# A novel Hybrid Inverter Drive for AC motors capable of bypassing the voltage source inverter when operating at grid frequency

Daniel Lendi, Reiko Raute, Cedric Caruana

► To cite this version:

Daniel Lendi, Reiko Raute, Cedric Caruana. A novel Hybrid Inverter Drive for AC motors capable of bypassing the voltage source inverter when operating at grid frequency. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0033 . hal-05075786

HAL Id: hal-05075786

https://utc.hal.science/hal-05075786v1

Submitted on 20 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# A novel Hybrid Inverter Drive for AC motors capable of bypassing the voltage source inverter when operating at grid frequency

Daniel Lendi, Reiko Raute, Cedric Caruana

University of Malta

E-Mail: Daniel.Lendi@um.edu.mt, Reiko.Raute@um.edu.mt, Cedric.Caruana@um.edu.mt

## Acknowledgements

Project "Hybrid Inverter Drive (HID)" financed by the Malta Council for Science & Technology, for and on behalf of the Foundation for Science and Technology.

## Keywords

Variable speed drive (VSD), Voltage source inverter (VSI), Hybrid, Efficiency, Space vector PWM (SVPWM).

## Abstract

Variable speed drives (VSDs) offer reduced power consumption for applications which allow operation at reduced motor speed. However, when continuous operation at the grid frequency is required, the VSD may become redundant, as it introduces conversion losses and electromagnetic interference (EMI), in contrast to grid-connected operation. It is hereby proposed to bypass the VSD for continuous operation at the grid frequency. This paper introduces a VSD, based on a two-level voltage source inverter (VSI), capable of transferring a motor load between grid- and inverter-connected operations, depending on the required motor speed. This paper presents a thorough analysis of the conditions under which the motor load may be transitioned from either operating mode. Thyristors are used as bypass devices. Simulation results for open and semi-closed transitions are presented, examining their impacts on the motor torque and stator currents.

## Introduction

There are currently around 8 billion electric motors in the European Union (EU) alone, which consume nearly 50% of the electricity generated in the EU [1]. Regulations, such as "Regulation on electric motors and variable speed drives (EU) 2019/1781" [1], are updated regularly, pushing for increased energy savings.

VSDs can operate motor loads at speeds independent from the grid frequency, resulting in application dependent energy savings by reducing the power of the motor load. However, VSDs introduce their own energy losses from their switching semiconductor devices. Extensive research is therefore carried out into the energy losses occurring in VSDs [2] [3], which consist of both conduction and switching losses [4] [5]. For continuous operation at the grid frequency, VSDs may become virtually redundant due to their conversion losses. Furthermore, VSDs introduce EMI [6] [7] and harmonic distortion [8] when compared to direct grid-connected operation. It would then be intuitively beneficial to bypass the VSD for prolonged motor operation at the grid frequency. Thus, it is proposed to limit the use of the VSD for motor operation at speeds different from the grid frequency and to bypass the VSD for extended operation at the grid frequency. In this manner, the motor load would always be operated at the most efficient mode of operation. This paper presents the concept of the Hybrid Inverter Drive (HID), which is a patented technology [9]. An overview of the HID topology is shown in Figure 1. The HID operates the motor load using the inverter with state-of-the-art Space Vector Pulse Width Modulation (SVPWM).

<figure><img src="image_2.png" /><figcaption>Figure 1: HID block diagram</figcaption></figure>

However, when the desired motor speed matches the grid frequency, the motor load automatically gets transferred to grid-connected operation. Critical applications where down-time due to VSD maintenance or failure is not acceptable, such as ventilation systems in hospital operating rooms, already utilize a bypass mechanism [10]. However, such bypass procedures make use of
‘open’ transitions, where the load is momentarily
left without a supply until the bypass path is
enabled. Open transitions cause interruption in
the motor supply, resulting in large disturbances.
Low inertia applications can consequently stall
due to such a disturbance. This paper presents a
‘semi-closed’ transition for the motor load from
inverter operation to bypass operation and vice
versa. For such a transition, the inverter output
voltage must first be synchronized to the grid
voltage in both phase and magnitude. The motor
load is then transferred between inverter driven to
direct-online operation during running. In the
developed phase current commutations the motor
load is briefly supplied by both inverter and grid
to minimize the disruption in the motor supply.
This paper uses a simulation model to
demonstrate in detail the transition algorithm.
Motor load transitions from inverter- to grid-
operation and vice versa are shown, using both
open and semi-closed transitions for performance
comparison. Thyristors are used as bypass
devices due to their fast turn-on speed and low
cost. To further reduce the energy losses in direct
online operation, bypass relays or a bypass
contactor can be integrated in parallel to the
thyristors.

