OPERATION OF THE VM-35N UNDER LOW-TEMPERATURE CONDITIONS

A. A. VOROBYOV, V. N. TITOV, N. B. BOGDANOVA¹

Introduction

At the Tomsk Polytechnic Institute of the Order of the Red Banner of Labor, systematic scientific research is being conducted on the behavior of electrical insulation and electrical apparatus under cold conditions [1].

In the present article we report some materials from an investigation of the VM-35N [2]. The nominal operating current of the circuit breaker is 600 A. This investigation was carried out at the request of the Elektroapparat plant. The program and methods of the work were agreed upon with the plant. The apparatus under investigation, with a PS-20 drive, was installed on a concrete foundation in the open air and was aligned in accordance with the rules of the installation instructions.

The VM-35N, being representative of a series of high-oil-volume circuit breakers and having wide application in outdoor distribution installations, was of great interest for investigation in the indicated respect.

Heating of the Circuit Breaker under Nominal-Current Load

Observations were made of the heating of the oil and contacts while a current, close in magnitude to the nominal value ($\simeq 600$ A), was passed through the VM contacts, at the following ambient-air temperatures: $+3^\circ$C, $-12^\circ$C, $-10^\circ$C, $-20^\circ$C, $-22^\circ$C, $-26^\circ$C, $-28^\circ$C, $-30^\circ$C, $-32^\circ$C, $-33^\circ$C. Temperatures were measured by means of iron–constantan thermocouples.

Figures 1 and 2 present the typical course of the heating curves of the oil circuit breaker, taken at various ambient-air temperatures.

Heating of the VM-35N oil circuit breaker at an outside-air temperature $t = -12^\circ$C and at a current strength $J = 600$ A.

Fig. 1

Heating of the VM-35N oil circuit breaker at an outside-air temperature $t = -32^\circ$C and at a current strength $J = 620$ A.

Fig. 2

It should be noted that when current was passed through the VM contacts at an air temperature of minus \(32^\circ\mathrm{C}\), after 4 hours the oil temperature in the arc-suppression chamber reached minus \(10^\circ\mathrm{C}\), and subsequently rose further to minus \(7.5^\circ\mathrm{C}\). As for the temperature of the contacts, after \(1/3\) hour from the moment the current was switched on it had already become positive, and after \(1^{1/2}\) hours it reached a value of about \(15^\circ\mathrm{C}\).

As can be seen from the curves shown in Figs. 2 and 3, the temperature of the contacts and of the oil became established with time and maintained a constant value. Obviously, the magnitude of this steady-state temperature will depend on the amount of heat released and on heat removal. The first quantity in our experiments was constant. The second quantity—the amount of heat removed—depends on the air temperature and atmospheric conditions (wind). Under our conditions the VM was installed in a yard enclosed on all sides and was not subjected to the action of wind. Thus, it may be roughly assumed that heat removal from the tank was determined only by the temperature of the external environment. It was then of interest to see how the established temperature of the oil and of the VM contacts depends on the air temperature.

Fig. 3. Dependence of the established temperature of the contacts and oil (\(T^\circ\mathrm{C}\)) of the VM-35N circuit breaker on the outside-air temperature (\(t^\circ\mathrm{C}\)).

The relationship between these quantities is presented in Fig. 3, where the abscissa axis gives the temperature of the outside air \(T\) in \(^\circ\mathrm{C}\), and the ordinate axis gives the established temperature of the contacts and oil \(t\) in \(^\circ\mathrm{C}\).

As can be seen from Fig. 3, the relationship between these quantities is linear. The points corresponding to the relationship between the established oil temperature and the air temperature lie well on a straight line whose equation is

\[ T = 0.55 t + 10.5. \tag{1} \]

According to the data presented in Fig. 3, for the established temperature of the contacts \(t\) as a function of the air temperature, equations of the same type may be written.

For the right contact:

\[ T = 0.42 t + 30. \tag{2} \]

For the left contact:

\[ T = 0.42 t + 31. \tag{3} \]

In view of the small difference between equations (2) and (3), for the heating temperature of the contacts we may give one general equation:

\[ T = 0.42 t + 30.5. \tag{4} \]

Observation of the heating of the VM at low temperature leads us to a conclusion of practical importance. The temperature in the VM tank, when it is loaded with its rated current, proves to be considerably higher than the ambient temperature. Under our heat-removal conditions the temperature of the contacts reaches a value equal to zero at an ambient-air temperature of minus \(13^\circ\mathrm{C}\).

