IEEE Standards

ANSI/IEEE C37.04-1979¹ and ANSI/IEEE C37.010-1979² are the standards relating to the rating structure and application of circuit breakers. There is no specific advice concerning low temperature. The standard service conditions are covered in C37.04¹, which includes altitude less than 1000m, and a temperature range of -30°C to +40°C. Other features are covered as well, from seismic to frequency. There is reference in C37.04.2.3.3¹ to low temperature as an unusual operating condition. And -60°C certainly is unusual.

SF₆ Circuit Breakers and SF₆ Basics

SF₆ has unique properties which render SF₆ a nearly ideal media for arc interruption and dielectric strength. The dielectric strength is greater than any other known media at the same density. The reason lies in the relatively large physical size and mass. The molecular weight is 146. Nitrogen and Oxygen are 28 and 32 respectively. The size and mass help reduce the propagation of free electrons. SF₆ also acts as an inelastic damper for collision mechanism. SF₆ is in the same class as freons which all have excellent dielectric properties. SF₆ has unusual thermal properties which contribute to its arc interruption. SF₆ has intrinsically higher thermal conductivity at low temperatures. Figure 1, which is a plot of the thermal conductivity in Watts/cm2/K° of SF₆ and N2 and temperature. The existence of this peak in conductivity results from a change of state of the gas. The disassociation requires a well defined amount of energy, much like the heat of vaporization. This enhances the thermal transfer at arc, helping reduce the arc temperature. Conversely in lower ambient the elements recombine and liberate this energy, transporting copious amounts of energy across the electric arc.

Figure 1

In other diatomic gases like O2 and N2, SF₆ becomes conductive when ionized. SF₆ remains basically an insulator during the disassociation of the arc, further aiding the dielectric and interruptive properties. The disassociation of SF₆ produces Fluorine, the most electronegative element, at 3.95, in the periodic chart (Figure 2). This enhances the capture of free ions, and capturing free ions reduces conductivity. As the temperature of the arc is reduced, Fluorine ions, with less than 1 percent the mean free velocity of electrons, capture free electrons and reduce the current density significantly. These two properties, high electro-negativity which results in high electron capture, and high thermal conductivity at relatively low temperatures, give SF₆ its ‘made for electrical apparatus’ properties.

Figure 2

Although the gas is nearly ideal, it does have one major disadvantage. The main disadvantage, from our perspective, is the relatively high liquefaction point of SF₆. Figure 3 The liquefaction occurs between -10°F and -30°F, depending on the density (70psig to 95psig ranges found in circuit breakers manufactured today³) of the SF₆. Here we have somewhat conflicting requirements; the need to extinguish the arc and provide dielectric strength, and the physics of SF₆. The higher densities and pressures increase the liquefaction temperature, a very unsatisfactory side effect. In locations where the wintertime temperatures fall below -10°F, for even an overnight excursion, the manufacturer or user must provide supplement means to maintain the SF₆ above -10°F. Figure 4 Sometimes the choice is made to maintain the gas warmer to allow a prudent reserve heat mass as a safety margin.

Figure 3

Figure 4

Reflecting on our operational experience, it would appear that cold weather testing to date does not adequately reflect environmental conditions. In many cases, Minnesota Power and other northern climate electric utilities have provided the test laboratories for cold weather.

Incidents in February 1996

The normal learning curve for this apparatus and problems associated with all the various styles are manageable, taken during the normal course of maintenance. Operations and Maintenance help to clarify the situation and the problems are remedied on a regular basis. Even with a few alarms on an overnight temperature sag, after operators gain experience with the thermal recovery of the tanks and density monitors, the situation is tractable. However, in the period January 19 through February 2 our service territory experienced temperatures in excess of -50ºC and several nights the temperature sagged to near -60°C. On the morning of February 2, MP had 22 SF₆ circuit breakers in alarm and three, 134L, 115MW and 762L, were locked out. The three that were locked out were all at the International Falls 115/15kV Substation. International Falls experienced -45°C that morning with a wind chill of -80°C. MP has produced composite charts from U.S. Weather Service data (5) of air temperature, average wind speed and nominal wind drill for three locations in our service territory (see Figure 5).

Figure 5

This station provides service to International Falls, Boise Cascade and the surrounding area, and is the interconnection with Ontario Hydro at Fort Frances. Any event which causes a transfer trip or breaker failure and cannot be cleared at the site has serious consequences to customers. MP serves International Falls and several smaller communities from this substation, as well as several major customers. An outage at this time of the winter is very undesirable. A disturbance on the line could have meant breaker operations at Little Fork/Running 230kV Substation. MP is relying at Little Fork 230kV Substation, as well, for SF₆ breakers to operate.

