# Analysis, Modulation and Control of a Ground Power Unit for Aircraft Applications

Beichen Duan, Marco Rivera, Patrick Wheeler, Javier Muñoz

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Beichen Duan, Marco Rivera, Patrick Wheeler, Javier Muñoz. Analysis, Modulation and Control of a Ground Power Unit for Aircraft Applications. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0136. hal-05099822

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# Analysis, Modulation and Control of a Ground Power Unit for Aircraft Applications

Beichen Duan

Marco Rivera

Patrick Wheeler

Javier Muñoz

School of Elec. & Elect. Eng.
Nanyang Tech. Univ. (NTU)

Singapore

ssybd1@nottingham.ac.uk

Dept. of Elec. & Elect. Eng.
University of Nottingham

Nottingham, U.K.

marco.rivera@nottingham.ac.uk

Dept. of Elec. & Elect. Eng.
University of Nottingham

Nottingham, U.K.

Dept. of Elec. Eng.
Universidad de Talca

Curicó, Chile

jamunoz@utalca.cl

## ACKNOWLEDGMENTS

The authors gratefully acknowledge the support provided by the Agencia Nacional de Investigación y Desarrollo (ANID) through the FONDECYT Regular Grant No. 1220556, ANID FOVI230169, and Fondap SERC 1523A0006. Additional funding was provided by the Consejo Nacional de Ciencia y Tecnología (CONACYT) under Research Project PINV01-743, as well as the ENNOBLE-R02401 initiative. The authors also extend their gratitude to the British Council for the Going Global Partnerships – Industry Academia Collaborative Grant (IND/CONT/G/23-24/14).

*Index Terms*—AC-DC converter, Control strategy, DC-AC converter, Electric aircraft, Modulation scheme.

*Abstract*—This paper presents a PLECS-based simulation of innovative AC-AC converter configurations tailored for Ground Power Units (GPUs), addressing the critical demand for airport electrification, a cornerstone of sustainable aviation and ground operations. The proposed design incorporates a Current-Synthesized Power Factor Correction (CSPFC) rectifier, operating with a suitable harmonic injection technique, and proper modulation strategies. Simulation results demonstrate the system's effectiveness in mitigating issues such as harmonic distortion, voltage ripple, and other operational challenges, underscoring its potential to enhance performance and sustainability in GPU applications.

## I. INTRODUCTION

With the rapid development of the aviation industry, the travel of aircrafts has become increasingly popular, leading to a growing demand for reliable and efficient energy systems. Traditionally, aircrafts rely on Auxiliary Power Units (APUs) during flight to meet their operational electrical needs.

However, APUs consume significant amounts of aviation fuel, up to 100 kg per hour, which contributes to air pollution and operational inefficiency [1]. To mitigate these issues, Ground Power Units (GPUs) have emerged as a viable alternative to supply power to aircraft while on the ground, reducing both fuel consumption and costs. GPUs are designed to deliver a sine voltage of 115 Vrms at 400 Hz over a power range of 30 to 180 kVA, and are also applied in marine and radar systems [2].

The primary applications of GPUs in aircraft include engine start-up, charging auxiliary systems, and performing inspections and maintenance [2]. Despite their advantages, GPUs face challenges related to output stability, harmonic distortion, and power factor efficiency, which affect the overall performance and reliability of aircraft electrical systems [3].

Traditional topologies for GPUs have commonly relied on Power Factor Correction (PFC) rectifiers to address these issues and ensure power quality. Key types of PFC rectifier include bridge rectifier, Vienna rectifier, and Current-Synthesized Power Factor Correction circuits (CSPFC) [4].

The uncontrolled bridge rectifier, though cost-effective and efficient, suffers from unsatisfactory harmonic distortion and a low power factor, making it unsuitable for high-performance applications like GPUs. The Vienna rectifier offers improvements in both harmonic reduction and power factor correction, with its three-level structure providing better performance at higher power levels [3]. However, CSPFC circuits have gained increased attention because of their balance of efficiency, cost-effectiveness, and ability to synthesize the desired power factor with precise control. These circuits, combined with advanced modulation strategies such as space vector pulse width modulation (SVPWM) and sinusoidal PWM (SPWM), are critical in ensuring stable and efficient power conversion [5].
To address these challenges, this paper develops a PLECS simulation of a novel AC-AC converter for GPUs. The proposed design integrates a variable frequency to constant frequency (VFCF) converter system and leverages advanced current-synthesized power factor correction (CSPFC) circuits, active current modulation, and harmonic suppression techniques [3].

