Hospital Transformer Failure Transient Investigation

Jan 6, 2017 | Blog

Hospital Transformer Failure Transient Investigation

By: Tom Grebe, Principal Consultant
tgrebe@enernex.com
865-770-4880

 

Power system apparatus, such as transformers, switchgear, and cables may be exposed to various types of transients. High-frequency transients and very steep overvoltages may cause problems for electrical equipment because they can cause local overstressing of the insulation system. Vacuum circuit breaker (VCB) opening-and-closing operations are a significant source of these high rate-of-rise (dv/dt) transients. In addition, dry-type transformers and motors are often more vulnerable to these transients due to their lower insulation level (BIL) ratings.

A high-frequency transient investigation was completed for a medical center in California. The purpose of the study was to investigate a transformer failure that occurred during commissioning of the facility. The principal objectives of the study were to determine transient overvoltages and evaluate mitigation alternatives for vacuum circuit breaker current chopping and pre-strike transients that were believed to have caused the transformer failure. The transient analysis was completed using the PSCAD®/EMTDC® simulation program.

The study included analysis of MOV surge arresters and other mitigation alternatives (e.g., R-C snubbers). Based on the analysis completed during the study, the hospital installed R-C snubbers (refer to Figure 1) at the switchgear terminals of the distribution circuits where the transformers were being switched. Properly-designed R-C snubbers reduce the initial rate-of-rise of the transient voltages, which is beneficial because severe dv/dt transient voltages can damage the first few turns of insulation of dry-type transformers and motors or excite internal transformer resonances producing severe overvoltages.

Vacuum circuit breakers are understood to be capable of initiating a phenomena described as current chopping. The physics of the vacuum circuit breaker allow for a smaller space to be utilized in the interruption of current in a vacuum.  It is well-known that these devices can interrupt (chop) current.  This is a different behavior than typical air circuit breakers, which normally allow current arcing following contact separation until a natural zero crossing occurs.

The current chopping phenomenon is generally not troublesome, however, there are specific circuit configurations that can cause problems. The most common configuration involves using vacuum interrupters to de-energize unloaded transformers or other highly inductive circuits. In this case, the inductive current to the transformer is interrupted, possibly causing high transient overvoltages. The current chopping analysis showed that severe transient overvoltages will occur if the vacuum circuit breakers chop current during opening.

Pre-strikes are a breakdown of the vacuum dielectric during closing of a vacuum circuit breaker. A pre-strike occurs when an arc current flows for a short period-of-time before mechanical contact closure. The pre-strike phenomenon is very complex and difficult to predict. The transient currents and voltages are dependent on many factors, such as circuit breaker characteristics, dielectric properties, surge impedance of the circuit components, and high-frequency current interrupting capability.

Transient voltages during pre-strikes are a result of the interaction (resonance) between a transformer inductance and a capacitive network. The susceptibility to the resonant frequency phenomena depends on cable lengths and other capacitive parameters and on the transformer inductance. The pre-strike analysis completed during the study showed the potential for high rate-of-rise transient voltages if the vacuum circuit breaker pre-strikes during closing.

The Power System Engineering group at EnerNex has extensive expertise performing power system studies investigating the high-frequency transients associated with vacuum circuit breaker operations. These studies often require use of sophisticated digital simulation tools (e.g., EMTP-RV, PSCAD). Computer simulations provide a convenient means to characterize transient phenomena, determine resulting problems, and evaluate possible mitigation alternatives.

Figure 1 – R-C Snubber Installed at Customer Facility

 

Smart Metering (SM) and Advanced Metering Infrastructure (AMI)

Smart Metering and AMI is a transformational process addressing multiple business and technical needs of the utility enterprise. This is more than just smart meters and communications networks; it includes all of the back end applications that can leverage the meter assets, such as outage notification, demand response, call center optimization, disputed billing process handling, pre-payment opportunities, and service connection management methods and procedures, to name a few.

Implementing SM and AMI faces the same business, engineering, and operational challenges as any other across-the-utility information technology endeavors – most notably risk associated with embracing proprietary technology, missing functionality and early obsolescence. Effective SM and AMI development, implementation, and operation relies on a marriage of electric power engineering with information technology expertise: a key component of EnerNex’s expertise and experience.

EnerNex provides an array of engineering and consulting services geared towards intelligent and effective implementation of SM and AMI. This covers all phases of project development, starting with capturing system requirements where our experts leverage a “Use Case” centric view of activities needed to be accomplished and their interaction with systems and other users. Subsequent project steps typically examine other critical areas, such as: modeling of business cases, building inter-department consensus, assembling and assessing system functional requirements and non-functional requirements, developing a system design, hardware and software specifications and standards, complete procurement services including RFI and RFQ process support, supplier rating system, response evaluation methodology, deployment management, and training of office and field personnel.

