Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Impact of Grid Forming Power Converters on the Provision of Grid Services through VSC-HVdc Systems

High Voltage dc (HVdc) transmission systems have gained increased popularity as a flexible and efficient power transmission option with higher grid controllability. Widespread adoption of HVdc systems for interconnecting power systems and integrating large renewable energy generation facilities such as wind farms, has forced the power system to undergo a transition from a predominantly ac system into a hybrid ac-dc system, especially in the high voltage transmission grid. This paper attempts to provide an overview on the role of Voltage Source Converter based HVdc(VSC-HVdc) systems within the evolving power system as a grid services provider. Special attention is paid to discuss the impact of Grid Forming converter control approach on the provision of such services through VSC-HVdc systems

Primary Frequency Regulation Using HVDC Terminals Controlling Voltage Dependent Loads

HVDC can provide frequency regulation during disturbances (e.g., faults) by controlling the power flow between two remote AC areas. While this action reduces the power deviation in the area affected by the disturbance, it causes a power imbalance in the other healthy AC area, leading to a frequency variation and endangering the system stability. In this work, a HVDC primary frequency regulation controlling voltage-dependent loads (PFRVDL) is proposed, where the HVDC terminal in the healthy area influences the grid voltage amplitude to shape (decreasing or increasing) the load consumption in order to cope with the power variation required by the fault-affected area. The PFR-VDL extracts the needed energy for the frequency support, not from the generators (with following frequency deviation) but from the voltage-dependent loads in the healthy area. This work analyzes the PFR-VDL performance, generalizing it with two possible HVDC connection cases: Asynchronous connection with single HVDC line, and embedded HVDC forming a parallel, hybrid connection with HVAC. The PFR-VDL application benefits and limitations are evaluated analytically and verified by means of PSCAD EMTDC simulations, and finally validated with a large interconnected IEEE 39 bus system

Potential and Impacts of Smart Transformer in Green Harbours

Harbour grids are undergoing rapid transformation due to the increased interest in green harbour initiatives such as ship cold ironing, renewable energy integration, battery-powered marine vessels, etc. In this scenario, better controllability over the power flow is important to maintain the voltage and current quality within the grid-code specified limits and ensure a stable and efficient power supply. This paper aims to explore the potential of the smart transformer (ST) in providing various support features to green harbours. The features include the integration and control capability of ST in accommodating renewable energy sources, electric vehicle charging stations and storage. In addition, the impact of the ST in the green harbour is analyzed with the focus of addressing the key issues and challenges such as voltage variations, peak loads and poor power factor

Scalable State-Space Model of Voltage Source Converter for Low-Frequency Stability Analysis

Low frequency instability phenomena in power electronic based power systems can originate both at converter and at power system level. At the converter level, the interaction between PLL, dc-link and ac voltage control in voltage source converters (VSCs) connected to a weak grid can lead to instability. At the power system level, the interactions among different parallel VSCs can produce oscillatory phenomena, and even result in instability. However, a model to study the instability phenomena at both levels is still under development. In this paper, a scalable VSC state space model, which captures the interactions among PLL, dc-link and ac voltage control is proposed. The proposed model is suitable for interconnection, and an example of power system modeling is shown. The model is then validated through simulations and experimental tests. Eigenvalue analysis is carried out to investigate the influence of the control parameters on the stability

Zero-sequence Circulating Current Suppression with Stand-alone Feedforward Control for Power Hardware-in-the-Loop System

A PWM converter based power hardware-in-the-loop (PHIL) system can provide rapid and low-cost alternatives for prototype testing of high-power converters for electric vehicles or smart grid industries. To achieve simple configuration avoiding an additional bidirectional DC supply, the power amplifier (PA) in the PHIL can share the DC-link of the device under test (DUT). However, due to the coupling of AC and DC ports and differences in the zero-sequence voltage (ZSV) of the two converters, an inevitable low-frequency zero-sequence circulating current (ZSCC) results. This paper proposes a stand-alone control method using ZSV feedforward control to suppress the ZSCC flowing through the PWM converters of the PA and DUT. The ZSV of the DUT can be estimated from the d-q voltage equation in the PA, and feedforward term is applied with a proportional resonant (PR) controller. The effectiveness of the proposed method is verified by experimental results

Analysis and Suppression of Zero-Sequence Circulating Current in Multi-Parallel Converters

The use of a multi-parallel converter system has many advantages in increased scalability, better maintenance, scheduling, and improved output current quality. However, a periodic zero-sequence circulating current (ZSCC) may occur due to the asymmetry of parallel-connected converters. ZSCC produces additional losses and possible instability of the system. Therefore, proper control must be applied to suppress this harmful ZSCC. In order to design an effective controller for suppressing ZSCC, it is necessary to analyze the cause of the circulating current generation. However, most of the existing studies have applied the controller without detailed analysis. Therefore, this paper mathematically analyzes the ZSCC spectrum using the Fourier series to identify which harmonics are included in ZSCC. From the analysis results, the necessity of multi-resonant controllers to suppress the ZSCC at specific harmonics is demonstrated. Simulation and experiments are conducted to validate the analysis results and the necessity of multi-resonant controllers

Online Estimation of Dynamic Capacity of VSC-HVdc Systems –Power System Use Cases

The dynamic capacity describes the capability of high voltage direct current (HVdc) systems to operate temporarily beyond their guaranteed active and reactive power (P/Q) limitations under specific conditions. In this work, the dynamic capacity is intended to be applied in various power system use cases to ensure a more efficient and secure grid operation. In contrast to previous works, the dynamic capacity is considered with a holistic view on the HVdc system’s components. Moreover, to overcome existing limitations considering only the HVdc system design, it is introduced to estimate the dynamic capacity based on real-time operational data. In principle, dynamic capacity could help for any power system use case where temporarily additional capacity is required. The article details five use cases, including congestion management, voltage support, frequency response, offshore wind overplanting and grid planning to be of high interest for such a feature. The main HVdc applications, embedded systems, interconnectors and offshore grid connection, and anticipated time frames for dynamic capacity are highlighted from power system perspective. Also, the time-criticality of the remedial actions is outlined

Smart transformer-based medium voltage grid support by means of active power control

In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid, due to the spread of non-dispatchable generation sources. This paper introduces a Smart Transformer (ST)-based Medium Voltage (MV) grid support by means of active power control in the ST-fed Low Voltage (LV) grid. The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer, which needs tens of seconds. This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid, controlling the ST output voltage. The last feature has two main effects: the first is to reduce the active power withdrawn from MV grid, and consequently the MV voltage drop caused by the active current component. At the same time, higher reactive power injection capability in the MV converter is unlocked, due to the lower active power demand. As result, the ST increases the voltage support in MV grid. The analysis and simulation results carried out in this paper show improvements compared to similar solutions, i.e. the only reactive power compensation. The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid