Current thermal action Short circuit. Thermal effect of short circuit currents

I.Conductor systems when current flows are experiencing electrodynamic interactions, accompanied by significant mechanical stresses. At the same current, the conductors are attracted, and if the currents are directed to the opposite sides, then repel.

The strength of the interaction of currents is determined by the formulas arising from the law of Bio-Savara. For two parallel conductors long l.located at a distance and from each other, it can be found from the expression

If currents are expressed in amperes, and the force F - in Newton, then the coefficient K is 2 × 10 7; coefficient k F.it takes into account the shape of the conductor and can be taken equal to 1 for round-section conduirers regardless of the distance between them and for the conductors of any form, if the distance in the light between them is greater than the perimeter of the transverse section of the current-carrying part.

The force F is distributed evenly along the length of parallel conductors. The specific force per unit length of the conductor for conditions is:

Electrodynamic interactions in three-phase installations alternating current They have a number of ocabilities. Efforts change in time by value and direction and have a oscillatory nature.

The force acting on the conductor with current is defined as the result of the interaction of it with currents in the conductors of the two other phases, while in the most difficult conditions it turns out the medium phase conductor. The greatest specific force on the medium phase conductor can be determined from the expression, N / m,

where I M -current amplitude in phase, a; but -distance between adjacent phases, m.

The interaction of conductors is significantly increasing in the CW mode, when the total CZ current reaches its the greatest value- shock.

To determine the specific effort at a three-phase KZ in the system of conductors, use the expression on the condition, then

where - the shock current of the three-phase KZ, A.

Above the interphasis efforts were considered. However, in real devices and tire structures there may be quite large forces of interaction of the currents of one phase. This occurs when the phase splitting into a number of parallel wires, and only when the conductors are not straightforward, and form loops, bend at an angle. Such forces take place in disconnectors, reactors and other devices.

To prevent mechanical damage under the action of efforts occurring in conductors when the currents of the CW currents occur on them, all elements of the current-carrying design must have sufficient electrodynamic resistance.

Under the electrodynamic resistance, usually the ability of devices or conductors to withstand mechanical efforts arising from the flow of CW currents, without deformations that impede them further normal operation.

For electrical apparatuses, the manufacturer indicates the warranty current of the KZ, which provides electrodynamic resistance. Most often in the catalogs on the equipment, the instantaneous value of the current of electrodynamic resistance is set. i. Dean (or. i Max,or i. Pr.CV). In the choice of devices guaranteed by the factory, the current is compared with the calculated shock current of the KZ. Condition must be fulfilled .

Electrodynamic durability of rigid tires, with the exception of complete conductive conductors and RC bus, is determined by the calculation of mechanical stresses in the material of the conductor at the KZ. The stability criterion is the satisfaction of the condition, where and is the permissible and calculated value of the mechanical voltage and the material of the conductor.

According to PUE, the electrodynamic resistance does not check the devices and conductors protected by fuses with fuses with fuses to the current up to 60 A, and the devices and tires of circuits of voltage transformers under the condition of their location in a separate chamber.

Do not count the mechanical stresses from the forces of electrodynamic interaction in flexible wires. However, with shock currents, more than 50 ka such wires need to be checked.

II.It is known that conducting system systems for currents of currents are experiencing electrodynamic interactions, accompanied by significant mechanical stresses.

With the same current, the conductors are attracted, and if the currents are directed to the opposite sides, then repel

Fig. 18.1. Electrodynamic interaction between two current-friendly parts in the consistent (A) and the oncoming (b) current directions.

The strength of the interaction of currents is determined by the formulas arising from the law of Bio-Savara. For two parallel conductors L Lena, located at a distance butfrom each other, it can be found from the expression

If currents are expressed in amperes, and the force F - in Newton, the coefficient K is 2 ∙ 10 -7; The coefficient K f takes into account the shape of the conductor and can be accepted with an equal unit for round-section conduirers regardless of the distance between them and for conductors of any form if the distance in the light between them will be larger than the perimeter of the transverse section of the current-carrying part. Otherwise, the coefficient K F is different from the unit and when calculating efforts must be pre-determined by special charts.

