UNIT-4
NEED FOR VARIABLE SPEED WECS
High Power conversion efficiency
Maximum power tracking for harvesting the highest possible energy from wind
Reactive power compensation
Lesser mechanical stress
Less variations in electrical power
Power – wind wind speed speed characteristics: characteristics:
A typical power curve is characterized b y three wind speeds: cut-in wind speed, rat-ed wind speed, and cut-out wind speed, as described in Figure 2-11, where P M is the mechanical power generated by the turbine and v w is the wind speed. The cut-in wind speed, as the name suggests, is the wind speed at which the turbine starts to operate and deliver power. The blade should be able to capture enough power to compensate for the turbine power losses.
The rated wind speed is the speed at which the system produces nominal power, power, which is also the rated output power of the generator. The cut-out wind speed is the highest wind speed at which the turbine is allowed to operate before it is shut down. For wind speeds above the cut-out speed, the turbine must be stopped, preventing damage from excessive wind. As the wind speed increases beyond the rated speed, aerodynamic power control of blades is required to keep the power at the rated value.This task is performed by three main techniques: passive stall, active stall, and pitchcontrol .The wind turbine should stop generating generating power and be shut down when the speed is higher than the cut-out wind speed
VARIABLE SPEED-CONSTANT FREQUENCY SYTEMS VARIABLE SPEED IS ACHIEVED USING SCIG, Synchronous generator,DFIG. generator,DFIG. VARIABLE SPEED USING SCIG:
In variable-speed squirrel cage induction generator (SC IG) wind energy conversion systems (WECS), full-capacity power converters are required r equired to adjust the speed of the generator in order to harvest the maximum possible power available f rom the wind. A typical configuration for the SCIG wind energy system is shown in Figure 7-1, where a back-to-back power converter configuration is normally employed. The generator-side converter (rectifier) is used to control the speed or torque of the generator with a maxi-mum power point tracking (MPPT) scheme. The The grid-side converter (inverter) is em-ployed for the control of DC link voltage and grid-side reactive power
Variable speed using DFIG(Doubly DFIG(Doubly fed Induction generator) generator)
The stator is connected to the grid directly, whereas the rotor is connected to the grid via reduced-capacity power converters [3]. A two-level IGBT voltage source converter (VSC) system in a back-to-back configuration is normally used. Since both stator a nd rotor can feed ener-gy to the grid, the generator is known as a doubly fed generator. The typical st ator voltage for the commercial DFIG is 690 V and power rating is from a few hundred kilo-watts to several megawatts. The rotor-side converter (RSC) controls the torque or active/reactive power of the generator while the grid-side converter (GSC) controls the DC-link voltage voltage and its AC-side reactive power. Since the system has the capability to control the reactive power, external reactive power compensation is not needed. The speed range of the DFIG wind energy system is around ±30%, which is 30% above and 30% below synchronous speed
Variable speed operation with Synchronous generator generator
The synchronous generator can be constructed with a large number of poles and op-erate at a speed that directly matches the turbine blade speed. Such a direct-drive sys-tem does not need a gearbox. This results a reduction in installation instal lation and maintenance costs and provides an advantage over induction generator (IG) based turbines where use of a gearbox is a must. The SG wind energy system is normally controlled by full-capacity power converters for variablespeed operation , ensuring maximum wind energy conversion efficiency throughout its operating range.
Schematic of variable speed operation with Synchronous Synchronous Generators(SG)
DOUBLY FED INDUCTION GENERATOR ( 16 marks)
A doubly fed induction generator is a, wound rotor induction machine with its stator windings is directly connected to grid and its rotor windings is connected to the grid through an AC/DC/AC converter. converter.
AC/DC converter connected to rotor
winding is called rotor side converter and another DC/AC is grid side converter. Doubly fed induction generator (DFIG), is used extensively for high-power wind applications (Figure. 2.1). DFIG’s ability to control rotor currents allows for reactive power control and variable speed operation, so it can operate at maximum efficiency ef ficiency over a wide range of wind speeds.
Figure 2.1 Doubly Fed Induction Generator
2.2 COMPONENTS OF DFIG
The DFIG-based WECS basically consists of generator, wind turbine with drive train system, RSC, GSC, DC-link capacitor, coupling transformer, and harmonic filters as shown in Figure 2.1 The stator of DFIG is connected to grid through transformer whereas the rotor connection to grid is done through GSC, RSC, harmonic filters and transformers. The rotor current is controlled by RSC to vary the electro-magnetic torque and machine excitation. Since the power converter operates in bi-directional power mode, the DFIG can be operated either in sub-synchronous or in super-synchronous operational modes.
2.3 OPERATION OF DFIG
Depending on the rotor speed there are two modes of operation in a DFIG D FIG (Figure 2.2)
Sub-synchronous mode in which generator operates below the synchronous speed
Super-synchronous mode in which generator operates above the synchronous speed
2.3.1 Sub-Synchronous Mode
In sub-synchronous operation (Figure 2.2b), the rotor receives power from the grid .both mechanical power |Pm| and rotor power |Pr | are delivered to the grid through the stator. The slip is positive in sub-synchronous mode. Although stator power is |Ps| the sum of |P m| and |Pr | it will not exceed its power rating since in the sub-
synchronous mode the mechanical power |P m| from generator shaft is lower than that in super-synchronous mode.
Figure 2.2 Operation modes of DFIG 2.3.2 Super-Synchronous Mode
In the super-synchronous operation mode (Figure 2.2b), the mechanical power |Pm| from the shaft is delivered to the grid through both stator and rotor circuit. The rotor power |Pr | is transferred to the grid by power converters, whereas the stator power |Ps| is delivered to grid directly. The slip is negative in supersynchronous mode.
Advantages of DFIG
Variable speed operation of WECS.
It has ability to control reactive power and decouple control of active and reactive power by independently controlling the rotor excitation current. So power factor control can be implemented in this system.
High energy conversion efficiency.
MODELLING OF DFIG This equivalent circuit is developed by adding the converter equivalent impedance to the SCIG steady-state model of Figure. The equivalent impedance of the converter is defined by,
where ws¡ is the angular slip frequency and Leq is the equivalent inductance of the RSC. Note that the frequency of the rotor current in the actual rotor winding flowing into the converter is (wsl not the stator frequency (ws)). In order to integrate the converter equivalent impedance into the steady-state model with the stator frequency ws, the impedance Zeq should be divided by slip s. The equivalent impedance referred to the stator side is then given by
Permanent magnet Synchronous generator: (CONSTRUCTION- you can Read what u studied special electrical machines)
MODELLING OF PMSG:
