As more renewable energy sources and battery storage units are specified and incorporated into power systems, consultant specifying engineers are faced with many challenges that could impact the overall design and performance of these solutions. Today, power consumers can become power producers as roof-top solar, behind-the-meter generators, and battery energy storage systems become more economical and ubiquitous. Additionally, concerns about the reliability of traditional utility electricity due to its aging infrastructure, extreme weather events, and more sensitive loads have led to wider adoption of microgrids.
What Is a Microgrid?
A microgrid is a local energy grid with control capability. Microgrids can be disconnected from the traditional grid and operate autonomously. Some microgrids may be entirely off-grid and be the prime power source for the end-users it serves.
Microgrids may incorporate a variety of traditional and renewable power sources, such as diesel or natural gas generator sets, wind turbines, solar panels, fuel cells, and energy storage. Working with a reputable power system supplier who fully understands the power system design space (Figure 1) can ensure a well-engineered solution that will achieve the project objectives.
Components Commonly Found in Microgrids
Inverter-based technologies and synchronous machines are both often incorporated into microgrids. Some components that could be utilized in a microgrid include the following.
Photovoltaic Panels. Photovoltaic, or solar PV, panels convert solar radiation into direct-current (DC) electrical energy. Solar panels require an inverter to transform the DC power to alternating current (AC) for use in most common electrical devices.
Solar panels require a large amount of physical space unobstructed by shade, but their costs have been falling steadily. Due to the extremely low or even non-existent marginal cost of generation, PV panels can be a valuable asset in a microgrid. They are, however, a highly intermittent power source and alone cannot provide uninterrupted power. As a result, they are increasingly being paired with battery energy storage systems to ensure greater utilization and reliability.
Battery Energy Storage. Battery storage when coupled with intermittent energy sources like PV panels can address the curtailment and irregularity of renewable energy sources. During the day when solar energy is abundant, energy produced in excess of the load demand can be stored in batteries to be discharged overnight rather than being curtailed. Batteries can also be charged by utility power, if allowed by the utility, when utility rates are lowest and discharged during high-rate periods or to avoid costly demand charges. As with PV panels, batteries are a DC power source and utilize inverters to charge or discharge to an AC electrical system.
Wind Turbines. Microgrids can also utilize wind turbines. Like PV panels, wind turbines are intermittent sources that produce power when there is adequate wind. Geographic location has a major impact on the viability of using wind energy in a microgrid, but in the right location, they can be very beneficial.
Fuel Cells. Fuel cell technology has been steadily improving with higher efficiencies, increased power density, and reduced costs. Having a good understanding of the expected operation and fuel availability can help aid in selecting the right fuel cell technology.
Inverters. To connect the DC output of solar, storage, and fuel cells to an AC network, the DC output must be inverted to AC power first. Inverters, which are semiconductor-based electronics, utilize Insulated-Gate Bipolar Transistors (IGBT) to invert the DC power to AC power by turning on/off the IGBTs at a pre-defined sequence and speed to generate pulses of currents, which is known as modulation. Additional filters and other electronic devices are used at different stages of the conversion to produce a clean sinusoidal waveform.
Inverters can be grid forming or grid following. In grid-following inverters, a voltage reference must be pre-established by another AC energy source (such as the utility or a generator set) in order for the inverter to synchronize. In grid-forming inverters, no reference voltage is required and the inverter can establish the voltage of the network independently. Solar inverters are most commonly grid following; whereas, storage inverters can be either grid forming, grid following, or both.
Synchronous Machines (Generator Sets). While PV panels and batteries are DC power devices, most electrical equipment utilizes AC power. A generator set, by its nature, generates AC power and can directly be used to power AC loads on demand and can be paralleled with other AC power sources such as the utility grid.
Incorporating a synchronous machine into a microgrid application has many benefits, as they are dependable, dispatchable energy sources, which, if fuel is available, can always be utilized to meet the electrical load demand (Figure 2). Generators, with their high-power density, can start quickly, accept large block loads, and provide a reference to grid-following components, such as PV or battery inverters. With intelligent controls in modern generator sets, autonomous synchronization and load sharing can be done at the equipment level.
There are a wide variety of generator sets available at various power levels that accept different fuel types, operate for various duty cycles, and produce a range of noise and emissions levels to meet specific project requirements. Generator sets also provide inertia to a microgrid system that inverter-based energy sources cannot provide, contributing to the overall stability of the system. Some microgrids, consisting of synchronous generators and PV energy, for example, will require a minimum capacity of synchronous generators to be always operating to maintain inertia.