The force F is distributed evenly along the length of parallel conductors. The specific force per unit length of the conductor for the conditions of Fig. 18.1 Equally:

. (18.2)

Electrodynamic interactions in three-phase AC installations have a number of features. In fig. 18.2 shows the effort vectors between the conductors of individual phases located in the same plane at various points in time for one period of alternating current. Efforts change in time by value and direction and have a oscillatory nature.

The force acting on the conductor with a current is defined as the result of the interaction of it with currents in the conductors of the two other phases, and in the most difficult conditions it turns out the medium phase conductor. The greatest specific force on the medium phase conductor can be determined from the expression, N / m,

, (18.3)

where I. M is the amplitude of the current in the phase, and; but - distance between adjacent phases, m.

The coefficient takes into account the phase shifts of currents in the conductors.

Fig. 18.2.Electrodynamic interactions in a three-phase conductor system:

a-B - the strength of the interaction for different moments of the period;

g -curves Changes Currents in Phase

The interaction of conductors increases significantly in the KZ mode, when the total CW current reaches its greatest value - shock. In assessing the interaction of the phases, it is necessary to consider a two-phase and three-phase KZ.

To determine the specific effort with a three-phase KZ in the system of conductors shown in Fig. 18.2, use the expression (18.3), provided then

, (18.4)

where ί y (3) - the shock current of the three-phase KZ, A.

In the case of a two-phase shortfall, the influence of the third (intact) phase is negligible, therefore expression (18.2) use to determine the specific effort (18.2), taking into account that Onceland

(18.5)

where ί y (2) is the shock current of the two-phase KZ, A.

Fig. 18.3. Epures of electrodynamic interactions within the same phase of the oil switch

Considering that It is not difficult to show that the interphasis force with a three-phase KZ is greater than with a two-phase. Therefore, the estimated type of KZ in the evaluation of electrodynamic forces is considered three-phase.

Above the interphasis efforts were considered. However, in real devices and tire structures there may be quite large forces of interaction of the currents of one phase. This occurs when the phase splitting into a number of parallel wires, and only when the conductors are not straightforward, and form loops, bend at an angle. In fig. 18.3 As an example shows the plot of efforts that occur within the current-host contour of the oil switch phase.

Such forces can lead to spontaneous disconnection of the switch, if you do not accept the appropriate measures. For example, at a current ί Y \u003d 50 kA, the MKP-35 of the moving contacts of the MCP-35 switch operates approximately 2000. Such forces take place in disconnectors, reactors and other devices.

To prevent mechanical damage Under the action of efforts arising in conductors when the currents of the CW currents are flowing on them, all elements of the current-carrying design must have sufficient electrodynamic resistance.

Under the electrodynamic resistance, usually the ability of devices or conductors to withstand mechanical efforts arising from the flow of CW currents, without deformation that impede their further normal operation.

For electrical apparatuses, the manufacturer indicates the warranty current of the KZ, which provides electrodynamic resistance. Most often in the catalogs on the equipment, an instantaneous value of the current of electrical dynamic dynamic dynamism is set, (or ί MAX, or ί ARC). When choosing devices, the current-guaranteed current is compared with the calculated shock current of the KZ. The condition ί DIN (MAX, PRKV) ί Y (3) must be satisfied.

σ additional σ determin

where σ additional and σ is calculated, respectively, the permissible and calculated value of mechanical stresses in the material of the conductor.

According to PUE, the electrodynamic resistance does not check the devices and conductors protected by fuses with fuses with fuses to a current up to 60 A, as well as devices and tires of voltage transformers, subject to their location in a separate chamber.

Do not count the mechanical stresses from the forces of electrodynamic interaction in flexible wires. However, with shock currents, more than 50 ka such wires need to be checked.

Other private cases are also stipulated in PUE when it is permissible not to check the devices and conductors on electrodynamic resistance at KZ.