Overview of the HID

Conditions for inverter to grid transition

This section introduces the grid supply voltage
states of the HID and the possible inverter output
voltage vectors. These are then used to explain
under which conditions a transition from the
inverter- to grid-connected mode of operation is
possible. The input stage of a VSD usually
converts a three-phase AC fixed frequency supply
voltage into a DC voltage. This conversion
commonly takes place through a six-pulse diode
rectifier as shown in Figure 2. Once this fixed
frequency supply voltage is rectified, it may then
be converted by PWM to a three-phase variable
frequency and magnitude output voltage by a VSI
through switching devices such as Insulated Gate
Bipolar Transistors (IGBTs).

<figure><img src="image_3.png" /><figcaption>Figure 2: Six-pulse diode rectifier</figcaption></figure>

In order to initiate a transition from inverter
driven motor operation to direct grid connection,
the inverter output voltage vector must be
synchronized to the grid voltage vector both in
magnitude and in phase. The precondition is
necessary to minimize the disturbance caused by
the transition. The following analysis shows
under what conditions a transition from inverter
to grid-connected operation is possible.

Consider the example of a three-phase supply
voltage of 400V/50Hz shown in Figure 3. The
figure shows, in addition to the three-phase grid
voltage, which diodes of the six-pulse rectifier
(identified in Figure 2) will conduct. Each diode
conducts for a total duration of 120°, resulting in
six unique diode conduction combinations for
every complete 360° cycle, referred to as sectors
in this paper. Each sector consists of a 60°
conduction period. Figure 4 represents both the
input states and the output voltage vectors of the
HID in a graphical manner under synchronized
condition. This means that the inverter output
voltage is in phase with the grid. The outer circle
represents the conduction periods for diodes D2,
D4 and D6, where the most negative grid phase is
connected to the negative DC link rail. The inner
circle represents the conduction periods of diodes
D1, D3 and D5 connecting the most positive grid
phase to the positive DC link rail. The hexagon
represents the standard inverter output voltage
vectors, consisting of six active and two null
vectors [11]. Each vector indicates the switching
state of the inverter. For instance, vector V100
shows that inverter phase leg A is tied to the
positive DC link rail, as the upper IGBT in the
inverter leg is turned on. Similarly, phases B and
C are tied to the negative and positive DC link
rails respectively. Having established the HID
input sectors and output voltage vectors, one can
start considering transitions from the inverter- to
the grid-connected operating mode.
<figure><img src="image_4.png" /><figcaption>Figure 3: Grid supply voltages with input sectors</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4: Synchronized inverter output vectors and input sectors</figcaption></figure>

For this, consider Figure 5, which shows a more detailed schematic of the HID. This consists of a six-pulse diode rectifier (D1-D6) supplying a DC link capacitor (C<sub>DC</sub>) and six IGBTs (T1-T6), which then supply a motor load. This is standard for any two-level inverter drive. However, the HID further contains six bypass thyristors (TH1-TH6) connected in anti-parallel for each phase. These allow the direct grid connection of the motor terminals. Once the HID input and output are synchronized, as previously shown in Figure 4, one may attempt a transition from inverter- to grid connection.

<figure><img src="image_6.png" /><figcaption>Figure 5: HID circuit diagram</figcaption></figure>

Figure 6 shows the HID circuit diagram under synchronized condition, when turning on all three bypass phases at the same time. This example transition takes place during sector I, where phases A and B supply the DC link voltage through diodes D1 and D6 respectively. The HID output voltage vector used for this example is V100, where transistors T1, T4 and T6 are turned on. In Figure 6, the grid phase voltages A, B and C are represented by yellow, blue and red respectively. It can be seen that T1 ties motor terminal M1 to the grid phase A voltage through diode D1, while T4 and T6 tie terminals M2 and M3 to grid phase B voltage through diode D6.

