CPF Continuation Power Flows DAE Differential and Algebraic Equations DC Direct Current DFIG Doubly Fed Induction Generator EESG Electrically excited synchronous generator FACTS Flexible
INTRODUCTION
The increasing environmental concern of the conventional electricity generation impacts raised the need to minimize it and to generate electricity from the renewable resources
Wind power is currently the most practical and economically viable renewable energy option for large‑scale deployment Global interest in wind energy is rising as electricity demand grows and fossil fuel reserves dwindle, especially in regions where hydropower is unavailable Nuclear power carries environmental hazards and leakage risks, prompting many countries to pause new nuclear builds and, in some cases, to phase out existing plants For example, Germany closed all of its older nuclear reactors and proceeded with a phased shutdown of the remaining units by 2022.
Wind power fluctuates by nature, and grid integration demands high reliability and availability As wind power penetration rises in the power systems of many countries, new requirements for connecting large wind farms to the transmission system have been established in grid codes issued by transmission system operators (TSOs) A key element across most grid codes is the fault ride through (FRT) or low voltage ride through (LVRT) capability of wind turbines, with LVRT designed to keep wind power generators connected during system disturbances by maintaining operation when voltage sags occur.
LVRT capability is the most crucial requirement for wind farms, especially with DFIG In DFIG machines, the stator windings are directly connected to the grid, and voltage dips or sags reduce the stator voltage, causing abrupt fluctuations in the stator flux This transient drives the stator currents up, creating an overcurrent that is transferred through the magnetic coupling between the stator and rotor windings These high transient currents can damage components, notably the semiconductors of the rotor-side converter (RSC), which are vulnerable to thermal breakdown.
A wind farm must stay connected to the grid during disturbances and continue operating, delivering both active power and reactive power under transient voltage conditions to comply with grid codes.
Wind power systems must meet grid-connection requirements, including effective fault ride-through capabilities and adequate reactive power support to handle faults and heavy loading If the system cannot meet reactive power demand during faults or high loading, voltage instability can occur, manifested as a decline in the voltage magnitude at one or more buses as reactive power injection rises This instability may cause a progressive voltage drop across the network, leading to local load loss, tripping of transmission lines and other equipment, cascading outages, and ultimately loss of synchronism among generators.
Stand-alone systems are easier to model, analyze, and control than large power systems in simulation studies A wind farm is a set of connected wind turbines placed in
A wind farm is a large facility that generates substantial electricity at a single site from many wind turbines arranged over a wide area, and each turbine produces power at varying levels because wind speeds differ These facilities are typically located in remote locations, which can create weak points in the transmission system and therefore require careful planning and specialized considerations when connecting wind generation to the grid.
Induction machines are highly sensitive to unstable operation, which can lead to stator overheating and significantly shorten the machine’s service life In addition, negative-sequence currents generate pulsations in the electrical torque, and these torque fluctuations can reduce the lifespan of the gearbox and blade assembly while increasing turbine noise.
To protect the machine, in some applications, Induction machines are disconnected from the grid when voltage unbalance occurred [5]
Doubly fed induction generators (DFIGs) are widely adopted in wind farms because they offer variable-speed, constant-frequency operation, reduced flicker, and independent control of active and reactive power Their partially rated power-electronic converters—typically 25–30% of the total machine rating—make DFIG systems economical and lead to lower converter losses than full-scale converter designs However, protection devices are required to safeguard the converters from over-current and over-voltage during grid faults.
Wind farms connected to a strong grid can quickly restore voltage and frequency after a disturbance, thanks to support from the power grid itself In contrast, a weak grid interconnection is unreliable because disturbances introduce a persistent risk of voltage instability.
Flexible AC Transmission Systems (FACTS) are widely used in power systems due to their ability to provide flexible power-flow control The main objectives of FACTS are to bring the power system under centralized control and to transmit power as directed by the control center, while allowing the transmission network to operate at its maximum thermal limits.
Reactive power support and voltage regulation offered by Mechanically Switched Capacitors (MSC), Static Var Compensators (SVC), and Static Synchronous Compensators (STATCOM) are essential for improving voltage stability and reinforcing weak power networks By delivering fast, controllable reactive power, these devices help mitigate voltage sags and swells, enhance grid reliability, and support secure operation of modern power systems.
MSCs can boost the system's voltage limit, but they are not highly sensitive to voltage changes and cannot track the day-to-day fluctuations in wind-power output Reactive power imbalance can significantly affect the connected grid, but it can be minimized by dynamic reactive power compensating devices such as SVCs and STATCOMs, which also support LVRT requirements.
Reactive power compensation capability of an SVC declines as the connected bus voltage decreases, reducing its effectiveness under low-voltage conditions A STATCOM, on the other hand, can deliver its full reactive power range even at very low voltages, preserving dynamic performance Therefore, to achieve the same dynamic performance, a higher-rated SVC is typically required compared with a STATCOM.
This thesis investigates the deployment of FACTS devices to empower wind farms to mitigate voltage at the Point of Common Coupling (PCC) with a weak grid under normal operating conditions and during disturbances and faults It emphasizes a grid operation strategy that leverages these devices for dynamic voltage support, improved voltage profiles, and enhanced system stability when voltages deviate from desired limits The work outlines practical considerations for integrating FACTS into wind-farm operations and describes methods to ensure reliable performance amidst faults and grid disturbances.
3 requirements to maintain the proper voltage at the PCC by regulating voltage This may increase grid stability and can assist wind power plants to follow the grid code
Four scenarios were tested to compare the performance of a wind farm connected to a weak grid without compensation, using large SVC and large STATCOM as compensation devices The first scenario examines high-load switching on the grid side, the second scenario considers a low-impedance three-phase short circuit on the grid side, and the third and fourth scenarios investigate short-duration voltage swelling and sagging, respectively The study then continues with smaller SVC and STATCOM units, evaluated both alone and in parallel with a constant capacitor.
