SELECTION AND APPLICATION OF GROUP OPERATED DISCONNECT SWITCHES David Childress, Marketing Manager-Power Switching Division Southern States, LLC 30 Georgia Avenue Hampton, GA 30228 phone: 770-946-4562, ext. 139 fax: 770-946-8106 e-mail: d.childress@southernstatesLLC.com website: www.southernstatesLLC.com Abstract—Group operated air disconnect switches are used in all types of substations—distribution, subtransmission, transmission, and extra high voltage (EHV). The correct selection of the type of group operated disconnect switch for a given application is key to the proper and desired performance of the installation, as disconnect switches are the substation component that isolates other pieces of substation equipment (such as circuit breakers, power transformers, etc.) when they need periodic, preventative, or emergency maintenance. This paper covers all of the types of group operated disconnect switches commonly used in the United States and Canada today, the various criteria which affect the selection of a specific type for a given installation, and the standards which govern these products. I. DEFINITION, FUNCTIONS, AND OBSERVATIONS Substation class group operated air disconnect switches can be defined quite simply as mechanical devices which conduct electrical current and provide an open point in a circuit for isolation of one of the following devices: circuit breaker, circuit switcher, power transformer, capacitor bank, reactor, or other. The three most important functions that disconnect switches must perform are: (1) to open and close reliably when called upon to do so, (2) to carry current continuously without overheating, and (3) to remain in the closed position under fault current conditions. The first important function listed above is taken for granted by some purchasers who deem that all disconnect switch types and their corresponding manufacturers are equal, thus reducing switches to a commodity item. In truth and point of logical fact, disconnect switches are the least commodity item of any substation, as they isolate the equipment which can be taken out of service when the equipment being isolated has a problem. The disconnect switch itself, if it has a problem, is very difficult to isolate from the system. This point has, in recent years, been erroneously overlooked by some key decision makers; making the selection of the correct disconnect switch for a given application a choice which should not be made before the decision maker is well informed about all of their options. II. STANDARDS WHICH GOVERN DISCONNECT SWITCHES Disconnect switches are governed by four key organizations—NEMA (National Electrical Manufacturers Association), ANSI (American National Standards Institute), IEEE (Institute of Electrical and Electronics Engineers), and IEC (International Electrotechnical Commission). Today each governs some specific aspects of disconnect switch design. NEMA governs wind loading design requirements and up until around 1971 also governed temperature rise and short circuit withstand requirements. Around 1971 ANSI/IEEE took over as the governing standard for temperature rise and short circuit withstand requirements. ANSI/IEEE also governs terminal pad loading requirements, altitude derating factors, seismic performance criteria, and switch overload capabilities. IEC governs all pertinent switch design criteria for switches manufactured outside the United States and Canada. III. COMPARISON OF GOVERNING STANDARDS AND OBSERVATIONS Although there are other differences between the various standards, their most significant differences as they relate to disconnect switches are their allowable temperature rise value and their required duration of short circuit withstand. NEMA standards allow only a 30 degrees C temperature rise; ANSI/IEEE standards allow a 53 degrees C temperature rise; and IEC standards allow a 65 degrees C temperature rise. NEMA standards require a 4 second short circuit withstand; ANSI/IEEE standards require a 3 second short circuit withstand; and IEC standards require only a 1 second short circuit withstand. For ease of comparison, these values are tabulated below. Table 1 Comparison of NEMA, ANSI/IEEE, & IEC Standards for Group Operated Disconnect Switches Allowable Temperature Rise Required Duration of Short Circuit Withstand NEMA 30 degrees C 4 seconds ANSI/IEEE 53 degrees C 3 seconds IEC 65 degrees C 1 second NEMA rated disconnect switches are the most robustly designed but are pricey when compared to ANSI/IEEE rated switches in most cases and are always very pricey when compared to IEC rated switches. For the criteria defined in the above table, if the disconnect switch can meet the NEMA requirements it can always meet the ANSI/IEEE and IEC requirements. Further, if the disconnect switch can meet the ANSI/IEEE requirements it can always meet the IEC requirements but cannot meet the NEMA requirements. IEC requirements are very marginal and thus the disconnect switches which meet only the IEC requirements are ill-suited for application in the Canadian and U.S. marketplaces which have high load currents, high fault currents, and highly developed electrical system networks. Disconnect switches designed to IEC requirements are best suited