A RETROFIT REPLACEMENT CIRCUIT BREAKER

Transcription

1 WHEN TO CONSIDER A RETROFIT REPLACEMENT CIRCUIT BREAKER BY PAUL H. GREIN, Circuit Breaker Sales Co. Inc. It is no secret that the majority of America s infrastructure is old. Speak with any field service technician, and you will hear a story about some piece of gear they recently worked on that Edison or Westinghouse must have commissioned himself. While it may be an exaggeration, it is commonplace for air-magnetic and even oil circuit breakers to be in service decades beyond their 20-year design life. The longevity of air-magnetic mediumvoltage circuit breakers especially General Electric s Magne-blast, Westinghouse s DH and DHP, and ITE s HK is a testament to the reliability of the original designs, materials, and manufacturing methods, as well as the performance of those that maintain them today and over the last 40 years. However, regardless of how well equipment is maintained, time eventually takes its toll. Eventually, maintaining antiquated switchgear becomes impractical and replacement options look appealing. This article discusses the options when that time comes, and specifically, when a retrofit replacement breaker should be considered versus the other options available. WHAT IS A RETROFIT, ANYWAY? The first logical step in discussing retrofit replacement circuit breakers is to define them. 52 FALL 2017

2 to provide a roll-in replacement for its legacy equipment. The primary advantages of direct replacement circuit breakers include maintaining interchangeability with existing equipment, allowing one breaker to be replaced at a time, and minimal downtime required during installation. While a commissioning procedure is recommended, in most cases, the legacy breakers can be racked out and the new breaker in using standard operating and safety procedures. Figure 1: Circuit Breaker Conversion Solutions Defining and explaining the various methods of converting switchgear is a subject unto itself, but basically, a retrofit is the product/ process of altering an existing power switchgear equipment design from any qualified design. While retrofit is the commonly used term in the industry, the IEEE accepted terminology is conversion, as there are many ways to convert a qualified design, retrofit being one of them. The following list defines and summarizes the methods switchgear conversions are performed. Roll-in/Direct Replacement A roll-in or direct replacement is, arguably, not a conversion. A direct replacement is a new circuit breaker that installs directly into the existing cubicle with no (or very simple) modifications. The breaker racks into the switchgear line-up and correctly interfaces with the existing compartment cell. The original racking mechanism, safety interlocks, primary/ secondary/ground disconnects, and MOC/TOC switches inherent in the original equipment design are maintained. The switchgear structure is not modified. While not always the case, it is common for the original equipment manufacturer (OEM) The primary disadvantages root from the original switchgear. Since the original switchgear components are maintained, if existing issues are related to the cubicle such as the unavailability of component s or misalignment of the primary disconnect, accessories, or interlock systems a direct replacement circuit breaker will not correct them, and in some circumstances, may amplify them. Retrofit (Conversion) A retrofit conversion, as it applies to mediumvoltage applications, is nearly identical to a rollin/direct replacement conversion so much so that the terms are often used interchangeably. The primary difference between the two conversions is the methodology used to qualify the design. Though used in an existing switchgear design, a roll-in/direct replacement breaker is completely new, requiring a medium-voltage roll-in direct replacement design tested in accordance with C37.09, the IEEE Standard Test Procedure for AC High-Voltage Circuit Breakers. A retrofit (Figure 2), though similar in construction, may use components from the existing breaker, such as the frame, mechanism, Figure 2: Retrofit Conversion Circuit Breaker NETAWORLD 53

3 or some portion of them. An original qualified design is converted, requiring a retrofit design to be tested in accordance with C37.59, the IEEE Standard Requirements for Conversion of Power Switchgear Equipment. C37.59 defines and specifies the qualification testing required depending on the nature of the conversion. For example, if the circuit interruption means is changed, the applicable design verification requirements of C37.09 are referenced. The certification testing is piece-wise, dependent on the nature of the conversion. The primary advantages and disadvantages of retrofits are very similar to those for roll-in/direct replacement conversions. Retrofill During a retrofill conversion (Figure 3), the existing switchgear structure and bus are modified to accept a new circuit breaker by installing a new cradle into the system. There are two primary retrofill methods. Commonly, a new cradle is hard-bussed into the system s existing main bus. Alternatively, in a cradlein-cradle retrofill, the cradle is installed with