RU1786585C - Method of protection of wires of electric power network against overheating - Google Patents

Abstract

Usage: electrical engineering, namely relay protection, and can be used to protect wires from overheating during overloads. SUMMARY OF THE INVENTION: measure the duration of n successively consecutive time intervals, starting from the moment the temperature of the wire exceeds a long-term acceptable value for each time interval. The absolute temperature of the wire T | is measured. The first electrical signal is generated proportional to a) riexp (C2-Ci / Tj), where Ci and Cr are constants that depend on the material and construction of the wire. A second electrical signal is generated proportional to the constant value of KZ, the values of the electrical signals of all successive time intervals t are summed. When this sum exceeds the value of the electric signal proportional to KZ, a signal is generated to turn off the network. The duration of each of the time intervals t is assumed to be at least 50 times less than the constant heating time of the wire. The value of the coefficient K3 is assumed to be less than unity. 3 n.p. Yo

Description

The invention relates to electrical engineering, in particular to relay protection, and can be used to protect the wires of an electrical network from overheating during overloads.

With overloads of the electric network, its wires are heated. Heating the wire leads to an increase in the sag of the sag and a decrease in its mechanical strength. For electric lines with a voltage of 6, 10, 35, 110 kV, due to the relatively short spans between the supports, an increase in the sag arrows is, as it turned out, not dangerous. For overhead lines there are no dangerous phenomena with regard to their dimensions, and overloading is possible by 30-40%, and in some cases up to 50-60%.

The higher the overload, i.e. the more the wire heats up, the faster its mechanical strength is lost. The loss of mechanical strength leads to wire breakage, i.e. to an accident in the electrical network. Such accidents are prevented by overheating protection. However, not every overload must be switched off immediately, even if the temp.VI 00 ON SL 00 SL

The wire size exceeds the standard long-term allowable value. A power outage leads to interruptions in the supply of electricity to consumers and significant damage. Therefore, if the overload duration is short and the wire is exposed to high temperature for such a short time that its strength does not decrease, then the line should not be disconnected. And, only in those cases when the wire has been exposed to high temperature for a long time, and this can cause a wire break, the line must be disconnected. It follows that during overloads, shutdown should occur with a time delay, and this exposure should be the less, the higher the temperature of the wire. A device is known in which a shutdown command is issued immediately when the temperature rises above a long-term acceptable value, i.e. without time lag. The method of forming a shutdown command during overheating without a time delay has the disadvantage of reducing the reliability and stability of the power supply.

A device is known for protecting the contact network from overheating by current, in which the time delay depends on the temperature of the protected wire. The description of this device does not formulate a method of forming a time delay, however, based on an analysis of the operating principle, this formulation can be as follows: A method of determining a time delay at which the temperature of a network wire is measured, nonlinearly convert it to a first electrical signal, form a second electrical signal , starting from the moment the temperature exceeds a long-term allowable value, increasing in proportion to the current time, the second signal is compared with the first and, when the command to shut down the network.

The analytical model (mathematical expression) of this method has the form:

, Gd T (g) .d1,

where t is the current time, starting from the moment when the temperature of the wire exceeds t. for a long time allowable value etc (t etc);

td - time delay is acceptable, i.e. period of time from the moment when the temperature of the wire exceeds the long-term permissible value, etc., until

forming a network shutdown command;

-dependence of the permissible time delay td on the temperature of the wire.

The method implemented in the device is adopted as a prototype. Its disadvantage is that it remains unknown how exactly, by what law f (t), the first electrical signal is converted to the required time delay for a given temperature. Unaware of this law, it is impossible to choose the time delay when using a device to protect networks consisting of wires of equal cross section and

different metal, since such wires have different thermal resistance and different dependence of mechanical strength on the duration of exposure to temperature. In connection with this,

the prototype does not provide conditions for accurate operation for wires of different sections of different metal. In addition, in the known method, it is assumed that the temperature of the wire, according to which the time delay is formed, exists all the time, starting from the moment the second electrical signal appears and it does not change. In fact, the temperature above the long-term allowed may vary,

for example, build up. This reduces the accuracy of determining the time delay, i.e. precision protection.

