Basic theoretical information. Protective equipment is a part of the system of transmission of q distributing electricity on the airplane

Protective equipment is a part of the system of transmission of q distributing electricity on the airplane. Protective devices are intended for storage of electrical networks, sources and electricity consumers from the failure of emergency operations, occurance of dangerous modes. As usually a malfunctions that occurred, are not eliminated, but the emergency areas are off from the operation.

To the protective equipment belong: thermal machines, fuses, various automatic devices and systems. During operation of the power network substation an emergency mode via wire break, short circuit and overload consumers can occur. The most dangerous for electrical systems are short circuits.

There are distinguished metal and short circuiting. Overloading current consumers is most often associated with violations of their normal operation. If overloading is contenuos, it could lead to breach the insulation of wires and to short circuit. Protection equipment has specific requirements: it must be characterized by selectivity, sensitivity and inertia.

The requirement of selectivity (selectivity) means that the protective equipment should only disable the damaged sections of the network. However, only the minimum number of consumers can feel the lack of electricity. The requirement for selectivity for the protective equipment is relatively easy to perform in radial DC networks.

Let the short circuit occurr at point A (Fig. 6.1). Then, in the presence of selective protection only automatic emergency protection cable works. Only consumer P3 is without electricity.

If a short circuit occurs at point B, then the network consumers of the section III must be opened and thus in the positive wire of generatorprotection 1 fails. Thus, the network protection should be implemented so that the first element in place breakdowns of short circuit and only if it disoperation - the protective element that is in the scheme closer to the source of electrical energy.

Fig. 6.1

The inertia of protection is required where there are circles in the current short-term fluctuations, such as different aircraft launches electric drive device. For must respond to continuos, although small overloads, i.e.to be characterized by high sensitivity. To protect networks and users we can apply various kinds of protection: Maximum jet, thermal, jet differential-directed, directed and others.

The most common protection is maximum jet that responds to excessive load size of current to the rated current. It is implemented using fuse and thermal (bimetallic) automatic circuit breakers. In power systems AC the maximum protection can be made at current transformers which operate in conjunction with electromagnetic spacecraft.

Advantages and disadvantages of fuses and bimetal automatic protection are determined by ampersecund characteristics. It is known that the conductor of heat Q that is released depends on the size and duration of current flow through a conductor:

,

where I is a current, А; r is resistance of a conductor (in this case the working element of protection device), Оhm; t is time, s.

Thus, the speed of operation of the thermal device depends on the size of the load current. Ampersecond characteristics PE express dependence of timing device on the value of current overloads. With ampersecund characteristics we can define the features of this unit protection, critical and nominal currents. The critical current protection device is called the smallest current at which the possible triggering device protection takes place. Rated current of the protective device is always less than the critical one:

I_ном = (0, 8…0, 5) I_крит.

This restriction prevents false positives protection when changing environmental conditions or scatter parameters of protection system.

Fuses. The main part of the fuse is a metal filament through which the current flows of the load circuit, which is protected. This filament burnout occur when currents of the rated load are in excess, and the presence of short circuit. On the aircraft we use fuses of the following types (Fig. 6.2):

- zone smelting ПВ;

- glass smelting fusesСП;

- heat-resistant fuses ТП;

- inertially-resistant fuses ІП;

- fuses of the type ПМ.

Fig. 6.2

Fuses ПВ are designed for currents of 24... 100 A and marked in accordance ПВ-2, ПВ-6 and etc. Zone smelting ПВ up to 40 A are produced in a closed performance, more than 40 and in the open. The zone smelting of the closed execution (Fig. 6.2, a) consists of a glass tube 1, two brass or copper caps with contact knives 2 and of fusible element 3 from calibrated silver wire. The wire is placed inside the glass tube and soldered to the cap.

Fusible inserts of the open construction (Fig. 6.2, b) does not have glass tube and silver wire is placed on the textolite plate 4. The fuse terminals are installed in special units of protection. Glass- heating fuses СП (Fig. 6.2, в) are produced in a closed performance and without contact carrying knives. They are designed for rated currents 1... 40 A. For currents up to 3 A fusible element are made of calibrated copper wire, for currents 5... 10 - from silver wire, the current 15... 40 A - from zinc plates. The design os fuses СП is similar to the joint venture fuse type ПВ. Launching thread design fuses zinc is increased with inertia.