connections interfacing with the existing primary disconnects, i.e. the new cradle is permanently racked in. In both retrofill methods, all new Figure 3: Retrofill Conversion racking mechanisms, primary/secondary/ ground disconnect connections, and MOC/ TOC switches are installed in the existing cell. The primary advantage of a retrofill conversion is utilizing the circuit breaker design of the OEM s standard product offering, making replacement breakers, parts, and support readily available. The breaker is unmodified, and thus maintains the original design qualification. The primary disadvantages are that the end-user loses interchangeability with existing breakers (unless the entire lineup is upgraded). Further, it is generally the most expensive conversion option, as it requires complete de-energization of the switchgear during the lengthy conversion process. While employed in medium-voltage, retrofill conversions are more common in lowvoltage applications. One of the challenges with discussing conversions is that different groups use varying terms to mean the same thing. Now that we are on the same page as far as terminology, take a look at the factors considered and the challenges faced during the conversion design process. VACUUM RETROFIT DESIGN PROCESS AND CHALLENGES There are many different types of conversions, but the majority of medium-voltage applications today consist of converting an air magnetic circuit breaker to an equivalently rated vacuum circuit breaker, and that is the focus of this article. As discussed, the two primary means of medium-voltage conversion are retrofill and retrofit. This section presents a streamlined explanation of a complicated process. It can guide the recommendations you make to your customer and assist troubleshooting efforts if the reader can gain an understanding of the challenges faced during the conversion process. The challenges associated with retrofills are generally not with the design; rather, they are presented during installation. In a retrofill, the switchgear is stripped, and the new cubicle is installed in the shell in most cases, the new breaker cubicle is hard-bussed into the system. 54 FALL 2017

4 While retrofill kits that make the process a straight forward step-by-step procedure are available for low-voltage switchgear, they seldom exist in medium-voltage applications. Note that the custom nature of fitting the new cubicle into the existing cell, fitting the bus to it, and wiring the controls in a plant setting is challenging. Outages are short, making preplanning paramount to successful retrofill conversions. Coordinating multiple outages one for assessment and at least one additional for installation is a cumbersome process for most utilities and industrial users, especially in medium-voltage installations. As the methodology varies between retrofill and retrofit conversions, so do the challenges associated with them. Perhaps the best way to introduce the design challenges is to give an overview of the retrofit process, from start to completion. What follows is a hypothetical but typical scenario. Midwest Utility Co. calls its local field service company, ElectroServ Midwest, because it is experiencing issues with their medium-voltage equipment again. ElectroServ arrives on the scene and quickly identifies the challenge. The breakers were manufactured by Centralized Atlantic Equipment Co., an obscure manufacturer that was only around for a decade, 40 years ago, and thus had a very limited installed base. To make matters worse, the breaker has high interrupt and continuous current ratings. The combination of an obscure manufacturer and rare ratings will make sourcing a replacement on the surplus market nearly impossible. Their spares (or the corpses of their spares) sit in the corner, having been robbed of parts over the last few decades to make the in-service breakers serviceable. Even if the utility could shut down long enough to replace the switchgear with new or to pursue retrofill options, the substation is on a mezzanine three floors up with plant expansions built around it. Keeping all of these factors in mind, ElectroServ makes the recommendation to install vacuum retrofits and starts gathering the necessary information for the converter to bid the project. The majority of the information needed to evaluate a circuit breaker for conversion is listed on the circuit breaker nameplate: Manufacturer Type Rated Voltage (kv) Rated Maximum Voltage (kv) Rated Impulse Voltage (kv) Frequency (Hz) Rated Continuous Current (A) Rated MVA Rated Short-Circuit Current (ka) Asymmetrical Rating Factor (k-factor) Rated Interrupting Time (cycles) Control Voltage (V AC /V DC ) In most cases, the breaker nameplate data is sufficient information for a retrofit provider to quote a replacement breaker, assuming it is a model that they have provided in the past and are familiar with. However, it is always a good idea to send photos of the switchgear cubicle (with the breaker removed) along with orthogonal views (front, rear, sides) of the breaker. Even if a retrofit conversion has been developed and in theory, the OEM designs are consistent there are sometimes optional accessories and