Welcome! This is my first blog post and I’ve found it to be the perfect opportunity to share my final year engineering project. Voliner is a name that came up by joining the Spanish words “Volante” (flywheel) and “Inercia” (inertia). The project is the Development and implementation of a flywheel energy storage system with applications in a smart power grid. I hope you like it!
Project scope:
- Creation of a flywheel energy storage system
- Design of the mechanical assembly
- Construction of a 3-phase AC-DC bidirectional electronic converter
- Implementation of software for the control system and bidirectional AC-DC conversion
- Development of a control strategy to stabilize voltage fluctuations
- Integration of active voltage regulation, start-up, shutdown and emergency subsystems
- Creation of a GUI with osciloscope-like plots and PID tuning tools
- Design and implementation of a commissioning routine
Motivation
The distributed location and intermittency of renewable energy sources (day and seasonal cycles, as well as climatic and meteorological dependencies) require reformulating three classic characteristics of interconnected systems:
- Unidirectionality of the power flow (from generation centers to consumers)
- Centralization of their management
- Generation based only on-demand requirements
In order not to waste the available renewable energy, excess generation must be injected into the system and/or stored until required, which requires multidirectional flow of power and decentralized management, located in each network and planned according to predictions of production and energy consumption. This requires real-time control systems to compensate for discrepancies between these forecasts and their actual manifestations. The concept of Smart Grid is a potential way to address these challenges.
Smart Grid
Summary
The work carried out is part of a project to expand the university’s intelligent micro-grid, by providing hybrid energy storage for it. The energy storage solution designed has three main components:
- A flywheel (mechanical device to store kinetic energy)
- A three-phase induction machine
- A two-way AC-DC electronic converter
The objective is to stabilize the DC bus voltage of the network, suppressing both the medium frequency and high frequency components of the currents circulating through the battery bank, conserving the batteries health and improving the overall operation of the network.
We’ll use the term induction machine and not induction motor because it operates as a motor (storing kinetic energy) or as a generator (returning stored energy) depending on circumstances.
The converter’s control strategies are implemented in a Texas Instruments F28335 DSP, programmed in C language.
Introduction
To increase the reliability and quality of the energy resource in a smart grid, energy storage is needed. The devices to achieve it can be divided into the following categories.
- Conventional: battery banks, capacitors
- Unconventional: reversible hydroelectric plants, hydrogen-based energy storage systems or systems based on flywheels.
Prior to the realization of this project, the smart grid of the Automation and Control Laboratory (LAC) had a single energy storage system, which is a bank of lead-acid batteries. Due to its nature, its main weakness was the early degradation that batteries suffer when subjected to micro cycles of charging and discharging, as well as repetitive current peaks.
To address this problem, we introduced of a second energy storage component, in the form of a Flywheel Energy Storage System (FESS), thus obtaining hybrid energy storage. This allows a separation, based on frequency, of the current of the storage devices:
- Low frequency –> battery bank
- Medium frequency –> FESS
- High frequency –> capacitors in the DC bus.
Since the whole system is connected by a DC bus, we can associate currents with power (P=IxV where V is constant)
The following figure shows a graphical representation of the resulting hybrid energy storage system, which is also the base system model used to create the control strategy:
Hybrid energy storage resulting from the combination of a battery bank and a FESS is a solution that combines the best of both worlds!
Advantages of Flywheel Energy Storage Systems
- Can withstand a high number of charges and discharges without degrading.
- Can provide high-value current peaks (High power capacity).
Disadvantages of Flywheel Energy Storage Systems
- Has a high self-discharge (mainly due to friction losses).
- Has a relatively low energy capacity.
All the disadvantages have less impact if we combine it with batteries!. Batteries have a low self-discharge rate and a relatively high energy capacity.
The characteristics of both storage devices complement each other. The presence of the FESS and capacitors allow both the voltage of the DC bus and the battery current to evolve smoothly towards a new steady-state value, not only increasing battery life, but also improving the overall operation of the smart grid.
If the duration of the disturbance is short enough, all energy is delivered or received by the FESS and both the DC bus and the current of the batteries remain unchanged.
