Static Var Generators (SVG) serve as core equipment for power quality optimisation in power grids. Their dynamic reactive power compensation capability directly determines the stability, transmission efficiency, and reliability of the power supply in the grid. The performance ceiling of SVG largely depends on its core main circuit topology. Diode—clamped multilevel converters have become the mainstream topology choice for next-generation SVG due to their advantages of a simple structure, low harmonics, and easy scalability. This article focuses on the in-depth integration of this topology with SVG, detailing how it empowers SVG to break through traditional technical bottlenecks and provide innovative solutions for the high-quality operation of power grids.
I. Core Challenges of SVG: Why Traditional Topologies Fail to Meet Modern Grid Demands?
With the diversification of industrial loads and the expansion of new energy grid connection scale, the performance requirements for SVG in power grids have become increasingly stringent. However, traditional two-level topology SVG has three core shortcomings:
1. Insufficient Response Speed to Keep Up with Load Fluctuations
High-frequency load fluctuations, such as motor startup/shutdown and frequency converter speed regulation in the power grid, require SVG to complete dynamic adjustment of reactive power within 20ms. Restricted by switching frequency (usually ≤ 2kHz), traditional two-level topology SVG has a response delay of more than 50ms, making it unable to balance reactive power in real time and prone to voltage fluctuations.
2. High Harmonic Content Increases Grid Burden
The total harmonic distortion (THD) of the output voltage of traditional SVG generally exceeds 8%, requiring additional LC filtering devices to meet national standard requirements (THD ≤ 5%). This not only increases the volume and cost of SVG but also may cause resonance risks due to mismatched filtering parameters.
3. Poor Scalability and Limited Adaptable Scenarios
From 10kV distribution networks to 500kV transmission networks, SVG systems of different voltage levels require differentiated designs. To improve capacity or voltage rating, traditional topology SVG needs significant modifications to the main circuit, resulting in poor compatibility and difficulty in quickly responding to diverse scenario requirements.
The emergence of diode-clamped multilevel converters precisely addresses these pain points for SVG, becoming a key breakthrough for SVG technology upgrading.
II. New Core of SVG: Working Logic of Diode-Clamped Multilevel Converters
Through the innovative design of diode clamping and capacitor voltage division, diode-clamped multilevel converters restructure the main circuit structure of SVG, with core principles and advantages as follows:
1. Topology Structure: Simple and Reliable, Supporting SVG Multilevel Output
Taking fully controlled devices such as IGBT and MOSFET as the core, combined with clamping diodes and voltage-dividing capacitors, the converter forms a three-phase bridge arm structure. Core features include:
- The DC side adopts series capacitor voltage division (e.g., a 5-level SVG requires 4 capacitors, with each capacitor voltage being 1/4 of the total DC side voltage). Through the clamping effect of diodes, the voltage across switching devices is limited to the range of a single capacitor voltage, avoiding overvoltage damage to devices;
- Each bridge arm can output multiple voltage levels through the on/off combination of different switching tubes (e.g., a three-level SVG outputs -E/2, 0, E/2, and a five-level SVG outputs -E, -E/2, 0, E/2, E), making the output voltage of SVG closer to a sine wave;
- Compared with the traditional two-level SVG with only 8 space voltage vectors, the number of vectors of this topology SVG increases significantly (27 for three-level), and the control accuracy of SVG over output current is improved by more than 3 times.
2. Modulation Strategy: PWM Technology Optimises SVG Output Quality
To maximise the advantages of the topology, this type of SVG adopts triangular carrier cascaded PWM modulation technology, with core logic:
- Taking a sine wave as the modulation wave (corresponding to the reactive current waveform that SVG needs to output) and multiple triangular waves as carrier waves, switching pulse signals are generated through comparators to precisely control the on/off of switching tubes;
- The higher the carrier frequency, the lower the harmonic content of the SVG output voltage—when the carrier frequency is set to 5kHz, the THD of the SVG output voltage can be reduced to 2.3%, meeting national standards without additional filtering;
- The carrier in-phase cascading method can specifically suppress low-order harmonics, such as 3rd and 5th harmonics, further reducing the harmonic interference of SVG on the power grid and adapting to precision manufacturing scenarios with strict power quality requirements.