The aim of the invention is to increase the accuracy of determining the permissible time delay for protecting the wires of the electric network from overheating by more accurately taking into account the relationship between temperature, time of exposure and mechanical strength of the wire.

The goal is achieved by the fact that in the known method, which consists in measuring the temperature of the wire, fixing the moment it exceeds a long-term acceptable value, non-linearly converting the temperature into a first electrical signal, generating a second electrical signal and generating a shutdown command based on the results of comparing the first and second electrical signals, measured, starting from the moment the temperature exceeds a long-term allowable value, successive intervals of time t , in every 1st time interval, the absolute

wire temperature T | and form the first electrical signal proportional to the value of a exp (C2-Ci / Ti), where Ci and C2 are constant, depending on the material and construction of the wire, form the second

an electric signal proportional to the constant value K3 sums up the values of the first electric signals ai of all successive time intervals m and, when this sum exceeds the value of the second electric signal, form a command to turn off the network.

In addition, the time intervals m take at least 50 times less than the constant heating time of the wire, the constant value K3 is taken positive with a value less than unity, and is taken for copper and bronze stranded wires Ci 11500, C2 23.84, for steel-copper stranded and contact wires Ci 16600, C2 35.15, for aluminum and steel-aluminum stranded wires Ci 30800, C2 50.28, for copper and bronze contact wires Ci 10100, C2 20.40, for copper low-alloy contact wires Ci 12130, C2 24.59, for bronze ones Toughened contact wires Ci 4800.02 3.99.

The analytical model of the proposed method has the form:

i, i 1

or

2 g iexp (C2-Ci / Ti) C3; KZ 1; Ti TD

i 1

where r is the duration of the 1st interval, which is part of the time delay; T | - absolute temperature of the wire, measured in the 1st interval; n is the number of consecutive intervals that add up to the time delay; TD - the absolute temperature of the wire is permissible for a long time.

Comparison of the content and sequence of operations in the known and claimed methods shows the following. In the known method, the first electrical signal is generated depending only on the measured temperature of the wire according to an unknown law. In the claimed method, the first electrical signal is generated according to a certain law depending on the absolute temperature of the wire and the duration of the received time intervals. In the known method, the second electrical signal is proportional to time; in the claimed method, the second electrical signal remains unchanged. In the claimed method, the first electrical signals are summed for all consecutive consecutive time intervals. In the known method, such an operation is not provided.

The proposed sequence of operations provides new security features. One of them consists in the fact that the dependencies, according to which the first and second electrical signals are formed, allow scientifically sound establishing (adjusting) such

operating conditions of protection, in which it takes into account the specific parameters and properties of any protected wire. The second is that the proposed method takes into account the actual thermal (total) effect on the wire at a changing temperature during all time intervals preceding the moment of protection operation. In the prototype, the total thermal effect

taken equal to that which occurs at a temperature corresponding to the instant of operation. If, however, before the temperature was triggered, the temperature changed, then the time delay in the prototype was not determined exactly.

When a wire is exposed to high temperature, its strength decreases after a certain time, and it may break. To prevent this from happening,

it is necessary to limit the period of time that the temperature is exposed to a value at which the mechanical strength of the wire is still maintained. This period of time determines the permissible time delay of the overheating protection. Therefore, in order to make a reasonable choice of time delay, one should turn to the theory of the thermal strength of metals. In particular, for copper, aluminum, their alloys,

for steel, i.e. materials used in the wires of the electric network, one of the widely used models, which is called the Larson-Miller dependence, has the form:

Tr exp (Ci / T-C2) (273 +1) -

where Tp is the time until the destruction of the metal, s; T 273 +1 - the absolute temperature of the metal, K; t is the temperature of the metal, ° C; Td - long-term allowable absolute temperature of the metal, K; Ci, C2 - constants, depending on the metal and construction of the wire.

However, for wires of electric networks, the values of these constants are not given in the known literature.