<figure><img src="image_7.png" /><figcaption>Figure 6: HID in sector I for output state V100 during transition from inverter to grid</figcaption></figure>

If during such an output voltage vector all three bypass phases are enabled through the bypass thyristors TH1-TH6, then a short-circuit between phases B and C will occur. While motor terminals M1 and M2 meet the correct voltages, at M3, grid phase C shorts to grid phase B as indicated by the red arrow in Figure 6. It hence becomes clear that phase C may not be transitioned in this sector. Phase A of the inverter may be transitioned, replacing it by its bypass thyristors TH1 and TH2. The same applies for phase B through TH3 and
TH4. Hence, a total of up to two phases may be bypassed in a sector. However, instances of the null vectors V000 and V111 can still cause a short circuit at motor windings M1 and M2 within this sector. Consider Figure 7, which shows the HID during sector II for the output state V100. It is assumed that no short circuits occurred during sector I. One may now activate the bypass path for phase C on motor terminal M3. This does not cause a short circuit for both sector II output states V100 and V110. However, a short circuit between phases B and C will now occur at motor winding M2, as indicated by the blue arrow in Figure 7. This shows that while an inverter phase may be bypassed by its corresponding grid phase, it should not remain connected in parallel with the grid phase. If the short circuit does not occur in the same sector due to a null vector, it will occur in the following sector due to a change in supply voltage on the DC link. Hence, once a phase is supplied directly by the grid through a bypass thyristor, the corresponding inverter output needs to be disconnected.

<figure><img src="image_8.png" /><figcaption>Figure 7: HID in sector II for output state V100 during transition from inverter to grid</figcaption></figure>

Figure 8 shows an example bypass procedure using the PWM- and gate signals for all IGBTs and bypass thyristors. This transition example is based on a phase-by-phase method, where a single phase is bypassed in each sector. The beginning of the sector is chosen as the motor current for the phase to be transitioned will be near zero. Furthermore, it is assumed that the motor current lags the motor voltage by no more than 30°, resulting in the motor current being of the same polarity as the voltage of the phase to be bypassed at the beginning of the corresponding sector. In Figure 8 during sector VI, the PWM signals for IGBTs T1 and T2 are shown. As the motor current is positive at the end of sector VI, it will be supplied by diode DT2 whenever T1 is

off. As soon as sector I starts, during which phase A is to be bypassed, the IGBT PWM signals of T1 and T2 are disabled and the gate signals of TH1 and TH2 are enabled. Hence a transfer of the phase A motor current $i_{MA}$ will occur between DT2 and the bypass thyristor TH1, as indicated in the figure. At the end of sector I, grid phase C is about to replace phase B to supply the negative DC link rail. Hence, the motor current for phase C is negative and supplied to the motor through DT5 when IGBT T6 is off. As soon as sector II starts, the PWM signals for phase C are disabled and the gate signals for TH5 and TH6 are enabled. This results in a transfer of motor current from DT5 to bypass thyristor TH6. The final phase, phase B, is then bypassed at the beginning of sector III, with a transfer of positive motor current taking place between DT4 and bypass thyristor TH3.

<figure><img src="image_9.png" /><figcaption>Figure 8: Sector timing diagram for transitions</figcaption></figure>

## Conditions for grid to inverter transition

Once the motor is grid-connected, it may be transitioned back to inverter-connected operation to resume variable speed operation. During grid-connected operation, the HID maintains tracking of the grid voltage vector to ensure that synchronism with the grid is maintained. If all three phases are transitioned at the same time in a synchronised state, a short circuit occurs on motor winding M3. This is shown in Figure 9 for sector I during output state V100. Furthermore, since thyristors are used as bypass devices, their turn-off characteristics must be taken into consideration. Thyristors, which self-latch, only turn off once their anode-cathode current drops below the holding current. A safety margin has to be included due to measurement noise and hence the thyristors are considered to be turned off only once their anode-cathode current is 0A for a number of measurement cycles.
<figure><img src="image_10.png" /><figcaption>Figure 9: HID in sector I for output state V100 in grid connection during transition from grid to inverter</figcaption></figure>

Consider Figure 10, which shows an example of the sectors for both motor voltages and currents. During grid-connected operation, these are supplied directly by the grid. The turn-off characteristics of the thyristors and the phase relationship between motor voltages and currents create two restrictions for the transition from grid-to inverter-connected operation, which are as follows:

1. The bypass thyristors gate signal may be required to be disabled in the previous sector depending on the motor currents phase shift.

