Multilevel Voltage Source Converter for HVDC Transmission System

Selvamaari I

, Sandeep Kumar S

, Sakthi Saravanan N

and Selvanayagam M

S A Engineering College, Anna University, Avadi-Poonamallee Road, Tamil Nadu, India

Keywords: High Voltage Direct Current (HVDC) Transmission, Multilevel Voltage Source Converter (VSC), Power

Quality Management, Uninterruptible Power Quality Conditioner (UPQC), System Cost Reduction in HVDC.

Abstract: This paper introduces a novel multilevel Voltage Source Converter (VSC) for High Voltage Direct Current

(HVDC) systems, featuring an integrated Uninterruptible Power Quality Conditioner (UPQC) to improve

power quality and performance. Unlike traditional Modular Multilevel Converters (MMC), which are

complex and costly, the proposed VSC uses series full-bridge submodules and an AC side unfolder to

minimize component requirements and reduce switching losses. The advanced control strategy regulates

active/reactive power and maintains capacitor voltage balance, enhancing stability and efficiency. Simulations

confirm this design's superior performance, highlighting its potential as an efficient, resilient solution for

modern HVDC applications.

1 INTRODUCTION

High Voltage Direct Current (HVDC) systems are

essential for modern energy infrastructure due to their

efficiency in transmitting large power loads over long

distances. This efficiency makes HVDC technology

highly suitable for integrating renewable energy

sources, such as wind and solar, into power grids,

thereby supporting sustainable energy solutions. By

minimizing transmission losses and stabilizing grids,

HVDC plays a crucial role in enhancing the reliability

and reach of energy networks. A common HVDC

technology is the Modular Multilevel Converter

(MMC), which offers scalability and flexibility in

power transmission. MMCs use numerous

submodules to manage high voltage levels, making

them suitable for scaling with power demands.

However, the high component count in MMC

systems leads to increased costs and complex control

requirements. Balancing voltage across submodules

is particularly challenging under variable loads, while

the large number of components contributes to

switching losses, affecting system efficiency and

reliability. These factors add to the operational

complexity and costs of MMC-based HVDC systems.

https://orcid.org/0009-0003-1152-9006

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https://orcid.org/0009-0003-5466-6019

https://orcid.org/0009-0003-9949-8418

The proposed solution introduces a novel

multilevel Voltage Source Converter (VSC) that

integrates full-bridge submodules and an AC side

unfolder, significantly reducing component count

compared to traditional MMC designs. This reduced

component structure simplifies the control system,

lowering costs and enhancing efficiency.

Additionally, by operating most switches in a soft-

switching mode, the design minimizes switching

losses and thermal stress, which improves the

converter’s reliability and longevity. This streamlined

design supports an HVDC system that is both cost-

effective and more efficient than conventional MMC

approaches.

The proposed VSC incorporates an

Uninterruptible Power Quality Conditioner (UPQC),

which enables effective management of power

quality disturbances such as voltage sags, swells, and

harmonics. This feature ensures a stable, high-quality

power supply, an increasingly important aspect of

modern grids. An advanced control strategy further

optimizes the VSC by balancing active and reactive

power, maintaining capacitor voltage, and adjusting

to dynamic load conditions. This approach offers a

I, S., Kumar S, S., Saravanan N, S. and M, S.

Multilevel Voltage Source Converter for HVDC Transmission System.

DOI: 10.5220/0013652100004639

In Proceedings of the 2nd International Conference on Intelligent and Sustainable Power and Energy Systems (ISPES 2024), pages 135-139

ISBN: 978-989-758-756-6

135

sustainable and resilient HVDC solution, crucial as

renewable energy sources continue to expand within

global power networks.

2 COMPONENTS

A Unified Power Quality Conditioner (UPQC)

system. Input power is processed through a Voltage

Source Converter (VSC) and regulated by the UPQC

to improve power quality for the load. A PIC

controller with a buffer manages the control signals,

supported by a 5V DC power supply.

