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Multi-source regenerative systems power conversion - REGENESYS

Final Report Summary - REGENESYS (Multi-source regenerative systems power conversion - REGENESYS)

Executive Summary:
The REGENESYS system forms part of the overall Cleansky Green Rotorcraft Integrated Technology Demonstrator.
The objective of REGENESYS is to develop the demonstrator of a flexible power conversion system incorporating multiple energy sources, as well as energy recovery and storage capability that can be further developed toward a new aircraft power distribution system which by the use of energy recovery, storage and subsequent managed distribution will reduce the energy footprint of the aircraft leading to reduced mass, maintenance, emissions and noise.
Specifically REGENESYS is intended to provide a technology demonstrator comprising:
• Bidirectional power converters (BDC) which interface to a nominal 270VDC power bus with the capability to connect at their output to either energy storage elements either loads
• Energy storage element(s)
• Variable loads
• CCU and associated SW

The goal is to demonstrate a system that will intelligently control all bidirectional converters interfacing loads or energy storage components of the system to provide a stable, flexible power distribution.
Following an initial phase of research into all aspects of the system requirements the key technologies required to implement the system have been identified.
As an outcome of this research a series of technical specifications have been defined for each sub-system of the REGENESYS Demonstrator.
Essentially the system has been subdivided into the following major sub-systems:
- 270V to 28V 1.2KW bidirectional converters (BDC’s)
- 270V to Energy Storage System bidirectional converter (BDCR)
- Energy Storage system capable of storing regenerative energy from a Rotor Brake (peak energy of 30KW) following landing, also energy storage from other regenerative loads
- A central Control Unit (CCU) for control of the Test Rig and the REGENESYS Demonstrator
- A comprehensive Test Rig accommodating the REGENESYS Demonstrator and providing for full system and sub-system testing
Each equipment has been designed, developed, built and tested in accordance with the defined specifications.
Following prototyping and characterisation of the BDC, the design has been engineered to a TRL6 level and five units assembled, tested and characterised.
The BDCR has been prototyped and characterised and then a second unit engineered at TRL5 level into a 6U 19” rack sub-assembly.
For the Energy Storage sub-system a Lithium Titanate battery solution has been identified, obviating the need for a supercapacitor pack as originally conceived. This has resulted in a five section battery pack, each section comprising 15 cells. The overall battery pack therefore comprising 75 13Ahr cells, providing a nominal voltage of 185V.
The Test Rig has been designed, built, tested and integrated. The REGENESYS Demonstrator equipment has been incorporated.
The REGENESYS System Demonstrator has been successfully tested against the defined System & Sub-systems Test Plan. The system of multiple BDC’s is stable, some minor problems associated with instability of the BDCR converter at high power.
Project Context and Objectives:
Within the ambit of more electric aircraft the objective of replacing existing functions with electrical equivalents is to reduce mass, volume, maintenance and pollution. This is achieved by the replacement of hydraulic and pneumatic actuation, for example landing gear, flaps etc, with electrical actuators such as motors and solenoids. This approach removes the use of corrosive hydraulic oils and significantly reduces the periodic maintenance necessary to maintain the functions in operative order. In addition the associated reduction in system mass reduces the aircraft fuel consumption and thus its environmental foorprint.
Within current aircraft the electrical distribution system is based on an AC generator for some specific loads and a DC supply for the rest. This DC bus distribution currently operates at a nominal +28V.
Evidently the more electrical loads implemented, the higher the bus current necessary to support them and the heavier becomes the cabling, negating some of the benefits of electrifying the aircraft.
If a higher bus voltage is introduced then the same power to the aircraft loads can be achieved with much lower bus current thereby reducing the associated cabling mass.
Further system enhancement is achievable where it is possible to recover energy on a continuous or periodic basis from aircraft functions. For example waste heat of engines, vibration sources, solar energy, rotating loads such as turbines, turbofans, helicopter rotors etc.
The Regenesys programme was conceived to demonstrate the feasibility of implementing such a system for aircraft use, specifically for a rotorcraft, although the outcome of the programme is equally applicable to fixed wing aircraft.
Given that the majority of electrical actuators and loads on an aircraft work at +28VDC, it is necessary to introduce DC to DC converters between the higher bus voltage, currently set within the industry at +270VDC, but with a view to moving to +540VDC (+/-270VDC) in the future, and the current loads.
If advantage is to be taken of the regenerative power derived from specific loads or sources then there needs to be a storage element within the system to retain that energy until it is required.
The call required that the proposed demonstrator system be aimed at the power distribution system of a small helicopter from which it was determined that a representative demonstration system would be as outlined in the following schematic:

The prime objectives of the Regenesys Demonstrator can be summarised as follows:
- To validate the feasibility of implementing an aircraft power distribution system incorporating regenerative energy sources
- To identify the preferred technology capable of fulfilling the requirements
- To assess the certification, safety & reliability aspects as applicable to future systems
- To specify the design requirements of the system and sub-systems
- To design, prototype build, test and characterise the 270VDC to 28VDC converter (BDC)
- To design prototype build, test and characterise the 270VDC to Energy Storage converter (BDCR)
- To design build, test and characterise the Lithium Titanate battery pack and associated battery management system
- To build, test and characterise 5off BDC converters to TRL6
- To build, test and characterise a compact sub-rack BDCR converter
- To design, build, test and integrate the Test Rig
- To integrate the Regenesys Demonstrator hardware in the Test Rig
- To perform scenario based testing of the Regenesys Demonstrator system

Project Results:
The following presents a brief outline of the main Science & Technology results from the REGENESYS programme.
Each of the main sub-systems is dealt with in turn.
i) 270VDC to 28VDC BDC
A 1.2KW bidirectional converter has been designed and realised. This converter comprises three main elements:
- The Power Cell that comprises all the power components of the dual active bridge DAB converter.
- The Converter Loop Controller (CLC) board, a fully FPGA based control platform that drives the power boards.
- the MicroController Module (MCM) board that manages the external communications and the control of the BDC unit.

Power Cell:

The Dual-Active-Bridge (DAB) topology has been chosen among different solutions, for the following main advantages:
(a) No additional clamp or snubber circuit required.
(b) Small magnetic components with further possibility of using the leakage inductance of the transformer instead of a series inductor.
(c) Zero-Voltage-Switching (ZVS) turn on of the power devices.
(d) 1st order system and current generator structure (ideal for parallel operation).
The DAB converter consists of two H-Bridges interfaced through a HF transformer. The leakage inductance and the series inductor LC are used to store energy and help achieve soft switching for all the power devices. Both the direction and the amount of power flow can be controlled by adjusting the phase shift Φ between the driver signals of input and output full-bridges.

BDC Converter block schematic
Control is implemented using a PI control loop with feed forward compensation, as outlined below:

The BDC control system block diagram is shown below. It consists of DC bus voltage control loop with a feed-forward current loop. The measured voltage VO is compared with the reference voltage (28V) and the error is fed through the PI controller to generate desired phase-shift between the primary and secondary square voltages. A feed-forward phase-shift compensation is added to make the system response faster.

BDC control scheme: voltage and current feed-forward loop.

This converter design has been prototyped and fully characterised and on the basis of the positive performance results has been re-engineered to a TRL6 level in conjunction with the MCM and CLC within an ATR type housing.
MCM:
The MCM provides the interface between the Arinc 825 serial communications multidrop bus and the bidirectional DC-DC converters.

Its function is to receive commands from the Central Controller Unit (CCU) for the set up and control of the system DC-DC converters and to provide error messages, alarms and status data (for both the converter and itself) to the CCU.
A secondary application of the MCM is to act as the interface to the Battery Pack management system (BMS).