3. Core Advantages: Targeting SVG Performance Pain Points
- Fast Response: The multilevel structure is compatible with higher switching frequencies (up to 10kHz), shortening the dynamic response time of SVG to within 15ms, which can track load reactive fluctuations in real time;
- Low Harmonics: Multilevel output reduces voltage jumps, and combined with PWM modulation, the harmonic distortion rate of SVG is significantly reduced, eliminating the cost of additional filtering;
- Easy Scalability: By increasing the number of bridge arm units, it can be flexibly upgraded to 5-level, 7-level SVG, adapting to different voltage levels from 10kV to 500kV, with scalability far exceeding traditional topologies.
III. SVG Simulation Verification: Actual Performance of Diode-Clamped Topology
To verify the effectiveness of this topology SVG, a 5-level SVG simulation model was built using Matlab/Simulink (parameters: modulation index 0.6-0.8, DC side capacitor 10mF, carrier frequency 5kHz, system frequency 50Hz). The test results are as follows:
1. Reactive Power Compensation Effect: Power Factor Quickly Approaches 1
In the initial simulation state, the grid active power was 200kW, inductive reactive power was 200kVA, and the power factor was only 0.75. After putting this topology SVG into operation, dynamic adjustment was completed within 0.05s, and finally, the grid power factor stabilised at 1.0, with reactive power approaching 0, completely offsetting the reactive power consumption of inductive loads.
2. Harmonic Suppression: Significant Reduction in THD
Under the same operating conditions, the output voltage THD of the traditional two-level SVG was 8.5%, while the THD of this topology SVG was only 2.3%, meeting the requirements of GB/T 14549-1993 Power Quality - Harmonics in Public Supply Networks without filtering, effectively avoiding equipment damage due to harmonic overheating.
3. Voltage Stability: Fluctuation Controlled Within ±2%
In the scenario of simulating sudden load changes (reactive power surging from 100kvar to 300kvar), this topology SVG responded quickly, controlling the grid voltage fluctuation amplitude within ±2%, which is far better than the ±5% of traditional SVG, ensuring the stable operation of sensitive equipment (such as precision instruments and PLC systems).
IV. SVG Application Scenarios: Topology Innovation Empowers Diverse Power Demands
Based on its excellent performance, SVG with diode-clamped multilevel topology has been widely used in three core scenarios:
1. Industrial Distribution Networks: Solving Nonlinear Load Problems
Nonlinear loads such as frequency converters and arc furnaces in industries such as steel and chemical engineering are prone to causing grid reactive power imbalance and harmonic pollution. This topology SVG can compensate for reactive power in real time (increasing power factor to above 0.95) and suppress harmonics (reducing THD to ≤ 3%), reducing electricity fines and equipment maintenance costs. A steel plant saved more than 2 million yuan in electricity bills annually after the application.
2. New Energy Grid Connection: Ensuring Grid-Connection Stability
The output fluctuations of photovoltaic power plants and wind farms are likely to cause voltage fluctuations and reactive power imbalance at the grid connection point. This topology SVG can quickly suppress fluctuations and maintain the stability of the grid connection point voltage, improving the grid-connection acceptance capacity of new energy power generation. A wind farm reduced its curtailment rate by 5% after applying this SVG solution.
3. High-Voltage Transmission Networks: Enhancing System Support Capability
In 500kV ultra-high voltage transmission networks, SVG with five-level and above topologies can provide dynamic voltage support, suppress line voltage sags and oscillations, increase transmission capacity by more than 20%, and ensure the reliable operation of cross-regional power grids.
V. Conclusion: SVG Topology Innovation Leads the Upgrade of Power Grid Power Quality
The application of diode-clamped multilevel converters enables SVG to break through traditional technical bottlenecks, achieving a performance leap of "fast response, low harmonics, and easy scalability", and becoming the core choice for power quality optimisation in modern power grids. From adapting to industrial load scenarios to supporting the stable grid connection of new energy, this topology SVG is driving the power grid towards efficient, reliable, and clean development through technological innovation.
If your power system faces problems such as reactive power imbalance, excessive harmonics, or voltage fluctuations, please feel free to inform us of the grid voltage level, load type, and core demands. HengRong Electric CO., LTD. will customize SVG solutions for you, helping SVG to fully leverage its performance advantages and ensure high-quality operation of the power grid!