The ability of devices, conductors and insulators to withstand electrodynamic and thermal impacts arising from passing through them the largest currents of the KZ are called electrodynamic and thermal resistance accordingly.

When CZ, with sufficient accuracy, the heating process can be adopted by adiabatic:

where i K.(t.) - a function that characterizes the change in the current of the CW in time; R. J - conductor resistance at a given temperature j; C j - the specific heat capacity of the conductor at a given temperature; G. - Mass of the conductor.

Considering that the resistance of the conductor and its specific heat capacity are temperature functions:

where r. 0 and C 0 - specific resistance and heat capacity of the conductor at the initial temperature J. H \u003d 0 ° C; a and b - temperature coefficients of resistance and heat capacity; S., l., G is the cross-sectional area, length and density of the conductor.

Separating variables and integrating in the required limits, we obtain the equation

which makes it possible to determine the final temperature of the conductor J to when it is heated by its CW current from the initial temperature J. n. However, the analytical solution of this equation is difficult, and therefore, for common conducting materials, the dependences of the values \u200b\u200bof the second integral from the final temperature are constructed ( J. H \u003d 0), which are presented in Fig. 2.8.

Fig. 2.8. Curves for determining the temperature of heating of the current-carrying parts for KZ

The first integral depending on the current of the CW and the time of shutdown t. Off, received the name of the Pulse of the Quadratic Current of the KZ B. Its approximate value can be expressed through the current values \u200b\u200bof the total current and its components

where The effective value of the total current of the KZ at the time of time t.; I. P, T. - the active value of the periodic component; I A, T - Aperiodic component.

Thus, the pulse of the quadratic current of the KZ is equal to the sum of pulses from the periodic B. P and aperiodic B. a component.

The pulse from the periodic component can be determined by the grafoanalytic method by replacing the smooth curve step with the orders corresponding to the average values \u200b\u200bof the current values \u200b\u200bof the current currents for each time interval:

In cases where the location of the closure is removed from the generators or is required roughly (with overgrowth), estimate the pulse from the periodic component, it can be assumed that the periodic component does not fade, i.e. .

The pulse from the aperiodic component of the CW current is equal to:

When found

Then the final temperature of the conductor will be equal to

In fig. 2.8 Sut the ordinate JN and according to the corresponding curve (point but) Find BUT n. Adding to BUT n (on the abscissa axis) B./S. 2, get BUT H and the temperature of the conductor J to (point b. on a curve).

The final temperature of the CW should not be higher than the insulation conservation permit or by the condition of mechanical strength (for uninsulated conductors).

The condition of thermal durability of the conductor:

Thermal resistance of the devices is taken to characterize the rated current of thermal resistance I. Ter with a certain duration of its passage, called rated thermal resistance time t. ter. To test the apparatus on thermal resistance, the value of the thermal pulse with the calculated plant is compared with the manufacturer. The thermal resistance condition of the device is formulated as:

The method of calculating the thermal and dynamic resistance of conductors and devices is in detail in the guidelines for calculating short circuit currents and the choice of electrical equipment of the RD 153-34.0-20.527-98

When conducting conductors electric current Conductors are heated. When the conductor is heated by the load current, the part of the selected heat dissipates into the environment, and the degree of dispersion depends on the cooling conditions.

When the current of the shortcut, the temperature of the conductors increases significantly, since the currents of the CZ increase sharply, and the duration of the KZ is small, so the heat released in the conductor does not have time to pass into the environment and almost everything goes to heating the conductor. The heating of the conductor under the CW can reach dangerous values, leading to the melting or charging of isolation, to deformation and melting of current-handing parts, and the like.

Criterion of thermal resistance of conductors are permissible temperatures Heating by their currents KZ (x additional, ° C).

The conductor or the device is considered thermally persistent if its heating temperature during the KZ process does not exceed the permissible values. The condition of thermal resistance in the general case looks like this, ° C:

x Kon? x additional (4.1.)

where x con is the end value of the temperature of the conductor in the KZ mode.

