2. Index
Abstract
Power Quality Standards
Power Quality
Causes and Consequences of Power Quality Issues
Power Imbalance
Power outages
Voltage Variation
Harmonics
Flickers
Challenges of Wind Power on Power System Stability
Mitigation Strategies for WECS Integration
References
3. Abstract
Day by day increasing demand of electricity forces us to invent
alternative sources for electricity generation leads to development of
renewable energy sources. Among various alternative sources wind is
gaining more attention due to its technical advancement and cost
effectiveness. Integration of wind farm in grid causes adverse effect
on grid leads to poor power quality and threat to power system security.
Power quality is one of the key factors in today scenario because
modern power electronics devices are very sensitive and poor power
quality result in high cost & loss of consumer. This presentation
focuses on various issues which are responsible for poor power quality
of grid.
4. Power Quality Standards
International Electro Technical Commission Guidelines :
The guidelines are provided for measurement of power quality of wind
turbine.
The International standards are developed by the working group of
Technical Committee-88 of the International Electro-technical
Commission (IEC), IEC standard 61400-21, describes the procedure
for determining the power quality.
5. The standard norms are specified:
1. IEC 61400-21: Wind turbine generating system, part21. Measurement
and Assessment of power quality characteristic of grid connected
wind turbine
2. IEC 61400-13: Wind Turbine - measuring procedure in determining
the power behaviour.
3. IEC 61400-3-7: Assessment of emission limit for fluctuating load
4. IEC 61400-12: Wind Turbine Performance.
The data sheet with electrical characteristic of wind turbine provides
the base for the utility assessment regarding a grid connection
6. Power Quality
Power quality problem is any power problem manifested in voltage,
current, or frequency deviation that results in failure or
malfunctioning of customer equipment.
Power quality and reliability cost the industry large amounts due to
mainly sags and short-term interruptions.
7. Causes and Consequences of
Power Quality Issues
In most of industry, more than 90% of the electric motor with inverter
driven application.
Poor power quality causes trouble in receptacle/transmission
equipment and electronic equipment malfunctions / Failure. Power
quality is a common problem for both electric power suppliers and
users.
Causes :
1. Power Imbalance
2. Voltage Variation
3. Harmonics
4. Flickers
9. Power Imbalance
Wind Energy Conversion Systems (WECS) generate electricity when
wind speeds exceed a certain minimum.
Wind speeds fluctuates over daily periods. Consequently, variations
in the output of WECS may increase the operating costs of the system.
The variability and the unpredictability of wind power can cause a
power imbalance on the grid. Their output power may not be available
to meet the demand when needed, while there could be an excess when
the demand is low, thereby causing an upset on the grid.
Extreme ramp rates recorded during storms, Reports of loss of
4000MW (58% of capacity) in Germany in December 2004 and a loss
of 2000MW (83% of capacity) within 6 six hours in Denmark in
January 2005 as a result of large changes in wind power output due to a
forecasting error.
A penalty cost is often attached to the deviation in the scheduled and
actual energy delivered to the grid to cover the loss.
10. Power outages
Power outages are total interruptions of electrical supply. Utilities
have installed protection equipment that briefly interrupts power to
allow time for a disturbance to dissipate.
Causes:
o Ice storms
o Lightning
o Wind
o Utility equipment failure
Effects: Complete disruption of operation.
11. Voltage Variation
Voltage fluctuations are changes or swings in the steady-state voltage
above or below the designated input range for a piece of equipment.
Results from the wind velocity and generator torque. The voltage
variation is directly related to real and reactive power variations.
The voltage variation is classified as under:
o Voltage Sags
o Voltage Swells
o Short Interruptions
o Long duration voltage variation
Variation, flicker, harmonics of voltage causes the malfunction of
equipments such as microprocessor based control system, adjustable
speed drives, etc.
12. Voltage Variation
Causes:
o Large equipment start-up or shut down
o Sudden change in load
Effects:
o Data errors
o Memory loss
o Equipment shutdown
o Flickering lights
o Motors stalling/stopping
13. Harmonics
It can be injected at the generation and the consumer end.
Causes:
- At consumer end
o Non linear loads such as television, personal computers, compact
fluorescent lamps, and so forth.
- At generation level
o Flexible Alternating Current Transmission System (FACTS) such as
reactive power compensators and power electronics devices.
Effects:
o Increase line losses
o Excessive heating of equipment , decreases lifetime
o Sub-harmonics could cause flickers
14. Flickers
It is periodic voltage frequency variations typically between 0.5 and
25Hz that cause annoyance from the incandescent bulb.
Flicker annoyance is severe at a frequency of 8.8Hz.
Measurement of flicker
o Probability short term (Pst) measure for a time span of 10 min
o Probability long term (Plt) measured for an average of 2 hours
For flicker free voltage, Pst = 0 & Pst=1 indicates that the flicker
pollution has reached the tolerable limit of an average person.
Flicker level for a medium-voltage grid is specified by 0.35(Pst) and
0.25(Plt).
Cause:
o Tower shadow
o Turbulence effect
• Effects:
o Uncomfortable visual effect on the eyes
o Imbalance & core saturation of transformers
o Thermal aging of induction motors
15. Challenges of Wind Power on Power
System Stability
Before the advent of wind power plants, power systems mainly
consisted of synchronous generators for electricity production. The
behaviour and control of these generators following a disturbance are
well understood by the utility operators due to their experiences
thereof over the years.
The advent of wind power introduces induction generators into
the power system for electricity generation because they are cheap,
robust and support variable speed operations. At the earlier stage
of wind power integration, there was little concern about its influence
on the overall stability of a power. With the increasing trend of wind
power integration, it may begin to have a significant influence on
the power system transient stability margin.
16. Induction generators :
o Operate asynchronously
o Characterized by poor reactive power control capability.
Surge in the input torque of a generator and a voltage dip beyond
the threshold limit at the point of common connection (PCC) can lead
into poor feeder regulation which can eventually cause voltage
collapse as a result of reactive power demand from the grid.
Fixed speed induction generators - provided with reactive power
compensator to cater for the large reactive power demand from the
network.
Variable speed generators - power electronics devices provide the
reactive power to the grid.
17. Mitigation Strategies for WECS
Integration
1. Geographical
Location
Use of wide geographical
location to smoothening
out the oscillations in the
output of the intermittent
energy sources.
18. 2. The Use of Smart and
Super Grid
Super grid allows the
transmission of wind
power from a site of
favourable generation to
places of high demand over
a long distance; this is
mostly practiced with the
high voltage direct current
lines (HVDC). It allows the
possibility of smoothing
out the intermittent output
power by geographical
location.
