Wind grid issues -power quality

1. EE8703
RENEWABLE
ENERGY
SYSTEMS
VII SEM -EEE
Unit 2- WIND ENERGY
D3– WPP- Siting, WECS
& Grid issues

2. UNIT II WIND ENERGY
SYLLABUS
 Power in the Wind
 Types of Wind Power Plants (WPPs)
 Components of WPPs
 Working of WPPs
 Siting of WPPs
 Grid integration issues of WPPs

4.  By nature, wind is not a steady source of energy, therefore, it cannot on its
own meet the needs of consumers at all times
 Necessarily, it has to be integrated with some other sources to provide a
constant backup
 Wind Electric Generators (WEGs) operate in one of the following three modes.
 Standalone mode
 Backup mode like wind–diesel
 Grid-connected mode
MODES OF WIND POWER GENERATION

5. STANDALONE MODE
 represents decentralized application of wind energy
 is characterized by the situation where an individual energy consumer or a group
of consumers install their own wind turbine
 A WEG with a capacity of 2.5 kW to 5 kW is useful for domestic power supply
 The two most promising applications of the wind energy conversion system are:
 Power supply for domestic use and battery charging
 Windmill water pump for irrigation and drinking purposes
 It operates independently with a battery and its charging equipment
 Such installations are useful for remote mountainous regions where the extension
of grid or supply of oil is a remote possibility.
 As the wind changes speed, the pitch of the blades is adjusted to control the
frequency of turbine rotation

6.  Wind energy, being intermittent, requires a backup of diesel generator to maintain
a 24-hour power supply.
 In areas inaccessible to grid power, the emergency loads of hospitals, defense
installations and communication services are met with a wind–diesel hybrid
system, while the general loads of domestic and commercial establishments are fed
by WTG
 As the wind speed drops, low tariff loads are automatically switched off to reduce
the demand.
 During the period of no wind, priority loads are fed by the diesel generator.
Backup Mode Like Wind–Diesel Power Supply

7.  A common arrangement for connecting medium capacity WTGs (250 kW) to ‘state
grid’ is shown in Figure 7.28.
 WTGs generate electric power at 400 V; it is then stepped up to make this voltage
compatible to the grid (11 kV).
 In India, grid-connected WEGs constitute wind farms where the generated power
is distributed among the nearby consumers and the excess power is exported to
the grid.
 Electrical energy is purchased (imported) from the grid during periods of no wind.
Grid Connected Wind Turbine Generators

8. WIND ENERGY CONVERSION SYSTEMS
 General block diagram WECS is as shown in Figure
 Turbine shaft speed is stepped up with the help of gear system to suit
generator speed
 Fixed gear ratio is preferred than variable gear ratio

9.  Generator 3 types
 large WTGs, - induction generators
 Medium capacity WTGs - synchronous generators
 Small capacity WTGs - permanent magnet dc generators
 Interface
 power electronic converters, transformers (high frequency), filters
 Control unit
 monitors and controls the interaction among various blocks

11. Microprocessor-based control system
 used with the grid-connected wind farms
 It is equipped with remote control and automatic call facility
 The controller can communicate with the wind farm through a PC and a
modem on a telephone line
 The microcomputer receives
 the input of wind speed and direction
 load /grid voltage and frequency
 It sends signals to the turbine to establish proper yaw (direction control),
blade pitch (pitch control) and to activate the brakes in high winds
 The microcomputer may turn on optimal loads in strong winds and can also
adjust the power conditioner to change the load voltage and frequency

13. GRID INTEGRATION
Wind Electric Generators are designed to operate satisfactorily
within the following grid parameters
 Voltage — 400/440 V ± 13%
 Frequency — 50 Hz, –3 Hz, +1 Hz
 Asymmetry current ±12.5%

14. Grid System and Properties

15.  Important parameter of a grid is the ‘grid short-circuit impedance
angle ratio’ i.e., X/R ratio.
 The voltage deviation at the PCC of a wind farm varies as a function
of the X/R ratio.
 Percentage deviation in voltage is low for
(i) Low X/R ratio and high short-circuit capacity (stiff grid)
(ii) High X/R ratio and low short-circuit capacity (weak grid)

16. GRID interface issues:
 Reactive power supply
 Voltage regulation
 Frequency control.

17. With conventional energy system, generators besides supplying active power, also
supply reactive power required by consumers to operate their electrical equipment.
But in case of WEGs (induction type), they require reactive power to start power
generation. To have availability of reactive power, each WEG is provided shunt
capacitors. These capacitors meet reactive power requirement of WEG and
maintain power factor at the rated value of 0.95
REACTIVE POWER SUPPLY

18. Voltage Regulation
 Voltage variation at the common coupling point should be within 15% when the
wind farm is connected or disconnected.
 Difficulty in controlling voltage regulation is accentuated when the wind farm is
located in a remote area and connected to the grid through the existing
transmission lines designed to serve only the load in the area
Solutions to voltage regulation are:
 alternative line arrangement
 addition of static or adaptive VAR controllers

19. Frequency Control
Utilities operating wind power plants connected to a weak, isolated grid, can
have difficulty in maintaining the normal system frequency of 50 Hz.
The system frequency shows fluctuations when gusting winds cause the power
output of wind plants to change rapidly.
Low frequency operation affects the output of WEGs in two ways:
 Several WEGs do not get cut in when the frequency is less than 48 Hz, thus
resulting in loss of output.
 The output of WEGs at low frequency operation is reduced due to low speed
of the rotor.