Quantitative evaluation of the degree of thermal exposure to the CW current of conductors and electrical apparatus are recommended using the Joule integral

where I KT is a complete current of the KZ in an arbitrary moment T, and; T Off is the calculated duration of the KZ, p.

The Joule integral is a complex function depending on the parameters of the sources of energy, the configuration of the original calculation scheme, the electrical remoteness of the KZ location from sources and other factors. For indicative calculations of the Joule integral in K in chains that have significant remoteness from power supply, you can use the formula 2 * C,

where - the active value of the periodic component of the CW current at the time T \u003d 0 from the equivalent source, ka; - the equivalent time constant of the attenuation of the aperiodic component of the current KZ, C; T Off is the calculated duration of the KZ, p.

The most difficult is the case of determining the joule integral with a short circuit near generators or synchronous compensators. But in the educational design and here you can use formula (4.1.3.), Since the value obtained in K will be somewhat overestimated, and conductors and devices selected in powerful connections (generator, communication transformer, etc.) under the conditions of the long-lasting mode. and electrodynamic resistance, have significant reserves of thermal resistance. Based on the above considerations, in formula (4.1.3.) As the T A.EKV, you can take the largest of the values \u200b\u200bof Ta and those sources that feed the place of the KZ, if there were several, because it leads to an increase in the calculated Joule integral and not Gives errors when checking the devices for thermal resistance.

When determining the Joule integral, it is necessary to determine to define T off. According to PUU, the calculated duration of the KZ T off is consisted of the operation of the main relay protection of this chain (T PB), taking into account the action of the APF and the full time of turning off the switch (T off), which is specified in the catalog of switches, C,

t off \u003d T PZ + T off (4.4.)

For chains of generators with r nom? 60 MW PUE It is recommended to take T off \u003d 4 s, i.e. By the time of reserve protection.

Plants manufacturers in directories lead the values \u200b\u200bof the guaranteed range of thermal resistance (T ter, ka) and the permissible time of its flow (T ter, C) for electrical devices (switches, disconnectors, current transformers, etc.).

In this case, the condition of thermal resistance of the devices in the KZ mode looks like that, ka 2 * s,

B k? T ter (4.5.)

When checking the thermal stability of the conductor having a standard cross-section of Q Stand, mm 2, a condition must be completed

q Stand? q min (4.6.)

In PUE, a number of cases are specified when it is permissible to check the conductors and devices for thermal resistance at KZ. This applies to the wires of air power transmission lines, devices and conductors of chains protected by fuses, etc.

If two parallel conductors proceed unidirectional Toki. ι 1 and ι 2, then these conductors are experiencing in relation to each other. power of attraction in the form of uniformly distributed solid mechanical linear load f [n / m] equal

F \u003d 2 ∙ 10 -7 to f, (6.32)

where ι 1, ι 2 - currents in the conductors, and;

a - distance between conductors, m 2;

k F - coefficient, taking into account the uneven flow of current distribution in cross section of the conductor (K Φ ≈ 1 for round, square and tubular sections at u< 6 кВ и для любого сечения при U > 6 kV; Under U.< 6 кВ для плоских шин к ф определяется по справочным кривым в зависимости от размеров сечения и расстояния между шинами).

With 3 phase KZ and the distribution of conductors in the same plane, the average phase is experiencing the highest force from the current. Maximum (shock) Linear mechanical load for this phase is equal to

F ud \u003d 10 -7 to f. (6.32)

Mechanical load causes a bending moment in hard conductors (tires). In the case when an infinitely long conductor is located on uniformly placed supports (Fig. 6.2), the bending moment is maximum on the support itself M Max, [N ∙ M] and equal

M Max \u003d, (6.33)

l - span between supports, m.

explorer fixed on uniformly arranged supports

Under the action of the bending moment in the metal, a mechanical voltage arises, σ, n / m 2 or MPa. The largest mechanical stress in the metal with bending is equal

where W is the moment of resistance, m 3.

