C. Rahmann

Justified Fault-Ride-Through Requirements for Wind Turbines in Power Systems

In this paper, a novel adaptive strategy to obtain technically justified fault-ride-through requirements for wind turbines (WTs) is proposed. The main objective is to promote an effective integration of wind turbines into power systems with still low penetration levels of wind power based on technical and economical considerations. The level of requirement imposed by the strategy is increased stepwise over time, depending on system characteristics and on wind power penetration level. The idea behind is to introduce stringent requirements only when they are technically needed for a reliable and secure power system operation. Voltage stability support and fault-ride-through requirements are considered in the strategy. Simulations are based on the Chilean transmission network, a midsize isolated power system with still low penetration levels of wind power. Simulations include fixed speed induction generators and doubly fed induction generators. The effects on power system stability of the wind power injections, integrated into the network by adopting the adaptive strategy, are compared with the effects that have the same installed capacity of wind power but only considering WTs able to fulfill stringent requirements (fault-ride-through capability and support voltage stability). Based on simulations and international experience, technically justified requirements for the Chilean case are proposed.

Control strategies of doubly fed induction generators to support grid voltage

The use of wind power has been increasing very fast in the last 10 years. Many new projects for the next 10 years including offshore and onshore wind farms are been developed and planned. The fast growing of the use of wind power has brought new challenges to the Transmission System Operators (TSO) in regions where wind power has reached significant penetration levels like Denmark, United Kingdom, Spain and Germany. According to new grid code requirements wind turbines must remain connected to the grid during grid disturbances and, moreover, they must also contribute to voltage support during and after grid faults. Dynamic models of doubly fed induction generator (DFIG) were developed to investigate the behavior off different converter control and protection strategies of the back-to-back IGBT-based converters during grid fault. The results have showed that reactive power injection by DFIG-based wind farms is limited when the rotor side converter is blocked.

Mitigation Control Against Partial Shading Effects in Large-Scale PV Power Plants

This study proposes a novel control strategy to allow partially shaded photovoltaic power plants (PV-PPs) to mitigate the detrimental effects on the frequency of power systems without the need for energy storage. The strategy divides the PV-PP into N sections operating in a deloaded mode with a specific reserve level. A central controller continually monitors each of these PV sections. When one or more sections are under shaded conditions, the control orders the unshaded sections to deploy their active power reserves to smooth the power output at the interconnection point of the PV-PP. The proposed control was tested in the isolated power system of northern Chile considering different PV scenarios and levels of deload. Results show that the control is effective in assisting frequency regulation, especially under large PV penetration scenarios. For these cases, and only on days with high irradiance variability, the benefits gained from the control strategy could be more valuable for the system than the forgone revenues due to the deloaded operation.

Special Protection Systems: Challenges in the Chilean Market in Face of the Massive Integration of Solar Energy

Special protection schemes (SPSs) are an efficient alternative to increase the flexibility in the operation of power systems. The implementation of SPSs is often motivated by technical rather than economic reasons. The aim of this paper is to propose a novel framework for the analysis and development of SPSs in a context from the Chilean experience that comprises both technical and economic incentives to enable high penetration of solar energy. With this purpose, this paper explores the new capabilities of SPSs based on maximizing the cost-effective integration of solar power into a power system, given a set of technical challenges (congestion and dynamic response in case of contingencies) that must be considered in their design. In this framework, three main drivers were identified and discussed. The discussion includes the mechanisms associated with each driver that promote the massive integration of solar power. Moreover, within the proposed technical-economic framework, a methodology for driver selection and SPS design is proposed. The methodology and the drivers identified are evaluated in the Chilean power system.

