RU2515121C1 - Method for determination of overload value and duration for oil-filled power transformer equipment - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: by means of temperature sensors a heat flow cooling the transformer is defined for different parts of the transformer. Degree of the transformer heating is defined against values of current passing through the transformer windings. Using these parameters the permitted time of the transformer overload is defined depending on the required level of overloading and the current thermal state of the transformer.

EFFECT: improving operational reliability of the transformer equipment due to more reliable determination of overload value and duration for oil-filled power transformer.

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Description

The invention relates to the field of electric power, in particular to automated control systems and diagnostics of transformer equipment of electrical substations.

Power transformers and autotransformers with oil-air or oil-water cooling are one of the most expensive types of electrical equipment of electrical substations. In such transformers, the active part (magnetic core and windings having pulp and paper insulation) is immersed in an insulating liquid - transformer oil. As a result of natural convection or forced circulation, the oil passes through the cooling channels in the windings and the magnetic circuit of the transformer, ensuring the removal of heat generated by losses in the windings and the magnetic circuit. If, due to insufficient heat removal, the temperature of the pulp and paper insulation is too high for a long time, structural changes in the pulp occur, the degree of polymerization of the paper decreases, which can lead to mechanical damage to the insulation and its electrical breakdown.

For safe operation of the transformer, it is important that the permissible values do not exceed the temperature in the hottest place of the winding (the so-called "most heated point", NNT). Power transformers are designed in such a way that at an electric load not exceeding the rated power of the transformer, the temperature of the NNT is below the maximum permissible. At the same time, the operating conditions of power systems require in some cases to transmit power exceeding the nominal value through a transformer.

Known technical solutions that provide the energy system operator with predictive information about the transformer overload levels acceptable at each time point and the permissible overload time depending on its size, for example, US Pat. No. 6,727,821 "Apparatus and method for predicting an overload trip for an electrical power transformer" dated 04/27 .2004.

The device contains an ambient temperature sensor, a load current sensor connected to one of the windings of the controlled power transformer, an analog-to-digital converter, to which these sensors are connected, and a computing device to which an analog-to-digital converter is connected via a digital interface. The computing device has discrete outputs for issuing respectively warning and alarm signals about unacceptable overload of the controlled power transformer. As a temperature sensor, the device can use an ambient temperature sensor, or a temperature sensor at a certain point in the oil volume in the transformer tank.

Using well-known relationships given, for example, in the standard IEC60067-7, the device calculates, using measured values of the ambient temperature and load current, the temperature of the most heated winding point (INT) and its rate of change. Under the assumption that the rate of change of temperature and resource consumption will remain unchanged, the device calculates the time value after which any of these parameters reaches the user-specified limit value of this parameter.

This method has a drawback: for the predictive calculation of the time change in the temperature of the most heated point of the transformer winding (NNT) at a given overload coefficient, based on which the remaining time until reaching the maximum permissible temperature of the NNT is determined, taking the temperature into one point is not enough.

The closest technical solution is a method for determining the permissible magnitude and duration of the overload of power oil-filled transformer equipment, implemented by a device containing a transformer winding current sensor, an ambient temperature sensor, an analog-to-digital converter (ADC), to the inputs of which these sensors are connected, and connected to it computing device with discrete outputs for issuing warning and alarm signals, an additional temperature sensor is introduced ry in the upper layers of transformer oil, as well as an oil moisture sensor, into which the active part of the transformer is immersed, and an oil temperature sensor inside the humidity sensor. Moreover, all sensors are connected to the corresponding inputs of the analog-to-digital converter, and the computing device connected to it is equipped with a second, digital interface for transmitting information to the operator about the permissible transformer overload time, depending on the required level of overload and the current thermal state of the transformer. For transformers with three or more windings, the introduction of two or more current sensors of different windings is provided, as well as the corresponding refinement of the algorithm for determining the permissible transformer overload time, depending on its current thermal regime, oil moisture level in the transformer tank and the predicted load level of all windings (RF patent No. 2453859, IPC G01R31 / 02, 02.27.2012).

The algorithm for accounting for the water content in the paper insulation of windings in the NNT area is based on measuring the relative humidity of the oil poured into the transformer, calculating its absolute humidity, calculating the corresponding equilibrium moisture content in the paper and introducing a correction coefficient for the permissible temperature of the NNT.