Critical current of glass-heating fuses:

I _кр= (1, 21…1, 37) I _ном.

Refractory fuses ТП (Fig. 6.2 g) are designed for large operating current of 400 to 900 A. These fuses have consumable element the copper plate 3 is encased in heat-resistant housing 5, which is made of asbestotsement composition. When overloads, when the plate is melted, the arc is formed which is extinguished with the gas released from asbestotsementn material. Critical current of the melting fuse is

I _кр =(1, 4…1, 7) I _ном.

Savety device of type ПМ are produced of the closed-produced and are designed for currents from a few to hundreds of amperes (Fig. 6.3).

Their feature is the high mechanical strength so they can withstand heavy dynamic loads that occur in electrical networks, especially in transient conditions. In addition, for ease of use in buildings set " window" - signaling 1. Signalling manufactured color and using wire hook 3 is attached to melting plate 4. When the fuse blows effort springs 2 from its socket is pushed the alarm. On this basis the state of fuses is determined.

Fig. 6.3 represents the design of low-power fuse ПМ, which are mounted on the aircraft, as well as links insertion, using blocks of protection. More powerful fuses (Fig. 6.3, b) have a tubular structure and are fixed with bolts similarly to the fuse of the type ТП.

All fuses of PE are quick-responsed by overloads and at twice the burn occurs almost instantly. Safety devices of the type ТП are more inertial. Fuses have the following dis advantages :
Fig. 6.3

- they are small inertia, so can not withstand large short-term overloads;

- rate of their operation is largely dependent on the temperature and density of air;

- it is impossible to check the performance of safety devices and in flight is not always possible to replace the burned-out fuses.

The benefits of fuses include simplicity of design, light weight, speed and reliability of operation at short circuits. More versatile is inertia-type fuses of the tipe ІП. They are unlike conventional fuses characterized by considerable inertia action of protecting electrical circuits, not only against overloads and short circuits, but also from small but prolonged overload. Safety devices ІП are the most convenient to protect circuits and electric motors and are manufactured for rated currents from 5 to 250 A.

Inertial fuse (fig. 6.4, a) consists of fibreboard case 1, to which there are attached the contact caps 3 placed inside the fusible element consisting of two parts:

Fig. 6.4

brass insertion 9 and a solder 7. At high overloads and short circuits the brass fuse 9 inserts.

In the case of small but continuos overloads the temperature of the heating element (spiral) 2 increases and heat is transferred to a copper plate 10 which serves as the inertial element. Upon reaching a certain temperature the solder 7 that holds the clip 6, melts and spring 5 pulls the bracket and electric circuit ibreaks. Attachment of inertial fuses on the aircraft is provided by using fasteners 4 and 8. Ampersecond characteristics of inertial fuse (Fig. 6.4, b) consists of two sections 1 and 2. Section 1 is relatively small current that is ampersecond characteristics of inertial part of thr safety device and is determined mainly with time heating of the copper plate 10. Section 2 features meet high short circuit at which the brass fuse 9 burns.

Inertia fuses largely satisfy the requirement of selectivity, sensitivity and inertia protecting small but continuos overloads.

Thermal circuit breakers are used to protect circuits against overloads and short circuits. Sensitive element of a thermal circuit breaker is a bimetallic plate consisting of two welded metal plates with different temperature coefficients of linear expansion. Plates with a low coefficient of linear expansion are made of Invar or platynit, and for plates with a large coefficient from non-magnetic steel, nickel, brass, constantan and other materials.

When the flow of current is present through the bimetallic plate it heats and flexes. This property is used to influence the kinematic protective device at the excessive increasing in the current flowing through the sensor and its contacts.

