The Electrical grid is undergoing a rapid change with the addition of renewable energy inverter-based resources (IBRs) offsetting and driving the loss of traditional synchronous generation.
TSOs (Transmission System Operators) are trying to balance between the rapid addition of IBRs and the removal of traditional synchronous generation resources. While power electronics used in IBRs enables them to respond to grid disturbances quickly and support grid reliability, due to their lack of inertia they are posing challenges for grid planning, operation, and protection. It’s important for Transmission Planners (TP), Reliability Coordinators (RCs), Planning Coordinators (PC), Generator Owners (GOs), Generation Operators (GOPs) and inverter manufacturers to ensure models used to represent these resources in steady-state power flow, dynamics, and short circuit studies sufficiently represent the actual behavior of these resources.
NERC strongly recommends that EMT modeling and studies be incorporated into NERC Reliability Standards to ensure that adequate reliability studies are conducted, to ensure reliable operation of the bulk-power system (BPS) moving forward. Existing positive sequence simulation platforms have limitations in their ability to identify possible performance issues, many of which can be identified using EMT modeling and studies. As the penetration of inverter-based resources continues to grow across North America, all TPs and PCs should have clear requirements to gather EMT models at the time of interconnection, and execute EMT studies to ensure proper ride-through performance for BPS fault events. Presently the approach taken by the industry is leading to modeling and study gaps, and consequently unreliable performance of inverter-based resources once interconnected.
For a steady state analysis phasor or an RMS based solution, from a power system planning and operations perspective, these methods have been traditionally used in models. They are relatively accurate for systems that are dominated by synchronous machines. The issues currently being faced are that the grid is transitioning away from rotating machines and leaning towards inverter-based generators that loads their controls. An EMT simulation might be preferable or necessary because it provides more realistic results, and is more likely to reliably predict things like control and stability.
Electromagnetic Transient (EMT) simulation provides a time variant instantaneous value output in waveform that matches the current/voltage waveform to be measured from a real system. It provides greater depth of analysis over a wide range of frequencies and has the ability to reproduce fast transients on the power system. The higher the sampling frequency of the simulation, the higher frequency results can accurately be reproduced. The period of sampling frequency or the gap between two consecutive outputs in a simulation is called a Timestep. Real Time Data Simulator (RTDS®) executes EMT simulation in real time. This real time aspect differentiates RTDS from offline or non-Real Time EMT programs, such as PSCAD. The typical Timestep of RTDS simulation is 25-50 microseconds (μs).
Efficiency is one of the advantages offered by RTDS as it demonstrates simulations in real time. Real time means that the computer needs to solve all the equations for the network in real world time equal to or less than the simulation time stamp. RTDS uses parallel processing hardware that enables it to complete these simulations in much less time than other offline EMT simulation programs. This means that the real time for a power system event to occur is equal to the simulated time. For example, a 3-cycle fault for 60 Hz system = 0.05 seconds. RTDS will simulate this fault in real time i.e., in 0.05seconds.
Real time operation of RTDS provides the ability to connect external hardware and update inputs and outputs in real time to the simulated environment, and then test it in a closed loop. This closed loop test allows us to observe the response of protection or control device to an imposed signal, but also allows interfacing external devices to respond back into the simulated network. This is also known as HIL (Hardware-in-the-loop) testing, that is not possible with offline simulation or open loop testing tools.
HIL simulations are normally closed loop where the device under the test receives signals from the simulation and provides signals back to the simulation. HIL simulations are divided into two types: Control HIL (CHIL) where HIL testing is done through power system protection and control equipment. Power HIL (PHIL) where the concept of connecting power equipment, renewable energy hardware, and converters, motors and loads to the simulator in closed loop has become popular. HIL also allows testing of multiple devices at the same time, more like a system level testing approach. It’s very valuable for modern systems with Distributed Energy Resources (DERs) and multiple tiers of controls, to be able to test interoperability and interactions between different systems. Nodal analysis with a timestep of 25-50 microseconds is a very intense process. The processing power required increases with the size and complexity of the network intended to be simulated.
Nor-Cal Controls recently purchased a RTDS which enables our team to perform hardware in loop (HIL) simulations. RTDS simulator was used for validating a PSCAD™ model of Nor-Cal Control’s PPC against its GE PLC based hardware controller. In addition, the testbed can also be used as a productivity tool for a wide range of functions, from testing a PPC concept in the R&D phase, to a System Acceptance Test (SAT) of a fully assembled PPC and protection system. This kind of approach for controller validation during factory or system acceptance test (FAT/SAT) has significantly improved Nor-Cal’s SCADA systems overall quality control.
The number of IBRs coming online is growing, which means the system is moving to a very low inertia and thus the system becomes more vulnerable and brittle as it can move around very quickly. It’s important to have realistic models during the planning stages. The need for EMT modeling will only continue to grow exponentially as it provides greater depth of analysis over wide a range of frequency and can reproduce fast transients on the power system.
Nor-Cal Controls is committed to staying current to industry requirements and helping our customers meet their project specific requirements. Contact us to discuss real time EMT model requirements for any of your existing or upcoming PV-Solar, PV-Solar + BESS or BESS only projects. The Nor-Cal team can also help with custom SCADA/EMS/MPC, DAS and MET stations requirements for your projects.
Collaborator: Bob Lopez