Two common concerns when utilizing generator sets for microgrids and prime applications are noise and emissions, with increasing pressure to be environmentally and socially responsible. Modern generator sets are available that produce exceptionally low emissions due to advancements in combustion and after-treatment technologies. Additionally, low-carbon fuels such as Renewable Natural Gas (RNG) and biogas can be used for carbon-neutral operation. Noise can be addressed through highly sound attenuated enclosures and buildings. Noise and emissions at the site level can also be minimized through intelligent controls, which optimize the use of each asset to minimize the duration and timing of operation.
Paralleling AC Power Sources
To parallel or connect alternating power sources, several characteristics of the AC output voltage must be the same or within an acceptable range. It is essential that the waveform and phase sequence are the same. Also, the frequency, phase angle, and voltage amplitude must be within the acceptable range to safely and effectively parallel the AC sources.
Synchronous generator sets naturally produce AC voltage and they can be either in load-sharing mode (analogous to grid-forming inverters) or load-govern mode (analogous to grid-following inverters). Load sharing is the proportional division of kW and kVAR total load among multiple generator sets in a paralleled system, and is essential to avoid overloading and stability problems. In load sharing, the generator sets regulate their voltage and frequency, and can either be isochronous, where voltage and frequency stay constant at 100% regardless of the load, or droop, where the voltage and frequency vary as the load varies.
Isochronous load sharing provides constant voltage and frequency, but requires the sources to communicate with each other. Droop control does not require communication, but results in varying voltage and frequency as the load varies.
The load-govern function applies when a generator set, or multiple sets, are paralleled to a utility or the grid. Because the utility voltage and frequency are fixed, the generator sets regulate their kW and kVAR output, instead of their frequency and voltage.
The synchronous generator’s onboard paralleling controls are expected to have paralleling and protection capabilities built in, and would be responsible for all the following:
- Paralleling functions.
- First start arbitration.
- Synchronizing (Ø, V, and Hz).
- Load sharing (kW and kVAR).
- Built-in safe manual paralleling.
Inverters are responsible for the lower-level synchronization, protection, and metering. However, one essential aspect when paralleling is load sharing across all the sources. Load sharing across synchronous generators is simply accomplished via the on-board paralleling control. When it comes to load sharing with inverters, one method would be to set the generator sets to operate as the utility (load sharing/grid forming) and the inverters would operate in grid following and act like a constant real-reactive power (PQ) source. And an external control system would send these PQ commands to the inverter.
At the heart of any microgrid power system there must be an autonomous controller. The microgrid controller is expected to do, as a minimum, the following:
- Optimize energy production from all energy sources to meet demand.
- Maximize the output power of renewable sources.
- Control loads via load add and load shed.
- Minimize emissions and fuel consumption.
- Achieve the lowest Levelized Cost of Energy (LCOE) and Total Cost of Ownership (TCO) for all assets.
- Allow for monetizing assets via grid support and demand response programs.
- Ensure assets in the system are best utilized.
- Adapt to changing weather conditions (such as cloud cover, wind speed, and other conditions).
- Operate the system entirely or partially off-grid.
- Monitor and/or control assets remotely with real-time notifications.
What is not expected from the microgrid controller is the lower-level machine-to-machine operations such as synchronizing, load sharing, and protection. By moving these operations to the individual machines, single point of failure is eliminated, and therefore, reliability is increased. Failure of the microgrid’s autonomous controller does not jeopardize the reliability of the microgrid power system. Any failures must be analyzed and mitigated for safe and reliable operations, which is typically performed by the microgrid controller provider.
Designing a Well-Functioning Microgrid
Microgrids can provide significant benefits for resiliency, sustainability, and cost savings. Every project should begin with a feasibility study and stability analysis to determine the best mix and sizing of assets to meet the goals of the project (Figure 3). The sequence of operation should be written to ensure system resiliency and power quality with mitigated failure modes. The integrated paralleling and protection controls for the synchronous generator sets as well as the microgrid controller are critical components to ensure seamless operation, system recovery, failure mode analysis, and system optimization.
As with all specifications, the microgrid specification should be based on function and performance. Working with a reputable power system supplier, who fully understands the power system design space, can ensure a well-engineered solution, which will achieve the project objectives.
—Jaimie Hamilton-Antonson is a technical advisor for the Energy Management Solutions team at Cummins Power Generation, focusing on microgrids and hydrogen applications. Prior to this role, she supported lean-burn gas generator sets since 2014, focusing on renewable fuel projects as well as unique and complex applications. Jaimie has a bachelor’s degree in Mechanical Engineering from Loyola Marymount University, and a master’s degree in Mechanical Engineering from the University of Minnesota, Twin Cities.