2. The bypass thyristors may only be considered as disabled after their current is zero.

Hence, if for example, with reference to Figure 10, phase A is to be transitioned back to inverter-connected operation during sector I, the thyristors for phase A must have their gate signal removed in the preceding sector VI. The zero-crossing of motor current A from negative to positive polarity is marked in Figure 10 during sector VI by the black arrow “a”. If the gate signal is removed in sector I instead, the conducting thyristor will remain turned on as it has already exceeded its latching current and a short circuit will occur. Hence, the gate signal is disabled during sector VI already, allowing the conducting thyristor’s current to reach 0A. This is followed by the inverter phase A IGBTs being enabled in sector I. The next current zero-crossing occurs for phase C from positive to negative polarity in sector I indicated by arrow “c” in Figure 10. Therefore, the thyristor gate signals of phase B are disabled at the beginning of sector I where its current zero-

crossing occurs and the inverter phase C is enabled in sector II. Finally, the gate signals for phase B thyristors may be disabled at the beginning of sector II, where its current zero-crossing occurs from negative to positive polarity, indicated by the arrow “b” in Figure 10. This is followed by enabling inverter output phase B at the beginning of sector III. Hence, given the turn-off characteristics of thyristors, a phase-by-phase approach must be used to transition back from grid- to inverter-connected mode. In comparison to the transition from inverter- to grid-connected operation, it is not possible to bypass two phases in a single sector.

<figure><img src="image_11.png" /><figcaption>Figure 10: Motor voltages and currents with corresponding sectors</figcaption></figure>

## Simulation parameters and algorithm

The HID was modelled in MATLAB Simulink using a 4kHz PWM frequency, implemented through an s-function. This simulates a microcontroller interrupt routine executed also at the 4kHz PWM frequency. Table 1 shows some general simulation model parameters and the parameters of the squirrel cage induction motor used in this simulation.
<table><caption>Table I: Simulation model parameters</caption><thead><tr><th>Simulation Parameters</th><th>Symbol</th><th>Value</th></tr></thead><tbody><tr><td>Grid resistance</td><td>R<sub>T</sub></td><td>30mΩ</td></tr><tr><td>Grid inductance</td><td>L<sub>T</sub></td><td>100μH</td></tr><tr><td>DC link capacitance</td><td>C<sub>DC</sub></td><td>2x1600μF</td></tr><tr><th>Motor Parameters</th><th>Symbol</th><th>Value</th></tr><tr><td>Nominal power</td><td>P<sub>n</sub></td><td>15kW</td></tr><tr><td>Nominal line-line voltage</td><td>V<sub>n</sub></td><td>400V</td></tr><tr><td>Nominal frequency</td><td>f<sub>n</sub></td><td>50Hz</td></tr><tr><td>Stator resistance</td><td>R<sub>s</sub></td><td>0.2147Ω</td></tr><tr><td>Stator leakage inductance</td><td>L<sub>ls</sub></td><td>0.991mH</td></tr><tr><td>Rotor resistance</td><td>R<sub>r</sub></td><td>0.2205Ω</td></tr><tr><td>Rotor leakage inductance</td><td>L<sub>lr</sub></td><td>0.991mH</td></tr><tr><td>Mutual inductance</td><td>L<sub>m</sub></td><td>64.19mH</td></tr><tr><td>Inertia</td><td>J</td><td>0.502kgm<sup>2</sup></td></tr><tr><td>Friction factor</td><td>F</td><td>0.0595Nms</td></tr></tbody></table>

The controller measures both the grid and inverter output voltage vectors through Phase Locked Loops (PLLs). The PLL structure is shown in Figure 11. The HID uses two separate but identically tuned PLLs to measure both the grid and inverter output frequency and phase. Two separate PPLs are implemented as the grid phase must be tracked independently to identify the voltage sectors and to maintain tracking of the grid voltage vector during grid-connected operation. The phase of the inverter output is subtracted from the grid phase as shown in Figure 11. A proportional controller then amplifies the resulting phase error. This produces the frequency deviation from the grid frequency required for the inverter output vector to catch up to the grid vector in phase, which is kept low to approach the grid vector phase gradually.

<figure><img src="image_12.png" /><figcaption>Figure 11: HID PLL structure</figcaption></figure>

The phase of the inverter output reference vector is obtained by integrating the reference inverter frequency as expressed in (1).

$$ \phi_{ref} = \int f_{ref} = \int (f_{grid} + f_{\Delta inv}) \quad (1) $$

Figure 12 shows the reference generation for the inverter output voltage. The inverter output is generated as a rotating voltage vector, whose phase is synchronized to the grid voltage vector. From the instantaneous reference voltage vector in the αβ-frame, SVPWM is used to produce the PWM signals for the inverter legs.