to countries which have light load currents, which have small fault currents, and which do not wish to have or do not require the same margin of safety desired and required in the U.S. and Canadian electric power grids. IV. TYPES OF GROUP OPERATED DISCONNECT SWITCHES There are six principal types of group operated disconnect switches used in the U.S. and Canadian electric power systems: vertical break, double end break, double end break “Vee”, center break, center break “Vee”, and single side break. Each of these has specific ratings, features, and characteristics which define the types of applications they are best suited for. IV.A. VERTICAL BREAK DISCONNECT SWITCHES Vertical break disconnect switches (See Figures 1 and 2.) are the most widely used group operated switch design. They are also the most versatile design, being easily adaptable for use with load interrupters, ground switches, and other accessory components. Vertical break switches can be installed on minimum phase spacing since their disconnect switch blades open upward rather than outward to the side. Due to their rotating blade design which pivots about its long axis, vertical break switches are excellent for application in environments which can experience large amounts of ice formation and are also excellent for installations which have large amounts of fault current due to their reverse loop contact design. The reverse loop contact design produces opposing magnetic fields in close proximity to each other, squeezing the disconnect blade in place in the closed position and holding it closed (See Figure 3.) so that the protective device (typically a circuit breaker or a circuit switcher) can clear the system fault. Vertical break switches encompass a ratings range of 15.5 kV through 800 kV; 600 Amps through 6500 Amps. Figure 1 A Horizontal Upright Mounted Vertical Break Switch Figure 2 A Vertically Mounted Vertical Break Switch Figure 3 Reverse Loop Contact Design IV.B. DOUBLE END BREAK SWITCHES Double end break switches (See Figures 4 and 5.) can be installed on minimal phase spacing, the same phase spacing that vertical break switches can be installed on; as the double end break switch’s disconnect blades, when in the open position, are disconnected from both the source and the load. In the open position the blade is not complete de-energized, but instead is at a floating potential of about 30% of system voltage. An advantage that double end break switches enjoy over vertical break switches is that they can be installed in locations which have minimal overhead clearance as the blades swing open to the side rather than lifting upward. An additional advantage that double end break switches enjoy over vertical break switches is that they do not require their blades to be counterbalanced as the blades are not lifted. (Vertical break switches typically require their blades to be counterbalanced at 69 kV and above.) Double end break switches are excellent for applications in environments which can experience large amounts of ice formation and are also excellent for installations which have large amounts of fault current due to their reverse loop contact design. In fact, double end break switches are even better performers in icy environments than vertical break switches are as the orientation of the jaw assembly of a double end break switch (a “C” configuration rather than the vertical break switch’s “U” configuration) reduces the amount of ice accumulation occurring in the jaw assembly area. Ice is inherently weak in shear but very strong in compression, making it much easier for any type of group operated disconnect switch to open under accumulated ice than to close into accumulated ice. Perhaps the second most overlooked performance characteristic of the double end break switch (behind only the fact that it can be installed on vertical break switch phase spacing) is that its design of two breaks per phase in series provides much greater line charging or magnetizing current interrupting capability with standard arcing horns or with quick break whip type arcing horns than does any single break type switch (Vertical break switches, center break switches, center break “Vee” switches, and single side break switches are all single break per phase type switches.). Double end break switches encompass a ratings range of 38 kV through 1100 kV; 1200 Amps through 4000 Amps. Figure 4 A Horizontally Upright Mounted Double End Break Switch In The Closed Position Figure 5 A Horizontally Upright Mounted Double End Break Switch In The Open Position IV.C. DOUBLE END BREAK “VEE” SWITCHES Double end break “Vee” switches (See Figures 6 and 7.) have all the same features and characteristics that double end break switches have but offer one additional and, for some installation situations, very key capability—the ability to not only fit on vertical break phase spacing but also to be able to fit on center break “Vee” switch style structures (single horizontal member structure with two or three vertical members [depending upon kV class]), producing the most compact disconnect switch installation possible. This capability can be particularly desirable when trying to shoehorn in a disconnect switch into an existing substation