Renewable Energy Systems (RES) provide an alternative to fossil-based electricity generation. Among the RES options available, wind energy systems are being implemented widely due to the maturity of the technology and the ability to provide bulk power (in a wind farm configuration). The integration of a wind farm to an electricity network is however considered to be significantly different to that of a fossil-fuel based power station. In particular, wind energy systems are coupled to the sub-transmission network by means of a power electronic converter, and typically operate in transient mode (time of rapid change) permanently because they're continuously trying to maximize the energy capture from incident winds. The sub-transmission networks are not considered as ideal infinite buses (like transmission networks), because they are susceptible (subject) to power quality (PQ) problems.
This project will involve the following: (this report investigates :) …… literature revue
Investigate the characteristics of the sub-transmission network to which wind energy systems are typically connected.
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Investigate the PQ problems prevalent on these networks.
Investigate the PQ problems on these networks when integrating wind energy systems. How do these vary as the percentage of wind penetration increases?
Identify the risks to the security and integrity of the network as the percentage of wind penetration increases. How can these risks be mitigated (moderated)?
What is the South African grid code relating to the integration of wind energy on these networks?
Use DigSilent or PowerWorld's Power Factory or Simulink's SimPower Systems Blockset to investigate the Fault Ride Through (FRT) and Low Voltage Ride Through (LVT) capability of Wind Energy Systems.
Sub-transmission is part of an electric power transmission system that runs at relatively lower voltages. It is uneconomical to connect all distribution substations to the high main transmission voltage, because the equipment is larger and more expensive. Typically, only larger substations connect with this high voltage. It is stepped down and sent to smaller substations in towns and neighborhoods. Sub-transmission circuits are usually arranged in loops so that a single line failure does not cut off service to a large number of customers for more than a short time. Most sub-transmission circuits are overhead. Many are built right along roads and streets just like distribution lines. Some, especially higher voltage sub-transmission circuits use a private right-of-way such as bulk transmission lines use. Some new sub-transmission lines are put underground in urban areas 
There is no fixed cutoff between sub-transmission and transmission, or sub-transmission and distribution. The voltage ranges overlap slightly. Voltages of 69 kV, 115 kV and 138 kV are often used for sub-transmission. As power systems evolved, voltages formerly used for transmission were used for sub-transmission, and sub-transmission voltages became distribution voltages. Like transmission, sub-transmission moves relatively large amounts of power, and like distribution, sub-transmission covers an area instead of just point to point
Characteristics of the sub-transmission network to which wind energy systems are typically connected.
Ideally, wind energy systems are connected to firm grids in order not to influence stability or power quality in a negative way. This is usually not the case, however. Wind power is usually connected far out in the grid, at sub-transmission or distribution levels, where the grid was not originally designed to transfer power from, the system ends back into the grid. Particularly when the grid is weak, unacceptable voltage gradients may occur . Wind farms are typically located in areas where wind resources are plentiful and can satisfy certain requirements. Most onshore wind farms are located in rural areas where the transmission system voltages are typically in the range of 69 kV to 161 kV. The nominal terminal voltages at the wind turbines range in value from 575 V to 4,160 V, depending on the turbine ratings .
Wind energy system (refer to bridge)
Early versions of wind turbine generators consisted of fixed-speed wind turbines with conventional induction generators. This class of machines was uneven but was limited to operation in a narrow wind-speed range. In addition, the conventional induction generator, which was directly connected to the electrical grid, required that reactive power support be provided locally to achieve the desired voltage level.
AC Induction generator
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The technology of induction generator is based on the relatively mature electric motor technology. Early developments in induction generators were made using fixed capacitors for excitation, since suitable active power devices were not available. This resulted in unstable power output since the excitation could not be adjusted as the load or speed deviated from the nominal values. This approach became possible only where a large power system with infinite bus was available, such as in a utility power system. In this case the excitation was provided from the infinite bus. With the availability of high power switching devices, induction generator can be provided with adjustable excitation and operate in isolation in a stable manner with appropriate controls
Induction generator also has two electromagnetic components: the rotating magnetic field constructed using high conductivity, high strength bars located in a slotted iron core to form a squirrel cage; and the stationary armature winding .
Doubly fed induction generator (DFIG)
Advances in power electronics have revolutionized wind turbine technology and led to the development of the doubly fed induction generator (DFIG) (Figure 2). The stator of the DFIG is directly connected to the grid, and the rotor winding is connected via slip rings to a converter, which only has to handle a fraction (20 to 30 percent) of the total power. The highly efficient, variable speed DFIG is designed to extract maximum energy from the wind, and it puts out electricity at a constant frequency no matter what the wind speed. 
