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TRANSCRIPT
Hello again! Welcome back to the converter features module.
In this lesson you will learn about the dead time violation detection feature - DTV detection - and
also about the Parallel Converter DTV Detector.Let's begin with the example of one IGBT leg.
The dead time refers to the amount of time when both switches of the same leg must be
off when transitioning states. Dead time is important to avoid the state in which
both switches conduct at the same time. If this is allowed, it may cause shoot-through,
a condition in which a low resistance path is created across the power supply leading to high
currents that can be destructive to the device.In Typhoon HIL devices, the deadtime violation
detection feature allows detection and reporting of destructive switching
states in the legs of a given converter.The DTV detection feature uses a dedicated
hardware unit on Typhoon HIL devices. This unit operates with the same time resolution as the
digital input sampling. This can be as low as 3.5 nanoseconds, as explained in a previous lesson.
The DTV detection feature operates based on the converter control inputs, and supports the
standard 2-level inverter as well as the 3-level NPC, and the 3-level T-type NPC topologies.
The switching states for these topologies are illustrated here
with destructive states highlighted. More information about the destructive states
can be found in the documentation available in the materials tab.
The destructive states are reported by the DTV detector using a flag in the HIL SCADA.
The DTV flag status can be retrieved using the HIL API.
DTV flags are also available as digital signals for each converter leg. This allows
you to use the shoot-through event as a trigger to capture and analyze the cause of the problem.
Now, let's go to the Typhoon HIL Control Center and check hands-on how the DTV detection works.
Here we are using the same example as the previous lesson, comprising
one IGBT leg with a resistive-inductive load.DTV detection can be enabled or disabled from the
IGBT leg's component properties by checking or unchecking the appropriate checkbox.
For this demonstration, let's set the deadtime period to 2 micro-seconds.
You can also go to the measurements tab and enable the current measurement of the switches to observe
the effect of the shoot-through conditions.In reality, destructive switching states are
most often caused by gate drive signals overlapping. The overlap can happen,
for instance, due to bugs in the controller or by having dead-time periods that are not long
enough to cope with the switching delays in the semiconductors. To illustrate the latter, you can
go to the timing tab and enable switching delays, defining a current dependent turn-off delay that
will intentionally conflict with the dead time period as you increase the modulating signal,
and therefore the current, through the switches.You can use the same variable delay curve created
in the previous lesson. By clicking preview, it is possible to see that the turn-off delay
will be longer than the 2 micro-second dead-time period for current values greater than 2 amperes.
Let's compile the model and open a SCADA panel like the one shown in the previous lesson.
Instead of the load current, let's now look at the current in one of
the switches and the PWM signals including the switching delays.
Let's also add another viewport on the scope to check the digital DTV flag.
In this demonstration, the simulation is running in real-time using a Typhoon HIL404 device but
remember that the DTV detection feature is available on all current HIL devices.
You can verify that the system is running properly, and that it is possible to vary
the current by adjusting the amplitude of the modulating signal in the slider.
By gradually increasing the current amplitude, you will see that there is a point at which the
DTV flag is raised due to the overlap between the top and bottom gate driving signals.
When this occurs, you can click on the DTV flag to see more information. The flag here
is informing us that a dead-time violation was detected in the component IGBT leg1.
By decreasing the current amplitude, you can return to an operating mode where there is
no dead-time violation. Then you can right click the DTV flag and choose to reset it.
As mentioned before, the digital signal from the DTV flag can be used to trigger a dead time
violation event and to observe the behavior of the system. To do so, let's change the combo box to
capture and import the scope settings. To properly configure the trigger, you can set the type to
digital, and set the DTV flag as the source.Now, let's enable the trigger and once again
increase the amplitude of the modulating signal. Once the dead time violation event
is triggered, you can see that the available waveforms are captured.
You can zoom-in on the PWM waveforms and see the overlap between the gate drive signals.
In this case, it is clear that the bottom switch turned on before the top switch turned off.
During this overlap, a shoot-through condition occurred where a very high current circulated
through the switch. This condition would be destructive for the converter.
Let's once again reduce the load current and reset the DTV flag.
Now, notice that you can also force a destructive switching
state using the switching block settings on HIL SCADA. This may be useful, for example,
if you want to test the response of your IGBT failure protection system.
For instance, if you enable the trigger again and then force the bottom switch to be high,
this will lead to a shoot-through condition and the dead time violation flag will be raised.
In the Module 2 lesson about semi-automated testing in HIL SCADA, we have shown how to
force the switch states using the HIL API.This concludes the DTV detection example.
Now let's move to the parallel converter DTV detector.
When you have parallel converters, shoot-through conditions can be harder to detect, since they
can happen between the legs of the parallel converters, even if there are no destructive
switching states in a particular leg.To illustrate this, consider the following
interleaved grid-connected converter, comprising a pair of two-level three-phase inverters in
parallel. In this topology, notice that there are 12 switches arranged in 6 legs,
with two legs in parallel for each phase. This can be better seen in this switch-level
representation of the topology.In phase A, you can see that the leg
containing switches 1 and 2 is in parallel with the leg containing switches 7 and 8.
Therefore, even if these legs operate individually with complementary switching states, note that a
shoot-through condition can be established by the parallel connection between the converters,
and a low impedance path across the power supply is created by switches 1 and 8.
To detect these conditions, you can use the Parallel Converter DTV detector component,
which supports the same topologies as the standard DTV detector.
Parallel converter DTV conditions are reported using the standard DTV flag available in both
HIL SCADA and the HIL API. This component has its own dedicated hardware unit and operates
independently and simultaneously with the single converter DTV logic.
The parallel DTV detector component is currently supported by Typhoon's HIL604 and HIL606 devices.
Each component can handle up to 4 parallel legs and you can use up to 3 components per device.
This means that 4 parallel 3-phase converters are supported per device.
Multi-HIL operation is also supported.
An example of the compiler log for the device specific hardware utilization
analysis when using three parallel converter DTV detectors is also shown in this slide.
A block diagram illustrating the logic of the Parallel DTV detector can be seen in this figure,
for the two parallel converters shown before.Notice that this component also offers an
option to tolerate a configurable overlap period between the legs of the different converters.
Unlike shoot-trough in a single leg, this configurable overlap period is allowed
since you may have some impedance between the legs, as in the filter inductance
of the interleaved topology shown here.The overlap period detection logic operates
with the digital input resolution, with a maximum supported overlap period of 5us.
More information about the logic diagram can be found in the parallel
converter DTV detection documentation by following the link in the materials tab.
Let's now go to the Typhoon HIL schematic editor and see how to
configure the Parallel converter DTV detector.We'll use the same interleaved converter shown
previously, which comprises a pair of three-phase inverters in parallel.
Since there are two legs in parallel for each phase, you will need to place one Parallel
Converter DTV detector for each phase. This component can be found in the Library explorer.
Remember that this component is only available if you select a
device that supports this component in the model settings, such as the HIL604.
You can now access the properties window of the detector component.
In the general tab, you can define the configurations of each detector unit,
such as the number of parallel legs and the leg type.
In the gate inputs tab, you can assign the digital inputs to the parallel converter DTV detector
components. The number of inputs varies with the number of parallel legs and leg type parameters.
For instance, according to the topology shown here, the digital inputs of the parallel legs
in phase A are given by 1 and 2 in the first converter and 7 and 8 in the second converter.
These are the digital inputs that must be set in the respective Parallel Converter DTV
Detector component tab. The same reasoning is valid for phases B and C and their respective
detector components.That's it for this lesson.
Thank you for your attention!For more information regarding this
lesson, check the additional documentation links available in the materials tab.