that was not originally designed to have one there, but can also be very desirable for use in any location where the cost of land is significantly high. Double end break “Vee” switches encompass a ratings range of 121 kV through 362 kV; 1200 Amps through 4000 Amps. Figure 6 Figure 7 A Pair of Horizontally Upright Mounted Double End A Horizontally Upright Mounted Double End Break “Vee” Switches In The Closed Position Feeding Break “Vee” Switch In The Open Position A Circuit Breaker IV.D. CENTER BREAK SWITCHES Center break switches (See Figure 8.) can be installed with the same minimal overhead clearances that double end break and double end break “Vee” switches can but need greater phase spacing than vertical break, double end break, and double end break “Vee” switches as when center break switches are in the open position one of the two disconnect blades per phase is still energized. An economic advantage of center break switches is that they only require a total of six insulators per three phase switch versus the nine required for vertical break switches, double end break switches, and double end break “Vee” switches. Like the double end break and double end break “Vee” switches the center break switch does not require a counterbalance for the blades as they swing out to the side rather than having to be lifted. A seldom realized characteristic of the center break switch is that it is the best available switch design for vertical mounting as its blades self-counterbalance each other on opening and on closing via the synchronizing pipe linkage which connects the two rotating insulator stacks on each phase. When a vertically mounted center break switch is opening the bottommost blade’s weight tends to push open the uppermost blade, and when closing the uppermost blade’s weight tends to pull closed the bottommost blade. Center break switches encompass a ratings range of 15.5 kV through 362 kV; 600 Amps through 6000 Amps. Figure 8 A Horizontal Upright Mounted Center Break Switch IV.E. CENTER BREAK “VEE” SWITCHES Center break “Vee” switches (See Figures 9 and 10.) have all the same features and characteristics that center break switches have but with one additional, space saving feature—the ability to be installed on single horizontal member; one, two, or three vertical member structures (depending upon kV class), producing the second most compact disconnect switch installation possible (second only to the double end break “Vee” switch). Center break “Vee” switches encompass a ratings range of 15.5 kV through 242 kV; 1200 Amps through 3000 Amps. Figure 9 A Horizontal Upright Mounted Center Break “Vee” Switch In The Open Position Figure 10 A Horizontally Upright Mounted Center Break “Vee” Switch In The Closed Position IV.F. SINGLE SIDE BREAK SWITCHES Single side break switches (See Figures 11 and 12.) can be installed with the same minimal overhead clearances that double end break, double end break “Vee”, center break, and center break “Vee” switches can but would need greater phase spacing than vertical break, double end break, and double end break “Vee” switches if the hinge end is energized when the single side break switch is in the open position as the disconnect blade would still be energized. The single side break switch enjoys the same economic advantage that center break and center break “Vee” switches do, six insulators versus the nine required for vertical break switches, double end break switches, and double end break “Vee” switches. Also, like all of the switch types except for the vertical break, the single side break switch does not require a counterbalance for the blades as they swing out to the side rather than having to be lifted. Single side break switches encompass a ratings range of 15.5 kV through 72.5 kV; 600 Amps through 2000 Amps; and are the most economical switch design available for 15.5 kV, 1200 Amps through 72.5 kV, 1200 Amps requirements. Figure 11 A Horizontally Upright Mounted Single Side Break Switch Providing An Isolation Point On The Secondary Side Of A Small Power Transformer At A Wind Farm Substation Figure 12 A Vertically Mounted Single Side Break Switch V. ADDITIONAL GENERAL GUIDELINES FOR DISCONNECT SWITCH SELECTION AND APPLICATION Thus far we have dealt with the kinds of installations that best fit the characteristics of each type of disconnect switch. At this time it is appropriate to address guidelines to prevent misapplication of certain types of switches and their accessories. Following are some key points that the decision maker should be aware of when designing an installation and when selecting the switch and its accessories for that installation. 230 kV rated double end break, double end break “Vee”, and center break “Vee” switches should not be installed in the vertical mounting position. 230 kV rated double end break and double end break “Vee” switches should not be installed in the vertical mounting position due to the strong potential for misalignment of the blades as a result of the blade’s weight and the weight of the 230 kV insulator that each blade pivots upon creating an overturning moment load that the switch’s rotating insulator stack bearing cannot compensate for completely. 