Description of the Flywheel Energy Storage System
An inertia flywheel energy storage system (FESS) is an energy storage device that uses a rotating element to store kinetic energy. This element is coupled with the shaft of an electrical machine that works as a transformer between the electrical and mechanical domains. The electrical machine can work as a generator or as a motor in the following situations:
- Motor: Increase its kinetic energy by accelerating the rotating element taking electric energy from the grid.
- Generator: Decrease its kinetic energy by slowing down the rotating element delivering electric energy to the grid.
Normally the speeds at which the high axis rotates are quite high (20000-50000 rpm) in order to store as much energy as possible, since the rotating kinetic energy is proportional to the square of the rotational speed. To decrease friction losses the rotating element is usually placed in a vacuum chamber, with the shaft upright and levitating magnetically. This project consists of a small-scale FESS construction and control, for teaching and experimental purposes, so it will not have the latter and the rotational speed it’s going to be lower. While its characteristics and dimensions are not the same as that of a FESS for higher power applications, it’s a device that serves well to research both energy management strategies and induction machine control.
Mechanical assembly
This section briefly describes on the design process of a series of structural components necessary for the correct and safe operation of the system. SolidWorks 3D modeling software was used to perform simple and visual representations.
The design process began with the modeling of the existing components. Those are, the inertia disks (two identical disks were used) and the induction machine.
Based on these 3D models all other components were designed:
- The mechanical coupling between the inertia disks (flywheels) and the electrical machine
- The mounting base to place and fix the FESS
- The protective acrylic dome that prevents accidental contact with the moving parts of the system.
Construction of the inertia flywheel
The solution adopted for the coupling consists of two parts, a commercial self-aligning coupling device (BLK 130) and a custom-made steel insert.
The steel insert fulfills two functions:
- To provide an adaptation between the external diameter of the self-aligning coupling and the internal diameter of the inertia disks
- To provide a structure that allows the fastening of the disks to each other, transforming them into a single rigid piece.
The self-aligning coupling can be adjusted with its radial expansion by tightening a series of bolts located on one of its faces. When the expansion occurs, the coupling exerts pressure on the motor shaft and steel insert, achieving the desired fastening.
By joining all the pieces, we obtain a rigid flywheel composed of two inertia disks that can be directly fixed to the motor axis.
Mounting base
The design of the mounting base serves as a support and elevates the machine so that the flywheel can rotate freely. The frame is wide and has the minimum required height in order to achieve great stability.
The dimensions of the frame were chosen so that there is enough space to fit flywheels in the front of the engine and in the back, allowing a possible expansion of the system in a future upgrade (since the fan in the back of the motor can be replaced with another inertia disk).
Structural pipe with a rectangular profile of 50x20x2mm was chosen for its construction.
Acrylic dome
The last component designed is an acrylic dome that covers the rotating parts of the system while providing a mounting solution for the encoder. For its creation, an acrylic plate of 5mm of thickness was carved by a CNC laser cutter.
Complete assembly
Finally here is a cross section view and a complete one of the whole system SolidWorks model.
Electronic Converter
The designed system consists of a 0.6kgms2 flywheel coupled to a 1.1KW three-phase induction machine and a 2KVA AC-DC bidirectional converter.
The converter can operate as a three-phase inverter or three-phase rectifier. It is based on the integrated circuit Mitsubishi PS21A79 that encapsulates the three-phase IGBTs bridge with their respective drivers and protections, and Texas Instruments F28335 DSP where control strategies are implemented.
Control Strategy
The control strategy implemented is an adaptation of An Innovative Control Strategy of a Single Converter for Hybrid Fuel Cell/Supercapacitor Power Source. That proposes a way to decouple currents in the frequency domain for a Fuel cell and a bank of supercapacitors. For this variant instead of the fuel cell, a battery bank is used, and the place of the supercapacitor bank is occupied by the flywheel (and the capacitors in the DC bus).
Let’s see a simplified schema of the implemented control topology, which is reduced to a number of nested PID controllers.
- At the inner level we can see the regulators that control the induction machine’s currents in the dq frame, i.e. id and iq.
- The setpoint of the id current is set to such a value that it generates the desired magnetization level for the machine.