To bring the International Falls substation back into service would have posed severe hardships. All systems, because of the extremely low ambient and wind chill, would cool quickly. Maintaining heat in the control house would have been difficult. At temperatures below 45°F, the capacity of lead acid batteries is reduced significantly from the nominal 77°F. IEEE C57.12.00-1993⁴ section 4.1.2.2 and 4.1.2.3 outline the standard operating and design conditions for transformers, and top oil temperatures or starting temperatures below -20°C (-4°F) are not considered usual operating conditions. Unless specified and designed to meet the lower starting temperatures, there may be restrictions on operating and loading the transformer. During this period, Duluth experienced 13 days in a row where the minimum temperature was -10°F (-23.3°C) to -39°F (-39.4°C)⁵. What is the best means to start a transformer at these temperatures? Many of the vendors recommend heating the core and coils. At the time of this event, MP was not prepared for this scenario. MP does have procedures in place for supplement heating should such an event occur.


Cold Climate Problems

Solutions

Operation

We began seriously to consider the effects of a failure to operate and failure of a breaker. We quickly realized that, as the breaker approached the magic alarm set point, the functionality of the circuit breaker didn’t vanish. Rather, under reduced gas pressure, the capacity of a circuit breaker to interrupt is diminished and a likewise similar condition for the dielectric strength. In reality, at lockout the interrupting capacity on our 115kV class of circuit breaker was reduced about 10 percent 6. The SF₆ breaker vendors of Minnesota Power indicate that their circuit breakers can withstand system voltage at 15psig⁷, and in some cases interrupt load current. Minnesota Power has procured exclusively 40kA circuit breakers over the years. At 10 percent reduction in interrupting capacity there still remains adequate margin, except at a few sites.

We investigated the failure mechanism of SF₆ circuit breakers. The vendors were not excited to discuss this condition. Apparently, there has not been any factory testing to simulate this event or anything similar. And most likely, failures have not been communicated to the vendors. The case we proposed was a breaker with a gross breach of the tank so the gas pressure was 15psig. Our concern was not for the final condition of the breaker but the failure mode. We were concerned about the self destruction of the tanks. Violent damage to the tank or porcelain could damage nearby apparatus or harm nearby workers. We were not able to learn of any catastrophic or eruptive failures of the designs in service at Minnesota Power. The worst case we uncovered was a utility operating, under load, a SF₆ circuit breaker with nearly atmospheric pressure in the tank. In this case there was a complete burn through of the tank wall.

Minnesota Power does not envision operating, and has not operated, breakers in this condition. However, we do plan to utilize fully the circuit breaker capacity to avert a possibly more harmful situation. We are willing to assume the risk of breaker damage or self destruction. Minnesota Power is prepared for damage to a circuit breaker if necessary to protect apparatus or reduce the risk of a more extensive outage. We are actively investigating means and methods to avoid this situation. We believe a greater risk exists in having inoperable circuit breakers during such an event. Warrantee claims were not a consideration.

Physical Changes

Figure 6

System Design Parameters

Objectives

• Total system to reduce maintenance
• Permanent installation
• Reduce or eliminate alarms and lockouts
• Easily retrofitted to various breaker vendors
• Materials, resistant to UV, solvents and non hydroscopic
• Standard available components
• Installed at most voltages and designs with breaker energized
• Inert chemically, electrically and aesthetically pleasing, as practical
• Heater and insulation system capable of providing 30°C delta heat input
  

The heater is silicone rubber design, presently in use by OEM breaker vendors. The heater is designed with a very low watt density less than 2 watts/in2. In discussion with the supplier of the heating system, the threshold value to assure 20 operations of the heater was a maximum of 2.5 watts/in.2⁶.

The controls are manufactured as a complete system with PID controller with optional heater circuit continuity annunciation. Heater control is a type K thermostat mounted on the center phase. Each heater element is protected by an over temperature relay.

Results

The objective is to show that the heat maintenance system provides four benefits:

1. More uniform circuit breaker tank temperatures.
2. Less total heat required to maintain SF₆ at optimal temperatures.
3. Eliminate all nuisance alarms.
4. Provide longer term operating history to satisfy that system does not deteriorate breaker performance in any manner.

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