The moment of resistance is determined by the size of the conductor and the direction of force acting on the conductor (the method of the tire location, Fig. 6.3)

Fig. 6.3. Location of tires on insulators:

a - Plafhmy; B - on the edge

At the location of the tires on the insulators, the plastics (Fig. 6.3, but), the moment of resistance is equal

When the tire is located on the edge (Fig. 6.3, b.) the moment of resistance is equal

The calculated voltage values \u200b\u200bin the metal tire metal should be less than the permissible value of the voltage σ additional for this material, i.e. The condition must be performed

σ ≤ Σ add. (6.36)

End of work -

This topic belongs to the section:

Summary of lectures on the discipline "Power supply of industrial enterprises"

Priazovsky State Technical University .. Department of Power Supplies of Industrial Enterprises ..

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Kolyada L.I.
Abstract lectures on the discipline "Power supply of industrial enterprises" for students specifically

Ways of development of the SES of industrial enterprises
Power supply systems (SES) of industrial enterprises are complicated as power consumption develops. When reconstructing (SES) and the design of new systems, the following main ass

Enterprises
The electrical energy receiver is the electrical part of the technological installation or a mechanism that receives energy from the network and consuming it to perform technological processes.

Characteristics of EP industrial enterprises
Consider the characteristic groups of electrical energy receivers of industrial enterprises. 1. Power general industrial installations. To this group of electrical energy receivers

Modes of operation of electrical acceptors
The correct definition of electrical loads (ENG) is a decisive and most important step in the design and operation of power supply systems. Electric loads Characteries

Methods for determining settlement loads
To calculate the electrical loads of industrial enterprises, two methods are used: the method of demand coefficient and the method of the calculated coefficient. To the auxiliary methods

Determination of electricity consumption
The total load (active, p ς and reactive, qς) on tires with a voltage above 1000 V is determined by the relations: Рς \u003d (σ

Elements of the electrical network
In the networks of industrial enterprises, about 10% of the transmitted electricity is lost. The magnitude of the loss depends on many factors, but primarily determined by the mode of operation of electrical receivers and the departure

Ways to reduce EE losses in power supply systems
Electrical receivers of industrial enterprises require for their work both active (P) and reactive (q) power. Reactive power is produced as active, synchronous generators

Energosystem
For industrial enterprises, the main source of power supply is the electrical stations combined into the energy systems. The number of electricity generated by

Industrial Power Station
Industrial power plants (factory power plants) refer to local sources of active power. The presence of local sources should be justified by technical economy

Power transformers in power supply system
Power transformers are the main electric equipmentproviding transmission and distribution of electrical energy from power plants to consumers. With the help of power trans

Neutral operation modes in power supply systems
Electrical installations and electrical networks with voltage above 1000 V, according to PUE, are divided into settings with large ground closures (single-phase short circuit current

Unlocked and closed networks
Unclosed (open) are called networks whose lines do not form closed contours. Such networks have one main power source connected to one of the network nodes.

Applied types of conductors
For execution electrical networks Uninsulated (naked) and insulated wires, cables, conductor are used. Naked wires do not have insulating covers. Them

Wiring with insulated wires
The electrical wiring is called a constant and alternating current network by voltage up to 1 kV, performed by insulated wires, also with small sections cables (up to 16 mm2).

Cable lines
Cables are used in industrial enterprises of all stresses (up to 110 kV inclusive) both within buildings and structures and in the territory of the enterprise and in external power supply.

Shinovrovod.
Shinovaya is called electricity transmission lines, whose conductors are rigid tires. Busbars can be open (uninsulated tires on the supporting from

Air lines
The electric power line (VL or VLEP) is called a device for transmitting electricity by wires. WL can be used in high and low voltage networks for distribution

Short circuits in electrical networks
Short circuit (KZ) is the intentional or accidental, not provided for by normal working conditions for the connection of two points of the electrical network through very small with

Calculation of the CW current with a constant periodic component
The periodic component of the CW current, in accordance with the permissible errors, can be considered almost unchanged in time if its changes remain in the range of 10%. If Ras.