site-specific differences between installations that interfere with standard designs. Eventually, the circuit breaker control schematic will also be required. The process of designing a retrofit replacement circuit breaker is simple in concept but challenging in execution. The stages of design include choosing a suitably rated replacement breaker, designing the interfaces between the original breaker and the new unit, and design verification testing. The first stage, choosing a suitably rated replacement breaker, is a straightforward process. The ratings for the replacement breaker must meet or exceed the corresponding rating of the existing breaker. For example, if the original airmagnetic circuit breaker is rated at a maximum voltage of 5 kv and 1200A continuous current, the replacement vacuum circuit breaker must be rated at least 5 kv and 1200A; however, a NETAWORLD 55

5 7.2 kv, 2000A-rated breaker could also be used. The MVA and short-circuit ratings are evaluated similarly, but two additional factors must be considered: the constant MVA to constant ka rating differences and the k-factor. In , circuit breaker ratings changed from a constant MVA to a constant ka rating method. This is important to consider in retrofit applications, as some constant MVA-rated equipment did not give the short-circuit rating in ka; it requires calculation. For example, a 15 kv, 500-MVA circuit breaker has a rated interrupt current of 19 ka at the maximum 15 kv design voltage: Note that this calculation is performed at the maximum design voltage. In the majority of constant MVA-rated circuit breakers, the physics of arc interruption are such that a given circuit breaker can interrupt a higher current at a lower voltage. However, there is a limit to how high the interrupt current can be increased, and that limit is defined by the k-factor. The k-factor is a dimensionless number that defines the range of voltage over which the interrupting current increases as the voltage decreases. In the 15kV, 500 MVA-rated circuit breaker example, if we include the k-factor rating of 1.3, the 19 ka interrupt current rating can be increased to 25 ka by lowering the maximum allowable voltage to 11.5 kv. When choosing a replacement circuit breaker for conversion applications, ensure that k-factor is considered and the maximum interrupt current is met or exceeded in the new design. Once suitably rated replacement circuit breakers are identified, the next step in the conversion process is to evaluate the best option from a mechanical perspective and begin the conversion process. The most obvious consideration is that the replacement circuit breaker must fit on the existing frame (or the new frame, if it s a direct replacement); in most cases, this is not an issue. The vast majority of medium-voltage conversions are air-magnetic to vacuum interruption. Vacuum circuit breakers are compact compared to their air-magnetic counterparts of equivalent rating. However, the lower kv/mva rating combinations can be a challenge, as the legacy frames are narrow (typically around 18 inches wide), and many modern vacuum circuit breaker designs have standardized across the 5 15 kv voltage ratings and are thus wider than necessary from a design limitation perspective. When a breaker has been identified, the conversion process can begin. The breaker is first mounted to the rolling chassis; then the position interlock and racking mechanism, spring discharge, primary disconnect and insulation system, secondary disconnects/controls, blast shield, and MOC systems are all integrated into the conversion. All of the systems have unique design challenges and risks that a technician commissioning or working on the conversion should be aware of. Position Interlock System The purpose of the position interlock is to prevent the insertion or removal of a closed circuit breaker in and out of the connected position. The position interlock interfaces with the circuit breaker racking and operating mechanisms. Typically, in medium-voltage applications, the position interlock locks the racking mechanism when the breaker is closed and results in tripfree operation unless it is in the connected, test, or disconnect positions. The conversion must match the fit and function of the original design, preferably with minor modification and without adding additional mechanical load on the new mechanism. Technicians should be familiar with the operation of the original position interlock system to ensure the conversion functions identically. The technician should also ensure that when the racking mechanism is locked, such as for a lock-out tag-out procedure, the breaker is held in the trip-free operating condition. The risks associated with improper conversions of the position interlock system are severe. Racking a closed circuit breaker into 56 FALL 2017