The oil temperature in the tank will be equal to zero at an air temperature of minus \(19^\circ\mathrm{C}\). The presence of a higher oil temperature near the contacts is desirable for ensuring normal operation of the VM. It may be supposed that insulating the tanks by covering them with heat-insulating materials would, in this case, prove a useful and sufficient measure for maintaining a positive temperature in the operating VM.

Viscosity and density of the oil

To judge the operation of the VM at various temperatures, it is essential to know the temperature dependence of the oil viscosity. For this purpose, measurements were made of the viscosity of the oil at different temperatures. These measurements were carried out with a Stokes viscometer [3].

The results of measurements of the viscosity of the oil filling the VM tanks are presented in Fig. 4, where the abscissa gives the air (oil) temperature in °C, and the ordinate gives the viscosity coefficient $\eta$ of the oil in poises. As can be seen from Fig. 4, as the temperature is lowered the viscosity of the oil at first increases slowly; then, when the temperature is lowered below minus 30°C, a very rapid increase begins. The change in the value of $\eta$ with decreasing temperature is described by the formula

\[ \eta = 0.74 e^{-0.11t}, \tag{5} \]

where $t$ is the temperature in °C, and $\eta$ is the viscosity coefficient in poises.

According to the experimental data, a sharp increase in the viscosity of the oil begins at temperatures below minus 30°C. It is obvious that, with strong thickening of the oil at these temperatures, one must expect considerable resistance to the motion of the VM traverse and a slowing of arc extinction by the viscous oil.

Stokes viscometer
Fig. 4

It is of interest to calculate at what outside-air temperatures the temperature of the oil in the tank will reach minus 30°C. Calculating by formula (1), we find that, under our conditions, the steady-state temperature of the oil of a VM loaded with nominal current will reach minus 30°C when the ambient-air temperature is minus 74°C. At the lowest ambient-air temperature of minus 50°C, the steady-state temperature of the oil in the VM tank will be minus 17°C. The viscosity of the oil at a temperature of minus 17°C, according to formula (5), will be 6 times greater than at 0°C.

Dependence of oil viscosity on temperature
Fig. 5

The density of the oil also depends strongly on temperature. The relation between the oil density and temperature is presented in Fig. 5. As can be seen from Fig. 5, the density of the oil increases linearly as the temperature is lowered. An analytical expression for this dependence can be represented by a formula of the form

\[ \delta = a - bt. \tag{6} \]

Influence of the quality of the lubricating material on VM operation

In order to determine the influence of the quality of the material used for lubricating the rubbing parts of the drive (solenoid armature, bearings), several types of lubricating material were tested. It was found that the quality of the lubricant has a great influence on the precision of operation of the switch and on the switching speed. Table 1 confirms this proposition.

The conclusion as to the best material for lubrication among those investigated is obvious from Table 1.

By the congealing temperature of the lubricant, noted in Table 1, is meant the temperature at which complete switching does not occur.

Table 1

of the switch when a current pulse is applied to the solenoid. The solidification temperature of tavot lies below minus \(41^\circ\mathrm{C}\). At this temperature the operation of the switch and of the drive lubricated with tavot took place distinctly.

Speed and Time of Motion of the VM Traverse as a Function of Temperature

These measurements were made with the aid of an electromagnetic vibrograph of very simple design.

The measurement of the time of motion of the traverse during closing \((\tau_{\mathrm{cl}})\) and opening \((\tau_{\mathrm{op}})\) as a function of the temperature of the outside air (oil) is presented in Figs. 6 and 7.

From Figs. 6 and 7 it is seen that both \(\tau_{\mathrm{cl}}\) and \(\tau_{\mathrm{op}}\) increase as the temperature decreases. The dependences in Figs. 6 and 7 were constructed with the drive and switch lubricated with tavot and with an operating-circuit voltage of 220 V. As the temperature decreases, an increase in the time of motion is observed.