Calculation of the CW current with a changing periodic component
If the condition x * ≥ 3 is not performed, then when calculating the CW currents, it is necessary to consider transient processes in the generators. It is simplified can be taken that these phenomena provide

Thermal (electrothermal) Current action KZ
The transition process (PP) of the heating of the conductor current of the KZ is characteristic of that its duration (τpp is a few seconds) is much less than the constant heating time of conductors T

Restriction of short circuit currents
For industrial electrical networks, the presence of powerful power sources and, accordingly, large values \u200b\u200bof the CW currents. This can significantly increase the cost of the power supply system by

Schemes of workshop transformer substations
CEE substations feed network NN. On shops transformer substations A voltage of 6-10 / 0.4 kV is used, as a rule, circuits without collective tires VN. Transformer schemes

Schemes of the main pioneering substations
For the reliable power supply of consumers I and II categories, the main pioneering substations (GPP and PGV) are usually constructed by two transformer. Food substations from power systems

Basic electrical equipment of substations
The main electrical equipment of the substations are: power transformers, switching devices, disconnectors, insulators and switchgear tires, measuring transf

Insulators and distribution tires
The current parts of the electrical installations are attached and are isolated from each other by means of insulators. Insulators are divided into linear, hardware, support and passing. Linear Isolators Pr.

Appointment of relay protection
Under the operation of electrical installations, damage is possible individual elements Power supply systems. A combination of special devices controlling the status of all elements of the system

Basic principles of relay protection
One of the signs of the occurrence of KZ is to increase the current in the line. This feature is used to perform relay protection (RZ), called current. Current RZ come into effect with uv

Enterprises
Relay protection is only part of the automation that has been used in power supply systems before other automatic devices. However, only relay protection cannot

Electrodynamic effect of short circuit currents.

With short circuits as a result of the occurrence of the shock current of the short circuit in tires and other designs of distribution devices, electrodynamic efforts arise, which, in turn, create a bending moment and mechanical stress in the metal. The latter should be less than the maximum allowable stresses for this metal.

In literature, the allowable calculated stress for aluminum is 80 MPa.

The electrodynamic force of the shock current of the short circuit with a three-phase short circuit is determined by the power of interaction between the conductors during the shock current.

where - the shock current at points K1, K2, ka,

The distance between the insulators of the same phase mm,

Distance between the conductors of the adjacent phases, mm

For CSO-366: mm; MM.

Calculate the strength of the interaction between the tires of the AT 15x3 tires on the side of 10 kV, according to formula (62):

Consider the tire as a uniformly loaded beam and calculate the bending moment created by the impact

where is the power of interaction, n

Distance between tires, m

Bending

To determine the mechanical stress in the metal, it is necessary to calculate the moment of resistance, given the location of the tires. Tires can be located either plafhmy or on the edge.

Figure 2.5.1.1. Location tire plaffers

Figure 2.5.1.2 Location of tires on the edge

In my courses, the tires are plaffed. At the same time, the moment of resistance is determined by the formula

where is the moment of resistance,

Tire width, cm,

Tire thickness, cm

We define the estimated tire tension:

From the condition we see that the tires of the brand AT (15x3) on electrodynamic resistance pass. Similarly, check the bins of the rectangular section of the AT (15x3) brand on the side of 0.4 kV.

Calculate the power of the interaction between the tires of the AT brand (15x3) on the side of 0.4kV, (63)

Calculate the bending moment created by the shock current (64):

Determine the estimated tires (62):

From the audit we see that the tires of the brand AT (15x3) on electrodynamic resistance pass.

Thermal effect of short circuit currents

The current parts, including cables, with short circuits can be heated to a temperature, much greater than during normal mode.