A driver circuit, powered by 12V DC, interfaces

the controller with the VSC. The system ensures

reliable and clean power delivery to the load.

2.1 Voltage Source Converter (VSC)

The Voltage Source Converter (VSC) utilizes a

topology of series full-bridge submodules, which

enhances efficiency while reducing the number of

components compared to Modular Multilevel

Converters (MMC). It employs power semiconductor

switches, such as IGBTs or MOSFETs, arranged in

full-bridge configurations to facilitate soft-switching,

thereby minimizing switching losses. The VSC is

designed to achieve a DC-link voltage 3.33 times

greater than the AC-side RMS voltage, ensuring

balanced stress across both the converter and

transformer components.

2.2 Uninterruptible Power Quality

Conditioner (UPQC)

Uninterruptible Power Quality Conditioner (UPQC)

integrates with the VSC to effectively mitigate power

quality disturbances, improving voltage regulation

and reactive power supply as needed. This system

includes both series and shunt converters, along with

voltage and current sensors and control circuitry,

which work together to ensure smooth operation

under varying load conditions, thereby enhancing

overall system performance and reliability.

2.3 Load

The load in the system can be either resistive or

inductive, representing the actual load conditions in

simulations. For this setup, it is specified at 12V DC,

simulating typical operational conditions that the

VSC and UPQC are designed to handle. This load

configuration is essential for evaluating the

performance and effectiveness of the overall system

under realistic scenarios.

2.4 Driver Circuit

The driver circuit's primary purpose is to convert

control signals from the PIC controller into gate drive

signals that activate the VSC switches. Operating on

a 5V DC supply, the driver circuit ensures

compatibility with the outputs of the microcontroller.

This circuit plays a crucial role in facilitating reliable

switching operations of the VSC, thereby maintaining

the overall efficiency and effectiveness of the system.

2.5 PIC Controller with Buffer

The PIC controller serves as the main control unit for

the system, executing an advanced control strategy to

regulate both active and reactive power. To enhance

stability and reliability, a buffer circuit isolates the

PIC controller from the high-power switching

elements of the VSC.

This isolation ensures that the controller can

effectively manage system operations without being

adversely affected by high voltage or current

fluctuations during switching events.

3 SIMULATION PROCEDURE

A The simulation showcases a high-voltage direct

current (HVDC) transmission system designed for

efficient long-distance power transfer. It connects two

230 kV, 50 Hz, 2000 MVA AC systems.

Station 1 (Rectifier): Converts AC to DC for

transmission using a Voltage Source Converter

(VSC).

HVDC Cable: A 75 km cable transmits DC power

with reduced losses.

Station 2 (Inverter): Converts the DC back to AC

for integration into the second AC system.

Both stations incorporate VSC pole control for

precise operation and data acquisition systems for

signal recording and system monitoring. A simulated

three-phase fault near Station 2 evaluates the system's

stability, fault tolerance, and response under

disturbance. This system highlights efficient power

transfer and robust fault management.

A high-voltage direct current (HVDC)

transmission system, an advanced and highly efficient

technology for long-distance power transmission.

HVDC systems are widely used to transfer bulk

power over long distances with minimal losses,

making them a preferred choice for interconnecting

ISPES 2024 - International Conference on Intelligent and Sustainable Power and Energy Systems

136

distant AC grids or for projects that involve

submarine or underground cables. The simulation

focuses on the key operational aspects, fault-handling

capabilities, and the stability of an HVDC system.

This system connects two large AC grids, each

operating at 230 kV, 50 Hz, and with a power rating

of 2000 MVA. The first AC grid supplies power to

Station 1, which functions as a rectifier, converting

the AC power into direct current (DC) for

transmission. The rectification process is controlled

using Voltage Source Converter (VSC) technology,

which allows precise regulation of the DC voltage

and current, ensuring smooth power transfer. VSC

technology also enables fast response to changes in

load or grid conditions, making the system more

stable and adaptive to fluctuations.