According to the known method, the currents of different transformer windings are measured, the temperature of the upper layers of transformer oil poured into the transformer tank is measured, the oil humidity and the oil temperature are measured at the point of oil moisture measurement, based on which the equilibrium moisture content in the paper insulation of the transformer windings and the corresponding value are calculated the maximum permissible temperature of the most heated point of the transformer winding, give warning and alarm signals.

The known method has several significant disadvantages.

Firstly, for the predictive calculation of the time change in the temperature of the most heated point of the transformer winding (NNT), use the ambient temperature, which does not accurately characterize the cooling process of the transformer: it does not take into account specific conditions - wind, rain, snow, air humidity, which significantly affect cooling process.

Secondly, they do not take into account the design features of a particular transformer model, which significantly change the cooling rate of the transformer.

The objective of the present invention is to increase the operational reliability of transformer equipment due to a more reliable determination of the permissible values and duration of the overload power oil-filled transformer.

The technical result is achieved by the fact that in the method for determining the permissible magnitude and duration of the overload of power oil-filled transformer equipment, in which the currents of different transformer windings are measured, the temperature of the upper layers of the transformer oil poured into the transformer tank is measured, the oil humidity and the oil temperature are measured at the point of measuring the oil moisture , on the basis of which the equilibrium moisture content in the paper insulation of the transformer windings and the corresponding content are calculated th value of the maximum permissible temperature of the most heated point of the transformer winding, give warning and alarm signals, according to the claimed invention

- additionally measure the temperature of the most heated upper part of the transformer tank and the temperature of the coldest lower part of the transformer tank,

- based on the data of temperatures and the measured temperature of the upper layers of the transformer oil, poured into the tank of the transformer, calculate the heat flow, cooling transformer,

- based on the heat flux cooling the transformer, and taking into account the measured currents of different transformer windings that heat the transformer, calculate the time dependence of the temperature of the most heated point of the transformer winding,

- on the basis of this dependence and the calculated value of the maximum permissible temperature of the most heated point of the transformer winding, the permissible overload time of the transformer equipment is calculated.

Thus, the technical result is achieved by direct measurement of the heat flux coming from the heated winding of the transformer into the cooling medium. For this, in addition to measuring the current of different transformer windings, measuring the temperature of the upper layers of transformer oil, measuring oil moisture and oil temperature in the specified humidity sensor, the temperature of the most heated region of the upper part of the transformer tank, the temperature of the coldest lower part of the transformer tank are also measured. Based on the measured temperatures of the transformer, the heat flow, the cooling transformer are calculated, and also taking into account this value, the dependences are calculated on the time of the temperature of the most heated winding point and, accordingly, the remaining permissible operating time of the transformer equipment until it reaches the maximum permissible temperature, depending on its current thermal regime, the moisture level of the oil in the transformer tank and the predicted load level of all windings.

The invention is illustrated by drawings, where in FIG. 1 is a schematic structural diagram of a device that implements the proposed method for determining the allowable magnitude and duration of an overload of power oil-filled transformer equipment, FIG. Figure 2 shows a tank with transformer windings and temperature sensors, and figure 3 shows a method for determining the permissible magnitude and duration of an overload of a power oil-filled transformer equipment in a diagram.

The numbers in the drawings indicate:

1A - temperature sensor of the most heated upper part of the transformer tank,

1b - temperature sensor of the coldest lower part of the transformer tank,

1c - temperature sensor of the upper layers of transformer oil,

2a - current sensor,

2b - current sensor,

3 - analog-to-digital Converter,

4a, 4b, 4c, 4d, 4e, 4f, 4g - inputs of an analog-to-digital converter,

5 - computing device

6 is the first digital interface,

7 - discrete output for issuing a warning signal,

8 - discrete output for issuing an alarm,

9 - transformer oil humidity sensor,

10 - temperature sensor for measuring the temperature of transformer oil in a humidity sensor,

11 is a second digital interface,

12 - communication line

13 - operator terminal,

14 - transformer tank,

15 - transformer oil,

16 - transformer windings,

17 - heat flow.

The method of determining the permissible value and duration of the overload power oil-filled transformer equipment is as follows.