6 or out of the connected position can result in an arc-flash event causing, equipment damage, injury, and death. Spring Discharge System The purpose of the spring discharge system is to ensure that when the circuit breaker is removed from the switchgear, all stored mechanical energy is absent. Typically, this is accomplished by holding the breaker in a trip-free operating state by engaging the trip latch and operating the close latch, thus releasing the energy from the close and open springs. Similar to the position interlock, the conversion must match the fit and function of the original design, preferably with minor modification and without adding additional load of the new mechanism. Technicians should be familiar with the operation of the original spring discharge and ensure the conversion functions identically. Technicians should pay particular attention to conversions that involve updating a solenoid-operated to a spring-charge mechanism, as there are no provisions in the legacy cubicle to trigger a spring discharge system. Converters will typically try to work around the issue by integrating the spring discharge with the racking mechanism, but this solution is not always feasible. In the cases where a workaround solution cannot be implemented, a label warning the operator of the risks is usually employed. The technician should be aware of the risks of a malfunctioning or absent spring discharge mechanism and ensure all mechanical energy is discharged prior to working on the circuit breaker. Primary Disconnect and Insulation System The purpose of the primary disconnect and insulation system is to make the high-voltage connections between the circuit breaker and the switchgear and isolate the primary voltage between phases and from phase to ground. The conversion process consists of aligning each of the line- and load-side connection locations of the legacy circuit breaker to the new breaker. The converter must ensure that when the converted breaker is in the connected position, the primary disconnect alignment matches the legacy unit in all three axes: horizontal (x-axis), vertical (y-axis), and depth (z-axis). The insulation system must match the fit and function of the original design without creating interference with the cubicle. The technician should be aware of the risks associated with converting the primary disconnect system. If the primary disconnects are not properly aligned in the x- and y-axes, it can introduce interference issues that prevent insertion of the breaker into the connected position. If the depth of the primary disconnects does not match the original, it can also cause interference or even prevent sufficient wipe across the stationary cubicle connections. If the primary disconnects are not correctly aligned and isolated from ground via the insulation system, a life-threatening arc flash from phaseto-phase or from phase-to-ground can occur. Insufficient wipe and/or inadequate contact pressure on the primary disconnects can also lead to catastrophic failures and should be verified during commissioning and periodically over its serviceable life. Secondary Disconnect/ Controls The purpose of the secondary disconnect is to make the control power connections between the circuit breaker and the switchgear. Similarly, the ground contact grounds the circuit breaker frame to the switchgear ground. The conversion process consists of ensuring the secondary disconnect matches the fit, form, and function of the original design. From the control standpoint, the breaker must function identically to the original breaker. The primary challenges are matching the original control schematic and ensuring the current draw of the new control components do not exceed those of the original (this is rarely an issue but is considered). There is also a special consideration for converting a circuit breaker equipped with a solenoid-operated mechanism to a spring-charge design. Spring-charge operating mechanisms NETAWORLD 57

7 require a constant supply of control power, whereas solenoid-operated do not. Though easily performed, the conversion will require modification to the existing switchgear control scheme to accommodate the charging circuit. Though seemingly trivial, locating the original control schematic is often problematic. Over the course of 25 years or more, the original documentation is often lost or outdated. Reverse engineering the control schematic from the converted breaker is sometimes required. The alignment of the ground contact is generally not a concern in retrofit applications, as it is mounted directly to the frame and is usually remanufactured and reapplied. The risks associated with the secondary disconnect and controls are less serious than the other systems but are the most common. Issues converting the secondary disconnect are easily avoided by attentiveness during the informationgathering phase of the conversion process and electrically testing/cycling the converted breaker prior to installation. MOC System An MOC is a mechanism-operated cell switch located in the switchgear cell but operated by the circuit breaker mechanism. It provides extra or redundant contacts for breaker status indication and other control functions. When equipped, an MOC operator on the circuit breaker engages an MOC in the switchgear directly, or more commonly, via a mechanical assembly. Driving legacy MOC systems with modern mechanisms is one of the most challenging barriers to overcome during the conversion process. Older switchgear MOCs were designed for the technology of the day. Air-magnetic