Once converted to DC, the power is transmitted

over a 75 km HVDC cable. The use of DC for long-

distance transmission offers several advantages over

AC systems, including reduced energy losses and the

elimination of reactive power issues. HVDC cables

are also more efficient for submarine and

underground installations, as they require fewer

conductors and have a lower footprint. This feature

makes HVDC systems particularly suitable for

applications such as cross-border power exchanges or

linking offshore wind farms to the main grid.

At the receiving end, Station 2 acts as an inverter,

converting the DC power back into AC for integration

into the second AC grid. Like the rectifier, the

inverter station also uses VSC technology to ensure

proper synchronization with the receiving grid. This

includes maintaining a stable voltage and frequency

while accommodating variations in load or power

demand. The combination of rectifier and inverter

stations ensures bidirectional power flow, which is

crucial for modern interconnected power systems that

require flexibility.

To monitor the performance of the system, both

stations are equipped with data acquisition systems.

These systems collect real-time operational data,

allowing engineers to analyze power flow, voltage

levels, and system stability. The data is critical for

identifying potential issues, ensuring reliable

operation, and optimizing the performance of the

HVDC system. Additionally, the simulation

incorporates a three-phase fault near Station 2 to test

the system’s fault-handling capabilities. This feature

is essential for evaluating how the system responds to

disturbances, such as short circuits or grid instability.

The fault simulation helps in designing robust

protection mechanisms, ensuring that the system can

recover quickly without compromising power

delivery.

The simulation provides a comprehensive

overview of an HVDC transmission system's

operation and reliability. It highlights the advantages

of HVDC technology, including its efficiency,

adaptability, and ability to handle faults. By using

advanced control systems such as VSCs and

incorporating fault analysis, the simulation

demonstrates the suitability of HVDC systems for

modern power transmission challenges, making them

an integral part of future energy infrastructure. This

setup is particularly relevant for projects that demand

high capacity, long-distance transmission, and

interconnection of renewable energy sources to the

grid.

3.1 Parameter Labels and Signals

The system includes key parameters such as the DC

pole-to-neutral voltage (Vdc PN), which ranges from

-10 to 10 volts for monitoring or setting purposes, and

measured.

DC voltage and power in per-unit (pu) terms

(<Udc_meas> and <Pdc_meas>, respectively). This

per-unit system standardizes values across the HVDC

setup, facilitating easier interpretation and

management of voltage and power dynamics during

operation.

3.2 Graphical Scales and Simulation

Status

Each parameter in the simulation has corresponding

graphical scales to visualize fluctuations, such as

voltage and power changes, dynamically. The

simulation indicates that it is currently "Running,"

with a sample time of 0.438 seconds, which affects

the model's responsiveness and accuracy during real-

time execution. For the BUS B1 STATION_1

component, the simulation tracks voltage, active

power, and reactive power measurements, which are

critical for maintaining power quality and system

balance.

3.3 Control Signals Station

The control_signals station focuses on managing the

direct (d-axis) and quadrature (q-axis) components of

current and voltage, essential for maintaining power

quality in the HVDC system.

Key parameters include the d-axis current reference

(lv_d Iref_d) and the q-axis reference (Iv_q Iref_q),

both displayed in per-unit terms, with a range from -

10 to 10. The modulation index (<Mod index>)

adjusts the converter's output voltage, while the

Multilevel Voltage Source Converter for HVDC Transmission System

137

reference voltage for three phases (Vref_abc) ensures

stable power delivery. This section also shows a

simulation status of "Running" with a sample time of

2.002 seconds, reflecting the need for less frequent

updates due to the stability of the managed

parameters

Figure 1. Working Procedure.

