An adaptive reclosing scheme based on phase characteristics for MMC-HVDC systems

To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC

Upstream Fault Detection in a Grid Tied Microgrid Setting

Microgrids have been introduced to integrate more renewable resources into the utility wgrid. Bidirectionality and low fault current levels are issues that cause fault detection problems for microgrids in both grid connected and islanded modes of operation. During an upstream fault condition the desired reaction for the microgrid is to island itself. This allows certain loads, such as critical loads, to remain uninterrupted during an outage on the main grid. Therefore, the maintenance crew can be safely dispatched to alleviate the problem without fear of back-feed from the microgrid side. Hence, a critical problem that still needs adequate solutions developed is the detection of upstream faults in the grid connected mode without reliance on communication systems. Protection methods to detect upstream faults through relay solutions are presented in this thesis. First, voltage collapse is used to detect faults on one particular microgrid topology. However, in power system protection design, it is generally not practical to use voltage as an indicator for faults. Hence, a novel protection method is proposed using second order harmonic magnitudes as a fault indicator. This protection solution detects faults based on voltage and current second harmonic magnitudes during fault transient periods. A microgrid system is built and validated based on an actual operational microgrid at Eaton Corporation in Warrendale Pennsylvania. Multiple case studies are simulated using PSACAD/EMTDC to verify the effectiveness of the proposed protection solutions

Fault Protection and Reduced-Order Modeling for Secondary Controller Design of Inverter-Based Microgrids

This dissertation studies fault detection in the case of low-fault current levels and reduced-order modeling of inverter-based microgrids. A phase-based fault detection method is developed that can detect faults regardless of fault current levels and without reliance on communication systems. The speed of this approach is increased by utilizing all the phases of the three-phase power system, effectively reducing the fault detection duration to one third of a cycle at most. Additionally, for any microgrid system configuration that would cause fault detection difficulties, a model-based fault detection approach is developed. This method can be used without communication for certain system constraints, which are derived analytically. Besides protecting the system properly, controllers are needed to stabilize the system post-faults or post-disturbance events. Model-based controller synthesis methods can be a plausible approach to this problem, but may result in high-order controllers. Using reduced-order models can lower the complexity of controller design. Hence, this dissertation also develops a reduced-order model for microgrids. A dq based reduced order model for secondary layer controller design is developed. The model has a significantly lower order with better accuracy than the current available models. A linear quadratic integral controller is designed based on the lower-order model to demonstrate the application of the proposed model. Simulations are performed to verify the proposed solutions in PSCAD and MATLAB/Simulink environments

Power control, fault analysis and protection of series connected diode rectifier and VSC based MTDC topology for offshore application.

A multiterminal high-voltage dc (MTDC) system is a promising method for transmitting energy generated from an offshore windfarm (OWF). The creation of MTDC systems became easier by the introduction of voltage source converter (VSC) due to the flexibility and controllability it provides. This technology is newer than the line-commutated converter technology (LCC). Power systems can include any number of windfarms together with converters for both offshore and onshore power conversion. Therefore, this thesis suggests a three-terminal MTDC model of two offshore windfarms and one onshore inverter. The electric energy generated by the two windfarms is rectified into dc and transmitted to the shore using dc cable. Although a VSC or a diode rectifier (DR) can convert ac to dc, a series connection of a VSC and two DRs was proposed at the windfarm side to convert the generated power to achieve controllability of the uncontrollable diode rectifiers and reduces the high cost of badditional VSCs. The proposed topology converts the ac power by dividing the windfarm power so that one-third is the share of the VSC and two-thirds is the share of the DRs. The same topology is used to convert the power produced from the other windfarm. Then, the dc power is transmitted via an undersea dc cable to the onshore location, and is then inverted into ac before it is supplied to the neighbouring ac grid using a grid-side VSC. The proposed topology has many advantages, including a significant save in windfarm VSC (WFVSC) capital cost and a significant reduction in the loss of power of the converter without losing the overall controllability. However, although this topology is suitable for windfarm applications, it might not be suitable for high-voltage direct current (HVDC) that requires bidirectional power flow unless making changes to the topology such as disconnecting the diode rectifiers. Furthermore, fault analyses were investigated, including dc faults and ac faults. Ac faults are categorised as symmetrical or unsymmetrical faults. For comparison purposes, a Simulink model was designed, implemented, and simulated as a reference model. The reference model can operate as VSC-, DR-based MTDC, or a mix of both in a way that any component can be added to or removed from the model at any time during the simulation run. The contribution to the dc fault current from various parts such as dc capacitor and the adjacent feeder was investigated thoroughly, and detailed mathematical formulae were developed to compute fault current from these contributors. In addition, the results of the system response due to both fault types are illustrated and discussed. Both symmetrical and unsymmetrical ac faults were initiated on the onshore grid side, and the system response results are presented for those faults. A generalised control scheme (GCS) was proposed in this thesis, which add the ability the model to control the reactive power and is suitable for both balanced and unbalanced ac faults conditions. A protection against faults was investigated and implemented using dc circuit breakers. The protection system was built to ensure safe operation and to fulfil the grid code requirements. Many grid codes are available and presented in the literature, such as Spanish, British, and Danish; however, a grid code by E.ON was chosen. The protection scheme in VSC-based MTDC networks plays a vital role during dc faults. It is vital that this protection be sensitive, selective, fast, and reliable. Specifically, it must isolate the fault reliably from the system within a short time after the fault occurrence, while maintaining the remaining components of the system in a secure operational condition. For optimal performance, the protection scheme discussed in this thesis employs solid-state circuit breakers. A literature survey relevant to the tasks mentioned above was conducted.PhD in Energy and Powe