Jan 23, 2015

Utilities are continuously looking for ways to optimize the use of their assets, drive efficiency through better infrastructure utilization, and generate cost savings. Using the voltage and VAR management solution recommended by Nexant’s Security Constrained Optimal Power Flow (SCOPF) software application, utilities have been able to achieve these goals. This case study presents the implementation of SCOPF to support the deregulated Swiss electricity market environment. It also discusses the system benefits SCOPF provides and its potential for use at other utilities to achieve similar results.

Energy Market Operation at Swissgrid

Switzerland plays a key role as Europe’s electricity hub, with cross-border energy transfer interfaces with neighboring countries Austria, Germany, France and Italy. Swissgrid is the transmission system operator (TSO) within Switzerland.

Switzerland is in the forefront of sustainable energy generation and Swissgrid supports sustainable energy generation and delivery to the electric power industry throughout Switzerland.

As a TSO operating the extra-high voltage (380/220 kV) transmission grid, Swissgrid coordinates energy exchanges and operational activities with neighboring utilities and is responsible for secure and reliable grid operation. Swissgrid has many operational goals, including maintaining voltages at various nodes of the grid within the operational limits and maintaining sufficient reactive reserves at all times. Both of these goals help improve system operation, reduce system losses, and generate cost savings. Reduction of system losses preserves MWs, which can be used for serving additional load.

Coordination with Power Plants

In the day-ahead time horizon, Swissgrid determines the voltage profile that power plant operators need to maintain for the 24 hours of the next operating day. Swissgrid sets an optimized voltage profile to maintain a secure grid voltage and minimize payments to grid operators. Plant operators participating in the power exchange are expected to utilize their reactive power capabilities to support the voltage setpoints suggested by Swissgrid and are compensated at the contracted rate. If plant operators deviate from the voltage schedule recommended by Swissgrid and the deviation is found to be counter-productive to the system needs, they are penalized for the deviation.

Coordination with Distribution Utilities

Distribution utilities are typically unable to provide voltage support to the bulk power system at 220/380 kV; however, they can support the transmission grid indirectly by running the distribution system efficiently by maintaining the power factor close to unity. Based on Swissgrid’s recommendations, some distribution utilities operate voltage regulation devices such as small generators, transformer taps, phase shifters and shunt devices in order to provide voltage support to the main grid. Thus, for coordination and effort to maintain grid reliability, distribution utilities may also be compensated by Swissgrid.

Nexant SCOPF Solution for Swissgrid

Swissgrid has many challenging questions to address, such as: What voltage profile should generators follow? What MVAR does each generator need to dispatch to maintain the desired voltage setpoint? What should the settings be for transformer taps or shunt devices to enable good power factor deliveries and to reduce losses? These overlapping objectives are not always mutually synergistic and pose optimization challenges.

To achieve all of these operational objectives in a coordinated manner, Swissgrid implemented Nexant SCOPF, a comprehensive optimization application. The application evaluated various operational objectives from multiple parties and recommended a balanced and optimized solution. There were three steps to the optimization process.

Data Preparation

As a first step, SCOPF analyzed all the input data such as load forecasts, generator operational limits, availability and MW requirements, and control device availability, settings, and operational limits. Control devices available for regulation were transformers, shunt devices and SVCs.

Reactive Optimization

Next, SCOPF optimized the available grid resources by performing a balancing act between two components of the objective function (e.g., MW losses and MVAR injection) by assigning appropriate weights to each of these components. The goal was to minimize transmission losses without depleting the reactive resources. Since maximizing MVAR reserve meant minimizing MVAR generated/absorbed by the generators, this was achieved by applying suitable cost curves to the MVAR injection.

Output Results

Nexant SCOPF has been in operation at Swissgrid since 2009. It runs every day in 24-hour look- ahead mode. The application also runs in an on-demand mode (e.g., following a major event). Objective function weights were adjusted to ascribe the costs associated with transmission losses to be higher (approximately 10 times) than the MVAR payments. The weights were determined based on past system operation experience and were then continuously adjusted to achieve the most optimal result.

In addition to improving reliability and enhancing grid efficiency, SCOPF also delivered a range of economic benefits to Swissgrid. SCOPF helped minimize transmission losses while improving voltage profiles and while significantly reducing the payments for generator MVARs. As a result of implementing SCOPF, Swissgrid’s costs savings were estimated to be up to approximately SFr 100,000 ($112,000) on a good operational day.

SCOPF also enabled power plant operators to know what their participation and payment structure will be ahead of time, thus incentivizing the plant operators for closely following the TSO voltage instructions.

SCOPF benefited distribution grid participants as well. The application recommended operational steps that reduced unwanted MVAR exchange with distribution grids by 50% while maintaining a power factor of 0.9 or better.

It was generally observed that the MVAR, supplied by the power plants to support the voltage profile, followed a linear trend thus increasing the confidence of the operators in the system planning process. Last but not the least; SCOPF was also instrumental in finding modeling errors. Whenever power plants were unable to follow the voltage profile recommended by the optimization application, the system operators dug deep looking for potential issues output data and discovered modeling mistakes.

The application amply justified the cost of its implementation by helping in reliable system operation, improving operational efficiency and saving money for the TSO as well as the grid operators.

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