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Posted on 02 September 2019

Soft Switching Three Level Inverter (S3L Inverter)

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New circuit topology for high-power PWM inverters with integrated du/dt filter and high efficiency

The S3L inverter is a novel circuit topology for three-level PWM inverters in the medium and high power ranges. Soft switching is used for all commutations, which therefore have no switching losses on principle. The result is significantly lower power dissipation and outstanding efficiency. There is also no need for an external du/dt filter.

By Manfred W. Gekeler, HTWG Konstanz University of Applied Sciences, Germany

Three-level PWM inverters have long been used for electric drives, as power grid infeed inverters for photovoltaic and wind power systems, and in power supplies. A common factor of the various circuit topologies currently used is that the power semiconductors in the inverter (IGBTs, GTOs and IGCTs) are hard switching. This means that very high voltages and currents are simultaneously present on the power semiconductors during switching. The resulting extremely high loss spikes add up to significant overall switching losses, which must be dissipated and which reduces the efficiency. In addition, high values of rate of rise of voltage du/dt result in undesirable effects, such as bearing damage in electric drives due to capacitive currents.

The S3L inverter

The soft switching three level (S3L) inverter topology, which avoids these disadvantages, was developed at HTWG Konstanz (Germany). All switching operations are soft and therefore loss-free on principle. This significantly reduces the overall power dissipation of the inverter. As a result, the cooling devices necessary for heat dissipation can be made considerably smaller, which is a major advantage in applications such as traction systems. The significantly higher efficiency is a key consideration for grid infeed inverters, such as are used with photovoltaic systems. Particularly for medium-voltage inverters in the megawatt range, the lower stress on the IGBTs, GTOs or IGCTs is very important because it allows higher utilization of these devices and increases their lifetime. The rate of rise of the output voltage can easily be limited to less than 1 kV/μs to prevent bearing damage in motors.

Basic circuit diagram of the soft switching three level (S3L) inverter

Circuit topology

Figure 1 shows the basic circuit diagram of a single-phase S3L inverter. The portion highlighted in green is the well-known circuitry of a three-level inverter in NPC II or T-type topology. The state table is shown in Table 1. The voltage applied to the load can assume the values +Ud /2 ,0 and -Ud /2.

State table of T-type and S3L inverters

This proven but hard-switching topology is augmented by a novel snubber circuit highlighted in red in Figure 1. As can be seen, it consists of simple passive (and therefore low-cost) components. The state table is as shown in Table 1. However, it should be noted that a direct transition from+Ud/2 to -Ud/2.is prohibited.

Operating principle

This snubber circuit is remarkable in operation. With the snubber in place, all switching operations are soft instead of hard. The processes during the individual commutations are fairly complex; please consult references [1] to [4] for more information. The operation of the S3L snubber circuit can basically be explained as follows.

Switch-off process: When an IGBT conducting the load current is switched off, one of the two snubber capacitors C1 and C2 limits the rate of rise du/dt of the collector-emitter voltage uCE of the IGBT that is switching off, while the current is immediately diverted into the capacitive bypass. This means that the collector current iC drops to zero almost instantaneously, while uCE only rises slowly. This largely avoids switching losses.

This du/dt limitation of the collector-emitter voltage also applies to the load voltage ULoad. This eliminates the need for an external du/dt filter, since the S3L inverter works as a highly effective du/dt filter. The value of du/dt can be limited to 1000 V/μs, 750 V/μs or another desired value by simply selecting appropriate values for the snubber capacitors C1 and C2 and the snubber choke L, as described below. No additional hardware is necessary, which spares the entire weight and volume of an external du/dt filter.

Switch-on process: When an IGBT is switched on and starts conducting the load current, the snubber choke L limits the rate of rise di/dt of the current in the IGBT that is switching on, while the collectoremitter voltage is reduced almost instantaneously to the on-state voltage drop. This is illustrated in Figure 2 by a comparison of hard and soft switching. This largely avoids switch-on losses.

Switch-on process: hard switching (left) versus soft switching (right) red: 200 V/div; blue: 50 A/div; time: 200 ns/div

In the same measure as the current in the newly switched-on IGBT only rises slowly, the current in the diode commutated into the off state drops with a limited du/dt. This has a positive impact on reverse recovery behaviour and reduces losses, in particular in the diodes of high-power inverters.

Practical aspects

Now that we have discussed the theory, it is time to consider a number of important practical issues.

What are the preferred application areas for S3L inverters? – PWM inverters with power ratings from 50 kVA to the MVA range and high switching frequencies.

