Chapter 2: Literature Review
Grid Integration of Wind Power
Wind Turbine Technology Trends
Wind turbines are available having different number, shapes and sizes of blades. A typical wind unit consists of three blades connected to the rotor assembly enclosed by a hub. A shaft connects the rotor hub usually through a gearbox, to rotate the generator as shown in Figure 2.1. The electrical voltage from the generator is then stepped up to be connected to the grid. In some turbine types it is achieved directly, whereas in others it is achieved through frequency convertors using power electronics to synchronise with appropriate grid frequency and voltage.
Figure 2.1: Example of modern wind turbine 
Wind turbines have, with the passage of time, improved in physical size and generation capacities. In the early 80‟s, wind turbine generators used to be typically 15-20 m in rotor diameter and produced around 50 KW. Now they can produce over 7MW of electrical power with a rotor diameter of over 100 m. The largest capacity turbines typically around 7-9 MW are being commissioned off-shore [3-5]. A 25 year comparative graphical representation for turbine sizes over time is shown in Figure 2.2. Increases in conventional fuel cost have been driving research and development in renewable technology, especially wind generation. Rapid advances in this domain have enabled manufacturers to develop larger and higher generation capacities leading to lower costs per megawatt; also resulting are improvements in the efficiency of conversion and additional safety and grid provision features, enabling a higher level of integration of wind energy around the globe. Table 2.1 outlines some of the world‟s major wind turbine manufacturers and the features of their turbines.
Large-scale integration for both on-shore and off-shore wind farms raises concerns and challenges for the electricity industry; these include generation, transmission, distribution customers and power retailers. A number of concerns need to be addressed to connect wind power successfully. These include, but are not limited to, grid codes, power quality and power plant capabilities; design and operational aspects such as reserve balance management, short-term forecasting of wind, energy management and storage, and optimisation of flexibility of the system; grid infrastructure issues such as extensions and reinforcements, off-shore grids and their interconnections; and market re-design issues such as market aggregation, adaption of market rules to increase market flexibility particularly for cross-border exchange and operating the system closer to the hour of dispatch .
China and India have recently progressed well into wind generation. By the end of 2013, China and the US had a share of 27% and 21 % respectively in total global wind generation. Europe has historically been the largest regional market for wind generation in terms of total installed capacity (77GW, or 33% of the world total). Total world wind generation installed capacities are shown in Figure 2.3. .
European countries contribute the most to the world‟s wind generation compared to the rest of the world. Denmark produces 19% of total power generation through wind, while Spain and Germany are at 14% and 8% respectively . The precedence of wind integration at a larger scale in these countries raises questions of standardised methodology for ride-through studies for other countries and also raises the question as to how much wind or renewable integration remains economical while fulfilling other expectations .
Grid Codes and Operational Requirements
Increased penetration of wind power into grid has led to the development of specific technical requirements for the connection of large wind farms in transmission system. As mentioned before, these requirements are referred to as „Grid Codes‟, requirements for which have been a major driver in recent development in wind technology.
Fault Ride-through (FRT) Requirements
FRT requirements are one of the essential elements of most of the existing grid codes. Because of the large capacity of wind connected to a transmission system requires wind farms to remain in operation during any network disturbance or fault. FRT means that generator or wind farms should be able to stay connected to the grid during a fault or disturbance across a specified voltage envelope. These requirements are specified in terms of voltage versus time characteristics of the connection point. Different countries have different specified envelopes depending on network voltages or characteristics of the network as shown in Table 2.2 below.
FRT or Low Voltage Ride-through not only require the wind turbines to remain in operation during a fault or disturbance but also needs swift restoration of active and reactive power after the fault. Certain existing codes, wind farms are required to support the system voltage through reactive power support during the fault.
With several available wind generation technologies, grid inter-connection and having concerns over their capability to support grid as effectively as conventional generation, transmission system dictates the development of stricter grid code requirements. New wind generation technologies claim to support transmission networks but existing large wind generation connected to a transmission system raises many concerns for not being able to fully support the system during system level disturbances. The response of a large wind farm during a voltage disturbance could possibly affect system stability [10-13]. With the increasing capacity of wind farms and the possibility of participation in the electricity market towards real-time frequency keeping and reserve management, wind farms cannot afford to go off-grid.
To ensure security of supply, increased penetration and emerging requirements in the form of grid codes different parts of the world including, USA, UK, Germany, Denmark, France, Netherland, etc., oblige wind farms to ensure certain specified capabilities and frequency response . These codes also demand existing and newly installed wind farms to stay connected during various voltage and frequency events.
To avoid disconnection of wind generators from the grid, the performance of protection schemes and the ability of available generator technologies are to be tested for compliance with the internationally available fault ride-through criteria. This is strictly monitored for adherence by the grid operators under situations where generators are disconnected during grid faults. The protection performance of wind farms, short-circuit strengths, fault responses and ability to ride through the faults, needs to be well understood as part of this research. Literature on FRT criteria is available but does not cover their development of criteria detailing the role of protection and wind technology types.
Some grid operators require participation towards voltage stabilization and system recovery during both a fault and the post fault period . FRT not only requires the wind turbines to remain in operation during a fault or disturbance but also needs fast restoration of active and reactive power after the fault. In certain codes, it is required to support the system voltage through positive reactive power during the fault. FRT is subdivided into Low Voltage Ride-through (LVRT) and High Voltage Ride-through (HVRT). FRT capability implies that generators must be able to remain connected during any fault event for defined voltage profile. In some countries, grid codes require all newly installed wind turbines to have FRT or LVRT capability [15-17], enough to satisfy voltage duration curves. These curves require wind farms to achieve certain voltage support, protection co-ordination and considerations with the system to ensure that they could ride through any fault event  [8, 19].
Chapter 1: Introduction
1.2 Objectives & Contributions
1.3 Thesis Structure
1.4 Publications & Presentations
Chapter 2: Literature Review
2.1 Grid Integration of Wind Power
2.2 Grid Codes and Operational Requirements
2.3 Special Protection Schemes & Wind Integration
2.4 Wind Generator Technologies and Grid Operation
2.5 The New Zealand Electricity Industry
2.6 NZ Experience of Large Scale Wind Generation
2.7 NZ Wind Farm Operational Experience
Chapter 3: Wind Generator Modelling
3.1 Wind Generator Technologies
3.2 Power System Modelling
3.3 Technical Challenges in Model Development
3.4 Detailed Model VS Aggregated Models
3.5 Wind Generators Modelling Experience
Chapter 4: Fault Analysis
4.1 Technical Background
4.2 Fault Type
4.3 Fault Impedance
Chapter 5: Fault Ride-through Assessment
5.1 FRT Criteria Development for New Zealand
5.2 Fundamental Considerations
5.3 Methodology & Assumptions
5.4 Analyses & Results
5.5 Compliance Tests
5.6 Recommendations and Future Direction
Chapter 6: Wind Farm Protection
6.1 Conventional Generator Layout
6.2 Wind Farm Layout
6.4 FRT Criteria and Protection & Control Coordination
6.5 Case Study
6.6 Wind Integration Dynamic Fault Studies
6.7 Significance of Results
Chapter 7: Developing Wind Generator Sequence Network Equivalent Models
7.1 Development of DFIG-based Wind Farm Model
7.2 Proposed Formulation using Sequence Components
7.3 Case Study
7.4 Discussion and Significance of Results
Chapter 8: Conclusion
8.1 Main Contributions
8.2 Significance of the Contributions
8.4 Future Work
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