The measurements are carried out by various sensors (Fig. 1): temperature sensor 1a of the most heated region of the upper part of the transformer tank, temperature sensor 1b of the coldest lower part of the transformer tank, temperature sensor 1c of the upper layers of oil, current sensors 2a, 2b connected to different windings of the controlled power transformer, oil humidity sensor 9, temperature sensor 10 for measuring the oil temperature in the humidity sensor 9.

An analog-to-digital converter 3 with inputs 4a ... 4g, to which respectively the outputs of these temperature sensors 1a, 1b, 1c, current sensors 2a, 2b, humidity sensor 9 and sensor 10 are connected, converts analog signals to digital signals.

Computing device 5, to which an analog-to-digital converter 3 is connected via the first digital interface 6, performs all calculations. The computing device has discrete outputs 7 and 8, to which respectively gives warning and alarm signals about unacceptable overload of the controlled power transformer, and a second digital interface 11, which transmits via the communication line 12 to the operator terminal 13 information about the permissible transformer overload time, depending on its current thermal conditions, the moisture level of the oil in the tank 14 of the transformer and the predicted load level of all windings.

To measure the heat flux coming from the heated transformer winding to the cooling medium, it is necessary to measure the temperature at two points of the transformer, which are located on the heat flux 17 from the heated transformer winding to the cooling medium.

The transformer tank 14 (FIG. 2) is filled with transformer oil 15. The transformer windings 16 are located inside the tank 14.

From the heated winding 16, the heat flows 17 go to the walls of the tank 14 and cool the winding 16. The temperature sensor 1c of the upper layers of the transformer oil (measured temperature T1) and the temperature sensor 1a of the most heated region of the upper part of the transformer tank (measured temperature T2) are located on the heat flow 17 .

Accordingly, the heat flux Q1 (first method), which cools the winding 16, is equal to:

$$Q\mathrm{one}=(T\mathrm{one}-T2)*\chi 12\text{(one)}$$

where: T1 - readings of the temperature sensor 1c of the upper layers of transformer oil, T2 - readings of the temperature sensor 1a of the most heated region of the upper part of the transformer tank, χ12 - thermal conductivity between the sensors 1a and 1s.

The transformer oil 15 heated by the winding 16, like any heated liquid, rises upward, therefore, the temperature sensor 1a of the most heated region of the upper part of the transformer tank shows a higher temperature than the temperature sensor 1b of the coldest lower part of the transformer tank (measured temperature T3).

Accordingly, the heat flux Q2 (second method), which cools the winding 16, is equal to:

$$Q2=(T2-T3)*\chi 23\text{(2)}$$

where: T2 - readings of the temperature sensor 1a of the most heated region of the upper part of the transformer tank, T3 - readings of the temperature sensor 1b of the coldest lower part of the transformer tank, χ23 - thermal conductivity between the sensors 1a and 1b.

The heat flux Q emanating from the heated winding 16 is equal to:

$$Q=(T41-T\mathrm{one})*\chi \mathrm{fourteen}\text{(3)}$$

where: T41 is the temperature of the winding 16, T1 is the temperature sensor 1c readings of the temperature of the upper layers of transformer oil, χl4 is the thermal conductivity between the temperature measuring points T1 and T41. In this case, the heat flux Q is equal to the measured value of the heat flux Q1, which cools the winding 16.

From this equality (Q = Q1) for the measured value of Q1, T41 is determined:

$$T41=T\mathrm{one}+V\mathrm{one}*(T\mathrm{one}-T2),\text{where v1}=\text{(}\chi \text{12/}\chi \text{14) (4)}$$

Similarly, from the equality (Q = Q2), T42 is determined from the measured values of Q2:

$$T42=T2+V2*(T2-T3),\text{where v1}=\text{(}\chi \text{23 /}\chi \text{24) (5)}$$

Temperature T5 of the most heated point of the transformer winding (NNT) is defined as the highest of temperatures T41 and T42:

$$T5=\max (T\mathrm{41,}T42)\text{(6)}$$

The difference (PQ) determines the heating or cooling of the transformer, where: P is the apparent power P = ∑ (Ri * Ii ² ), which is emitted by the currents And passing through the windings of the transformer 16, Ri is the loss resistance of the windings of the transformer 16. In the stationary state (when temperatures are constant) (PQ) = 0 (heating power is equal to the heat flow of cooling), and thus χ12 is determined (for the first method of measuring the heat flow of cooling):

$$\chi 12=P/(T\mathrm{one}-T2)\text{(7)}$$

Similarly, for the second method of measurement in a stationary state, χ23 is determined:

$$\chi 23=P/(T2-T3)\text{(8)}$$

The algorithm for accounting for the water content in paper insulation of windings in the NNT area is based on measuring the relative humidity of the oil poured into the transformer, calculating its absolute humidity, calculating the corresponding equilibrium moisture content in the paper and introducing a correction coefficient for the permissible temperature of the NNT - calculating the maximum value T5max.

