Modulation methods for converters used as static synchronous compensators
Ruusila, Arttu (2023)
Ruusila, Arttu
2023
Automaatiotekniikan DI-ohjelma - Master's Programme in Automation Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
Hyväksymispäivämäärä
2023-05-12
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202305115624
https://urn.fi/URN:NBN:fi:tuni-202305115624
Tiivistelmä
A static synchronous compensator (STATCOM) is a device that is capable of compensating reactive power from the grid or supplying active power into the grid if connected to energy storage. The main component of the STATCOM is a voltage source converter, in this thesis a multilevel modular converter (MMC) in a double-star configuration.
The MMC consists of submodules that can be inserted or bypassed. Two main types of MMC submodules exist. A half-bridge (HB) submodule (SM) consists of 2 semiconductor switches and is capable of producing 2 voltage levels. A full-bridge (FB) SM on the other hand consists of 4 semiconductor switches and is capable of producing 3 voltage levels which allows a much broader range of operation.
The modulation methods for the HB-MMC are well-known. Although FB SM has significant advantages and is widely used, the available modulation methods for the FB-MMC include only phase-shifted carriers pulse width modulation (PS-PWM), optimized pulse patterns, space vector modulation, and hybrid nearest level modulation (NLM). Out of these methods, only PS-PWM enables the use of a variable dc-link voltage and boosting the dc-link voltage. Based on the modulating signals of PS-PWM phase disposition (PD) PWM and NLM for FB-MMC are developed. All these methods can produce 2N+1 voltage levels and boost the dc-link voltage.
In addition to choosing the number of SMs to be inserted at any given time step, the SM capacitor voltage balancing has to be considered. The balancing can be done with a sorting algorithm or the modulation can have inherent natural balancing ability. In this thesis, two sorting algorithms are examined. The natural balancing ability of PS-PWM for HB-MMC and FB-MMC is also addressed.
The performance of the modulation methods is evaluated with simulations based on their current quality, device switching frequency, apparent switching frequency, and peak-to-peak amplitude of the circulating currents. For a fair and meaningful comparison, all methods have effectively the same device switching frequency. The performance of PS-PWM and PD-PWM is identical when \(2N+1\) modulation is used and almost identical with \(N+1\) modulation. These carrier-based methods outperform NLM in all performance metrics.
The MMC consists of submodules that can be inserted or bypassed. Two main types of MMC submodules exist. A half-bridge (HB) submodule (SM) consists of 2 semiconductor switches and is capable of producing 2 voltage levels. A full-bridge (FB) SM on the other hand consists of 4 semiconductor switches and is capable of producing 3 voltage levels which allows a much broader range of operation.
The modulation methods for the HB-MMC are well-known. Although FB SM has significant advantages and is widely used, the available modulation methods for the FB-MMC include only phase-shifted carriers pulse width modulation (PS-PWM), optimized pulse patterns, space vector modulation, and hybrid nearest level modulation (NLM). Out of these methods, only PS-PWM enables the use of a variable dc-link voltage and boosting the dc-link voltage. Based on the modulating signals of PS-PWM phase disposition (PD) PWM and NLM for FB-MMC are developed. All these methods can produce 2N+1 voltage levels and boost the dc-link voltage.
In addition to choosing the number of SMs to be inserted at any given time step, the SM capacitor voltage balancing has to be considered. The balancing can be done with a sorting algorithm or the modulation can have inherent natural balancing ability. In this thesis, two sorting algorithms are examined. The natural balancing ability of PS-PWM for HB-MMC and FB-MMC is also addressed.
The performance of the modulation methods is evaluated with simulations based on their current quality, device switching frequency, apparent switching frequency, and peak-to-peak amplitude of the circulating currents. For a fair and meaningful comparison, all methods have effectively the same device switching frequency. The performance of PS-PWM and PD-PWM is identical when \(2N+1\) modulation is used and almost identical with \(N+1\) modulation. These carrier-based methods outperform NLM in all performance metrics.