Previous studies have proposed solutions such as active current-modulated three-phase rectifier topologies [4] and choke-based common-mode methods to minimize circulating currents between variable and constant-frequency systems [3]. This work aims to further enhance these approaches by simulating and optimizing the converter design for improved power quality and efficiency, validated through simulation in PLECS.

This paper provides a detailed description of the converter design and discusses its effectiveness in meeting the requirements of modern aircraft power systems.

## II. PROPOSED AC-AC TOPOLOGY FOR THE GROUND POWER UNIT

The schematic diagram of the topology and control circuit proposed in this paper is shown in Fig.1. It is important to note that the CSPFC rectifier and the PWM inverter are controlled topologies, while the rest are uncontrolled components. This section presents the design of the Current-Synthesized Power Factor Correction (CSPFC) rectifier, which was chosen for its superior performance in power factor correction and efficiency over traditional rectifiers such as bridge rectifiers and Vienna rectifiers. It includes topology, control, and modulation strategies, as well as key circuit components such as RC snubber circuits and harmonic current injection.

The design of a parameter selection for the input filter, the DC-link, the three-phase PWM inverter, and the output filter will follow classical methods available in the literature [6]–[9], as the main focus of this research is placed on the CSPFC rectifier.

### A. CSPFC Rectifier Topology

The CSPFC rectifier topology is a significant improvement over traditional rectifier designs, offering better efficiency and power factor correction. This topology is proposed in the literature [5] and is an improvement of the Vienna rectifier. The CSPFC rectifier solves the complex drive logic of the injection circuit part of the Vienna rectifier, which needs to drive six high-frequency switches, and improves the efficiency.

As shown in Fig. 1, within the CSPFC rectifier the bidirectional switches (i.e., injection network) and the bridge rectifier form a low-frequency switch, and the two

semiconductors with diode and inductor form a high-frequency switch, which is also known as the shaping network [4]. The current shaping network is used to generate a third harmonic current that is approximately synchronized with the input AC voltage. The current injection network is used to selectively inject the harmonic currents generated into each phase [4]. This method is also known as the third harmonic current injection method. This part of the switching element involves a complex modulation process and will be described in the next section.

The CSPFC rectifier was selected for this article because of its high efficiency, despite the additional components that increase manufacturing costs and equipment size. This trade-off is justified given the improved performance that the CSPFC rectifier provides in terms of reduced harmonic distortion and improved power factor correction, compared to simpler bridge rectifiers and more complex Vienna rectifiers [4].

### B. RC Snubber Circuit

During switching, resonance between junction capacitance and stray inductance can lead to harmful oscillations that can damage circuit components. To address this issue, RC snubber circuits are employed in the CSPFC rectifier. These circuits are connected in parallel with the switching elements to dampen the oscillations. Previous research has shown that increasing the resistance in RC snubber circuits can effectively suppress these oscillations, though it also increases switching losses due to slower rates of change in voltage and current [10], [11]. Given the complexity of designing RC snubber circuits, an estimation method is used to select the necessary parameters for this paper, as detailed in Table I. This approach strikes a balance between minimizing switching oscillations and managing switching losses within acceptable levels [12].