Absolute increase—

Dependence of the oil density \(\delta\) on temperature.

Fig. 6

Dependence of the intrinsic closing time on temperature for the VM-35N with PS-20.

Fig. 7

Dependence of the intrinsic opening time on temperature for the VM-35N with PS-20.

Fig. 8

in the time of motion of the traverse during opening \(\tau_{\mathrm{op}}\) in the temperature interval from zero to minus \(41^\circ\mathrm{C}\) is 0.07 seconds, changing from 0.145 at \(0^\circ\mathrm{C}\) to 0.215 at \(-41^\circ\), i.e., by 50%.

Correspondingly, for \(\tau_{\mathrm{cl}}\) we have an increase of \(0.025''\) in the temperature interval between zero and minus \(36^\circ\mathrm{C}\); at a temperature of \(0^\circ\), \(\tau_{\mathrm{cl}}\) is \(0.150''\), and at \(36^\circ\mathrm{C}\)—\(0.175''\) (an increase of 16%).

Thus, down to a temperature of minus \(36^\circ\mathrm{C}\) during closing and a temperature of minus \(41^\circ\mathrm{C}\) during opening, the change in the time of motion of the VM traverse is insignificant, and under the worst conditions \(\tau_{\mathrm{cl}}\) remains less than the time guaranteed for this type of switch (0.25 sec.).

It is of interest to compare the temperature behavior of \(\tau_{\mathrm{cl}}\) and \(\tau_{\mathrm{op}}\) with the temperature behavior of the viscosity of oils. In Fig. 8 the dependence of \(\tau_{\mathrm{cl}}\) and \(\tau_{\mathrm{op}}\) on temperature is plotted.

As can be seen from Fig. 8, there is no direct dependence between the value of $\tau_{\mathrm{on}}$ and $\eta$, which should be considered quite natural, since in this case the motion of the crosshead occurs under the action of the powerful drive force and is fully determined by it.

A considerably greater influence of the viscosity of the oils should be expected on the switching-off time.

Figure 9 shows the dependence between the value of $\tau_{\mathrm{off}}$ and the oil temperature, and also between the value of $\eta$ and temperature.

As can be seen from Fig. 9, both dependences have the same course in the temperature interval $0$–$30^\circ\mathrm{C}$. Hence it follows that in this temperature interval the time of motion of the crosshead during switching off is determined to a considerable degree by the viscosity of the oil. The same conclusion was also drawn by A. K. Krasin [5], who had previously measured the dependence of the motion time of the VM-14 crosshead on temperature during switching off. The dependence of $\tau_{\mathrm{off}}$ for the VM-35N on the viscosity of the oil is shown in Fig. 10. From Fig. 10 it is seen that the increase in $\tau_{\mathrm{off}}$ lags behind the increase in the viscosity of the oil. The dependence of $\tau_{\mathrm{off}}$ on the magnitude of the oil viscosity may be weakened still further by increasing the force acting during switching off.

Fig. 9

Fig. 10

According to the results of our experiments, a dependence was constructed between the value of the maximum speed during switching operations and temperature. Figure 11 shows the dependence of the maximum speed of motion of the crosshead during switching on on temperature. As can be seen from Fig. 11, the maximum speed during switching on decreases only slightly as the temperature is lowered. In our case this change is characterized by the following values. At a temperature of minus $35^\circ\mathrm{C}$ the maximum speed is 10% lower than the maximum speed at a temperature of $0^\circ\mathrm{C}$. This circumstance is in agreement with the weak dependence, indicated above, of $\tau_{\mathrm{on}}$ on temperature.

Figure 12 shows the dependence on temperature of the maximum speed of motion of the crosshead during switching off. A decrease in speed is observed as the temperature is lowered and the viscosity of the oil increases. When the temperature is lowered from zero to minus $35^\circ\mathrm{C}$, the speed of motion decreases by 26%, and when the temperature is lowered to $-41^\circ\mathrm{C}$ the speed of motion decreases by 30%. From consideration of these data it is evident that the maximum speed during switching off decreases with decreasing temperature more slowly than the total time of motion of the crosshead.