3.4 Filter Bus Station 2

The filter_bus station_2 block is designed for filtering

and measuring voltage and current parameters, which

are vital for stabilizing power transmission and

minimizing harmonic content. It includes filtered

voltage measurements (<Uf_meas>) and three-phase

voltage outputs (Uf_abc), as well as filtered three-

phase current (Iv_abc) and unfiltered voltage

(Uv_abc) for comparison. The simulation status

indicates it is "Running" with a sample time of 2.284

seconds, suggesting that the filtering process requires

less frequent updates, which aligns with its role in

ensuring clean and stable signals for the HVDC.

3.5 Simulation Outcome

The system begins with AC System 1, which serves

as the power source at the sending end of the HVDC

link. This AC system operates at 230 kV and 50 Hz,

with a capacity of 2000 MVA. The electrical power

generated by AC System 1 is delivered to Station 1,

which functions as a rectifier. The primary role of

Station 1 is to convert the alternating current from AC

System 1 into direct current (DC). This conversion is

accomplished using advanced power electronics,

such as thyristors or insulated-gate bipolar transistors

(IGBTs), which are controlled by a Voltage Source

Converter (VSC) system. The VSC control system

ensures efficient and stable conversion while

managing power quality and minimizing losses.

Additionally, Station 1 includes data acquisition

systems that monitor and control the conversion

process in real-time. Once the power is converted to

DC at Station 1, it is transmitted through a 75 km

cable to Station 2. This transmission cable is a critical

component of the HVDC system, designed to handle

high voltage and current with minimal losses.

Depending on geographical and environmental

factors, HVDC cables can be overhead lines,

underground cables, or submarine cables.

The 75 km distance highlights the system's

capability to transmit power over significant

distances with high efficiency. Upon reaching Station

2, the DC power is converted back into AC power

suitable for use in AC System 2, which operates at the

same voltage and frequency as AC System 1. Station

2 functions as an inverter, utilizing VSC technology

similar to Station 1 to ensure efficient and precise

conversion from DC to AC. Like Station 1, Station 2

is equipped with data acquisition systems for

continuous monitoring and control.

The AC power is then delivered to AC System 2

at the receiving end of the HVDC link. AC System 2

also operates at 230 kV and 50 Hz, with a 2000 MVA

capacity, and receives the transmitted power for

distribution. The schematic also includes various

protection and control mechanisms to ensure safe and

reliable operation. Notably, a three-phase fault is

indicated on the AC System 2 side, which is used to

test the system's stability and response to faults.

ISPES 2024 - International Conference on Intelligent and Sustainable Power and Energy Systems

138

Figure 2: Simulation

These tests ensure that the HVDC system can

handle unexpected disruptions and maintain stable

operation. Overall, this HVDC transmission system

exemplifies advanced electrical engineering

techniques used to achieve efficient long-distance

power transmission.

4 CONCLUSIONS

In conclusion, the proposed multilevel Voltage

Source Converter (VSC) for High Voltage Direct

Current (HVDC) systems with an integrated

Uninterruptible Power Quality Conditioner (UPQC)

presents a significant advancement in power

transmission technology. By utilizing series full-

bridge submodules and reducing component count,

this VSC design offers enhanced efficiency,

reliability, and improved power quality for HVDC

systems. The innovative control strategies

implemented ensure stable operation by dynamically

managing both active and reactive power, while

maintaining voltage balance across the capacitors.

The incorporation of UPQC further strengthens the

system’s ability to mitigate voltage sags, swells, and

harmonics, ensuring high-quality power delivery.

Additionally, the reduced DC-link voltage stress and

soft-switching operations contribute to the overall

efficiency and longevity of the system. The design

has been validated through simulations and

experiments, demonstrating its effectiveness in real-

world scenarios. Ultimately, this VSC design

contributes to the ongoing efforts to enhance HVDC

transmission technology, making it more adaptable to

modern energy demands, particularly in the context

of integrating renewable energy sources. Its

scalability, reliability, and cost-effectiveness offer a

viable solution for future power grid applications.

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