• Energy Research

Three-phase three-leg inverters represent the majority of inverters that are integrated into low voltage power grids. They can develop positive- and negative-sequence components during unbalanced conditions including asymmetrical faults, but not zero-sequence components. This indicates that a sequence-based control strategy is required for comprehensive control of inverter currents and voltages. Positive- and negative-sequence components must be controlled separately with their own dedicated controllers in such a scheme. Given this critical need, a sequence-based control technique for grid-forming and grid-feeding inverter-interfaced distributed generators is proposed. As a necessary part of the fault ride-through capability of inverter-interfaced distributed generators, a current limiter is developed, which can limit inverter currents at a pre-defined threshold for both balanced and unbalanced faults. Whilst the current regulators work in the synchronous reference frame with simple PI controllers, the limiter works in the natural reference frame and thus treats each phase current separately, which results in current limiting at the threshold for all phases. The advantage of the proposed limiter is that it acts instantaneously such that the inverter current can be limited from the start of the disturbance. The simulation results in MATLAB/SIMULINK reveal promising performance of the proposed control strategy with the proposed current limiting.

This research investigates third harmonic injection applied to a modular multilevel converter (MMC) to generate a higher DC voltage. This is achieved using a proposed novel control scheme that activates existing submodules (SMs) in the converter arms. The technique is fundamentally different to, and does the reverse of, the well-known third harmonic injection techniques utilized to increase the AC output voltage in three-phase converter systems for a given DC link voltage. In the proposed scheme, the number of inserted SMs in each converter leg is greater than the number of SMs per arm, whence the MMC can operate with a higher DC link voltage while the SM number per arm and their capacitor voltages remain unchanged. This lowers the DC current and the DC transmission loss is significantly reduced by 22%. Station conduction losses with the operational scheme are lowered by 2.4%. The semiconductor current stresses are also lowered due to the reduced DC component of arm currents. Additionally, the phase energy variation is reduced by 18%, which benefits circulating current control. The operating principles are presented in detail and mathematical models for conduction losses, energy variation, and circulating voltage are derived. Simulation of a point-to-point HVDC system demonstrates the effectiveness of the proposed MMC operational scheme.

This paper proposes a multi-timescale volt/var optimization for the optimal dispatch of battery energy storage system in smart distribution grids. It aims to coordinate the substation on-load tap changer operation on slow-timescale (hourly basis) with the photovoltaic inverters and battery storage operations on fast-timescale (15 min basis). This coordination is achieved by using two-stage stochastic programming and implemented via model predictive control. The power loss and energy purchase cost are reduced while maintaining voltages within limits. The forecasting uncertainties are modeled by generating a large number of random scenarios and then subsequently reducing scenario numbers to establish a tradeoff between computational burden and accuracy of the solution. The mixed-integer second-order cone program (MISOCP) is formulated with reduced scenarios, which achieves global optimum. Simulation results demonstrate the effectiveness of proposed MISOCP model in keeping the voltages within limits under forecasting uncertainties.

Abstract This paper studies the feasibility of using the vanadium redox flow battery (VRB) for power quality control applications. This work investigates the dynamic voltage and current responses of the VRB to load changes over a range of frequencies (up to 5 kHz), through experimental studies on a laboratory scale testing system. Experiments were carried out under different operating conditions to examine the effects of system SOC, discharging current and temperature. The analysis shows that the magnitude of battery impedance is higher at low frequencies but lower at high frequencies. These results suggest that the VRB has the ability to handle charging-discharging power fluctuations in a frequency range up to a kHz level. By using the concept of fractional order systems, the transient behaviour of the VRB cell was modelled as an equivalent circuit that utilises a constant phase element to represent the electrochemical double layer and a Warburg element to describe the effect of concentration polarisation. This equivalent circuit model is useful for electrical interface design and power flow control applications.

The modeling of electric sources in the sequence domain helps understand their response during faults when they are integrated into power systems. Since the fault response of inverters is significantly different from that of synchronous generators, their equivalent sequence models must differ as well. While the sequence models of synchronous generators are well-known and verified, practical sequence models of inverters remain an area of research. This manuscript addresses the modeling of grid-forming inverters in the sequence domain where the focus is on three-leg three-phase inverters. To generalize the analysis, two different control strategies used with grid-forming inverters are considered. The equivalent sequence models of these inverters with both strategies are discussed in detail and numerical simulations are used to further clarify the analysis. It is observed that grid-forming inverters with different types of control strategies show a consistent behavior in the positive-sequence circuit during faults for a given voltage drop. Conversely, the negative-sequence model of grid-forming inverters is heavily influenced by the control strategy. The analysis and results are useful to develop reliable protection schemes in inverter-based grids, as understanding the characteristics of electrical elements in the sequence domain is essential for any protective function.

The AC voltage control of a DC/DC converter based on the modular multilevel converter (MMC) is considered under normal operation and during a local DC fault. By actively setting the AC voltage according to the two DC voltages of the DC/DC converter, the modulation index can be near unity, and the DC voltage is effectively utilized to output higher AC voltage. This significantly decreases submodule (SM) capacitance and conduction losses of the DC/DC converter, yielding reduced capital cost, volume, and higher efficiency. Additionally, the AC voltage is limited in the controllable range of both the MMCs in the DC/DC converter; thus, over-modulation and uncontrolled currents are actively avoided. The AC voltage control of the DC/DC converter during local DC faults, i.e., standby operation, is also proposed, where only the MMC connected on the faulty cable is blocked, while the other MMC remains operational with zero AC voltage output. Thus, the capacitor voltages can be regulated at the rated value and the decrease of the SM capacitor voltages after the blocking of the DC/DC converter is avoided. Moreover, the fault can still be isolated as quickly as the conventional approach, where both MMCs are blocked and the DC/DC converter is not exposed to the risk of overcurrent. The proposed AC voltage control strategy is assessed in a three-terminal high-voltage direct current (HVDC) system incorporating a DC/DC converter, and the simulation results confirm its feasibility.