The most common in aircraft device are АЗР and АЗС. By type of kinematic scheme the thermal automatic of the first group are made of free uncoupling of the control body and the contact system, and of the second group have no free uncoupling. After triggering of АЗР it is not allowed to re-lock their jet contact with a level until the bimetallic plate does not cool to a certain temperature. As permitted filling of supporting the enforcement of current contacts in the closed state the lever regardless of the current flowing through the machine. They are designed for rated currents of 2, 5, 10, 15, 20, 30 and 50 A.

Any machine without uncoupling free in its design consists of the following parts (Fig. 6.5): plastic casing 6, the control knob 2, pin lever 4, contacts 5, thrust pin with a spring 3, bimetallic plate 9, spring stopper 10, spring 7, mobile plastic block 12 copper cords 8 and 11. The automat is switche on with handle Fig. 6.5 2 that is turned to the left. At this time the pin lever 4 at the back holder 1 and simultaneously closes the contactds 5. The block 12 is shifted to the right and fixed in this position a spring stopper 10. If this knob 2 is back to its initial position, the contacts 5 open, but the block 12 will not change its position. After switching on, АЗС current flows through the conductive bus, contacts 5 and bimetallic plate. When overloading the bimetallic plate bends down and spring stopper 10 releases the block 12, which is under the influence of return spring 7 and is moved to its initial state. This upset both handle 2 to " off", and contacts of the devise opens.
Circuit breakers of the type АЗР (Fig. 6.6) are produced at rated currents 6, 10, 15, 20, 25, 30, 40, 150 A. The design of this type of machines consists of the following main parts: a manual control lever 1 mounted on an axis sear earrings 2 with movable axis 3, thrust lever 4, auxiliary spring 5, returning spring 6, bimetallic plate (L-shaped) 7, damping spring 8, the contacts 9 and auxiliary lever 10.

To turn the machine lever 1 we must return all the way to the left. This movable axis 3, moving in the trigger lever 2 is the first auxiliary compression spring 5. In unstretching of the spring it moves the auxiliary lever 10 in the far left, and the associated movable electrical contacts close. The spring 6 is stretched in the construction machine and accumulates potential energy, and the trigger lever 2 rests on its end a stopping lever 4. Therefore the moving machine can return to its initial position.

а б

Fig. 6.6

If you enable АЗР to return the lever 1 to the right, under the action of the springs 5 and 6, the lever 10 also moves to the right and the electrical contacts 9 will be opened. In inadmissible overload of the bimetallic plate 7 flexes up and presses the ceramic ball thrust lever 4, which leads to its rotation. As a result, the trigger lever 2 is released under operation of springs 5 and 6, the lever 10 is moved to the right, and contact system opens. It should be noted that while the bimetallic plate 7 will not return to the initial position and the stopping lever 4 fits into clutch lever 2, turn on the machine possible.

For three-phase AC networks power protection machines are designed which are devices such as АЗ-3, which will be triggered under the conditions of short circuits and overloads in the phases of the line that is protected. The principle of the three-phase machines is similar to the principle of the bimetallic machines АЗС and АЗР.

Thermal machines have much greater thermal inertia than fuses. But they have less inertia than the fuse of the type ІП. Timing of operation АЗС and АЗР at the double rated current (І = 2 І _ном) is 25... 40 C in the conditions of the previous heating nominal current. Unlike fuses, the bimetallic machines have several advantages: multitiming of actions, usage as switching equipment, the ability to check the characteristics of the operation, availability of necessary inertia, allowing large transient currents jumping; ability to respond to small but lasting overload currents. Disadvantages of bimetallic protection are automatic design complexity and a significant dependence of ampersecond characteristics on environmental parameters.

Chosing PE. Considered protection devices are widely used to protect the transmission and distribution networks. However, they can not always make full protection of any network. Let us consider the question of choice of protection devices for the distribution network. It is necessary to take into account the nature of electricity consumers.

If the load current of consumers at all modes of operation does not exceed the nominal value, the protection may be provided with fuses or thermal machines.

Rated current of the protection device is chosen by rated current iof the consumer, taking into account safety factor:

І _зах = К _з І _ном > І _ном,

where І _зах – is the rated current of the circuit breaker; К _з – is the safety factor; І _ном – is the rated current of the consumer. In practice, they choose the closest larger nominal current device protection.