In the ever-evolving landscape of energy production and distribution, power system studies play a pivotal role. Historically, large electricity grids were characterized by conventional synchronous generation connected to the distribution system via a high-voltage transmission network. However, with the ongoing transformation of the grid, distributed energy resources (DERs) embedded in the distribution system are becoming the norm. These DERs can include wind, solar, and batteries, as well as conventional diesel and hydro-power units.

With the added grid complexity due to the increased penetration of distributed and renewable generation, utilities, transmission operators, and independent system operators need to model the power system. By performing static and dynamic simulation and analysis, utilities and operations can ensure that the power system’s stability and resilience are maintained.

Straddling art and science, power system modeling involves developing mathematical representations of various components, including specific wind and solar equipment, and conventional generation. As we transition toward cleaner energy, these models are indispensable for maintaining economic and reliable grids.

Here, we highlight some of the challenges and complexities of integrating DERs into power system models.

Basics of Power System Studies

Power system models are mathematical representations of the physical and operational characteristics of an electrical power system. They help simulate the behavior of the physical grid assets (generators, transmission lines, and loads) under different conditions and contingencies, including outages and disturbances.

For developers, the results of these models need to be submitted to their utility as part of stringent processes of grid connection. For utilities, these models help them identify vulnerabilities, predict the impact of introducing an additional generation asset into the grid, formulate mitigation strategies, optimize the dispatch of resources, and maintain grid reliability.

Generators need different models to capture only the most relevant features for a particular problem setting:

• Load-flow (or power-flow) models are simplified representations of generators, used to study bus voltages and power flows in transmission lines, in a power system under steady-state conditions. These models are appropriate for analyzing the impact of power injections at a point of interconnection on the power grid, for a specific operating condition.
• Short-circuit models aim to reproduce the generating unit response a few cycles within a disturbance and are used in protection coordination studies, as well as for equipment sizing. Conventional generators are usually modeled as a sub-transient impedance behind a voltage source, while inverted-base generation is more complex, requiring specialized models to mimic the uniqueness of inverter controls during disturbances.
• Root mean square (RMS) dynamic models are more detailed representations, as they include the dynamics of the machines, mechanical systems response, and controls. These models are suitable for system-wide studies involving multiple generators when subjected to transient events, such as faults, line outages, and disconnection of loads or generators.
• Electromagnetic transient (EMT) dynamic models capture the full electrical dynamics of the machines. Being a computationally intensive modeling approach, it is used for studies restricted to a narrowed area of interest, requiring also a detailed representation of the grid, loads, and other relevant equipment.

The PSC team typically obtains a generalized model from the client’s equipment vendor. PSC then fine-tunes this generalized model based on various specifications including location and type of power source. Additional sensitivity analyses may be carried out at various stages to establish the robustness of the system.

Complexities of modeling renewable energy projects

With more renewable energy generation projects entering the grid, analysts conducting power system modeling studies need to ensure that they are representing the real project integrating into the real grid. Among other things, this means reflecting the correct control schemes, dynamic parameters, turbine type (in the case of wind energy), and active and reactive power capabilities of renewable energy resources.

Shifting from generic models to user-defined models (UDMs)

There is a growing trend of moving from generic models to UDMs, which are provided to project developers by the equipment vendors. Generic models are standard library models that represent the typical behavior of an electrical component. While generic models are still widely used as a starting point for grid planning and reliability studies, they may not capture the specifics of the projects’ technologies and may result in inaccurate dynamic responses under certain grid conditions.

User-defined models (UDMs), on the other hand, are developed by equipment manufacturers and custom-made to represent the specific characteristics of a particular component. UDMs are useful for modeling complex or specialized equipment like inverter-based renewable resources and are often necessary to accurately capture the unique factors and challenges involved in integrating renewable energy into the grid.

There are downsides to UDMs, however. UDMs are often called “black box models” because equipment manufacturers do not make the proprietary details underlying these models accessible. UDMs also require continuous maintenance, since they can become outdated when the software used for analysis is updated.

Accurately capturing the type of wind turbines

When modeling wind energy projects, analysts need to ensure that they are using the appropriate model depending on the types of wind turbines. Wind turbine types determine how the turbines interface with the grid. In type 3 turbines (i.e., doubly fed induction generators), the rotor is connected to the grid through a partial-scale power converter, which provides some control over the active and reactive power output to help stabilize the grid. In type 4 turbines (i.e., full converter), the generator is decoupled from the grid by a full-scale power converter that handles the entire power output. While most wind turbines today are still type 3, type 4 turbines offer full flexibility in providing reactive power capability which helps to regulate voltages where they connect to the grid. From a modeling perspective, type 4 turbines are indistinguishable from solar PV.

Active vs. reactive power

Renewable energy resources like solar PV and wind turbines possess both active and reactive power capabilities, which need to be represented in power system models. Active power is the real, useful electricity that powers motors, lights, or heating elements. A constant balance between active power produced and consumed in a system is necessary due to the intrinsically fluid nature of electricity. This balance is mostly achieved by proper control of generating units, which need to adjust their output quickly and automatically when such imbalances occur to prevent system instability.

The active power control of renewable energy resources helps the grid maintain stability and reliability by adjusting the active power output of generators in response to changes in grid frequency.

Reactive power, on the other hand, is the portion of power in an AC circuit that is not converted into useful work and is associated with the storage and release of energy in inductive and capacitive circuit elements including transformers and motors. Reactive power is highly correlated with grid voltages; therefore, reactive power control plays a key role in ensuring that equipment connected to the grid receives adequate voltage levels to operate as intended.

Reactive power control capabilities of renewable energy resources, including through the use of various devices, can improve the power transfer capability of transmission systems and help maintain grid voltage stability and power quality.

The PSC Advantage

PSC has the expertise to model the changing landscape of energy for developers, utilities, and independent system operators who need independent, third-party consultants. PSC’s Power Networks engineers have deep experience in the modeling, analysis, and planning of transmission and distribution networks, and we have developed a strong reputation with our clients. With an increasing demand for network analysis in the electricity distribution sector, PSC has a team ready to meet the requirements of these complex studies.

Find out more about PSC’s capability in this area and contact us to talk about the first steps.