<figure><img src="image_13.png" /><figcaption>Figure 12: Inverter output reference generation</figcaption></figure>

An example of the synchronization between the grid and inverter voltage vectors obtained from the PLLs is shown in Figure 13. The inverter output frequency increases from 50Hz to 50.6Hz until both voltage vectors are in phase. In this scenario, the inverter output frequency is increased as it lags the grid vector. The two voltage vectors are considered synchronized when a phase difference of 0.01 radians is achieved between them. Once both vectors are in phase, the transition from inverter- to grid-connected operation is initiated after a 100ms settling time. This settling time is required to allow any disturbances to decay that occur when returning the inverter output to the grid frequency of 50Hz.

<figure><img src="image_14.png" /><figcaption>Figure 13: Inverter frequency and voltage phases</figcaption></figure>
For this paper, two types of transitions were implemented and tested:

1. Open transition

2. Semi-closed transition

The open transition most closely resembles the types of transitions that already exist [10]. Once the inverter output is synchronized to the grid, all three inverter legs are disabled. After some off-time, where the motor is supplied by neither the inverter nor the grid, all three sets of anti-parallel thyristors are enabled. During such a period, the produced motor torque will drop to 0Nm. The motor will drift out of synchronism with the grid voltage vector, resulting in a significant disturbance during the transition. An off-time of 1ms was used for the open transitions in this paper. The semi-closed transition eliminates the 1ms off-time. Once synchronized, the motor is supplied by both the grid and the VSD for a short instance. Throughout this paper, for all semi-closed transitions, sector I, where phases A and B are supplying the DC link, is chosen as the sector to begin all transitions.

# Transition Results

## Open transition

Figure 14 shows the motor stator currents and torque for an open transition from inverter- to grid-connected operation. The motor is initially operated under full load in inverter-driven operation, which results in visible PWM switching harmonics in its currents and a torque ripple. When all bypass thyristors are enabled after the 1ms off-time, a large disturbance in both motor currents and torque can be observed. This disturbance is significant as the motor loses synchronism with the grid voltage vector. After approximately 100ms, this disturbance decays. The motor currents are now sinusoidal and the torque no longer contains a ripple, due to the grid-connected mode of operation. During the transition, all motor currents drop to 0A and the motor torque drops to 0Nm. This represents a disturbance of 100% with respect to the nominal values. During the transient disturbance, an overshoot of 150.6Nm is seen. This represents a deviation from the nominal 104Nm torque of 44.81%.

<figure><img src="image_15.png" /><figcaption>Figure 14: Inverter to grid open transition</figcaption></figure>

Figure 15 shows the open transition from grid- to inverter-connected operation. To initiate the transition, all bypass thyristors are disabled. This results in all three motor currents decreasing to 0A. After a 1ms off-time, all three inverter outputs are enabled. Once again, the motor torque can be seen to drop to 0Nm during the transition representing a deviation of 100% from its nominal value. After the inverter outputs are enabled, an overshoot in the motor torque occurs, peaking at 170.9Nm or 64.33% with respect to the motor nominal torque.

<figure><img src="image_16.png" /><figcaption>Figure 15: Grid to inverter open transition</figcaption></figure>

## Semi-closed transition

The inverter to grid and grid to inverter connected transitions are repeated using the semi-closed transitions explained previously. A phase by phase approach is used in Figure 16 for the inverter to grid transition. The instances of each phase transition are indicated with arrows. Phase A is transitioned to grid connected mode first, followed by phase C and lastly phase B. This semi-closed transition results in a much less
significant transient disturbance than the open transition previously observed in Figure 14. The motor torque momentarily drops to 62.95Nm, followed by a peak overshoot of 122.9Nm. These correspond to values of 39.47% and 18.17% with respect to the nominal torque of 104Nm.

<figure><img src="image_17.png" /><figcaption>Figure 16: Inverter to grid semi-closed transition</figcaption></figure>

Further improvements were made to this semi-closed transition by transitioning phases A and B together in sector I, as indicated by the arrows in Figure 17. The torque overshoots with a peak value of 125.5Nm or 20.67%. This is followed by an undershoot to 93Nm or 10.58%.

<figure><img src="image_18.png" /><figcaption>Figure 17: Inverter to grid semi-closed by initially commutating two phases in one sector</figcaption></figure>

Figure 18 shows a semi-closed transition from grid to inverter connected mode. As previously explained, it is here only possible to transition a single phase at a time due to the turn-off characteristics of the thyristors. The first phase to transition is phase A. The motor current $i_{MA}$ can be seen to reach 0A and remain at this value for a short amount of time. It then increases in amplitude as the inverter leg is enabled. The same

procedure is repeated for phases C and B until all three motor currents are supplied by the inverter legs, as indicated by the arrows in Figure 18. The duration for the motor phase currents at 0A is approximately 0.5-1ms. This time is affected by the update frequency of the s-function, which needs to detect a 0A current. Furthermore, this time needs to be extended due to noise in the current reading, which could indicate a false thyristor turn-off. The motor torque can be seen to momentarily drop to 33.04Nm and overshoot to a peak value of 154.9Nm. These correspond to values of 68.23% and 48.94% respectively.

