A Comprehensive Study On Control And Design Techniques For SPWM-Based Controllable Current Source Converters

High-voltage direct current(HVDC)transmission technology is an essential approach for achieving long-distance.large-capacity power transmission.The converter serves as a critical core component of HVDC transmission.Traditional line commutation converters,reliant on AC system voltage and strong AC system for commutation and lacking the ability to actively interrupt the current,are susceptible to commutation failures.In severe cases,these can cause large-scale power outages at the sending end and AC system breakdowns at the receiving end,threatening the safe operation of the power grid.Conversely,controllable current source converters(CSC),which operate independently of the AC system voltage to commutate,can actively interrupt the current,thereby resolving the issue of commutation failure.Moreover,they are capable of flexibly controlling active and reactive power,thus emerging as a new research area in the DC transmission field.Nevertheless,their practical application remains challenging due to intricate system control coupling,complex converter topology,and a scarcity of high-power devices.This dissertation introduces a controllable current source converter developed using Reverse Blocking Integrated Gate Commutated Thyristor(RB-IGCTs),capable of bidirectional voltage blocking and controllable switching.The research proposes a power decoupling control strategy based on constant DC voltage,a specific carrier synchronous overmodulation strategy,and an "equivalent capacitor voltage clamping" active voltage balancing technique.These solutions address the severe fluctuations in the converter DC-side output voltage,high switching frequency of the valve arm,and challenges in controlling consistency of series devices.Firstly,the dissertation outlines a power non-linear decoupling control strategy based on a constant DC voltage.This includes the derivation of active and reactive power coupling and analytical mathematical equations of the converter.From this basis,an inner-loop control strategy is proposed,targeting the converter filter capacitor voltage,as well as an outer-loop control strategy based on active and reactive power.This approach illuminates the dynamic power exchange mechanism of the converter’s AC and DC systems,defines the power operation range constrained by DC voltage and modulation ratio,and enables independent regulation of DC voltage,active power,and reactive power.The work presented overcomes the control structure limitations of the converter control system for "constant DC current,"reducing DC reactor from "henry-level" by more than 50%,enhancing system regulation speed,and lessening the complexity of inductor fabrication.Secondly,the dissertation proposes an improved saturation adjustable synchronous sinusoidal pulse width modulation method using a specific "triple frequency" carrier frequency of 150Hz,based on dual Fourier transform.This analysis reveals the elimination principles of the "zero state" in the valve arm switch state and the commutation mechanism between valve arms.Mathematical equations for the transient processes of valve turning-on and turning-off are derived,revealing the coupling rules of transient voltages between valves during the commutation process,and deepening understanding of the energy redistribution mechanism during abrupt changes in the commutation transient circuit.This results in the resolution of issues related to high switching frequency and substantial valve losses due to straight-through conduction of the bridge arm,and significant DC voltage drops at the valve output.Compared with traditional modulation methods,the device switching frequency has been reduced from 550Hz to 150Hz or 50Hz,losses have decreased by 72%,DC voltage utilization has improved from 83%to 100%,and the rate of transient valve voltage change has dropped by 32%.Lastly,the dissertation proposes an "equivalent capacitor voltage clamping"series voltage balancing circuit,based on variable frequency hysteresis control.It analyzes the causes of static and dynamic voltage imbalances in series connected IGCTs.By paralleling a diode and capacitor voltage control circuit with the resistor,capacitor,and diode(RCD)snubber circuit,this solution collaborates with voltage slope control,microsecond-level impedance switching,and active clamping of the isolated capacitor voltage.This effectively diminishes the challenge of voltage balancing,instigated by the equivalent impedance disparity in the series RB-IGCTs during the valve turning-off process.Experiments demonstrate that the inconsistency is less than 7.8%,which resolves the contradiction in selecting the snubber capacitor and resistor ratings,thus avoiding the risk of transient oscillation caused by too small damping coefficient and reducing the resistor power to 13%.This dissertation establishes a hybrid current source converter system simulation model and corroborates the capability of the controllable current source converter to mitigate commutation failure and to exercise flexible control over power.The work in this dissertation advances a compact,low-inductance integrated series module based on RB-IGCTs and constructs an equivalent test platform for the RBIGCT module and series components.Experiments were conducted on the RB-IGCT turn-off,along with the synchronized turning-on and turning-off of the series modules.The results indicate that the proposed control,modulation,and voltage sharing methods can provide support to overcome existing bottleneck issues.