circuit breaker mechanisms were massive with high forces and inertias, which resulted in slow travel times and velocities. Legacy MOC switches were designed to be robust. In a typical conversion process, the MOC actuator is the only factor that affects the dynamics of the new circuit breaker, as it alters the mechanical load the mechanism must drive. Modern vacuum breakers are smaller, lighter, and compact. Vacuum contacts travel a much smaller distance than their air-magnetic counterparts and at a much faster speed 10 times faster on average. These force, inertia, velocity, and travel time mismatches between legacy and modern MOC systems present a number of challenges for converters. In addition to matching the fit, form, and function of the original MOC system, the conversion has the potential to change the operating speed of the new mechanism. The increased energy can cause MOC switch overtravel, bounce, wear, and damage. Since the MOC is installed in the switchgear and not the breaker, it presumably will not be replaced during the conversion process. Poor MOC condition has the potential to be compounded by the converted circuit breaker. Some installations choose to replace aging MOC systems with relaying. The design and operation of legacy MOC switches varies considerably between OEMs, as do MOC conversion solutions. Of all the challenges associated with the conversion process, the risks associated with MOC systems are the most important for technicians to consider and mitigate. Worst case, a badly worn or broken MOC in the switchgear can stall the breaker, leading to a catastrophic failure. Technicians must ensure MOCs and their actuators are operating correctly and are in good condition, and must take them into account when recommending and working on converted equipment. Design Testing ANSI Standard C37.59, IEEE Standard Requirements for Conversion of Power Switchgear Equipment governs the requirements for conversions of previously qualified equipment. The standard was created in 1991 to address the lack of design verification of some conversions. Many forms of conversion exist; several have been discussed in this article. The standard provides the logic that should cover all forms of power switchgear conversion existing at the time of its publication. The latest version was published in 2007, but it is 58 FALL 2017

8 currently undergoing revision by the IEEE PES Standards Committee and will be published within the next year. The latest revision adds conversions that were introduced since 2007, such as the addition of infrared inspection windows and remote racking devices. The document is organized by the category of equipment, then by type of conversions. For each conversion method, the standard instructs which portions of the original design verification testing to perform. Here is an excerpt from the standard for the process described in this section: Section b) Conversions utilizing a modular assembly may require alterations to the original circuit breaker frame to mount the modular assembly as well as to provide connections to the existing or new primary bushings, and the conversions may require revisions to insulation structures or components. The modular assembly shall be subjected to the complete series of design tests in accordance with IEEE Std C The design test data from the modular assembly tests may be utilized for design verification if clearly applicable. Additional design tests shall be made on the complete conversion and shall include dielectric withstand, momentary current, shorttime current, continuous current, interlock, and other operational tests including tests to verify correct function with MOC switch assemblies, if applicable (see and IEEE Std C ). When a conversion is performed and design verified in accordance with the latest revision of C37.59, the risks discussed in this section have been mitigated but not eliminated. Conversions should only be performed by a qualified provider with the necessary knowledge, capability, and experience. With an understanding of the primary challenges encountered during the retrofit process, you should know when and which form of conversion is a suitable option. Conversion, however, is not the only option available. As equipment ages, you can remanufacture it, replace it, or convert it. The next section discusses when a conversion should be considered. WHEN TO CONSIDER A MEDIUM-VOLTAGE CONVERSION The previous section touched on when a medium-voltage conversion should be considered. To expand on and address this question, consider the product lifecycle of medium-voltage switchgear. The first stage in the product life cycle is the commercialization period. The commercialization period is when the switchgear is launched onto the market and is offered for sale through the OEM s approved sales channel. This discussion focuses on air-magnetic to vacuum conversions. The average commercialization period for air-magnetic switchgear was years (modern equipment is much less, averaging years). There are no mediumvoltage air-magnetic switchgear models still in the commercialization period (though airmagnetic switchgear may still be produced for special applications). For the