuit breaker, condition (3) is satisfied, because t a1 (AZ) > t start. max , but not for the fuse, because t a1 (R)< t пуск. max.

If it is still necessary to select a fuse, then in order to fulfill condition (2), it is necessary to increase the rated current of the fuse. Then condition (2) will be written in the form I n.Pr1 > I n.pot. and the ampere-second characteristic of such a fuse (Pr1) will shift to the right (Fig. 4) in relation to the initially selected fuse Pr and now condition (3) is satisfied, i.e.

t a1 (Pr1) > t start. max. But such a solution has significant drawback. Let there be an overload current I overload, i.e. . I n.Pr1 > I overload > I n.pot.

This will lead to the fact that I n.Pr1 > I overheat fuse Pr1 will not work. But because I overload > I n.pot., then due to overload the consumer will fail. Thus, in the current range I n.Pr1< I >I n.pot. the consumer is not protected. Therefore, it is recommended to install fuses in circuits where there is no overload.

If for some reason it is necessary to install fuses, then they must be selected so that the maximum value of the rms starting current does not exceed half the fuse operating current determined by protective characteristic for a time equal to t start. max , i.e.

in accordance with fig. 2b.

To protect consumer feeders with intermittent or pulsed loads, the rated current of the protection devices must be selected from the condition:

Where I rms.u– root-mean-square current strength of the consumer during the cycle of action of an intermittent or pulsed load, A;

The response time of the protection device according to the time-current characteristic for the environmental conditions in which the protection device is located, at ( I rms.u) max ;

(t u) max – time at which the root-mean-square current of a pulsed or intermittent load has a maximum value, s;

(I rms.u) max – maximum value of the root mean square current of a pulsed or intermittent load, A.

(t u) max and ( I rms.u) max are determined from the curve of changes in the root-mean-square load current over time. For any moment in time ( I rms.u)t determined from the oscillogram of the current strength of a pulsed or intermittent load using the formula:

Where I rms 1 ,…,I rms.k– root mean square values ​​of the pulse current, A;

t 1 ,…,tk– pulse duration, s;

t c– cycle time of pulsed or intermittent action

loads.

I rms 1 ,…,I rms.k are determined by a formula similar to (4), and n in this case will denote the number of equal intervals in the pulse current section.

Fuses must be selected so that the maximum values ​​of the root mean square current of a pulsed or intermittent load do not exceed half the fuse response current determined by the protective characteristic for a time equal to (t u) max (Fig. 5).

To protect feeders supplying a group of consumers, the rated current of the protection devices must be selected taking into account the rated current of the consumers and the simultaneity of their operation in accordance with the condition:

Where I n.pot.– rated current strength of simultaneously operating consumers.

Analysis of failures and non-rated operating modes electric machines allows you to highlight following types accidents that often occur in practice:

Short circuit(short circuit) on the machine terminals or in the stator winding;

Locked rotor when starting the engine (short circuit mode of the engine, especially common when starting it directly);

Phase failure of the stator winding (often found when protecting windings with fuses);

Technological overloads that occur when the load increases during engine operation;

Cooling failure caused by system malfunction forced ventilation engine;

A decrease in insulation resistance that occurs as a result of insulation aging due to cyclic temperature overloads.

Emergency modes in the circuit asynchronous motor can cause either a short-term increase in current by 12... 17 times compared to the rated one, or a long-term flow of current 5... 7 times higher than its rated value.

To protect electrical circuits from short-circuit mode, circuit breakers, current relays and fuses are widely used. In case of overcurrent, another protective equipment. Thus, when one of the phases of an asynchronous motor breaks, the most effective are minimum current and temperature protection; less effective, but workable - thermal protection(thermal relays). When the rotor is locked, maximum current relays and temperature protection are very effective; thermal protection is less effective. In case of overload, temperature protection gives the best results. Thermal relays are also effective. If engine cooling is impaired, only temperature protection can prevent an accident.

A decrease in the insulation resistance of the stator winding of the motor can provoke both an overload in the circuit and a short circuit.

Protection in case of such an accident is carried out by special devices for monitoring the insulation resistance of the motor winding.

The main emergency mode in lighting installations is short circuit. Overload protection is required only for lighting installations operated indoors and in explosive and fire hazardous environments. The most common protection device for lighting installations is the circuit breaker. When incandescent lamps are turned on, a short-term surge of current appears, 10...20 times the rated current. In approximately 0.06 s the current decreases to the nominal value. The value of the inrush current is determined by the power of the lamps. When choosing the type of protection for incandescent lamps, it is necessary to take into account the peculiarities of their starting characteristics.

Due to the widespread use of power semiconductor technology, its protection requires the use of effective devices. One of the main disadvantages of power semiconductor devices is their low current overload capacity, which imposes stringent conditions on protection equipment (in terms of speed, selectivity and operation reliability). Currently, to protect power semiconductor devices from short circuits (both external and internal), high-speed circuit breakers, semiconductor switches, vacuum circuit breakers, pulse arc switches, high-speed fuses, etc. are used. The feasibility of using one or another protection for power semiconductor devices is determined by specific conditions of their operation.

A special place is occupied by the protection of electrical circuits. Currently, networks with voltages from 0.4 to 750 kV are widely used. The main, most dangerous and common types faults in networks are short circuit between phases and phase-to-ground short circuit.

The bulk of consumers receive power from distribution networks with a voltage of 0.4; 6 and 10 kV (v Lately found wide application network voltage 0.66 kV). For powering stationary power consumers and lighting installations general purpose Three-phase four-wire networks with a voltage of 380/220 V with a solidly grounded neutral are used. Power consumers are connected to line voltages of the network, and lighting- to phase ones. Powerful power consumers, for example electric motors with a power of 160 kW and above, have a voltage of 0.66; 6 and 10 kV.

Main emergency modes in such networks are: single-phase short-circuit (up to 60% of accidents), three-phase short-circuit (up to 10%), two-phase short-circuit to ground (up to 20%), two-phase short-circuit (up to 10%).

Protection of electrical networks with voltages up to 1000 V is carried out, as a rule, by protection devices, and networks with voltages over 1000 V have relay protection.

The most common network protection devices are circuit breakers and fuses. If it is required to have protection with high speed, sensitivity or selectivity, then relay protection is used, made on the basis of relays and circuit breakers.

Electrical networks with voltages up to 1000 V indoors must also have overload protection, usually made on the basis of automatic circuit breakers with thermal or combined releases.

The main task faced when choosing equipment for protecting consumers and electrical networks is to coordinate the characteristics of protection devices with the maximum load characteristics (dependence of the permissible current on the duration of its flow) of various consumers and networks (wires and cables). For each specific type of consumer, the most complete agreement can be achieved by using a certain type of protection devices. In case of full agreement, the current-voltage and time characteristics of the protection device on the graph are higher and as close as possible to the load characteristic of the consumer.