<table><caption>TABLE I: RC snubber circuit parameters</caption><thead><tr><td>Component</td><td>Resistance (Ω)</td><td>Capacitance (nF)</td></tr></thead><tbody><tr><td>Diode</td><td>500</td><td>250</td></tr><tr><td>IGBT with diode</td><td>10<sup>5</sup></td><td>1</td></tr></tbody></table>

## III. MODULATION AND CONTROL STRATEGIES

The CSPFC rectifier employs a combination of classical modulation methods and independent control systems for injection and shaping networks, which together provide efficient power factor correction and harmonic mitigation.
<figure><img src="image_2.png" /><figcaption>Fig. 1: Proposed topology diagram.</figcaption></figure>

## A. Modulation

The modulation of the rectifier follows the method proposed in [4]. In the shaping network, a third-harmonic current is generated by a half-bridge inverter approximately synchronised to the AC supply. Bi-directional switching is controlled by an injection network operating in the low frequency range, which enables the injection of harmonic currents to compensate for the waveforms generated by the bridge rectifier. There are a number of optimisation methods based on this approach that offer improvements in areas such as harmonic content. For example, [13] has researched the use of ripple current cancelation (RCC) techniques to achieve quasi-zero current ripple in rectifiers while reducing the high-frequency harmonic components on the AC side. This approach simplifies the constraints in the design process and extends the operating range of the converter to some extent. Considering the complexity, the classical modulation method is used in this design.

## B. Control

In terms of control strategy, considering the design requirements and complexity, the classical control method is chosen, where the control systems of the injection and shaping networks are independent. The block diagrams are shown in Fig. 2.

1) *Injection Network Control:* the injection network uses an open-loop control strategy, with multiple comparators determining the switching signal. The switches open and close approximately $\pi$ radians before and after the zero crossing point of the corresponding phase voltage. This control method ensures minimal voltage spikes, which can occur due to inductive loads, by slightly overlapping control signals [6]. The three-phase gating

signals $S_a$, $S_b$, and $S_c$ are generated for the injection network switches through the phase selection block. It is worth mentioning that complementary patterns are also required to achieve the bidirectional behavior of the switch.

2) *Shaping Network Control:* the shaping network is controlled using a closed-loop system that tracks the input voltage phase and calculates the optimal output voltage. A reference current is generated on the basis of this input, and the difference between the reference current and the actual inductor current is processed by a proportional integral (PI) compensator. This compensator generates the semiconductor control signal, which adjusts the third-harmonic current injection and switching ratio. This method optimizes performance while maintaining simplicity in design [4]. The gating signals $T_+$ and $T_-$ are generated for the top and bottom switches of the injection network, respectively.

<figure><img src="image_3.png" /><figcaption>Fig. 2: Block diagram of CSPFC rectifier controller</figcaption></figure>
Although more advanced techniques such as rip-
ple current cancelation (RCC) have been researched to
reduce high-frequency harmonic components [13], the
classical modulation method is preferred in this paper for
its balance between complexity and performance. RCC
techniques would extend the operational range of the
rectifier and reduce current ripple, but increased design
complexity is not warranted for this application.

IV. GPU POWER CONVERTER SIMULATION

Fig. 3 presents the results obtained from the topology proposed in this paper, corresponding to the PLECS simulation of the circuit shown in Fig. 1. In addition to validating the overall circuit performance, the results include waveforms generated using both SVPWM and SPWM modulation techniques, allowing for a clear comparison between their effects on the inverter’s performance.

As shown in Fig. 3 (a), the incorporation of an input filter significantly reduces distortion, bringing the current waveform closer to a sinusoidal shape, with the presence of dominant low-order harmonics due to the diode bridge. Furthermore, coordinated operation of the CSPFC rectifier ensures that current and voltage are in phase, providing a unitary displacement power factor.

Fig. 3 (b) shows the current in each phase of the injection network. As can be seen, the waveforms are phase-shifted by 120°, appropriately distributing the current $I_L$. On the other hand, minor fluctuations are observed in the DC voltage $v_{dc}$ waveform, as can be seen in Fig. 3 (c).

Fig. 3 (d) shows the closed-loop tracking of the in-
ductor current $I_L$. Thanks to the network shaping control
strategy of Fig. 2 (b), the current can satisfactorily follow
its reference with some minor differences at the extreme
values.

When comparing Fig. 3 (e) and Fig. 3 (g), no
significant differences in the load voltage waveforms are
evident. Under SPWM modulation, the output current
THD is 0.20%, slightly better than the result under
SVPWM modulation (0.25%).

