By Francis Luces, Ric Austria, Cherry Bautista

(Pterra has provided support to developers on most aspects of grid interconnection, including pre-feasibility/site assessment, modeling, interconnection, and engineering design studies.)

In today’s rapidly evolving energy landscape, the reliability and efficiency of the power grid are more crucial than ever. An increasing penetration of inverter-based resources (IBR) to meet clean energy targets will also mean an IBR-dominated dynamic behavior of power systems. It is extremely important that a usable power system model representation of IBR, conventional machines, loads, and other components is submitted to utilities to support the analysis of the reliability of the transmission system.

Our Services to Meet Planning Model Submissions

Pterra offers modeling and reporting services to help conventional, or IBR plant owners comply with ISO/RTO or utility-specific interconnection application requirements and/or regulatory-mandated model submission guidelines:

Phasor-based Steady-State and Dynamic Models

PSS/E is the program used by most utilities and ISOs based in the East Coast while PSLF is used by the Western Region. Regardless of which software is being used, a usable phasor-based steady-state and dynamic model of the Plant is required for submission to aid planning engineers in their interconnection or network studies.

Oftentimes, IBRs are represented using generic dynamic models in these two well-known programs due to their simplicity. These models offer flexibility and low maintenance on larger database sets, but it is important to recognize that they have limitations. When standard models do not reflect the actual behavior of the equipment, a user-defined model (UDM) should be used and must be properly parametrized according to Project specifications.

Electromagnetic Transient (EMT) Models

Traditional use cases for electromagnetic transient simulations were insulation design, surge protection, controller design, and equipment rating adequacy studies. In the studies of IBR interconnection, EMT modeling and simulations are now needed to verify responses of IBR such as fault ride-through, P/Q priority, voltage and frequency step, phase angle jump, among others. Unlike phasor-based models, the EMT model is the closest representation of the actual equipment and controls present in the field, thus considered a high-fidelity model for system studies. ISO/RTOs have recognized the need to include PSCAD or EMTP-RV model submissions of IBRs to augment offline power system studies that were traditionally performed by phasor-based simulations.

Model Benchmarking and Validation

Model benchmarking is an exercise to compare model responses across different software platforms. Typically, the simulation results obtained from PSS/E and PSCAD are overlaid in one plot to allow comparison. This approach helps ISO/RTO engineers to understand model behavior and helps planning engineers to create and make better engineering solutions. The model validation is also a benchmarking effort which focuses on comparing inverter-level EMT model response against field recordings, such as those obtained from hardware in-the loop (HIL) simulations or actual measurements from grid events.

The table below shows the summary of model requirements and the corresponding references stating such requirements.

Model Quality Testing

Model quality testing for IBRs in PSSE and PSCAD are a set of prescribed tests by utilities to ensure the accuracy and reliability of models submitted. Typically, the tests may include flat run, ringdown, ride-through, small step inputs on voltage and frequency controllers, and short-circuit ratio () calculations. For projects that will provide grid services or as part of power purchase agreement (PPA), additional tests such as deadband, high and low frequency disturbance, or nighttime reactive power capability are called for to demonstrate capability and compliance. Finally, new model tests such as loss of last synchronous machine (LLSM), black start capability, interaction, and phase angle jumps are being required for projects that are equipped with grid forming (GFM) control topologies such as virtual synchronous machine (VSM) or droop-based control.

NERC MOD Compliance

NERC issues a set of reliability standards, known as (Modeling, Data, and Analysis) Standards, that aim to ensure consistent modeling data reporting and validation of equipment owned by Generator Owners (GO) or Load Serving Entities (LSE). The MOD standards are:

• MOD-032-1 — Data for Power System Modeling and Analysis
• MOD-033-2 — Steady-State and Dynamic System Model Validation
• MOD-026-1 — Verification of Models and Data for Generator Excitation Control System or Plant Volt/Var Control Functions
• MOD-027-1 — Verification of Models and Data for Turbine/Governor and Load Control or Active Power/Frequency Control Functions

Typically, Transmission Planners (TP) or Planning Coordinators (PC) sends a notification to GOs and LSE for updating of power system data, model submissions, and reports of model validation against field measurements. The MOD compliance will require modeling and simulations of the Plant using phasor-based programs which can be performed by plant owners or oftentimes, third-party consultants like Pterra.

On the Model Accuracy and Usability

With the release of FERC Order 901, the accuracy of models and their usability is more important than ever to address existing and potential reliability issues associated with IBR interconnections. Our modeling efforts focus on the following key points to deliver an accurate and usable model:

1. The models can be parametrized according to Project, and the model response is consistent with the field measurements.
2. The parameters (e.g. control gains) applied are within the acceptable range and can be equally applied in the field.
3. The models comply with ISO/RTO or utility-specific model testing requirements and quality assurance.
4. The models used are not in the list of unacceptable models and represent the required control features, deadbands, and protection settings as prescribed by NERC.

Conclusions

Increasing IBRs applications results in backlogs on interconnection queues but model accuracy and usability cannot be comprised. While planning model submissions are only a small subset of a long list of interconnection application requirements, its impact to ensuring system reliability is significant.

Pterra, as a consultant, endeavors to maintain the key points mentioned above and establish collaborations with inverter original equipment manufacturers (OEM), utilities, and project developers to meet and deliver accurate, usable, and high-quality power system models.