The cross section of the cable or tire when checking on thermal resistance is checked by the formula:

where VK is a thermal impulse,

art - coefficient depending on the material of the conductor, is taken according to PUE: Art \u003d 85 for aluminum lived; st \u003d 88 for copper lived

Pre-define the thermal impulse:

VK \u003d · T Off, (68)

where i pc is a current of the periodic component, I pc \u003d i pk1 \u003d ka \u003d 2350 a

t Off - Disconnection time with short circuit,

t off. \u003d T Off .. + t z, (69)

where t off. - the response time of the switch; C, T off \u003d 0.2С,

t h is the triggering time; C, T s \u003d 1.1c

t off. \u003d 0.2 + 1.1 \u003d 1.3C

We define the heat impulse for the airline and tires on the side of 10 kV (68):

In k1 \u003d 1,3 \u003d 7179250

We define the minimum cross section of the CL grade ASBG (3x16) (67):

F min \u003d\u003d 31.52 mm

According to the condition of the inspection on the thermal resistance of the selected cross section of the grade

Asbg (3x16) must be more either equal to the minimum calculation section

F min s extra (70)

31.52 mmi 16 mmi

From the condition, we see that the chosen cross section of the CL grade of ASBG (3x16) does not pass, taking away the larger cross section of the ASBG brand (3x35):

30.72 mm 3 35 mmi

From the condition you see that the selected cross section of the CL grade ASBG (3x35) passes

We define the minimum cross section of the AT 15X3 tire (66):

F min \u003d\u003d 31.52 mm

Checking condition (70):

31.52 mmi 45 mm

From the condition we see that the tires of rectangular cross section on the side of 10 kV brand AT (15x3) pass

Check on the side of 0.4 kV produce the temperature to compare the temperatures for this will be Table 2.5.2.1 The parameters of the current-generating parts

Table 2.5.2.1 Parameters of current-handing parts

To check the CL AAB 2 (4x25) on the thermal resistance on the low side, we will specify the temperature of the heating in the normal mode of operation. The heating current does not coincide with a long-permissible current.

h \u003d 0 + (additional one - 0) · () 2; (71)

h \u003d 15 + (65-15) · () 2 \u003d 15.69c

We define the thermal equivalent for the normal mode of work on graphics. 3.13 literature

An \u003d 0.12 · 10 4 A 2 · C / MMI

Determine the actual time of the short circuit current

t acts. \u003d T B + T s, (72)

where t off. - the response time of the switch; from,

t h is the triggering time; from

t Action \u003d 0.2 + 1.1 \u003d 1.3C

Determine the present time of the aperiodic component of the short circuit current

t PR \u003d 0.003 · ", (73)

where "\u003d; Because Ipko \u003d IPC, it means "\u003d 1

t PR \u003d 0.003 · 1 \u003d 0.003 with

We define the present time of the periodic component of the short circuit current in Figure 3.12 of the literature: T AP \u003d 0.85 C

Determine the total present time:

t PR \u003d T Pra + T AP (74)

t pr \u003d 0.003 + 0.85 \u003d 0.853 with

We define the thermal equivalent with a short circuit:

And k \u003d a n +, (75)

And k \u003d 0.12 · 10 4 + \u003d 0.205 · 10 4 a 2 s / mmi,

consequently, the heating temperature is 30C

Must be implemented:

The condition was followed, consequently, the CL passes through thermal resistance.

Check the tires on thermal resistance:

To check the rectangular section of the AT (15x3) brand (15x3) on thermal resistance on the low side, we will refine the heating temperature in normal operation. The heating current does not coincide with a long-permissible current (71):

h \u003d 25 + (88-25) · () 2 \u003d 48.69c

We define the thermal equivalent for the normal mode of work on graphics. 3.13 literature, an \u003d 0.38 · 10 4 A 2 · C / MMI

We define the heat equivalent with a short circuit (75):

And k \u003d 0.38 · 10 4 + \u003d 0.76 · 10 4 a 2 s / mmi,

consequently, the heating temperature is 110c

Condition must be executed (76):

The condition is performed, therefore, the Tires of the brand AT (15x3) pass through thermal resistance.