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Submersible Transformers and Switches, Advanced Monitoring and Control


The weeks following a major storm provide an opportunity for a utility to evaluate its response and look for ways to improve. Superstorm Sandy was no exception. The size of the storm and the nature of the damage presented grid operators with many new challenges to deal with.

The following technologies can mitigate the effects of large storms and speed the recovery process. It’s worth noting that while some of these involve cutting-edge products that are still in the pilot program stage, others are already in widespread use.

Submersible Transformers and Switches

Two products that fall into this category are submersible transformers and switches. As the term implies, these are essentially the same as the transformers and switches used in distribution grids all over the U.S. The difference lies in the capability to operate underwater.

Such devices are used primarily in cities where much of the power infrastructure is below street level. It’s important to note that the vaults that house such equipment may lie well above the flood plain, but are nevertheless susceptible to localized flooding during exceptionally heavy rains. Transformer vaults, in particular, typically have a grate at street level to allow heat to escape, but this also means they are exposed to street level runoff.

Submersible transformers utilize a variety of materials and design features to ensure continuous operation: a sealed tank, less corrosive steel, corrosion-resistant paint, high resistance to short circuits, increased capacity to support overloads, and the ability to withstand seismic events. The tanks are also designed to direct fluid downward in the unlikely event of a rupture to minimize the expulsion of material upward to street level. Some transformers use no fluid insulation, thus eliminating the risk of leakage or fire associated with oil insulation. The design of submersible switches is similar in terms of materials and performance.

While submersible devices have been used for years, they have become more common recently. Switches in particular offer a good example.

While historically distribution grid operators could manage outages at the substation level, the application of switches provides a much finer level of control. In other words, instead of taking down large sections of a city, a utility could isolate flooded areas more precisely. Having submersible switches in place means that the utility can continue to operate those devices—remotely—even when the surrounding area is completely flooded.

In the wake of Hurricane Katrina (2005), utilities began to reassess their ability to handle flooding on a wide scale. ConEd in New York elected to install submersible switches at key points on its distribution grid, and employed them during Sandy, shutting down sections of lower Manhattan. While power is being supplied via alternative pathways, the submersible switches help speed the recovery process by allowing power to be restored to the primary circuit as soon as the surrounding equipment is determined to be safe.

Advanced Monitoring and Control

“Smart Grid” has been a mantra within the utility industry, but while most public discourse has centered on smart meters, perhaps the most compelling aspects of Smart Grid technology lie in their ability to make the grid more resilient in the face of disasters. When it comes to monitoring and control systems, the key is to increase operators’ situational awareness. This is not just about data, but rather speaks to the availability of actionable information presented through an effective user interface.

Since the 1970s, computers have played an increasingly important role in the monitoring and control systems that run transmission and distribution systems. At the heart of them is SCADA/EMS1, but due to the latency of readings and calculations, these systems provide a view of “what just happened” on the grid as opposed to what is happening currently. Real-time or near real-time monitoring is the goal of phasor measurement units (PMUs) that combine readings from disparate points on the grid with GPS time stamps and sophisticated algorithms to provide grid operators with a more detailed picture of grid conditions with fraction-of-a-second latency.

Wide area monitoring systems collect data from phasor measurement units and then use it in a variety of applications, some of which are especially relevant for storm situations.

  • Phase angle monitoring and power oscillation monitoring (POM)—Disturbances can be detected by monitoring the phase-angle relations between strategically chosen substations, even if they occur outside the system operator’s region; the same can be done for power oscillations.
  • Voltage stability monitoring— Assessing voltage stability of an important power transfer corridor in real-time relies on phasor measurements from both ends of the corridor. (Currently installed at Hrvatska Elektroprivreda, Croatia.)
  • PMU-assisted state estimator (PMUinSE)—Network manager’s state estimator can make use of PMU data for improved state estimate accuracy.

In order to be effective, a system protection and control function must have an array of countermeasures ranging from opening/tripping and closing primary devices to more subtle actions like controlling static VAR compensators (SVCs), and less surgical ones like activating a braking resistor on a generator. A good example is POM triggering use of an SVC. Most grids operate on n-1 criteria and can handle the loss of a major line, but will still experience oscillations as the system shifts to a new configuration. A system operator equipped with these technologies could avoid system-wide disturbances while making a smooth transition.

Presently, PG&E is conducting a proof-of-concept project in its multi-vendor interoperability, system integration, and application validation laboratory, equipped with the latest advancements in synchrophasor technology.

While phasor measurement holds great promise for transmission system integrity, the fact remains that most outages occur at the distribution level. Fortunately there is a wide variety of solutions available today not only to mitigate the impact of major storms but to accelerate recovery efforts as well. These fall under the umbrella term of distribution automation and include applications such as:

  • Fault Location—Reduces the time to locate faults, and has yielded a 20-minute reduction in SAIDI2 for some utilities. Operators can communicate the possible fault location to field crews, expediting repairs.
  • Fault location, isolation, and restoration (FLISR) system—FLISR goes a step further to determine and evaluate available isolation and service restoration switching actions, then prioritizes them according to multiple criteria.
  • Other advanced applications can assist operators with restoration such as line unloading, unbalanced load flow, and simulations.

The value of these applications is often multiplied by their integration with other utility systems such as outage management systems, mobile workforce management, and automated metering infrastructure. All of these Smart Grid technologies provide the utility with enhanced business intelligence when the organization needs it most.

Substation automation is another area where technology advances have produced solutions for storm management/recovery. Remote terminal units represent one of the important elements used to gather important data digitally or through hard wiring in either distributed or centralized configurations. Data is sent via local or wide area networks to a central intelligence unit where it is analyzed and information is presented to grid operators to take actions in a timely manner.

Fault record analyses, sequence-of-events information, and alarm indications help in identifying “what/when/how it happened” so that the utility operations crew can make fast and informed decisions on restoration actions. The system also provides a greater visibility of substation assets and facilitates efficient monitoring and control under normal or abnormal conditions.

Finally, it’s worth noting that the title of “Smart Grid” could be applied to a variety of technologies that don’t necessarily involve IT. One example of this is optical instrument transformers, which use fiber optics to measure current instead of the insulated copper wire used in conventional devices. They are a fraction of the size of their conventional counterparts and a tenth of the weight. In addition to their smaller footprint and exposure to wind shear, they are also oil free and thus carry no risk of fire or groundwater contamination. Also, while the surrounding electronics would obviously be damaged by flooding, the fiber optic cabling will not. These devices are in commercial use, but as yet make up a relatively small share of the market.

Roadmap Recommendations

Utilities already must adhere to legally binding reliability standards. However, there is more to be done within the industry itself to arrive at agreement on technical standards for technologies like PMUs. More work also must be done to ensure fail-safe operation for real-time applications. Likewise, the ever-increasing communication and data storage demands of data-intensive solutions will continue to put pressure on the IT systems that support them. Support for research in these areas would likely accelerate the development and adoption processes.

Next | Submersible Automated Switchgear

 


1 Energy management system

2 System average interruption duration index