In order to manage the demonstrator the functionalities of the MCM, considering a future implementation of a real aircraft system and ensuring expandability and performance, the core of the MCM is based around a Texas Instruments TMS320F28335 Delfino Floating Point Series microcontroller with associated peripherals to provide the communications interfaces, analogue & digital I/O and PBIT/CBIT circuitry. .
The design may be divided into the following active blocks:
o Control Logic
o Analogue Inputs
o Analogue Outputs
o Digital Inputs
o Digital Outputs
o Temperature Sensors
o Power Supply
o Serial Communications

The MCM comprises a single printed circuit board assembly.

CLC:
The CLC interprets the commands received from the MCM and closes the converter control loop to ensure stable operation of the converter at its defined operating point. It acquires analogue feedback parameters of the converter (voltage and current) and via a PID algorithm controls the MOSFET switching via PWM.
Moreover it provides status data to monitor converter functional status and its own status. In the event of an alarm, the CLC takes appropriate autonomous action to safeguard the system e.g. power limit, shutdown, disconnect.
The interface from the CLC to the converter module utilizes fiber optic drivers in order to minimize EMC/EMI related problems.
The design is based around a reprogrammable FPGA.
ii) 270VDC to Energy Storage Converter (BDCR)
A 5KW bidirectional converter has been designed and realised. This converter comprises three main elements:
- The Power Cell that comprises all the power components of the two phase interleaved converter
- The microprocessor/FPGA based control platform that drives the power boards and closes the control loop
- A MicroController Module (MCM) board that manages the external communications and the control of the BDCR unit.

Power Cell:

The power cell comprises a two phase interleaved converter based upon Silicon Carbide (SiC) power mosfet’s, as illustrated below:


Control is implemented using both voltage and current control loops, the following schematic illustrates


In voltage control mode, the BDCR will take care of the DC bus voltage regulation to maintain it at the desired voltage reference u*dc, (270V). The output of the PI voltage controller is fed as an input (reference current) to the PI current controllers of the DC/DC converter phase currents. The reference current I*ph1 and I*ph2 are limited according to the value of Ilmt that is set by the supervisory controller (the user, i.e. by the MCM).
In current control mode, the voltage controller is disabled and the reference currents I*ph1 and I*ph2 are set directly equal to Ilmt. The value and the sign of Ilmt will determine the value and the direction of the battery current (charge or discharge).
The output of the PI current controller is added up to the measured battery voltage (feed forward control) to produce the reference voltage u*ph1 and u*ph2 of the DC/DC converter phases. The reference voltages and the measured DC bus voltage udc are fed to the PWM generator to produce the switching pulses for the DC/DC converter semiconductor switches.
Controller:
An existing design based upon a microprocessor and FPGA board
MCM:
Identical to that referred to above.
iii) Energy Storage System
The energy storage system was designed to fulfill the following three requirements
- Minimum energy over a 30 minutes flight
- Maximum discharge power and energy during engine start, enough for 3 engine starts
- Maximum power and energy during rotor braking (helicopter stopping), peak of 30KW

During the course of the initial research phase it was determined that the use of a Lithium Titanate battery type could fulfil the requirements of fast & deep charge and discharge rates, thus it was not necessary to pursue the implementation of a supercapacitor pack as originally conceived.
The Lithum Titanate technology has a lower energy density than the more common Lithium Cobalt variety, and apart from the 20 fold increase in current handling for charge/discharge, is significantly safer having a much higher thermal runaway temperature.
The resulting battery pack is based on a 13Ahr soft cell. 75 cells are employed separated into 5 sections. Each section is mounted within a mechanical fixture along with separators and temperature sensors.