<figure><img src="image_19.png" /><figcaption>Figure 18: Grid to inverter semi-closed transition</figcaption></figure>

Table 2 summarizes the results of both open and semi-closed transitions. In the open transition from inverter to grid, the motor torque was seen to drop to 0Nm. This disturbance was greatly improved in the semi-closed transition bypassing only a single phase at a time, resulting in an undershoot of 39.47% for the motor torque, compared to the 100% observed in the open transition. A reduction in the overshoot was observed from 44.81% to 18.17%. Further improvements to these disturbances were observed when bypassing two phases in the same sector for the semi-closed transition. While the resulting torque overshoot slightly increased from 18.17% to 20.67%, the resulting torque undershoot greatly improved from 39.47% to 10.58%. This shows that bypassing two phases simultaneously in the same sector when transitioning from inverter- to grid-connected operation results in a smoother transition.

Lastly, when transitioning from grid- to inverter-connected operation, improvements were again observed. However, due to the limitations set by the thyristor bypass devices, they were not as significant. The torque resulted in reduced over-
and undershoots from 64.33% to 48.94% and from 100% to 68.23%.

<table><caption>Table III: Summary of transition results</caption><thead><tr><td>Transition</td><td>Torque over- and undershoot [%]</td></tr></thead><tbody><tr><td>Open inverter-grid</td><td>44.81, 100</td></tr><tr><td>SC inverter-grid 1 phase</td><td>18.17, 39.47</td></tr><tr><td>SC inverter-grid 2 phase</td><td>20.67, 10.58</td></tr><tr><td>Open grid-inverter</td><td>64.33, 100</td></tr><tr><td>SC grid-inverter</td><td>48.94, 68.23</td></tr></tbody></table>

## Conclusion

In this paper, a semi-closed transition strategy was proposed to transition a motor load between inverter and grid connected modes of operation for a simulation model of the HID. Results were presented and analyzed for both open and semi-closed transitions. Even for a minimized off-time of 1ms in the open transition, which would take 50-100ms with a mechanical contactor, a large disturbance was observed in the motor currents and torque. This disturbance was greatly reduced by the semi-closed transitions, which is a critical improvement for low inertia applications, as these could stall from such a transient disturbance. Finally, to remove their conduction losses, the thyristors may be bypassed through a mechanical contactor or electromechanical relays when steady state grid-connected mode is established.

## References

[1] European Commission [Online]: Electric Motors and variable speed drives; Ecodesign and Energy Label

[2] J. W. Kolar and F. Casanellas: Losses in PWM inverters using IGBTs, 1994 IEE Proceedings, vol. 141, no. 5, pp. 235-239.

[3] K. Berringer, J. Marvin and P. Perruchoud: Semiconductor power losses in AC inverters, IAS 1995

[4] T.-J. Kim, D.-W. Kang, Y.-H. Lee and D.-S. Hyun: The analysis of conduction and switching losses in multi-level inverter system, PESC 2001

[5] D. Cholewa, W. Mazgaj, Z. Szular and B. Woszczyna: Reduction of switching losses in three-phase three-level voltage source inverters, WZEE 2018

[6] H. Bishnoi, P. Mattavelli, R. Burgos and D. Boroyevich: EMI Behavioral Models of DC-Fed Three-Phase Motor Drive Systems, IEEE Trans., vol. 29, no. 9, pp. 4633-4645, 2014

[7] B. Revol, J. Roudet, J.-L. Schanen and P. Loizelet: EMI Study of Three-Phase Inverter-Fed Motor Drives, IEEE Trans., vol. 47, no. 1, pp. 223-231, 2010

[8] K. Lee, H. Shin and J. Choi: Comparative analysis of power losses for 3-level NPC and T-type inverter modules, INTELEC, 2015

[9] University of Malta: Commutation between grid and inverter, Israel Patent 311048

[10] ABB: ACH550 E-Bypass Drives

[11] D. G. Holmes and T. A. Lipo: Pulse Width Modulation for Power Converter, Wiley - IEEE Press, 2003