Experimental and theoretical studies of the authors have established that for copper and bronze stranded wires Ci 11540, € 2 23.84; for steel-copper stranded wires Ci 16600, 02 35.15; for aluminum and steel-aluminum stranded wires iCi 20820, C2 50.28; for copper and bronze contact wires Ci 10100, Ci- 20.40; for low-alloy contact wires Ci 12130, € 2 24.59; for bronze heat-resistant contact prb waters d 4800, C2 2.99; for steel-copper contact wires Ci 1600, Ca

35.25: -

When exposed to a temperature wire, bW will not be destroyed if the exposure time r is less than the time before destruction tp (r gr). It follows that at a constant temperature T, it is maximally small, but the permissible time delay r can be determined from the conditions t Kz tp # p7 where e K3 is the safety factor. This stability is possible at K3 1. Thus, at K3 1 and T const, the maximum allowable exposure time can be calculated by the formula g / Tr E: K3. Substituting the last formula: (2), we obtain the condition T, observance of which ensures the greatest permissible time delay, which does not lead to destruction of the wire (taking into account the known dependence 1 / ex ex):

r / exp (Ci / Ti-C2) g exp (C2-Ci / T) K3,

KZP; T TD

.; .- "v.

(3)

Formulas (2) and (3) are valid for one constant temperature T. Under real conditions, the temperature of the wire changes with a change in the current value in the electric network. It is proposed to extend conditions (2) and (3) to the changing temperature T as follows. As soon as the temperature of the wire T exceeds the long-term admissible value of Td, the time t starts counting. In this case, the time t is divided into separate consecutive small intervals m, where i is the number of the interval (i 1, 2, 3, .... p ) Within each small interval m, the temperature of the wire of the transformer is measured, which is assumed to be constant during this interval. For each time interval m, aj -t exp (C2-Ci / Ti) is calculated.

where ai is the fraction of the thermal stability resource expended in the TI time interval. We find the full resource in the form of the sum of the quantities ai for successively the following intervals of m. In this case, condition (3) takes the form:

10

Z pehr (C2-C1 / TO K3, K3 1; Ti TD, (4)

where n is the number of small intervals m following each other at which condition (4) is satisfied. Expression (4)

identically equal to (1), which proves the positive effect and achievement of the goal by the proposed method. Expression (4) is all the more accurate the shorter the duration of each interval is selected

P. Calculations show that in order to obtain technically reasonable accuracy at varying temperatures, each of the time intervals should be at least 50 times less than the constant time

heating the wire.

Expression (4) proves that the proposed sequence of operations in comparison with the prototype, provides increased accuracy in determining exposure

protection time by taking into account the parameters and properties of specific forbidden wires, by taking into account the actual law of the thermal strength of the metal and by taking into account the actual total resource

thermal resistance of metal wires under varying load.

Claims (4)

SUMMARY OF THE INVENTION 1. A method of protecting wires of an electric network from overheating, in which the temperature of the wire is measured, the moment it exceeds a long-term acceptable value is recorded, the temperature is nonlinearly converted to the first electrical signal, a second electrical signal is generated and a shutdown command is generated based on the results of comparing the first and second electrical signals, characterized in that, in order to improve accuracy by more accurately determining the time delay, measure, starting from the moment of exceeding Temperature values of the permissible duration of the following succession of intervals of time t, and each In the 1st time interval, the absolute temperature of the wire P is measured and a first electrical signal is generated proportional to ai riexp (C2-Ci / Ti), where Ci and C2 are constant, depending on the material and construction of the wire, form the second an electric signal proportional to a constant value sums up the values of the first electric signals ai of all successive time intervals m and, when this sum exceeds the second electric signal, form a command to turn off the network. 2. The method according to claim 1, characterized in that each of the time intervals m is taken at least 50 times less than the constant heating time of the wire. 3. The method according to claim 1, characterized in that the constant is taken 0 5 positive with a value less than one. 4. The method of claim 1, characterized in that it is adopted for copper, bronze wires SC 11500, C2 23.84, for steel-copper stranded and contact wires Ci 16600, C2 35.25, for aluminum and steel-aluminum stranded wires - Ci 20800 Cz 50.28, for copper and bronze contact wires -Ci 10100, Ca 20.40, for low-alloy copper contact wires Ci 12130, C2 24.59, for bronze heat-resistant contact wires-Ci 4800, Ca 3.99.