sake of discussion, if the switchgear were still offered by the OEM, a conversion would not be a recommended solution except in rare circumstances where the switchgear is of poor design, was incorrectly applied, or the application requirements have changed. The next stage in the product life cycle is the support period. Following the commercialization period, OEMs offer support through parts and service. The length of the support period varies between OEMs and models, but on average, medium-voltage equipment is supported for years. Many models, however, are supported far beyond that especially those with the highest market share (largest installed base), as well as those used in special applications like the nuclear industry. Once the OEM withdraws full parts supply for its equipment, it is considered at the end of its lifecycle. However, a product at the end of its life cycle does not mean end of service. At the end of the product lifecycle or even during the support period, the OEM will often sell the intellectual property to what becomes the authorized aftermarket solutions provider that NETAWORLD 59

9 will continue to support the switchgear until it is no longer profitable to do so. The surplus switchgear market also supports the industry. Only when support, replacement parts, and spares are no longer available should conversion solutions be presented as a viable option the end-of-service point. The end-of-service point is not a fixed date; it comes gradually. Eventually, as discussed in the example in the previous section, supporting legacy equipment becomes cost prohibitive or outright impossible and must be remanufactured, converted, or replaced. Remanufacturing power equipment is an extensive subject; this article focuses on when it should be considered versus conversion. Remanufacturing programs have increased over the last decade and for good reason. If the switchgear itself is in good, serviceable condition and parts/spares are readily available, a remanufacturing program should be considered over conversion and replacement. A properly performed remanufacturing process on an airmagnetic circuit breaker returns it to new, or better than new, condition. Organic insulation can be upgraded to modern materials. Modern, high-performance lubricants can be applied, and hazardous materials can be replaced with benign equivalents (technically, this is a conversion). A remanufacturing program can solve most issues with aging equipment. Arguably, it can solve everything but obsolescence at a fraction of the material and outage cost and with less risk compared to replacing the switchgear or performing a conversion. When obsolescence is the primary issue and support no longer an option, consider the two remaining solutions: replacing the switchgear or converting it. Choosing to replace the existing switchgear versus converting it usually comes down to cost and how easily the end user can support the extended outage it will take to replace the gear. In almost every facet, a conversion will be dramatically less expensive than replacing switchgear, especially since the breakers can be performed one or several at a time as budget factors allow. The choice between converting switchgear and replacing it is a straightforward decision, as the two solutions are on the opposite ends of the spectrum in regards to time and cost. CONCLUSION When addressing needs that may include raising interrupt requirements, the limited availability of parts and support, or challenges with antiquated technology such as the insulation system, a medium-voltage vacuum conversion may be the answer. To simply modernize, you may better serve your customer by recommending remanufacturing the existing equipment and upgrading the switchgear controls and insulation. However, we ve seen generations of converted circuit breakers good and bad and it s clear that not all converters are equal. Too many designs do not pass basic design and safety testing. Remember, no level of testing can compensate for poor design. When recommending a conversion option, take time to develop a detailed specification that references the required ANSI/IEEE design and testing requirements. Finally, if you are ever in doubt whether to convert or maintain, remember this: The legacy equipment you are working on has been in service for 30, 40, or even 50 years. Don t rush to replace it. Paul Grein is Vice President of Circuit Breaker Sales and has been with GroupCBS since 2008, working primarily in the Dallas area. Paul s primary responsibilities include business development, engineering design and management, technical expertise, standards, and project management. He has worked with industrial electrical equipment for over 20 years, beginning in the Navy as a Nuclear-Qualified Electrician on the submarine USS Topeka SSN 754 from 1996 through 2002, followed by positions in the steel industry through Paul has a BSEE from the University of Texas at Dallas (2007) and an MBA from the University of North Texas (2014). He participates in the IEEE/ANSI PES C37 Standards Committee, which publishes and maintains the design and testing standards that govern the industrial power equipment industry. 60 FALL 2017