Each section is also fitted with a Local Monitoring Unit (LMU) as part of the Battery Management System (BMS).
A Battery Management System is essential in this kind of application in order to ensure that the battery pack is safely handled.
The BMS performs the following functions:
- Measures State of Charge (SOC)
- Enables Pre-charge & Main charge contactors
- Balances all cell voltages
- Monitors all cell temperatures
- Monitors battery current & voltage
- In the event of an overcurrent, overtemperature, cell failure etc, automatically isolates the battery pack
- Reports status & alarms to host controller
The BMS is interfaced to the demonstrator system via another MCM and is controlled over a CAN bus.
The energy storage system based upon Lithium Titanate batteries is currently the optimum solution. Although it does not have the same energy density as a standard lithium cobalt battery it is significantly safer for an avionic application having a much higher thermal runaway temperature. However the battery industry is fast moving with new discoveries and designs being published every week or month. It is therefore inevitable that alternative energy storage solutions will arrive in the foreseeable future which will result in a smaller physical storage cell or one with substantially increased energy density and further improved safety.
This will have very little impact on the system since the battery pack is isolated from the aircraft bus, it may have some effect upon the BDCR and BMS designs but this will need to be considered at some point in the future.
iv) Test Rig
The Test Rig comprises four 19” 42U racks of equipment and a separate host computer.
The Test Rig comprises the following main items:
i) Main Generator Simulator
ii) 28V Loads & Actuators
iii) Regenerative Load
iv) CCU host computer
v) 270VDC power bus
vi) Data Acquisition system

The objective of the Test Rig is to enable the interconnection of the REGENESYS system elements i.e. Energy Storage System, 270V to Storage System Bidirectional Converter and 270V/28V Bidirectional Converters (4) in a representative small helicopter power distribution system and permit the overall testing and performance evaluation of the REGENESYS system.
In order to accomplish this, the Test Rig provides for the modelling of all generators and loads (active, passive & regenerative) using commercial instruments. It also provides a communication control link to the Regensys elements based upon the Arinc 825 communications protocol.
To enable efficient system set up and control all of the commercial instruments are linked via Ethernet to a hub for control and monitoring by the host computer (CCU).
In addition, to supplement sensors already incorporated within the REGENESYS system elements a series of distributed temperature, voltage and current sensors are installed and linked back to a bespoke data acquisition system for conditioning and measurement.
The output of this data acquisition unit is in turn coupled to a National Instruments PXI data acquisition and control unit, also managed via the host computer (CCU).
This host computer (CCU) provides, via a comprehensive labview HCI, the ability to configure all system parameters, operate the system, record all alarms and monitored data and make such data available for later analysis.
The following provides a brief summary of the Test Rig capabilities:
Main Generator Simulator – the current power supply provides up to 15KW output capacity (nominally 55A at 270V Bus), there is space within the Control Rack for the accommodation of a second power supply to raise the capacity to 30KW if required.
LISN – the design will adequately cope with greater than 100A line current.
Active Load – the active load provides a maximum 1.5KW capacity (60V max, 600A max)
4 Quadrant Supply – the 4 quadrant power supply, used as load or regenerative load has a capacity of +/-50V, +/-20A.
Regenerative Load – the power supply provides up to 1.5KW capacity (40V, 38A).
Resistive Loads – 2 resistive loads are capable of dissipating up to a maximum of 5KW at 270VDC bus, 2 to a maximum of 1.2KW at 28VDC. The overall Load Rack can sustain full power with all loads active. Each resistive load can be incremented in steps of 1/5 load capacity.
Data Acquisition Unit – has a maximum capability of 60 PT1000 temperature sensors, 12 voltage sensors, 16 current sensors.
PXI Sub Rack – has the capacity for 64 analogue channels @ 250kS/s, 16 analogue channels @ 1.25MS/s and 144 Digital I/O channels.
Ethernet Hub – has a capacity of 16 channels.
CCU – provides for the set up and control of all Test Rig Instruments, provides for the acquisition of all distributed sensors, provides for the control of the Arinc 825 communications to the REGENESYS system, provides for the visual display of all system set-up, control and monitor values, provides for the set-up and initialization of REGENESYS system test scenarios, provides for the storage of all received data.
v) Communications
Within the overall Test Rig & REGENESYS Demonstrator system there are a number of diverse communications links implemented.
These include CAN bus, Ethernet and Arinc 825.
In conjunction with the Topic Manager it has been determined that the preferred communications interface for the REGENESYS system is ARINC 825.
ARINC 825 is a derivation from the CAN aerospace data bus which in turn is a derivation from the original CAN (Controller Area Network) bus designed and defined by Robert Bosch GmbH in 1983.

