Mechanical Power Inversion for Islanded Solar Generation

Project description

Suggested Project Scope To design, build, and test a power inverter that can operate without an external voltage reference and is resilient to demand adjustments, based on mechanical power conversion. Project Purpose Preliminary research has indicated that there does not appear to be a power inverter available to the public that is capable of grid-forming based only on a solar (or any other variable DC supply resource, for that matter), without a battery acting as an energy store and backup source of power supply. Such a device could be used in situations where an AC power supply is required, but demand can be varied as the supply power resource allows – for example, for off-grid desalination using electrolysis, irrigation pumping, or for any remote location that only requires occasional power supply. This project would develop a product to fill this market gap, allowing the avoidance of expenditure on the battery component currently required by such products. Top-Level Concept Converting the DC power supplied from variable renewable sources is theoretically easy using a relatively simple IGBT switching stack, coupled with capacitors and inductors to approximate a sine wave output. However, such a system intrinsically does not supply any significant quantity of system strength (the ability to maintain a consistent voltage waveform through a disturbance) or inertia (the ability to maintain the output frequency at a consistent value through changes in demand). Traditional (i.e., thermal or hydro) power generators supply these properties by virtue of having large spinning masses included in the process of converting the energy supply to AC power. Thus, a conceptually simple method of allowing a variable renewable power source of supplying these properties is by first converting the supplied DC power to rotational power (i.e., a DC motor) and from rotational power to AC power (i.e., an AC generator). Such a system clearly needs a means of controlling the supply and demand of the system so as keep the power throughput in balance, and therefore the system frequency stable. It is proposed that this could be achieved through controlling both the supply and demand independently, through methods such as:

• To ensure DC supply does not exceed AC load:
  • Control of DC supply through direct control of the power point tracking of the solar input (assuming the power source is indeed solar), acting by shifting the solar supply away from the maximum power point as required
  • Control of the rotational speed of the system through DC motor speed control (PWM)
• To ensure AC load does not exceed DC supply
  • Communication to an external load management system, instructing it what is the maximum available power capacity
  • An automatic under-frequency load shedding scheme, like what is in place in large-scale power grids

The system controller should be capable of at least one method of reducing the power supply, and one method of controlling the power demand in order to keep the system in balance. Some difference between the supply and demand is of course acceptable for a short duration, due to the inbuilt design of system inertia, but this clearly cannot be maintained indefinitely, and as such the system response to imbalances in supply and demand will need modelling based on the selected control system and configuration parameters (e.g., values of gains for PID control of PWM motor speed control). Depending on the final design, it is possible that the mechanical components of the system (the DC motor, AC generator, and mechanical connections) may have sufficient inertia to maintain an electrically islanded power system, however it is considered likely that a flywheel will also be required to provide enough inertia that the system controller has sufficient time to react to significant changes in load (within the limits that can be supplied at the time by the variable DC supply). The scale of the inertia requirement would of course need to be calculated for any given load requirements (i.e., for a given step change in load, what maximum deviation of frequency and what ROCOF might be acceptable varies by load). Conceptually, the power and control of the system is shown in the below diagram, with electrical power shown in black, mechanical power in blue, and control objects/signals in red.

Industry involvement

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