Green power roundtable: Exploring green impacts of electrical distribution systems
Q: What electrical system reliability issues have you seen, and how are your technologies addressing these?
Loucks: Switching inductive loads can produce voltage transients, which are initiated whenever there is a sudden change of circuit conditions. We have seen situations where dry-type transformers have been damaged due to switching transients. These switching transients produce both steep-front wave and end-of-turn damage to transformers and motors, but they also excite resonances that occur within a core-and-coil assembly of a motor or transformer. A transient voltage study is highly effective at identifying the corrective measures that are required to avoid system downtime and equipment damage. Typical corrective measures include surge arrestors, damping resistor-capacitor combinations, and modified switching resistors.
Powell: Power outages are occurring more frequently than ever and lasting longer with devastating effects. Siemens standby generators protect homes and businesses from unpredictable weather and unforeseen outages. They are available from 8 kW to 150 kW and are powerful enough to back entire homes or businesses without the cost of an expensive configured system.
Smith: When powering equipment from dc with a direct connection to a backup battery, failures in the upstream ac generation and distribution network do not impact the critical load for a period of time dictated by the capacity of the battery system. Additional isolation from electronic failures is provided by provisioning redundant elements in the system architecture. Multiple redundant battery strings are employed to protect from battery element failures. The same redundant approach is taken in renewable energy systems, with multiple solar panel strings being used to feed the dc storage bus and battery elements through independent converters.
Walker: CES attacks reliability issues head-on and results in a new paradigm for level of service. The CES units are deployed near the customer and contain an energy reserve for reliability. If a facility outage occurs, the storage unit immediately isolates from the grid and continues to supply the local customers. The interruption can be avoided altogether as long as the grid is returned to service within a few hours. Even the momentary interruptions typically associated with a line sectionalizing event are avoided due to the immediate transfer of load from the utility system to the energy storage battery system.
Q: What impacts are green power sources having on system reliability, and how is your company addressing these issues?
Loucks: There is voltage imbalance due to variable single-phase distributed generation. But even 3-phase distributed generation is causing issues. The Texas Competitive Renewable Energy Zone project is adding 20 GW of wind from West Texas and the Panhandle to highly populated metropolitan areas of the state. Due to the variable nature of renewable energy and to reduce flicker, substantial additional power electronic hardware is being added.
As the percentage of renewable power increases, grid stability is reduced. A solution is to provide more transmission interconnects to distribute sources and loads over larger areas. However, there is a lot of resistance to building more towers where people live, work, and play (and rely on electrical power to do so). To address these issues, Eaton is working with energy storage solutions that mitigate fluctuations due to the variable nature of renewable energy. Integrated, grid-tied storage is helping to make renewable energy a dependable, on-demand power that is deliverable with controllable power ramp rates.
Powell: Renewable energy can be hard to control because it generates power intermittently. However, our control systems—together with storage—minimize the adverse impact arising out of these on the micro grid as well as the macro grid.
Smith: Many sources of green power, such as solar and wind, are inherently intermittent. The traditional architecture of a dc power system with battery backup provides protection from the intermittent absence of the green source by storing backup energy for use during these times. Proper understanding of the availability of the sources, combined with appropriate provisioning of the power generating elements, sizing of the storage battery, and provisioning alternative or backup sources, allows for maximum reliability and minimum cost to be achieved.
Walker: One of the concerns of highly distributed, small solar generators is their power output fluctuation, which results from rapidly varying sunlight. The power output variations of solar generators across a distribution feeder are tightly synchronized, causing voltage swings that are faster than traditional voltage regulation can compensate. CES units that are deployed in the same area will provide the necessary compensation to stabilize voltage. The CES units operate in four quadrants, which are defined as real and reactive power both in and out. They can be configured to perform power smoothing where real power is exchanged, stored, or released to compensate solar generator output variations. The CES units also consume or produce reactive power as required to directly control local voltage magnitude swings.