Advanced power converters for flexible AC transmission systems

Power systems are undergoing significant changes for two reasons which are the widespread and growing use of power electronic based equipment and the fast penetration of renewable energy sources and distributed generation systems which have unpredictable generating nature. Whilst these are beneficial for meeting the forever increasing power demand and environmental requirement, they bring problems of injecting harmonics into the power lines and proliferating unbalanced current flow. Combining these with the usual issues of power systems such as faults caused by voltage distortion and unbalance, resonances, line impedance mismatch, etc, maintaining high quality power supply to customers can be challenging. Flexible AC Transmission Systems (FACTS) have been applied for reactive power compensation, voltage control and power flow regulations. Recent development also demonstrates the attention of engineers and researchers being moved toward deploying FACTS devices for harmonic cancellation and reactive power compensation under unbalanced grid voltage conditions. Nevertheless, effective control techniques capable to track and eliminate the power line voltage/current distortions and harmonics accurately, in real-time are yet to be developed.

This work investigates advanced harmonic measurement and control techniques for both Static Compensator (STATCOM) and Synchronous Series Compensator (SSSC), to realize high performance harmonic cancellation and unbalanced voltage/current compensation. The converter topologies explored are all based on modular multilevel (voltage/current) types without a transformer.
The work has led to the development of a new adoptive harmonic cancellation scheme for Modular Multilevel cascaded converter STATCOMs. This scheme uses the discrete wavelet packet transformation (DWPT) algorithm to identify dominant harmonic elements in the measured non-stationary load current in real-time. To overcome the limitations of DWPT due to the effect of data array boundary, a data expansion technique is proposed. By applying the identified harmonics to update the parameters of a chain of notch-filters, the reference currents are generated for MMCC-STATCOM to inject into the grid thus eliminating the unwanted current elements.

Operating under an unbalanced current, the sub-module capacitor voltages of an MMCC-STATCOM may become unbalanced and hence not able to function. A novel technique to mitigate such a problem is created which relies on injecting a common mode voltage to the phase current models when applying the model predictive control (MPC) method. Not only this approach ensures each modules’ capacitor voltages are at the required level, it also extends the unbalanced current compensation range. Meanwhile when implementing MPC, an adjusted branch and bound (B$\&$B) algorithm is proposed to minimize the cost function in order to find the optimal solution. This method has been shown able to produce accurate results whilst giving reduced computational burden compared with traditional methods.

The final contribution in this work lies in the area of SSSC for power flow control. A new SSSC topology – the parallel Transformer-less SSSC is investigated which has the advantages of increased power rating and control capability, fast dynamic response and high efficiency. To suppress the current circulating between parallel converter branches associated with this topology, a novel MPC is proposed for the current control. This inserts a differential current term in the cost function apart from the common reference current tracking terms. This scheme has been validated successfully when two parallel converter branches are used.