Power Electronics
The electronics perform the following functions:
- Control the force that is required to unload the flywheel.
- Rotate the flywheel.
- Regulate the DC bus voltage.
- Generate the voltage and the current that is used in the event of an outage.
There are four electronic assemblies: static switch, utility inverter, field coil drivers and flywheel inverter.
Static Switch
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| Illustration 1 | g03404378 |
Static switch assembly | |
The static switch assembly is composed of three discrete thyristor modules. One module is connected to each of the input phases. Each module has two devices. In the system, the devices are connected in parallel. However, the devices are connected in opposite directions. The functions of the static switch are in the following list:
- Isolation of the input node during a discharge
- Synchronization of the signals of the inverter node to the signals of the input node upon restoration of the input signals after an outage .
- Initial charge of the DC bus
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| Illustration 2 | g01007660 |
Sine wave of the thyristor for phase A | |
One of the parallel sets of thyristors is turned on at a time. The thyristors only conduct current when the thyristors are forward biased. This is true for all three of the phases. Refer to illustration 2.
Gate Signals (Static Switch)
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| Illustration 3 | g03404380 |
Gate signals of the thyristor | |
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| Illustration 4 | g01007665 |
Gate signals of the composite static switch | |
During start-up, the system monitors the AC voltage that is on each phase of the input node. The thyristors are turned on by the system at the proper phase angle in order to slowly increase the DC bus voltage. Refer to illustration 3. Once the DC bus charges to approximately 350 VDC, the gate signals will be enabled for the entire portion of the phase. Refer to illustration 4.
Utility Inverter
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| Illustration 5 | g03404381 |
Simplified utility inverter | |
The utility inverter is used in order to charge the DC bus. The utility inverter contains a three-phase full wave insulated gate bipolar transistor (IGBT). The insulated gate bipolar transistors (IGBTs) rectify the AC voltage from the filter node. The insulated gate bipolar transistors (IGBTs) invert the DC voltage back to AC voltage during discharge.
Field Coil Drivers
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| Illustration 6 | g00802180 |
Field coil circuit (1) Top field coil (2) Bottom field coil | |
The insulated gate bipolar transistors (IGBTs) for the field coil are connected to the ±400 VDC bus. The field coils are used to generate magnetic fields in the flywheel which perform the following functions in the system:
- Provide a lifting force on the rotor in order to prolong bearing life.
- Generate an electromotive force that enables the flywheel to spin during start-up.
- Generate an electromotive force during discharge that provides energy to the bus.
The amount of current through the top field coil and the bottom field coil is controlled by the two Insulated Gate Bipolar Transistors (IGBTs) that are connected across the field coils.
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| Illustration 7 | g00690678 |
Gate signals of the field coil | |
One driver is always on. When both of the drivers are on, the current that is in the coil is increased.
The unloading force and the speed of the flywheel determine the current that is applied to the field coils. As the speed of the flywheel increases, the field current that is needed decreases.
Inverters
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| Illustration 8 | g03404388 |
System inverters | |
There are two inverters in the system, the flywheel inverter and the utility inverter. The inverters are used for the reasons in the following list:
- Develop the current that is necessary in order to rotate the flywheel.
- Develop the voltage that is used during a discharge.
- Maintain the DC bus Voltage.
Illustration 8 shows the way that the inverters are connected together. The system uses a ± 400 VDC split bus.
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| Illustration 9 | g01089620 |
Power flow (standby) | |
Power is provided by one inverter at a time to the DC bus. Illustration 9 shows the flow of power for the motoring mode and the standby mode. The utility inverter maintains the voltages for the DC bus. The utility inverter regulates the output voltage. The flywheel inverter extracts three-phase power from the bus. This power is used to spin the flywheel.
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| Illustration 10 | g01089627 |
Power flow (Discharge) | |
Illustration 10 shows the power flow during a discharge. The flywheel inverter rectifies the three-phase AC signals from the armature windings. The flywheel inverter also maintains the voltages for the DC bus. The utility inverter extracts power from the DC bus. The power is converted into a three-phase AC voltage. This is delivered to the load that is attached to the output of the system.
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| Illustration 11 | g03404389 |
Inverter modules | |
Illustration 11 shows the components that make up the flywheel inverters and the utility inverters. The components are Insulated Gate Bipolar Transistor IGBT modules. Three modules make up each inverter. The utility inverter modules are designated "A, B, and C". The flywheel inverter modules are designated "1, 2, and 3".
Each of the inverter modules have an internal driver board. The driver board provides an output signal. The output signal is a representation of the current flow in the inverter module. The driver board also provides error signals for the system.