In December 2022, the New York State Energy Research and Development Authority (NYSERDA) published a report on the Great Lakes Wind (GLW) energy feasibility based on a study conducted by the National Renewable Energy Laboratory (NREL), Advisian Worley Group, Brattle Group and Pterra Consulting.  This study (link to the full report) was intended to complement the options for renewable resources such as land-based wind, solar, hydro and offshore wind that would help meet New York’s renewable energy portfolio and decarbonization goals under the New York State Climate Act.

New York State is bounded by two of the Great Lakes, Lake Erie to the west and Lake Ontario to the north. The potential for developing fixed and floating wind turbines on the lakes using both existing and emerging technologies was the focus of the feasibility study. The study examined myriad issues, including environmental, maritime, economic, and social implications of wind energy areas in these bodies of freshwater and the potential contributions of offshore GLW projects.

Pterra’s role in the study (link to interconnection report) was to conduct a feasibility assessment for potential interconnections of GLW generation with the New York Bulk Power System (NYBPS). To perform the assessment, Pterra developed power flow models to represent the NYBPS in 2030 with an assumed renewable generation buildout.

To provide a measure of interconnection capacity, the capacity headroom definition and calculation method described in recent New York State Public Service Commission orders were selected. Potential points of interconnection (POIs) on the existing NYBPS substations located within 20 miles of either the Lake Erie or Lake Ontario shoreline were initially selected for analysis. These were filtered down to a few representative POIs for more detailed analysis. (Headroom represents the potential capability for GLW to interconnect; however, it also represents the capacity that is available to any other generation resource that may want to interconnect at the same POI. The nature of the NYISO market for any new generation is competitive and GLW is expected to compete with other resource development modeling analyses to utilize the available headroom.)

Lake Erie abuts the New York counties of Erie in the north and Chautauqua in the south. For Lake Erie GLW, the available POIs showed combined capacity headroom of 270 megawatts (MW) without transmission upgrades. Applying a set of simple transmission upgrades costing some $68.8 million can increase the Lake Erie total headroom capacity by 60 MW to 330 MW.

New York State has a longer shoreline along Lake Ontario compared to Lake Erie. Several New York State counties border the lake, including Niagara, Orleans, Monroe, Wayne, Cayuga, Oswego and Jefferson. For Lake Ontario GLW, several POIs in Monroe and Oswego counties showed solo headroom capacity in the range of 850 to 1100 MW without the need for transmission upgrades. At most, there is a total headroom capacity of up to 1140 MW for the Lake Ontario POIs. The total headroom capacity may be increased by 140 MW by implementing simple upgrades costing some $236.6m. In Jefferson County, the studied POIs showed no solo headroom capacity. Simple transmission upgrades costing at least $164.5 million may open about 50 MW of headroom capacity.

Pterra’s interconnection assessment found that there is some headroom capacity on the NYBPS for which GLW can compete for the delivery of energy to the grid. In order to access the POIs with headroom, other reliability issues relating to transient voltage, stability, short circuit, deliverability, transfer capability and higher-level contingencies would also need to be considered.

By Francis Luces, Ric Austria

Power projects planning to participate in the wholesale market are required to undergo impact studies as part of the interconnection application process. The studies, as a minimum, evaluate the performance of the projects under instantaneous, steady-state and transient conditions. The timescales of phenomena and equipment studied are as illustrated in Figure 1.

In the specific case of transient studies, the impact of a proposed project on the voltage and frequency control capability of the overall grid is evaluated. Traditionally, it was sufficient to consider a timeframe of 0.5 to 10 Hz (10-100 msec) for a type of study known as transient stability. The computer models (and software, such as PSS/E and PSLF) used to conduct these studies are known as phasor-based models. These models capture phenomena limited to the target timeframe.

By R. Tapia, M. Gutierrez and M. Infantado

Introduction

Independent System Operators (ISO) constantly face the challenge of assessing the impact of facility additions to the power grid. They normally require a system impact study for any proposed interconnection of a large generating plant or transmission project. The purpose of this analytical study is to determine the potential adverse impacts of the interconnection of transmission facilities to a power system and whether it would cause any of the following:

• Post-contingency thermal overloading on transmission lines and transformers,
• Voltage criteria violations on substations,
• Negative impact on the dynamic response of power system facilities,
• Degradation on the transfer limit of transmission interfaces,
• Increase in substation short circuit current that could possibly exceed the fault duty of existing circuit breakers.

The system impact study determines the impact of the proposed project by comparing simulation results of the case with the project in service against the case without the project. If adverse impacts were to be found, appropriate solutions to mitigate the violations would be required, except for the extreme contingency assessment which is performed for information purposes on issues such as avoidance of widespread load interruptions, uncontrolled cascading, and system blackouts among others.

by Francis Luces

Introduction                                                               

For developers of power plants, one of the important factors to consider is where and how to interconnect a plant to an existing transmission network in order to reliably deliver its full output. For conventional power plants (i.e. coal, oil, natural gas, etc.), the availability of fuel supply and environmental permitting are the main considerations for siting. In the case of solar photovoltaic (PV) projects, given the availability of land area for mounting solar panels and sufficient solar irradiance, the point of interconnection (POI) to the grid can be the determining factor for siting. An assessment of the thermal capacity at potential POIs provides an effective screen for potential sites. Using transmission capacity injection analysis, developers can swiftly determine the capability of the existing network to support additional power from a new source such as a PV project. With this type of analysis, solar power project developers can know fairly early in the development process if the selected site and POI can support the plant’s output.