- the iq current setpoint can be generated by two different controllers depending on the operating mode in which the system is located.
- During normal operation the iq reference is generated by the voltage regulator so that, for example, if you want to increase the voltage of the DC bus you get power from the flywheel by applying a negative torque (or a negative quadrature current).
- For complementary system operation (start up, shutdown and limitations) the iq setpoint is generated by the speed control controller.
- The voltage regulator reference is driven by the recovery regulator, responsible for
- Keeping the flywheel spinning at a certain speed when the system is in stand by
- Restoring that speed after an event in which the system delivers or absorbs energy.
The speed reference is adjustable and should be chosen based on a trade-off, since represents the amount of energy that the system has stored and ready to deliver, but it also conditions the amount of energy that the system can absorb (due to the mechanical and electrical limitations of the machine).
Experimental results
Steps and pulses of 4A load current were performed using a resistor bank to verify the correct operation of the designed FESS. The results obtained in each test are compacted into a figure that shows:
- Components of the stator current in the dq frame with their respective setpoints.
- DC-side currents of the converter, particularly the current by the battery (ibat), the current by the charge (iload), and the current delivered/absorbed by the converter (iFW).
- DC bus voltage (VDC), its setpoint (VdcRef) and voltage evolution with FESS disabled (natural). It should be noted that this last curve arises from a second test and not from a simulation or estimation.
- Rotation speed of the flywheel and its stand by speed (natural).
Step test
We begin by analyzing the system response to a 4A load step.
- When the step occurs the system reacts by producing a negative flank of current iq while the current id remains constant.
- The evolution of iq is reflected in iFW which implies that the charging current flank is delivered by the FESS and not by the batteries.
- The currents iq and iFW then evolve smoothly towards their resting value.
- The iFW current flank and its slow extinction results in a smooth evolution of the battery current and voltage of the DC bus towards its new regime values. It’s observed that the voltage evolution is much smoother than when the FESS is disabled (natural).
Pulse test
Let’s analyze now the system response to a 4A load pulse of 300ms duration. The FESS does not completely eliminate the disturbance, but reduces the amplitude of battery current and DC bus voltage variations by approximately 75%.
Conclusions
- The built system proved to have remarkable structural robustness without the need to add external bearings, thus managing to maintain the levels of energy loss by friction at relatively low values.
- The use of a field oriented control (FOC) played an important role in the control of the induction machine. While it is often used to control speed or torque, it turned out to be versatile and robust enough to control the power flow between the FESS and the smart grid.
- With regard to the operation of the FESS as part of a smart grid, the hybridization of the energy storage achieved by introducing a second storage device significantly reduced the presence of medium and high frequency components in the current circulating through the battery bank and in the voltage of the DC bus. As a result, when there are abrupt changes in the smart grid load, these variables evolve smoothly towards their new steady-state value, improving battery life and overall operation of the smart grid.
- The FESS has a limitation in its system response time. It cannot react fully in the first milliseconds after a disturbance. The response time is formed by the combination of the electrical constants of the induction machine currents found in the most internal control loop and by the control structure itself. Adding capacitors is an effective way to compensate this.
The voltage disturbance compensation time is between 5 and 10 times longer than the current control response time because disturbance compensation is performed in a loop outside the current control loop in a cascading control scheme. Having a smaller ratio would cause oscillations due to interference between control loops and even instability.
- The broad DC bus voltage range (240V-300V) introduced an additional degree of complexity the system dynamics depends on the voltage value. As a result, PID regulators had to be adapted to perform properly across the smart grid operating range. The variation of this voltage also conditions the ability of the id and iq current regulators to act, which results in a limitation in the system power capacity (remember that P=VxI).
- The self-discharge of the constructed FESS is much greater than that of its commercial equivalents because those have several constructive advantages, the main ones being magnetic levitation (which significantly reduces the losses present in the motor shaft bearings) and the vacuum environment (which eliminates friction with air). This difference doesn’t have a significant impact in the system dynamics, so the results obtained are valid and scalable to commercial FESS. However, this has an impact on the energy consumption needed to keep the flywheel spinning at stand-by speed.