Essentially ARINC 825 is a standardization of the CAN aerospace bus to ensure uniform application within the aerospace industry and ensure interoperability between diverse systems and other airborne networks.
CAN is a linear multi-drop bi-directional data bus conforming to international standard ISO-11898.
CAN transmits data, in a half-duplex mode, across a shared shielded twisted pair media and has advantages in terms of weight savings at the aircraft integration level. The CAN Physical Layer protocol specification provides a built-in message priority scheme coupled with error recovery and protection mechanisms that make this data bus standard applicable to avionic applications.
The CAN bus utilises a Carrier Sense Multiple Access/Collision Detection-Carrier Resolution (CSMA/CD-CR) format.
- Carrier Sense – each node waits for the bus to be inactive (bus idle) before attempting to transmit
- Multiple Access – all nodes have equal access to the bus for transmission
- Collision Detection – resolved by bit-wise arbitration

The bus comprises screened twisted pair cable with a characteristic impedance of 120Ω +/-10% (as defined in ISO11898-2) terminated at either end in a resistor of 120Ω.
The Arinc 825 protocol has been implemented within the MCM interfaced to the serial bus using an avionic Arinc 825 integrated transceiver.
At the Computer Control Unit (CCU) the Arinc 825 interface has been implemented via a commercial printed circuit board assembly and the communications protocol implemented on the host computer.
vi) Unit & System Performance
The BDC was designed as 270V to 28V 1.2KW bidirectional converter.
The selected design approach has proven to be more than adequate for the application. The converter readily achieves it maximum output power and with minor future modification will reach 1.5KW. The converter switches on with no overshoot and can be switched instantaneously from passive load to active regeneration without any problems. The efficiency of the converter is typically around 94% with some margin for improvement in a future design iteration.
The BDCR was designed as a 270V to Energy storage (185V nominal) 30KW peak bidirectional converter.
The selected design approach has been proven to fulfil the requirement. Step load in regenerative mode performance from 2KW to 30KW provides for minimum overshoot on the bus voltage.
System testing has proved that the interconnection on the same power bus of four BDC’s, a BDCR and LISN functions as envisaged. The BDC’s work stably together in both passive/active load combined with regenerative load even with rapid switching on a converter from one mode to the other.
The BDCR & Battery pack function correctly in conjunction with the BMS. Some problems arise with instability of the BDCR at high power due to the complex impedance it sees on the power bus. Some future work is required to optimise the control loop of this converter in order to ensure engine start and rotorbrake regenerative functionality.
Potential Impact:
From the results of unit and system testing to date of the REGENESYS Demonstrator the concept of a regenerative distributed power bus system for a rotorcraft or fixed wing aircraft has been proved.
It is evident that further systems development would be required to arrive at a system which could be installed on an aircraft.
However the BDC converter is already at a TRL6 level and with a further design iteration to improve electronics integration and overall packaging could rapidly arrive at a series production level.
The development of an aircraft level system could provide significant advantages
- Reduction in weight and volume of installed electrical distribution system, especially relevant as the overall electrical power requirements on aircraft increase
- A consequent reduction in the fuel consumption due to the recovery of regenerative energy. Therefore a reduction in pollution
- More intelligent control over system loads via the Arinc 825 interface
- Significantly improved status, monitoring and alarm information
- The possibility of improved system availability & safety via communications redundancy and alternative power bus structures
At the outset of the programme it was determined that the results of the programme would not be disseminated outside of the partners, topic manager and Clean Sky.

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