Introduction to Wind Power Plant technologies in the world

Renewable energy is the main concern of the global energy sector to face the energy crisis occurred with the depletion of fossil fuel. Wind energy is one of the prominent renewable sources of energy currently used globally. Following the global trend, Sri Lanka too has wind power plants connected to the national grid that consist of three main types of generators which are variable speed synchronous wind generators, doubly-fed induction wind generators and fixed speed asynchronous wind generators (FSAWD). This article talks about the main construction modes of each type of wind power plant available globally.

Fixed speed asynchronous wind generator

These types of generators are operated with less than 1% variation of rotor speed, which is also the reason to be called as fixed speed wind generators. They are equipped with a squirrel cage induction machine which is directly connected to the power grid (Figure 1). The speed of the rotor is determined by the frequency of the network not from the wind speed. It is done with the speed multiplier ratio and by the generator type. In order to increase the power production, most of the wind generators use two coils for low wind speed and high wind speed. There are 8 poles and 4-6 poles available respectively for each type of coils. These generators have used soft starters as for starting up of the generators since starting current is very high and it might also be a cause for voltage variation in weak power system network.

Mainly there are two types of fixed speed wind turbines available in the industry as pitch control and stall control. In pitch control type turbines, blades are not fixed to the hub so it can be rotated a few degrees to fully confront the wind in order to produce full power or be in line with wind direction to extract no power. In stall control type turbine blades are fixed to the hub rigidly and they are designed in a way that the airflow over the blades is a laminar air flow to turbulence flow at high speed.  But the drawback is, rigidly fixed blades limit the mechanical power extracted from the wind at high speed to protect the machines from overloading.

Wound rotor induction generator

Wound rotor induction generators use variable rotor resistance control in order to achieve output power control. These types of wind turbines can extract wind power in a optimum way, compared to squirrel cage induction wind generators so they are generally employed with variable speed wind turbines. They are stall controlled wind turbines (blades are rigidly fixed to the hub) to focus on the rotor resistance control.

Main objective of the rotor resistance controller is to obtain the operating point with maximum possible wind power extraction without exceeding machine limits. Wound rotor induction wind generator is illustrated in figure 2.

Variable speed doubly fed induction wind generator

Due to high energy efficiency and controllability, variable speed doubly fed induction wind generator has become more popular these days. This model is called doubly fed induction generator because the grid is powered by two feedings, shown in Figure 3, as one from the stator, which is connected to the grid directly while other is from the rotor connected to the grid via an AC/DC/AC converter. Converter of this turbine handles only 30% to 40% of the generator output.

Ability to change of rotor voltage allows control of operating conditions of the generator as in low speed drop in rotor speed direct the generator into a sub synchronous operating mode by absorbing power from the grid and also during high wind speed, the DFIG wind turbine operate at super synchronous speed delivering power originated from the rotor through the converters to the power system. Ultimately rotating speed of the DFIG rotor determines if the power is delivered to the power system via the stator only or via the stator and rotor.

Full power converter wind turbine generators

The ability of effectively decoupling the generator from the grid, improved fault response, operating at wide speed range has led to improve the popularity of full power converter wind turbine generators in the industry. There is a converter connected to the turbine as shown in Figure 4, to handle the entire output of the generator.

Introducing the new technology, full converter wind turbines are equipped with a permanent magnet alternator. This type of wind turbines with permanent magnet generators (PMGs) are excited by permanent magnets and it can also be excited by generator-side converters. These PMGs are normally connected to the grid via frequency converters. This makes a DC link from the power grid to the generator, as shown in Figure 4 and there is no any reactive power exchange between generator and power grid. As such the power factor of the wind power plant output is 1. The AC-DC converter is a diode-bridge rectifier and a buck-boost converter which controls the DC link voltage.

Reference:
O S D De Silva, H K C O Dayarathne, V I P Dasanayake, J G D S De Silva and A S Rodrigo; Wind Generator Dynamics: Modelling of Fixed Speed Asynchronous Wind Generator using PSS/E
ISSN: 2545-9557

Article By: Team SOID

Posted in: Sustainable Energy and Policy