In the amplified view of the AC voltage in Fig. 3
(f), the waveform peaks at 116.149V and drops to a
minimum of 113.013V, resulting in a voltage variation
($\Delta v_{out}$) of 3.136V. In contrast, the AC voltage peaks at
116.776V and reaches a minimum of 113.496V in Fig.
3 (h), resulting in a $\Delta v_{out}$ of 3.28V, marginally higher
than under SVPWM. Overall, SVPWM does not offer a
notable advantage over SPWM in this simulation.

While SVPWM theoretically provides lower har-
monic distortion, reduced switching losses, and faster dy-
namic response, the simplicity of the circuit and control

used in this simulation may limit its benefits in this case.
In more complex circuits or real-world applications,
the advantages of SVPWM could be more pronounced.
Thus, both the potential benefits of SVPWM and its
practicality should be considered when selecting the
optimal design for specific applications.

V. DISCUSSION AND FUTURE WORK

In the future, GPUs for use in aircraft will continue
to require in-depth research. With the swift growth of the
aviation field and the increasing appeal for environmental
protection at the societal level, the new configuration
schemes must not only ensure the stability and reliability
of the aircraft system but also adapt to the context of
green fuel.

Based on the above development trends, combined
with the research results presented in this article and the
issues identified in the simulation, the following aspects
can be pursued for the subsequent optimization of GPUs
applied to aircraft:

* Replace the original second-order filter with a Phase-Locked Loop (PLL). By tracking and synchronizing the grid frequency and phase, the filtering performance can be effectively improved.

* Replace the diode part of the CSPFC with a controllable rectifier. This allows for better performance regulation by controlling the switching elements.

* Adopt an NPC inverter instead of a two-level inverter. The modulation method should be based on SVPWM combined with predictive modulation techniques. The NPC inverter offers advantages in terms of output voltage quality and harmonic content. Predictive modulation techniques enable more accurate control by predicting the demand for the next cycle.

* Replace the ideal parameters in the PLECS model with actual parameters for simulation to perform practical tests. Once the simulation results are at an acceptable level, the actual parameters of the components will be measured using instruments such as LCR and added to the PLECS model for more accurate simulation. If the simulation results with actual parameters also meet the requirements, further consideration can be given to testing the proposed circuit in the laboratory.

* In line with the trend of sustainability, a redesign of the converter section can be considered to integrate green fuel technology into the GPU power system.
<figure><img src="image_4.png" /><figcaption>(a) Voltage and current in the input source (phase a)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(c) DC voltage $v_{dc}$</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(e) Three-phase AC output voltage (SVPWM)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(g) Three-phase AC output voltage (SPWM)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(b) Three-phase current in the injection network</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(d) Shaping network current ($I_L^{ref}$ vs. $I_L$)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(f) Amplified AC voltage segment (SVPWM)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(h) Amplified AC voltage segment (SPWM)</figcaption></figure>

<caption>Fig. 3: Converter waveforms (comparison between SVPWM and SPWM)</caption>
## VI. CONCLUSION

In this article, the AC-AC power converter is thor-
oughly examined in the context of a GPU (Ground
Power Unit) applied to aircraft, a critical component
of the aircraft power supply system. The primary goal
is to achieve a stable 115V, 400Hz output voltage for
aviation applications. A novel AC-AC converter con-
figuration is developed and implemented by integrating
an actively modulated CSPFC rectifier, an SVPWM-
modulated PWM inverter, and a second-order passive fil-
ter. Exhaustive simulations performed in PLECS verified
the accuracy of the output waveforms, demonstrating the
effectiveness and viability of the proposed design.

The paper systematically presents the proposed con-
figuration, covering aspects such as circuit topology,
modulation strategies, control methods, and their inte-
gration. By optimizing the circuit design and employing
advanced modulation techniques, such as active current
modulation and SVPWM, the scheme effectively reduces
harmonic distortions and noise interference. Addition-
ally, a comparison between SVPWM and SPWM was
conducted. The results indicate that both techniques
achieve good performance in the output voltage, with
THD levels appropriate for the application.

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