In this build level the current loop
for the PFC is checked. Lab 6 is primarily meant for tuning the current loop and
optimizing its performance. It is always safe to start Lab 6 at low voltage and low
power as starting at higher power without a supervisory voltage loop can boost the
voltage due to overcurrent events and can blow away the switches. Hence, it is very
important to carefully set the TINV_idRef_pu variable so as to avoid overcurrent
trips and high voltages at the DC terminals. Also the TINV_idRef_pu is defined with
a negative sign for PFC mode of operation and with a positive sign for inverter mode
of operation.
Figure 3-21 describes the software flow for running Lab 6.
Figure 3-21 Lab 6 Software Diagram
Set the project to Lab 6 by changing
the Lab Number in the <settings.h> file, (this will be changed by powerSUITE
GUI when using powerSUITE project)
In the user settings.h file some
additional options are available, but the following are used for the tests
documented in this user guide.
In this check the SW is run on the
hardware, or the HIL platform, or both.
Refer to the hardware test set up section for actual details of the equipment used
for configuring the test. Do not supply any HV power yet to the board.
First launch the main.cfg and
select Lab 6 in the project options. The compensator style (PI compensator) and
the tuning loop (current loop) will be automatically populated. Now click run
compensation designer icon and compensation designer tool will launch, with the
model of the current loop plant with parameters specified on the powerSUITE
page.
The current compensator coefficients used for running the control loop are shown
in the following code. The user can modify these coefficients to meet the
necessary loop bandwidth and phase margin. The ideal coefficients with resistive
load are slightly different than the one used for grid connection because the
grid impedance is very low. The compensator design transfer function and
response are shown in Figure 3-22.
Figure 3-22 Compensator Design GUI
- Current Loop PI Coefficients
Once satisfied with the proportional and integral gain values, click on Save
COMP. This will save the compensator values into the project. Close the
Compensation Designer, and return to the powerSUITE page.
Turn on the auxiliary power supply and set it to 15 V. Build and load the code,
use the lab6.js file to populate the watch variables in the CCS window.
Set the load resistance to a high value 3.18 kΩ.
Set the AC input voltage to 30 VRMS with appropriate current
limit.
After turning on the AC power supply, immediately turn on the relay by writing a
1 to TINV_allRelaySet. Ensure that the relay is turned on immediately (within
3-4 seconds) after turning on the ac supply.
Set TINV_idRef_pu to –0.013 pu
To start the PFC mode, enter "1" on TINV_startPowerStage variable, the current
should now be drawn from the grid as a sinusoidal signal (with some harmonics as
it is at low power) and boost action seen on the vBus. The output voltage will
boost from 75 V to around 180 V as shown in Figure 3-23.
The current will become sinusoidal as the load is increased. This verifies start
up of PFC at 30 VRMS.
The Figure 3-23
shows PFC closed current loop start up at 30 VRMS. Scope
signals: Channel 1 - AC voltage (blue), Channel 2- DC voltage (light
green), Channel 3 - AC current (dark green).
Figure 3-23 PFC Current Loop Start-up at 30 VRMS
Now repeat the previous steps to verify start up at 220 VRMS.
At
220 VRMS, with the TINV_idRef_pu set to –0.013 pu, the bus
voltage jumps from 550 V to 640 V at 155 W power as Figure 3-24
shows.
Figure 3-24 shows
PFC closed current loop start up at 220 VRMS. Scope signals:
Channel 1 - AC voltage (blue), Channel 2 - DC voltage (light green), Channel 3 -
AC current (dark green)
Figure 3-24 PFC Current Loop Start-up at 220 VRMS
As Figure 3-24
shows, running this lab at higher input voltage without a supervisory voltage
loop results in big overvoltage across the DC terminals. Hence it is always
advisable to start this lab at low voltage and low power as previously described
for safety reasons and then slowly ramp to the desired voltage for closed
current loop tuning. Starting at 230-VRMS AC input voltage and
boosting DC voltage to 800 V directly is shown in Lab 7.
In case any overcurrent trip is observed which causes the PWMs to switch off,
please refer the notes in Lab 5 to debug this condition.
A soft start scheme is implemented at the start-up to ensure overcurrents are
reduced at start-up. For this the duty is restricted to the PWM module by
adjusting the dead band set Figure 3-25 shows
the PWM configuration for this setup where the dead band is set to large value
and slowly reduced to the nominal value to limit the current spikes.
Figure 3-25 Dead Band Soft Start PWM Configuration
Figure 3-26 shows
the effect of the start-up scheme without dead band implemented at start-up.
Without soft start, a huge current spike appears which causes over current trip
and the DC bus voltage also collapses. Figure 3-26 shows
overcurrent without soft-start implementation. Channel 1 - AC voltage (blue),
Channel 2 - AC current (light green), Channel 3 - DC voltage (dark green)
Figure 3-26 Without Soft Start
Figure 3-27 shows reduced current spike with soft-start implementation. Channel 1 -
AC voltage (blue), Channel 2 - AC current (light green), Channel 3 - DC
voltage (dark green).
Figure 3-27 Soft Start With Adaptive Dead Band
SFRA is integrated in the software of this lab to verify the designed
compensator provides enough gain and phase margin by measuring on hardware. To
run the SFRA keep the project running, and from the cfg page, click on the SFRA
icon. SFRA GUI will pop up.
Select the options for the device on the SFRA GUI. For example, for F28377D
select floating point. Click on Setup Connection. On the pop-up window
uncheck the boot on connect option, and select an appropriate COM port. Click
OK. Return to the SFRA GUI, and click Connect.
The SFRA GUI will connect to the device. A SFRA sweep can now be started by
clicking Start Sweep. The complete SFRA sweep will take a few minutes to finish.
Activity can be monitored by seeing the progress bar on the SFRA GUI and also
checking the flashing of blue LED on the back on the control card that indicates
UART activity. Once complete, a graph with the open loop plot will appear, as in
Figure 3-28
and Figure 3-29
which corresponds to plant and loop response measured by SFRA GUI respectively.
This verifies that the designed compensator is indeed stable
Figure 3-28 PFC SFRA Plant Response for Current Loop
Figure 3-29 PFC SFRA Loop Response for Current Loop
The frequency response data is also saved in the project folder under an SFRA
data folder and is time stamped with the time of the SFRA run.