New Ways of Interfacing Wind Turbines into Isolated Grids using
Variable Speed Turbines
Juergen Zimmermann
Powercorp Pty Ltd
3406 Export Drive
Trade Development Zone
Northern Territory 0822
AUSTRALIA
Telephone:
+61 (0) 8 8947 0933
Facsimile:
+61 (0) 8 8947 0925
E-mail:
powercor@ozemail.com.au
Abstract
Effective integration of large Wind Turbine Generators into isolated diesel powered grids
has still a long way to go. The key issue is how can a high wind penetration be achieved
without compromising power supply reliability and quality of the diesel grid. This article
describes the integration of large WTG into Wind/Diesel grids using the concept of variable
speed Wind Turbines in combination with a power station automatic control system and a
battery energy buffer.
1
INTRODUCTION
Wind/Diesel Systems have been installed in various remote sites throughout the world for the last 10 years.
Many of these installations in the 100 kW to MW range are pilot projects only and have not progressed to the
stage of commercial serial production. The best known site of a Wind/Diesel System in Australia is the
Esperance Wind Farm where nine 225 kW pitch regulated Wind Turbine Generators (WTG) supply power to
the local diesel grid. Other sites in Australia for Wind/Diesel Systems are on Thursday Island (Queensland),
King Island (Tasmania) and Denham (Western Australia).
The inter-connection of WTG to large grids in Europe and United States has proven to be technically simple
and economical, whereas the connection to isolated “small” diesel grids has led to a number of problems.
•
•
•
Ineffective use of WTG caused by incorrect system design and control strategy (dump loads,
unnecessary shut-down of WTG, etc.)
Increased stress on, and decreased performance of, interconnected diesel generator sets
(under/overloading, reverse power, high number of start/stop cycles)
Decreased power supply quality in terms of frequency/voltage stability (harmonics generated by
thyristor inverters that are not compensated, flicker and frequency instability due to variation in wind
speeds)
The new type of variable speed WTG and the latest power electronics technologies in IGBT for
battery/converter systems make it possible to achieve a high wind penetration and economical usage of WTG
in isolated diesel grids.
This article reflects on experiences of POWERCORP in the past with wind/diesel systems and shows how
the new generation of variable speed WTG makes it possible to fully integrate these turbines into diesel
powered stations.
This article has also been submitted to the Wind Energy Workshop in Melbourne 1997. Minor changes have
been done to this revision.
1
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
2
J. Zimmermann
ISOLATED GRIDS-IDENTIFYING THE PROBLEM
Most wind turbines or wind farms are connected to the main grid. Usually the contribution of wind turbines
compared to the generation capacity of the total grid is minor. However, experience with large wind parks
has shown that the grid interference of the wind park at the connection point (substation) due to uncontrolled
power output fluctuations can be substantial and therefore requires upgrading of the substation or limitation
to the number of wind turbines that can be installed.
In Wind/Diesel Systems the problem of connecting wind parks with high penetration is even bigger, because
the impact of power output fluctuations of the wind park has a much higher impact on other generating sets
due to the smaller grid. A number of additional factors have to be considered in isolated grids, as follows:
• Ratio of installed Wind Capacity to installed Diesel Capacity
How many turbines can be installed on the grid without compromising their utilisation during low load
periods?
• Grid Stability
How can WTG be integrated without compromising the stability and quality of the grid? Can the WTG limit
its power output in cases of wind gusts to prevent voltage/frequency fluctuations?
• Spinning Reserve
Who provides the spinning reserve on the grid to compensate for wind speed/WTG power output
fluctuations?
• Reactive Power
Who is going to provide the additional consumer reactive power demand on the grid when generators are
shut down because of high wind penetration?
• Impact on Diesel Generator Lifetime
What is going to be the long term effect on the life of the diesel generators due to mechanical stress caused
by periodic short term wind power fluctuations (eg tower wind shadow effect)
Figure 1 Typical Layout of Wind/Diesel/Battery System
2
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
3
J. Zimmermann
EXPERIENCE WITH WIND/DIESEL SYSTEMS
POWERCORP has had experience with Wind/Diesel systems for more than 8 years. From the first
installations to those of today, a number of modifications and innovations have been implemented to achieve
higher penetration of renewable energy and better quality of power supply. The POWERCORP Wind/Diesel
System called IPS (Intelligent Power System) was developed in cooperation with SMA/Germany. In all
installations, pitch regulated WTG with conventional drive train systems were used.
POWERCORP never considered the installation of stall regulated WTG in Wind/Diesel Systems because of
their inability to control the power output of the WTG.
Stall regulated machines can also not achieve a very high wind energy yield in part load operation because
they can not change the angle of the rotor blades.
The following description of three Wind/Diesel System designs reflects on the way the wind turbines were
controlled and highlights the advantages and disadvantages of each configuration.
3.1
System I: Wind/Diesel System Controlled by Frequency (droop mode)
The first generation of Wind/Diesel Systems consist of two diesel generators with one pitch regulated wind
turbine (30 kW). In order to be able to control the power output of the wind turbine (specifically at low load high wind periods) a power output control based on the grid frequency has been implemented. If the load on
the grid increases the speed of the diesel generators drops due to their droop characteristic and therefore the
grid frequency decreases (see Figure 2 Droop Curve of Wind/Diesel System)
Pdiesel
P_WTG
Wind Turbine Droop Curve
0.5
Pdiesel
Diesel Generator Set Droop Curve
49 Hz
50 Hz
51 Hz
Figure 2 Droop Curve of Wind/Diesel System
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155
3
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
J. Zimmermann
The droop characteristic shows that the power output of the wind turbine is not decreased until the diesel
generator power output is less than its minimum loading (eg 50%) threshold. In this case the WTG controller
would sense any rise in frequency and start to decrease the power output of the turbine by pitching the rotor
blades, hence increasing the load on the diesel generator. The grid frequency is used as a measure for the
loadsharing between the WTG and the diesel generator.
Advantages:
• WTG can be installed without communication link at any point of the grid.
• Minimum load on the diesel generators is ensured.
Disadvantages:
• Wind speed fluctuations affect the grid frequency (f grid = k * v3).
• The need to provide spinning reserve to balance wind speed fluctuations.
• Control dynamic problems occur during low load periods when the WTG power output must be
decreased to maintain the minimum loading on the diesel generator set.
3.2
System II: Wind/Diesel/Battery System Controlled by Frequency
(droop mode)
The next Wind/Diesel System introduces an energy buffer in the form of a battery/converter system. The
reason for introduction of this energy buffer is to balance the wind power output and achieve higher
penetration in the long term.
Figure 1 shows the three generation sources; Wind, Diesel and Battery storage contributing to supply of the
consumer demand.
The Battery/Converter system in Figure 1 provides additional backup power in case of wind speed
fluctuations. On the other hand, oversupply by the WTG is used to recharge the battery during high wind
speeds periods.
Advantage:
• Higher penetration of wind energy.
• Increased reduction in fuel and maintenance cost of diesel sets.
• Increased stability of the grid.
• No communication link to WTG required.
Disadvantage:
• Higher investment cost for battery/converter system.
• Wind speed fluctuations are still injected into the grid and cause mechanical stress on the generator sets
(Pwind = k x v 3).
• Problem of “Tower Shadow Effect” - resonant frequency with governor system (reduction in power
output when blades go through tower).
• Additional battery maintenance cost.
• Dynamic control problems during low load periods.
3.3
System III: Wind/Diesel/Battery System with WTG Setpoint Control
(isochronous mode)
The next system generation of IPS Wind/Diesel Systems is based around isochronous operation (eg constant
grid frequency of 50 Hz). The diesel generators are equipped with load share and speed control. The engine
speed controllers together with the engine governor control the speed of the engine to a setpoint of 50 Hz.
The load share units ensure that the load between the diesel generators is shared proportional to the size of
the generator sets.
4
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
J. Zimmermann
To be able to control the power output of the WTG the power station controller is equipped with the
capability to give setpoints to the Wind Farm.
Advantage:
• Increased power quality (50 Hz isochronous).
Disadvantages:
• Higher cost to install communication link to WTG.
• Fluctuations due to wind gusts are still noticeable in frequency and voltage (specifically when only a
small generator is on line at night and high wind energy output).
• Depending on the ratio of wind capacity to diesel capacity, wind fluctuations still cause some
mechanical stress on the diesel engines.
3.4
System IV Wind/Diesel/(Battery) System with Variable Speed WTG
With the appearance of variable speed WTG on the market the level of possible wind penetration into diesel
grids has been increased. Variable speed WTG offer many features that suit ideally the integration of such
WTG into isolated diesel grids. As a result the combination of ;
•
•
•
Variable Speed Wind Turbine Generators (see section 4 for further explanations),
Central automatic power station control ( see section 5 for further information) and
Battery/Converter system as short-term energy buffer
is today’s best technical solution to achieve high penetration of wind energy in diesel grids.
The variable speed WTG offers precise power output control by buffering wind speed fluctuations in the
rotor speed. As a result the rotor speed adapts to the change in wind speed and not to the grid frequency (as
required for conventional drive train type systems). Variable speed WTGs are even able to support the grid
by changing the power factor of the WTG hence stabilising the grid voltage. In summary this leads to the
following advantages:
Advantages:
• Higher wind penetration due to more stable power output.
• Better grid quality due to support of grid voltage by WTG power management system.
• Less mechanical stress of the WTG and diesel generators due to constant precise power output of the
WTG.
• Lower fuel and maintenance cost of diesel generators, because diesel sets can be switched of
(Battery/Converter System provides spinning reserve)
Disadvantages:
• Communication link to WTG is required.
• Additional maintenance cost of batteries.
3.5
Summary
Many years of experience in the area of Wind/Diesel Systems made it clear that the key to high penetration
of WTG into diesel grids is
•
•
•
The capability to precisely control the power output of the WTG ( eg variable speed WTG).
Central automatic control of the power station and WTG.