230 kV rated center break “Vee” switches have two rotating insulator stack bearings per phase, each of which would have a significant overturning moment load imposed on them as a result of the weight of the blades and the weight of the insulators, creating even more potential for blade misalignment than on double end break and double end break “Vee” switches. (The problems mentioned here associated with vertically mounting these switch types at 230 kV do not manifest themselves at 161 kV and below.) 345 kV and higher kV rated switches of any type should not be vertically mounted as the weight of the live parts (hinge assembly, blade assembly, jaw assembly) and the weight of the insulators produces far too great an overturning moment load on the rotating insulator stack bearings to permit proper switch alignment. Manual operation of group operated disconnect switches via a swing handle operator should generally be limited to applications of 69 kV and below, 1200 Amps and below. For manual operation of group operated disconnect switches above 69 kV and/or above 1200 Amps continuous current, a manual gear operator should generally be used. If desired, it is also possible to select a combination manual operator which consists of a manual gear operator, a decoupler above the manual gear operator which allows the manual gear operator to be disconnected from the vertical operating pipe, and a manual swing handle operator above the decoupler. This combination manual operator is selected for applications where the effort required to operate the switch justifies a manual gear operator but the switch may need to be closed under iced conditions where speed on closing is essential to dislodge the accumulated ice. The required speed on closing necessary to break accumulated ice is generally not achievable via a manual gear operator but can be achieved using the manual swing handle operator while the manual gear operator is decoupled from the vertical operating pipe. This combination manual operator (See Figure 13.) is typically called a manual gear operator with auxiliary ice breaking swing handle and decoupler. Auxiliary Ice Breaking Swing Handle Would Be Inserted Into This Sleeve Figure 13 Manual Gear Operator With Auxiliary Ice Breaking Swing Handle And Decoupler (Swing Handle Not Current Installed Since Gear Operator Is Currently Coupled) Manual gear operators should generally not be installed on single side break switches as these switches are required to slam-seat the blade into the jaw contact to achieve full blade/contact engagement. Manual gear operators, as previously mentioned, do not generate enough speed to slam-seat the blade into the jaw contact. For group operated switches requiring motor operators, the higher the kV rating of the switch the slower the motor operator needs to operate. For example, a 4 second operating time motor operator is well suited for use on a 115 kV switch but is far too fast for operation of a 345 kV switch. Higher kV rated switches require slower operating time motor operators to maintain full control of the switch blades during the entire operational travel. The installation of a group operated switch should allow for 180 degrees of operating travel of the outboard bearing as the switch goes from full open to full closed and vice versa with an additional 5 degrees of overtoggle at the end of the opening and at the end of the closing operations (for 190 degrees of total travel). The 180 degrees of operational travel provides for a much better mechanical advantage on the switch than does any set up which has less than 180 degrees of operational travel, resulting in less effort being required to operate the switch. Quick break whip type arcing horns should not be installed on 230 kV switches due to the strong possibility of the whips generating corona (either visible and/or audible). [Quick break whip type arcing horns do not generally exhibit any corona problems at 161 kV and below.] Group operated switches which are required to interrupt charging current or magnetizing current using arcing horns (either standard arcing horns or quick break whip type arcing horns) should not be installed in the underhung mounting position as any arc drawn as a result of interrupting charging current or magnetizing current in air will travel upward as the air becomes superheated. This upward arc travel could track across the switch’s insulators since the insulators are above the live parts on an underhung mounted switch. All group operated switches equipped with group operated ground switches should have the switch vertical pipe and the ground switch vertical pipe interlocked in some fashion; using either a mechanical cam-action interlock, a pair of key interlocks, or some other type of interlocking scheme; to prevent the possibility of closing the switch when the ground switch is closed and to prevent the possibility of closing the ground switch when the switch is closed. If the switch and its integral ground switch are both manually operated then the interlocking scheme need only be able to prevent manual closing of one when the other is closed; but if either the switch, the ground switch, or both are motor operated then the interlocking becomes more complex. If the switch is motor operated