Demand Response (DR)

Demand response can be as simple as load interruption directed by the energy supplier in response to severe demand requirements, to complex customer defined load management in response to price signals. DR is one of the components of a “Non-Wires Alternative” that many utilities are effectively using to avoid expensive distribution fortification or upgrade.

 

Often the success and/or failure of demand response programs can be linked to program implementation challenges such as rate/tariff design rate structures communication (e.g. price signals) or ineffective incentives used by utilities to encourage customers to accept operational change. The issues of program design, rate structure and customer impact have a tremendous influence on the success or failure of load management initiatives. Demand response has traditionally been used as a tool of the energy industry to ensure system stability. However, the introduction of microelectronics, communications, home automation and the Internet of Things (IoT) has led to the development of cost effective solutions that have the capability to allow the consumer to take control of managing their energy load and ultimately, the price they pay for energy.

EnerNex has the experience and skills to turn your DR program into a successful operational asset and customer engagement process that can deliver value to all parties.

Energy Assurance Planning

Natural and man-made disasters cause an estimated $57B in average annual costs for all parties; large single events have resulted in losses of $100B or more. Events, such as the World Trade Center disaster, Hurricane Katrina, and most recently Hurricane Helene, have demonstrated an acute need to revisit, revise and implement an effective energy assurance plan. Energy assurance plans assess the functionality and interdependencies of buildings and infrastructure systems and the role they play in sustaining service and rapidly restoring critical services to a community following a hazard event.

 

EnerNex assists our clients in developing comprehensive energy assurance plans that mitigate and minimize the impact of energy disruptions. Our experts assess critical infrastructure risks and evaluate appropriate mitigation strategies and can help in developing an effective business continuity/disaster recovery (BC/DR) plan for utilities and your customers.

Microgrid Development

As the electric grid becomes more distributed and interactive, microgrids are playing an increasingly important role in our energy future. Decision makers at military bases, corporate and institutional campuses, residential communities and critical facilities across the world are exploring and implementing microgrids to meet economic, resiliency and environmental goals. Utility-grade microgrids are being deployed to meet transmission constraints, reliability requirements and safe-havens in the event of a significant storm event.

Microgrid_development Graphic steps to support grid modernization

Bringing together a portfolio of distributed energy resources into a controllable, islandable microgrid comes with its own set of challenges. The key to solving these challenges is in architecting a system to support information exchanges between components across well-defined points of interoperability (interfaces) in a technology independent manner. This interoperability ensures that the system is resilient to technology change. Modern systems engineering techniques must be employed to ensure that individual sub‐systems are clearly identified, their functions enumerated, their data requirements known, and the points of interoperability clearly specified, along with the commensurate monitoring, command and control that is needed to ensure grid stability. With such architecture, we can apply best of breed technology available today to support those information exchanges at interface boundaries but be free to upgrade / change the implementation technology later without causing a ripple effect throughout the system.

Enterprise Architecture

Enterprise Architecture focuses on aligning an organization’s business strategies with its anticipated, desired and planned technology enhancements. Enterprise Architecture provides a framework to cost-effectively transition from a current “as-is” technology to future enterprise-wide technological solutions. An effective Enterprise Architecture program aligns business investments with long-term business strategies while minimizing risk and providing superior technological solutions. EnerNex’s key asset is its highly skilled and experienced staff who are closely connected to both the smart grid and EA standards and practices. We provide clients with the insight necessary to operate a fully functioning smart grid, which is flexible, scalable, and vendor independent.

Grid Modernization Roadmap

Utility companies across the globe are continually modernizing their grid. Each company often has different rationales, objectives and priorities. Frequently, smart grid plans are developed for individual, incremental initiatives, rather than as a part of a whole, intelligent and interoperable infrastructure. Planning may be developed around technology choices rather than business and technical requirements. The result of incremental and flawed planning leads to increased cost and risk, lost opportunities, disconnected expectations and dead ends.

 

EnerNex’s approach to grid modernization roadmap development follows a proven, industry-standard approach to grid modernization planning by collaboratively working with the utility to develop a set of prioritized and time-phased grid modernization initiatives unique to its business strategy and objectives. The roadmap developed is holistic, requirements-based, business value driven and actionable. It often builds on and leverages existing applications and infrastructure, and incorporates industry standards to ensure interoperability, flexibility and reduced cost and risk.

Utility Communications

Utility communication and control systems are increasingly interconnected to each other and to public networks and as a result, they are becoming increasingly more susceptible to disruptions and cyber attacks. EnerNex has experience with the various issues relating to development, implementation and optimization including feasibility analysis, design, software development and customization, project management and acceptance. Our expertise extends from being involved in the development of the fundamental standards that support utility communication and automation, through deployment and securing of those resources. EnerNex personnel were heavily involved in development of such standards and protocols as IEC 61850, IEC 60870-5 and DNp3. Our staff played a key role in the EPRI Utility Communication Architecture (UCA) project and the IntelliGrid Architecture effort.

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