Introduction of an energy buffer (battery/converter system)
If these requirements are implemented in a wind diesel system a high quality of electricity supply to the
consumers (frequency and voltage fluctuations are within the utility standard) and high wind penetration can
be achieved without compromising one another.
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155
5
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
J. Zimmermann
The following two sections explain in more detail which technical solutions are available to achieve the
above requirements.
4
VARIABLE SPEED WTG (eg ENERCON E40)
The ENERCON E40 wind energy converter has a three-bladed rotor, active pitch control and variable speed
operation, with a rated power output of 500/600 kW. With its rotor diameter of 40 m and tower heights of 42
to 63 m, it efficiently makes use of the prevailing wind conditions at its respective sites of operation for the
generation of electrical energy.
Power control with variable speed allows for operation of the E-40 at optimum efficiency whilst minimising
operating loads, even in the partial load range. Moreover it prevents the occurrence of undesirable power
spikes. A good energy yield and a high quality of the power that is fed to the grid are therefore guaranteed.
The E40 variable speed WTG offers the following features in the application of Wind/Diesel Systems
•
•
•
•
•
•
•
5
Decoupling of rotor speed and grid frequency through AC/DC/AC system
Precise power output control (1%)
Control of dP/dt (ramping up and down the power output)
Precise reactive power control (very important for high penetration to supply reactive power to
consumers)
Minimal harmonics content (THD <1%)
Optional mode of power control through voltage sensing
Full communication to control and remote access the WTG from the Station Management System (SMS)
POWERCORP IPS STATION MANAGEMENT SYSTEM
The POWERCORP Intelligent Power System (IPS) control systems have been designed to control and
automate the following system configurations
•
•
•
5.1
Multiple Diesel System
Wind/Diesel System
Wind/Diesel/Battery System
Interface WTG <-> Station Management System
Figure 3 on the following page shows how the WTGs are connected to the Station Management System
(SMS). A common communication bus is used to transmit setpoints to the WTG and to monitor the actual
status of the WTG from the SMS.
This setup allows the following control features:
• Setpoint power output control of the WTG (active power control, reactive power output control)
• Monitoring of WTG status (contactor, active power output, reactive power output etc.)
• Start/Stop Commands to individual WTG
• Alarm information
If sufficient wind speed is available the variable speed WTG essentially can be controlled like a diesel
generator set by the SMS.
6
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
5.2
J. Zimmermann
Control Philosophy of Station Management System
The Station Management System (SMS) controls the WTG, diesel generator sets and optional the
battery/converter system. It makes decisions based on the following parameters:
•
•
•
•
•
•
•
Number, size and availability of diesel generators
Consumer load
Setpoint for spinning reserve
Actual power output of wind turbines
Generator priorities set by operator
Additional capacity call up of external loads (large pumping station)
Wind speed
Power Station and Wind Turbine Control System
Schematic Diagram
Maintenance Management System
Distribution Maintenance and Control
System
Power Management
- Power output control
- Reactive power control
- Feeder control
- Reserve power management
- Power station start/stop
Multiple Power
Station Control
System
Monitor
Visualisation
System
Monitor
Fault and Data
Analysis
System
Process
Visualisation
Printer
Monitor
Data trending & analysis
Fault and event mgmt
Wind Turbine Control System
Wind Turbine Control System
Power Station Control System 1
Grid
Management
System
Power Station Control Computer
Generator Control Units
Monitor
RTU
Turbine
Control
Unit
Feeder Circuit Breaker Control and
Monitoring Units
Grid
Management
System
RTU
Turbine
Control
Unit
Generators
Precise power output control C/B control and monitoring
- kW, voltage, power factor
- Auto operation
- Trip information
H.V. Feeders
Turbine Control
- Automatic start/stop
- Power output control by active
pitch, rotor speed and yaw
control
Turbine Protection, safety
and Monitoring
- Mech and elect sensors
- Lightening protection sys.
- Independent blade pitch
braking
Powercorp Pty Ltd
Figure 3 Typical Control Wind/Diesel Control System
The main control functions of the SMS Station Management System are
• Maintain minimum loading of generator sets
• Maximise wind penetration at day and night time
• Minimise fuel consumption of diesel generators
• Minimise operating hours of diesel generators (turn generators off)
• Battery charging and discharging control
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155
7
New Ways of Interfacing Wind Turbines into Isolated Grids Using Variable Speed Turbines
6
J. Zimmermann
CONCLUSION
The focus of the design of Wind/Diesel Systems is on the selection of the correct WTG and its capability to
interface with the soft diesel grid. Clearly variable speed WTG like the ENERCON E40 offer the best
features for this application.
In order to achieve high penetration of wind and generate the savings in fuel and maintenance of diesel
generators, a central automatic control system (Station Management System) must be installed.
Depending on consistency of wind speed and the level of wind penetration the installation of a
battery/converter system as an energy buffer to provide spinning reserve should be considered. Diesel
generator maintenance savings can only be achieved if diesel sets are shut down and this is an important
factor in economic justification of the WTG as savings in this area may be equivalent to savings from
reduced fuel consumption.
8
Proceedings of Solar’97 - Australian and New Zealand Solar Energy Society
Paper 155