When (P-Q)> 0, the temperature of the transformer increases, and for the time interval dt, the temperature T5 increases by dT5:

$$dT5=(P-Q)*dt/C\text{(9)}$$

where: C is the heat capacity of the transformer. In a linear approximation, we determine the time tmax, during which the temperature T5 increases to the maximum value T5max:

$$t\max =(T5\max -T5)*C/(P-Q)\text{(10)}$$

Moreover, the estimate by formula (10) is the lower bound for tmax, since the rate of temperature increase T5 will slow down as the temperature of the transformer increases. In Fig.3, a method for determining the allowable magnitude and duration of the overload of power oil-filled transformer equipment is presented in the form of a diagram.

From the international standard IEC 60354: 1991 (E), numerical estimates can be made. At a temperature of the cooling medium Θа = 20 ° С, excess of the oil temperature in the lower part of the winding ΔΘbr = 33 ° С, excess of the oil temperature at the outlet of the winding ΔΘor = 55 ° С, temperature gradient of the most heated point Hqr = 23 ° C. Assume that the temperature of the transformer tank is approximately equal to the temperature of the oil at the corresponding point. Then, with a load factor (ratio of transformer current I to rated current) K = 1, we accordingly obtain: temperature of the coldest lower part of the transformer tank T3 = Θа + ΔΘbr = 53 ° C, temperature of the most heated region of the upper part of the transformer tank T2 = Θа + ΔΘor = 75 ° С, temperature of the most heated point of the transformer winding (ННТ) T5 = Θa + ΔΘor + Hqr = 98 ° C. Accordingly, (T2-T3) = 22 ° C, (T5-T2) = 23 ° C, and the ratio V2 = (χ23 / χ24) ≈1.

This shows that equation (5) is well suited for measuring the temperature of the most heated winding point of the transformer T5: at V2 = 1 (5) it is simplified:

$$T5=2*T2-T3\text{(10)}$$

If, to estimate the temperature of the oil at the outlet of the winding (Θa + ΔΘor), use the readings of the temperature sensor of the upper layers of transformer oil T1, and to estimate the temperature gradient of the most heated point of the transformer winding, use the difference (T2-T3) (taking into account that V2 = ( χ23 / χ24) ≈1), then expression (5) can be written as:

$$T5=T\mathrm{one}+T2-T3\text{(eleven)}$$

Thus, in the proposed method for determining the permissible magnitude and duration of the overload of power oil-filled transformer equipment, the heat flow directly from the heated winding of the transformer to the cooling medium is directly measured, and the allowable time of transformer equipment overload is calculated.

Using the proposed method will improve the operational reliability of transformer equipment due to a more reliable determination of permissible values and duration of overloading a power oil-filled transformer due to direct measurement of the heat flux coming from the heated winding of the transformer to the cooling medium.

Claims (1)

The method of determining the permissible magnitude and duration of the overload of power oil-filled transformer equipment, in which the currents of different transformer windings are measured, the temperature of the upper layers of transformer oil poured into the transformer tank is measured, the oil moisture and oil temperature are measured at the point of measurement of oil moisture, based on which the equilibrium content is calculated moisture in the paper insulation of transformer windings and the value corresponding to this content of the maximum permissible temperature n the warmer point of the transformer winding, give warning and alarm signals, characterized in that they additionally measure the temperature of the warmest upper part of the transformer tank and the temperature of the coldest lower part of the transformer tank, based on temperature data and the measured temperature of the upper layers of the transformer oil, poured into the transformer tank calculate the heat flux cooling the transformer, based on the heat flux cooling the transformer, and taking into account the measured currents ra GOVERNMENTAL transformer windings which heat transformer calculated temperature dependence of the hot spot of the transformer winding time on the basis of this dependence, and the calculated value of maximum permissible temperature of the hottest point of the transformer winding is calculated allowable transfer time of the transformer equipment.

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