How much additional complexity is necessary? – The control software can be the same as for a T-type inverter, although small adjustments (e.g. eliminating dead time) are a good idea. On the hardware side there are the additional components of the S3L snubber. The snubber capacitors and snubber diodes are very small, as can be seen from the 20 kVA prototype in Figure 3a and 3b.

Prototype of a 20 kVA S3L inverter and

What are the benefits? – That depends on the application. If efficiency is a primary consideration, such as with inverters for photovoltaic systems, S3L inverters can provide significant benefits in comparison with hard-switching topologies. If instead heat dissipation, and with it the weight and size of the cooling devices, is an important consideration – such as with traction inverters – then the lower overall power dissipation is an advantage. Fig. 4 shows a direct comparison between a hard-switching NPC inverter and an S3L inverter, with both inverters built with low-cost standard IGBTs. If du/dt limiting is necessary, for example with three-phase drives, the built-in du/dt limiting can be useful because it completely eliminates the need for external du/dt filters.

Comparison of the relative losses of inverters with standard IGBTs. The S3L inverter is clearly better

Are there any restrictions with regard to soft switching, such as operating conditions that must definitely be avoided? – Switching transitions directly from +Ud /2 to -Ud /2 are not allowed. Otherwise there are no restrictions. All switching operations are soft.

Is that also true with different load phase angles? – Yes. All load phase angles from 0° to 360° are possible. This means that both synchronous and inductive motors can be operated in motor and generator mode without restrictions.

Is compliance with specific times, such as a minimum pulse duration, necessary for generating the control signals? – It is a good idea to respect minimum values for pulse duration and interpulse time to ensure that the processes in the snubber circuit are completed before the start of the next switching operation. However, there are no serious consequences if this does not happen. It does not cause component damage or disrupt the operation of the snubber circuit. The only effect is a slight increase in switching losses.

What switching frequencies are possible? – With the 20 kVA prototype shown in Figure 3, 34 kHz was achieved with no problem. Lower values should be used with higher-power inverters, but they can still be significantly higher than with hard-switching inverters.

Does all of this still work when parasitic inductances are present? – Yes. The only effect is a slight increase in switching losses. However, as has been shown by simulations, there are no voltage spikes above Ud on the IGBTs and the diodes. Although inductive voltage spikes occur when the IGBTs switch off, the collector-emitter voltages do not exceed Ud because a charged or discharged snubber capacitor is always present in the relevant circuit.

What about di/dt limiting? – For all switching operations, the current rate of rise di/dt remains limited in V1 to V4 and D1 to D4. Practical experience shows that with inverters rated at about 100 kVA the value is approximately 30 A/μs, and with inverters in the MVA range it is approximately 100 A/μs. This makes diode current decommutation non-critical.

Why is there no need for an external du/dt filter? – The snubber circuit of the S3L inverter limits the rate of rise of voltages directly in the switching process in two ways: by the action of the snubber capacitors C1 and C2 and by suitable gate control of the IGBTs. Practical experience shows that values in the range of approximately 750 V/ìs to 1000 V/ìs are reasonable. External filter components, such as chokes or capacitors, are not necessary.

How can the values of C and L be dimensioned? – That depends on the optimization goal. For example, if the main interest is built-in du/dt filtering, the value of C can be calculated as follows:

Capacitance calculation for SL3 inverter

If is also specified, you obtain:

Inductance calculation for SL3 inverter

Depending on the characteristics of the IGBTs that are used, the calculated values can be adjusted to optimise the efficiency.

References
[1] Manfred W. Gekeler: Soft switching three level inverter with passive snubber circuit (S3L inverter); Proceedings of the 2011 14th European Conference on Power Electronics and Applications (EPE 2011, Birmingham); ISBN 9789075815153, pp. 1–10
[2] Manfred W. Gekeler: Weich schaltender 3- Stufen-Pulswechselrichter mit verlustfreiem Entlastungsnetzwerk; International ETG Convention 2011 (ETG Proceedings 130 Part B); 2011, ISBN 978-3-2018-3376-7, pp. 264–270
[3] Manfred W. Gekeler: Soft switching three level inverter (S3L inverter); Proceedings of the 2013 15th European Conference on Power Electronics and Applications (EPE 2013, Lille)
[4] 3-Stufen-Pulswechselrichter mit Entlastungsnetzwerk; German patent DE 10 2010 008 426 B4; application date: Feb 18, 2019; publication date of patent award: Sept. 1, 2011; patent holder: HTWG Konstanz; inventor: Manfred W. Gekeler

 

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