The unit transformer at each wind turbine steps up the voltage and feeds power into a collector system that operates at voltages ranging from 12.5 kV to 34.5 kV. The high side node of the collector system is then connected to the main substation transformer for the wind farm, which again steps up the voltage to the desired level and connects the wind farm to the transmission system in the geographical vicinity .
Most modern wind farms have DFIGs and are available in ratings that range from 1.5 megawatts (MW) to 4.5 MW. Newer generations of wind generators, which have permanent magnet synchronous generators and fully rated converters, have a range of control over both real power and reactive power for varying wind speeds.
permanent magnet synchronous generator wind turbine
From all the generators that are used in wind turbines the PMSG's have the highest advantages because they are stable and secure during normal operation and they do not need an additional DC supply for the excitation circuit (winding) . This type of wind turbines is combined with synchronous permanent magnet generator and AC/DC/AC converter with a rating of 100% of the rated wind turbine power. Since it does not need the gear box, the weight at the hub height can be lowered a lot, and the operation and maintenance of the gear box are not needed .
Impact of wind energy system on the network
Low voltage ride through
Voltage dips falls under short duration voltage variations which comprises of three categories namely under voltage, voltage dips and swells.
Voltage dip is a power quality problem and it damages the integrity of the power quality to the end user . According to Bollen et al , Voltage dip is defined as a sudden reduction of the supply voltage to a value between the ranges of 10% to 90% of nominal voltage followed by a voltage recovery after a short duration usually from 10 ms up to one minute. While an interruption it is whereby the nominal voltage magnitude goes to zero volts .
Most countries have grid codes which requires wind generators to stay connected to the grid during a fault. This is of concern to transmission operators and regulatory agencies that ensure wind turbines remain connected during a network fault, this is because if wind farms are off-line due to faults, the combined effect of fault and sudden loss of generation might cause severe decrease in voltage and voltage instability . The figure below shows a LVRT curve.
Figure: LVRT for a voltage admissible at connection point  
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With reference to the figure above, wind generators are required to remain connected for 0.625s if the voltage drops down to 15% of the nominal value. Beyond this the wind turbine should be disconnected from the network. However if the voltage falls to 0.9 the wind energy system can remain connected 
Fault ride through
Grid codes define two main issues regarding fault ride-through: The properties of the voltage dip during which the generating system has to maintain operability and limits or requirements for the short circuit current during the grid fault .
Figure: Voltage and current definitions of grid codes 
Voltage dip duration and retaining voltage b) Short circuit current during voltage dip
The grid codes request minimum retaining voltages between 15% and 25% with maximum voltage dip durations between 500 ms and 3000 ms, depending on the retaining voltage. Short circuit current requirements address the maximum transient short circuit current and the current to be fed to the grid during the voltage dip. The transient short circuit current becomes more and more an issue for the erection of wind turbines at locations with low short circuit grid power 
This section deals with the voltage quality problems that wind generation in distribution networks can produce to the loads supplied from the same grid. A wind farm can be connected to low voltage, medium voltage and higher voltage networks. In the case of smaller installations connected to weak electric grids such as medium voltage distribution networks (22 kV), power quality problems may become a serious concern because of the proximity of the generators to the loads. One of the power quality problems is voltage dips. In developed countries, it is known that from 75% up to 95% of the industrial sector claims to the electric distribution companies are related to problems originated by this disturbance type 
The voltage variation issue results from the wind velocity and generator torque. The voltage variation is directly related to real and reactive power variations. The voltage variation is commonly classified as under :
Voltage Sag/Voltage Dips.
Long duration voltage variation.
Because many electrical devices are not designed to maintain their normal operation during a voltage dip, these disturbances are thus a big problem. The behavior of a wind turbine to a voltage dip is affected by the type of technology. In case of a fixed speed induction generator, a voltage dip initially decreases the active power supplied to the grid, while the reactive power consumed by the generator also decreases due to the demagnetization of the machine. When the voltage recovers, the main effect is the absorption of reactive power in order to recover the magnetic flux, extending the duration of the voltage dip . The following figure shows the typical voltage dip experienced at the load terminal and the delay in the voltage dip, caused by the induction generator.
Figure: Instantaneous voltage sag caused by a fault  
As seen in the figure above, the dip is extended for another 0.75s after the fault is cleared, causing the load voltage to return to its nominal value after 1.25 s