The dependence of the maximum speed of motion on temperature, according to our data (Fig. 12), can be represented by the formula

$$ V_{\max}=2.7+0.5t, \tag{7} $$

where $V_{\max}$ is the maximum speed of motion of the crosshead during switching off in m/sec, and $t$ is the temperature of the air and oil in $\sim^\circ\mathrm{C}$.

It is essential to know the dependence of the closing time on the voltage at the drive terminals. We recorded this dependence at various temperatures. Figures 13 and 14 show two characteristic dependences of the intrinsic closing time on voltage at temperatures of minus 18°C and minus 23°C. The drive and the switch were lubricated with ta-

Dependence of the opening time \(t_{\text{open}}\), sec., on the viscosity of oil \(\eta\) in poises.

Fig. 11

Change in the maximum speed of traverse motion during closing as a function of temperature.

Fig. 12

vot. At a temperature of minus 23°C, under our operating conditions, the minimum closing voltage is 142 V. At a voltage on the drive of less than 142 V, closing does not occur at all. At drive voltages in the range from 142 to 160 volts, in 30% of the solenoid operations the switch does not close. Thus, a voltage of 160 volts should be regarded as the minimum voltage for reliable operation of the switch at an air temperature of minus 23°C.

Change in the maximum speed of traverse motion during opening as a function of temperature.

Fig. 13

Dependence of the intrinsic opening time on voltage.

Fig. 14

Determination of the minimum closing voltage at temperatures above minus 23°C showed a slight decrease in the value of the minimum closing voltage (to 138 volts).

The value of the minimum voltage corresponding to reliable operation of the switch during closing is 160 volts. Table 2 gives the re-

results of measuring the minimum pick-up voltage at various temperatures.

Table 2

Measurement of Insulation Resistance

The dependence of the drive insulation resistance on decreasing temperature is shown in Fig. 15. Here the air (insulation) temperature is plotted on the abscissa axis, and the insulation resistance in megohms is plotted on the ordinate axis. The insulation resistance was measured with a galvanometer at a voltage of 180 volts direct current. In Fig. 15, the points corresponding to the temperature dependence of the insulation of the closing and opening solenoids practically coincide. The points corresponding to the insulation resistance of the signaling circuit are absent from the drawing, since its resistance over the entire temperature range proved to be infinitely large. It follows from Fig. 15 that the insulation resistance increases as the temperature decreases; moreover, in the interval from minus 3°C to −35°C, the dependence between these quantities is well expressed by the formula

Dependence of intrinsic pick-up time on voltage.

Fig. 15

Dependence of drive insulation resistance on temperature.

Fig. 16

\[ R = 0.71 e^{-0.1t}, \tag{8} \]

where \(t\) is the insulation temperature in °C, and \(R\) is the insulation resistance in megohms.

Conclusion

As a result of the experiments carried out, it follows that, when the VM is loaded with operating current, the temperature near the contacts remains at a sufficiently high value.

The article gives formulas for calculating the thermal regime of the VM-35N under certain cooling conditions.

A considerable hazard to the normal operation of the VM is posed by an unfortunate choice of drive lubricant. In operation, appropriate attention must be paid to the selection of a lubricant with the necessary low-temperature properties and to maintaining the required calculated temperature of the drive. The article presents data showing the change in the travel time of the cross-arm during switching on and switching off as the temperature changes.

REFERENCES

1. A. A. Vorob'ev. Electric Stations. 2, 14, 1941.

2. A. A. Vorob'ev, V. N. Titov, and N. B. Bogdanov. Report on the work “Testing the VM-35N under low-temperature conditions.” Tomsk Industrial Institute, 1941, part II.

3. A. A. Vorob'ev and N. A. Prikhod'ko. Elektrichestvo. 8, 66, 1940.

4. Eng. V. N. Titov, postgraduate G. E. Pukhov, and postgraduate V. A. Vorob'ev. Report on the work “Testing the VM-35N under low-temperature conditions.” Tomsk Industrial Institute, 1940, part I.

5. A. K. Krasin. Elektrichestvo. 12, 52, 1939.

1. Preparatory work and the first measurements were carried out by graduate students G. E. Pukhov and V. A. Vorobyov. In carrying out the work we made use of consultations from senior engineer S. A. Chirov of the Elektroapparat plant, for which we express our gratitude to him. ↩