Power Electronics
The electronics perform the following functions:
- Control the force that is required to unload the flywheel.
- Rotate the flywheel.
- Regulate the voltage that is on the DC bus.
- Generate the voltage and the current that is used in the event of an outage.
The four electronic assemblies are in the following list: static switch, field coil driver, utility inverter and flywheel inverter.
Static Switch
Refer to illustration 1 in order to view the electronics for the static switch. 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 1 | g00655027 |
Static Switch Assembly | |
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.
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| Illustration 2 | g00690667 |
Sine Wave of the Thyristor | |
Utility Inverter
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| Illustration 3 | g00690668 |
Simplified Utility Inverter | |
The utility inverter is used in order to charge the DC bus. The utility inverter contains a three-phase full wave rectifier and other electronics. The electronics rectify the AC voltage from the inverter node and the electronics invert the DC voltage back to AC voltage for the inverter node.
Gate Signals
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. The gate signals operate at a frequency of 16 kHz. The gate signals have a duty cycle of 30 percent. The thyristors will begin to shift just before crossing zero. The thyristors are enabled for a period of time just before crossing zero. This period increases in order to increase the DC bus voltage. Refer to illustration 4. 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 5.
During discharge or loss of utility, the utility inverter will invert the DC bus voltage to AC voltage that is 480 volts and 60 Hz. This power will be identical to the voltage that is provided by the utility.
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| Illustration 4 | g00690670 |
Gate Signals of the Thyristor | |
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| Illustration 5 | g00690674 |
Gate Signals of the Composite Static Switch | |
Field Coil Drivers
The field coil that is on the top of the flywheel is connected between the positive rail and the neutral rail. The bottom field coil is connected between neutral and the negative rail. 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.
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| Illustration 6 | g00729581 |
Field Coil Circuit | |
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
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 an outage.
- Maintain the DC bus Voltage.
Illustration 8 shows the way that the inverters are connected together. The system uses a ± 450 VDC split bus. The utility's neutral wire is the reference potential. This neutral is also connected to the neutral post on the armature of the flywheel.
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| Illustration 8 | g00690715 |
System Inverters | |
Power is provided by one inverter at a time to the DC bus. Illustration 9 shows the flow of power for the monitoring mode and the standby mode. The utility inverter rectifies the power from the utility. The utility inverter also maintains the DC bus voltages. The flywheel inverter extracts power from the bus. The power is inverted into a AC signal that is 3 phase. This signal is used to spin the flywheel.
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| Illustration 9 | g00690718 |
Power Flow (standby) | |
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 DC bus voltages. The utility inverter extracts power from the DC bus. The power is inverted into an 480 volt AC that is three-phase. This is delivered to the load that is attached to the output of the system.
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| Illustration 10 | g00690721 |
Power Flow (Discharge) | |
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" and the flywheel inverter modules are designated "1, 2, and 3".
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| Illustration 11 | g00690723 |
Inverter Modules | |
Each of the inverter modules have an internal driver board. The driver board provides an output signal that is a representation of the current flow in the inverter module. The driver board also provides error signals for the system.