by Nisheeth Singh, Swissgrid, and Herminio Pinto, Nexant
Utilities continually look for ways to optimize their assets, drive efficiency through better infrastructure utilization and generate cost savings.
Utilities such as Swissgrid have achieved these goals by using voltage and VAR management solutions.
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 that operates the extra-high voltage (380/220 kV) transmission grid, Swissgrid coordinates energy exchanges and operational activities with neighboring utilities and is responsible for secure, reliable grid operation.
Swissgrid has many operational goals, including maintaining voltages at all nodes of its 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 megawatts, 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 who participate in the power exchange are expected to use their reactive power capabilities to support the voltage set points 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 counterproductive to the system needs, they are penalized for the deviation.
Coordination With Distribution Utilities
Distribution utilities typically are 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 to provide voltage support to the main grid.
Thus, for coordination and effort to maintain grid reliability, distribution utilities also may be compensated by Swissgrid.
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 set point?
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’s Security Constrained Optimal Power Flow (SCOPF) software application.
The application evaluated operational objectives from multiple parties and recommended a balanced, 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 megawatt requirements, and control device availability, settings and operational limits.
Control devices available for regulation were transformers, shunt devices and SVCs.
Reactive optimization. Next, the software optimized the available grid resources by performing a balancing act between two components of the objective function (e.g., megawatt losses and MVAR injection) by assigning appropriate weights to each component.
The goal was to minimize transmission losses without depleting the reactive resources.
Because maximizing MVAR reserve meant minimizing MVAR generated by the generators, absorbed by the generators or both, this was achieved by applying suitable cost curves to the MVAR injection.
Output results. Swissgrid has used the software 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 system operation experience and then were adjusted continually to achieve the most optimal result.
In addition to improving reliability and enhancing grid efficiency, the software also delivered a range of economic benefits to Swissgrid.
The software helped minimize transmission losses while improving voltage profiles and significantly reducing the payments for generator MVARs.
As a result of implementing SCOPF, Swissgrid’s costs savings were estimated to be up to 100,000 Swiss francs ($112,000) on a good operational day.
The software also enables power plant operators to predict what their participation and payment structure will be, thus incentivizing the plant operators for closely following the TSO voltage instructions.
The software benefited active distribution grid participants, as well.
Active distribution grid participants provide an additional reactive power capacity of some ±200 MVAR and are being compensated financially for the compliant exploitation of their reactive resources.
Last, SCOPF was instrumental in finding modeling errors.
When power plants were unable to follow the voltage profile recommended by the optimization application, the system operators dug deep for potential issues, analyzed the 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 and grid operators.
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