Novel Methodology for Selecting Representative Operating Points for the TNEP

As part of transmission network expansion planning (TNEP), a technical and economical assessment of several planning alternatives must be performed in order to ensure fulfillment of the network security criteria and to estimate the alternatives’ expected operating costs. This task requires performing load flow calculations for different operating points (OPs) of the power system. Due to the high computational burden, considering all possible OPs is simply not possible. As a consequence, only a set of representative OPs is usually taken into account. Most works in the TNEP focus on issues related to optimization algorithms and modeling, neglecting the selection process of the representative OPs. Furthermore, most works only consider a few OPs, providing little or no insight about the criteria used in the selection process or about the error made when evaluating planning alternatives using a limited number of OPs. In this work, a novel methodology for selecting representative OPs to consider within the TNEP is presented. The proposal pays special attention to critical situations, where the network security may be endangered. Furthermore, the methodology allows quantifying the error made when evaluating network operation using a limited number of representative OPs.

Protection strategies for rotor side converter of DFIG-based wind turbine during voltage dips

Modern wind turbines operate with variable speed and most of them are based on doubly fed induction generators (DFIG), with a back-to-back power converter. During voltage dips, the stator terminals can cause overcurrents in the rotor windings, which could threaten the converter integrity. The use of crowbar systems is the most common technique to avoid this situation. The crowbar activation disables the control of the rotor side converter during a short period of time. Once the rotor side converter is blocked, the DFIG operates like a typical induction generator and thus consumes reactive power. In this paper, the performance of the well-known crowbar protection is compared with a current compensation strategy. Results show that the current compensation strategy can have positive effects on the terminal voltages during the fault without disconnection of the rotor side converter.

Effects of inertia emulation in modern wind parks on isolated power systems

Inertia issues due to increased use of modern wind turbines (WTs) have gained increased attention during the last years, especially in isolated power systems with limited frequency control capabilities. Therefore, different control strategies to allow inertial response from WTs have been proposed, either by considering deloaded operation or by using part of the kinetic energy stored in the blades. Other control strategies including additional equipment like flywheels or supercapacitors can also be found in the literature. In this paper, a comprehensive analysis of main effects of modern WTs with inertia emulation on the dynamic performance of a medium-sized isolated power system is presented. Simulation results indicate that inertia emulation in WTs can improve system frequency response and avoid the activation of under-frequency load shedding schemes during large power imbalances, thus significantly improving system security.

AC interconnection between longitudinal power systems - The Chilean case

This paper presents a comprehensive study about low frequency oscillations and transient stability when considering the interconnection of an extreme longitudinal power system with a second system through a tie line. The evolution of critical low frequency oscillation modes is investigated as the power transfer through the tie line increases. Time domain simulations are also conducted in order to verify some of the obtained results in the linear analysis. The simulations are based on detailed models of the two main power systems in Chile. The tie line model is based on technical data of the AC interconnection project which is under construction in Chile today.

Frequency stability support requirements for WTs in slow-response thermal power systems

A review of the latest publications and updates of international grid codes shows that the future trend regarding frequency stability support by wind turbines (WTs) during disturbances will be to impose stringent requirements. Nevertheless, system dynamic response depends in a complex manner on wind power network development and power system characteristics and therefore, no general conclusions for all power systems can be made. This paper presents a comprehensive analysis of main effects of modern WTs able to support frequency stability on the dynamic performance of slow-response isolated thermal power systems. A general roadmap including key factors to consider for the determination of proper grid requirements for WTs is presented. The study is carried out for different wind power penetration levels and strategies for frequency support by WTs. The investigation is applied on the isolated power system in Northern Chile characterized by a thermal generation matrix with slow response generation units. Simulation results indicate that the adoption of severe grid requirements regarding frequency stability support by WTs is already justified for penetration levels above 5%, in which case the system performance even improves compared to the scenario without wind power.

Large induction motors in distributed wind power generation

Recently, the interest in the use of wind energy has increased considerably in the worlds energy markets. In the same way there is a visibly tendency in distributed generation due to market deregulation, technological advances, governmental incentives and environment impact concerns. Considering these facts, it is important to understand the different impacts of using wind generation on several technical issues. This work aims to study the behavior of induction generator based wind farms during the connection of large induction motors to the network. Moreover, the influences on voltage sag, (LVRT) capability and short circuit characteristic are discussed.