Defence Plan Design for Liberia

The CLSG interconnection line (1,300km 225kV), linking Côte d’Ivoire, Liberia, Sierra Leone and Guinea, is currently being constructed and its commercial operation is expected to gradually start as from early 2021.

While the entire interconnection is not in operation and in case of tripping of one section of the CLSG line, the electrical system of CLSG will be split into two parts. The utilities of Liberia and Sierra Leone will import electricity through the CLSG line and therefore in case of tripping of the CLSG line they will face unbalance between load and generation. In order to avoid a partial or total blackout, in case of tripping of the CLSG line, it is necessary to implement a Defence Plan, which will shed an amount of load, allowing to reach a new equilibrium between generation and demand.

The objective of this study concerns the operations when Liberian power system will only be connected to the CLSG line. The study consists in designing an under-frequency load shedding (UFLS) scheme for the network of Liberia in order to reduce the impact on system operation following generation or interconnection contingencies (islanded and interconnected operation, respectively).

Smart Flow was employed in this study in order to perform the static and dynamic analyses that supported the design of the UFLS scheme. A total of 36 different operating conditions were analysed, considering different import levels from CLSG to Liberia. The following types of simulations have been carried out using the different modules of Smart Flow:

  • AC Load Flow;
  • Optimal Power Flow (IPSO);
  • Short-Circuit Current Calculation (SHOCC);
  • Advanced dynamic simulations (Eurostag).

An extensive use of the advanced Application Program Interfaces (APIs) of Smart Flow in this project was key in order to allow the simulation of large quantities of operating conditions and events.

H2020 Research Project Promotion

The EU’s Horizon 2020 Research project PROMOTioN focuses on the development of meshed offshore HVDC grids. Meshed HVDC grids are not a mature technology yet and the right tools have to be used in order to analyze the behaviour of the combined AC and meshed HVDC system under normal and emergency operating conditions. For this reason several functionalities have been included in a prototype version of Smart Flow:

  • Modelling of voltage droop characteristics of HVDC converters
  • Include DC equipment contingency in the Security Analysis
  • Improvement of the Tabular Output to include variables of the HVDC grid
  • Possibility to include dynamic modelling of DC lines

Several improvements in our OPF module IPSO are also ongoing in order to be able to perform hybrid AC/DC OPF.

Smart Flow and Eurostag has been intensively used in the PROMOTioN project for the following applications:

  • Load flow and Security Analysis
  • Dynamic simulations of the converter controls and comparison with EMT simulations

The first application aimed at verifying the technical viability of HVDC grids proposed by a dedicated Optimum Transmission Expansion Planning algorithm.

While the second application allowed to verify the adequacy of the Eurostag tool for performing dynamic simulations of large meshed HVDC grids and contingencies.

Energy Systems of the Future

The project “Energy Systems of the Future: Integrating Variable Renewable Energy Sources in Brazil’s Energy Matrix” aims at studying the impacts of the integration of large shares of variable renewable energy (VRE) sources to the Brazilian interconnected power system (SIN).

The study shows how the Brazil’s power system needs to be prepared – in terms of operation and expansion – to support increasing participation of variable renewable energy sources, indicating the critical points and scenarios for insertion of these sources. The technological resources available to mitigate the impacts of penetration of these sources in the National Interconnected System in the mid- and long-term will also be evaluated in the study.

In this project, Smart Flow was employed to perform the power system studies. The power system analyses aim at assessing in detail the suitability and/or need to adapt the Brazilian system in order to ensure its operability and maintain the targeted level of reliability with increased VRE integration.

One of the greatest challenges for the application of Smart Flow is related with the size and complexity of the system, summarized as follows:

  • About 300 static simulations carried out, considering normal operation and about 2500 possible contingencies for each operating condition;
  • About 9000 dynamic simulations performed;
  • A total of about 450 GB of static and dynamic simulation input and output data produced
  • Power system simulation model size:
    • ~11000 nodes
    • ~9000 transmission lines
    • ~7000 transformers
    • 7 CSC HVDC bipoles
    • ~3000 generating units with dynamic model

Smart Flow proved to be a robust, efficient and accurate tool for the simulation of large-scale power systems with complex dynamic phenomena induced by the long electrical distances of this system and the high amount of VRE penetration.

Power Management System

Power Management System

The Power Management System (PMS) developed to Emirates Global Aluminium – Al Taweelah site (EGA-AT), is a complex system capable of balancing generation and load following incidents within the plant.

The PMS ensures the stability and reliability of EGA-AT plant by monitoring power flows in the interconnectors, system frequency and equipment circuit breaker status and acting according to the identified incident by reducing demand or generation.

To develop the basic design of the PMS, the static and dynamic characteristics of EGA-AT plant was modelled in Smart Flow. That includes the dynamics of potline, which is basically a DC bus behind a rectifier, but with very complex control logics. Besides that, the PMS itself was modelled including:

  • Signal acquisition: measurement of voltage, current, frequency and breakers’ status signals which are used as input to the logics.
  • Feedback logics: based on user-defined protection relays capable of detecting power flow or frequency violations and triggering actions like the runback of potlines, tap down or trip of rectifier transformers, or tripping of gas turbines.
  • Feed forward logics: based on user-defined protection relays capable of detecting pre-coded incidents (loss of full combined cycle or loss of multiple potlines) and triggering pre-defined actions.
  • Equipment protections: over-current and direct power protections were modelled using macroblocks, while over/under voltage/frequency were modelled using the existing protection relays.
  • Grid MW control and isochronous load sharing: these two functionalities were added directly to the gas turbines’ governor model with the objective to control the power exchange with the grid and to control the frequency when the system is isolated from the main grid

Once the complete model was implemented, transient stability analysis were performed in batch mode to analyse the system under different network configurations facing all possible combination of incidents, resulting on more than 1000 cases. For the cases presenting stability issues or unexpected trips, the logics were adjusted and the simulations re-run until all the cases were acceptable.

The final logics were translated into a Technical Specification and shared with the Original Equipment Manufacturer to be implemented under a dedicated controller. The PMS was finally commissioned on site and it is now under operation in EGA-AT plant.