The dynamic model of the DFIG is available in the MATLAB-based Power System Analysis Toolbox (PSAT) package version 2.1.6 was used for the simulation
BACKGROUND
V Curves
Power-voltage (P-V) curves are the most widely used method for predicting voltage instability in power systems They indicate the MW distance from the current operating point to the critical voltage, providing a tangible measure of how close the system is to instability A typical P-V curve is shown in Figure 2-4.
This PV curve depicts how the voltage at a specific bus varies with the total active power supplied to loads or sinking areas, highlighting a stable region followed by a rapid voltage decline as power increases It can be seen that at the knee of the curve the system transitions from gradual voltage changes to sharp sag, signaling the onset of voltage instability.
The PV curve reveals a critical point where voltage drops rapidly as load demand increases, and load-flow solutions beyond this point indicate the system becomes unstable This critical point defines the minimum operating voltage and the collapse margin, making the PV curve a key tool for determining the system's critical operating voltage and stability limits Generally, operating points above the critical point correspond to a stable system, while those below it indicate an unstable condition.
PSAT generates PV curves using continuation power flow (CPF) [27], which employs predictor-corrector steps to ensure convergence of the nonlinear algebraic equations describing the power system and to avoid Jacobian singularity near the maximum loading point Stability studies around the PV curve equilibria are performed by analyzing the eigenvalues from the linearization of the differential-algebraic equations that describe the system dynamics [28].
Voltage sag (also called voltage dip) is defined by IEEE 1159 as a drop in the rms voltage level to between 10% and 90% of its nominal value at the power frequency, typically shown in Figure 2-5, and lasting from half a cycle to one minute It is classified as a short-duration voltage variation and is one of the general categories of power quality problems.
Voltage sag durations are categorized as instantaneous (half cycle to 30 cycles), momentary (30 cycles to 3 seconds), and temporary (3 seconds to 1 minute), as shown in Table 2-3 These durations align with typical protective device operating times and with divisions recommended by international technical organizations Voltage sags can cause system shutdowns or reduce the efficiency and lifespan of electrical equipment such as motors, heating elements, lighting, control devices, speed drives, and other power electronics In industrial settings, where device malfunctions can lead to substantial financial losses, managing voltage sags helps prevent downtime and equipment damage.
Instantaneous 0.1 - 0.9 pu 0.5 to 30 cycles Momentary 0.1 - 0.9 pu 30 cycles to 3 sec Temporary 0.1 - 0.9 pu 3 sec to 1 min
Voltage sags are typically caused by weather conditions and utility equipment problems, which commonly lead to faults in the transmission and distribution system These faults disrupt power delivery and cause voltage levels to dip across feeders and substations For example, a fault on a parallel feeder circuit can result in a voltage drop at the substation, affecting downstream customers.
On a 13-bus power system, a fault at one bus can affect all connected feeders until the fault is cleared by protective devices Most faults on utility transmission and distribution networks are single-line-to-ground (SLG) faults, but the same concept applies to faults anywhere on the transmission system, where the fault path interrupts power flow until clearance.
Starting large motors or switching heavy loads can cause voltage dips in power systems An induction motor can draw six to ten times its full-load current during starting, creating a transient that contributes to the dip When the current magnitude at a point in the power system is larger than the available fault current at that point, the resulting voltage dip can become significant [32].
In addition, voltage sags can affect large areas, particularly if the fault occurs upstream; such events usually start on the transmission or distribution system - faults and switching
Voltage swell, as defined by the IEEE 1159 standard, is an increase in the RMS voltage level to 110%–180% of the nominal value at the power frequency, lasting from a half cycle up to one minute It is the opposite of voltage sag and represents a short-duration voltage variation phenomenon, one of the main categories of power quality problems, as shown in Figure 2-6.
Swells can be subdivided into three categories: instantaneous, momentary and temporary as shown in Table 2-4
Instantaneous 1.1 to 1.8 pu 0.5 to 30 cycles Momentary 1.1 to 1.4 pu 30 cycles to 3 sec Temporary 1.1 to 1.2 pu 3 sec to 1 min
Voltage swells are usually linked to system fault conditions, similar to voltage sags, but they are much less common This is especially true for ungrounded or floating delta systems, where a sudden change in the ground reference causes a voltage rise on the ungrounded phases An SLG fault on the system can produce a temporary voltage swell on the unfaulted phases, with the elevated voltage persisting for the duration of the fault.
Deenergization of a very large load can cause a voltage swells The abrupt interruption of current can generate a large voltage, per the formula: 𝑉 = 𝐿 𝑑𝑖/𝑑𝑡, where
L denotes the line inductance, and di/dt represents the rate of change of current Voltage swell can also arise from energizing a large capacitor bank, although it more often produces an oscillatory transient [32].
Although the effects of sag are more noticeable, the effects of a voltage swell are often more destructive Although the effect may be a gradual and accumulative effect, it may cause breakdown of components on the power supplies of the equipment It can cause control problems and equipment failure, due to overheating that could eventually result in shut down In addition, electronics and other sensitive equipment damage are more likely to happen in voltage swells
The increasing number of wind turbines connected to the grid nowadays shifted the focus of the research towards grid stabilization When integrating large wind farms to the power system grid it raises stability and control issues The potential problems and power quality issues are studied in many literatures to identify and to develop measures to mitigate them
Low Voltage Ride Through (LVRT) is the capability of wind turbines to stay connected and operate reliably during short periods of low grid voltage caused by disturbances The ability of wind farms to support the grid during faults and to maintain voltage and power stability is established in national guidelines in several countries, including Germany [33], the United Kingdom [34], and Denmark [35].