and the ground switch is manually operated then the switch must be blocked via interlocking from being closed either manually or electrically if the ground switch is closed. If the switch is motor operated and the ground switch is motor operated then each must be blocked via interlocking from being closed either manually or electrically if the other is closed. In some instances where switches are required to be installed in contaminated environments (salt spray, cement dust, etc.) there is a tendency on the part of the purchaser to want to use a higher kV rated insulator on the switch (for example, a 54 inch tall 138 kV rated insulator on a 115 kV rated switch designed for a 45 inch tall insulator) to increase the insulator’s creepage distance to compensate for the degradation of the line-to-ground insulation as a result of the contaminated environment. If this is done then the inherent coordination of having a greater open gap distance on the switch than its phase-toground distance could be defeated completely and, at a minimum, will be marginalized. The potential consequences of this are dire, as if the open gap dimension of the switch is less than the phase-to-ground distance then a flashover, if one occurred, would take the shortest distance, jumping the open gap of the switch rather than going phase-to-ground. An open gap flashover with personnel working on the piece of substation equipment that the switch is supposed to be isolating could result in personnel injury or death. To avoid this it is necessary to re-establish the inherent coordination of the open gap distance and the phase-to-ground distance. This is accomplished via the use of spill gaps (also known as rod gaps, arc gaps, and spark gaps). Spill gaps are bolted on either below the bottom of the insulator, above the top of the insulator, or both below the bottom of the insulator and above the top of the insulator; and they make the insulator look electrically the same height that it would otherwise look electrically if it were furnished with the correct height insulator for that kV rating. A better solution to handle contaminated environments that does not produce this hazard to personnel and which does not require spill gaps to be used is to choose resistance glazed insulators. These insulators have a semi-conductive coating baked into them during the firing process of making the insulators. This semi-conductive coating creates a small leaking current across the insulator, heating the insulator and preventing moisture from accumulating on it. If moisture were to accumulate on the insulator the airborne contaminants would be even more likely to deposit onto the insulator, creating a greater potential for flashovers. VI. SUMMARY The chart below summarizes information previously outlined in text form regarding the various features and characteristics of the group operated disconnect switches covered in this paper. Table 2 Summary of Features & Characteristics of Group Operated Disconnect Switches Vertical Break Double End Break Double End Break “Vee” X X X X X X X X X X X 1 2 9 Installable on minimal phase spacing Installable with minimal overhead clearance Excellent choice for icy environments Excellent choice for sites having high fault currents Number of breaks per phase Number of insulators Does not require a counterbalance Excellent at minimizing structure cost and land space consumed Center Break Center Break “Vee” Single Side Break X X X 2 1 1 1 9 9 6 6 6 X X X X X X X VII. CONCLUSION The selection of the proper group operated disconnect switch is key to the success of any substation’s long term reliable performance. Many different types are available to meet the varied needs of each specific installation. Considerable care should be taken when choosing a group operated disconnect switch to meet a given installation’s needs. If the decision maker is uncertain about which offering best meets their required criteria for a specific installation then it is highly recommended that the switch manufacturer be consulted for guidance as both the purchaser and the seller have a strongly vested interest in assuring that the chosen product completely meets the requirements and expectations of the installation; providing the desired long, trouble-free operational life needed from all disconnect switches. VIII. BIOGRAPHY David Childress received his Bachelor of Science degree in Engineering from Mississippi State University in 1991. He joined Siemens Energy & Automation in 1991 as an application engineer responsible for circuit switchers and disconnect switches and later joined Southern States in 1997 holding positions of regional manager, international sales manager, and product manager. He is presently the marketing manager of Southern States Power Switching Division responsible for all Southern States SF6 products including circuit switchers, load and line switchers, and capacitor switchers. He is a member of IEEE; a multi-published technical paper author; an author of over 100 catalog flyers, catalog bulletins, and other technical/product related documents; and has recently co-authored a chapter for Electric Power Substations Engineering Book-Second Edition entitled “High Voltage Switching Equipment”.