Content
The market for wind-diesel systems is found in regions, where it would
be expensive to supply bigger power-plants with fuel and/or the public
grid is not available or not stable enough or the electrification is not
yet realized.
The use of diesel-generators together with a wind turbine is based on the need of storage elements for energy in autonomous systems. Bad expierience with battery/inverter-systems lead back to the use of the conventional diesel technology because this is more economical. In a well set up installation the diesel-engine is only used as a backup for the wind turbine. The first priority is: use as little diesel fuel as possible.
The main technical problems in our development work which needed a lot of practical expierience to be solved were situated in a high-reliability regulation of the wind turbine without being led by a stable electric grid and in the system management using the Diesel engines.
The construction of a larger electric network is possible with such a system by just adding new energy supply units.
The
Wuseltronik system is based on a number of autonomous energy supply units
with their own controls. Each module runs independently of other generators.
It can be used for stand-alone systems, weak electrical networks or a national
grid with the quality of the North-European network. A failure of one unit
does not influence the function of the others.
The basic concept of this control (in the simplest case only one diesel-generator or wind turbine in stand-alone mode) was designed to give the possibilty of adding new energy supply units if more power is used or a higher reliability is necessary. Step by step a stand-alone system can be supplemented to a wind-diesel system and combined with other small systems to a regional electrical network or be connected to the national grid.
One essential quality is to dispense with a central system management. In other systems a central computer controls each energy supply unit, collecting measurement data about the electric power and generating on/off commands for each unit. In the concept of Wuseltronik the autonomous control units get all information necessary for running the system throughout the frequency of the common grid.
Depending on varying conditions for the operation of an energy system there are different possibilities for managing an energy supply unit. The customer is able to program different preset-values for system management by using simple commands on a small keyboard. In this way a modification and optimization of settings within a wide range is easily done.
In this system there is only one energy supply unit (i.e. wind energy converter) running and feeding the electrical network. There is no connection to other supply units (i.e. diesel generator) or to the public mains.
The rotation speed of the wind turbine which is proportional to the frequency of the grid is kept constant. Measurement of the revolutions-per-minute is done by counting the frequency of the grid. If a preset-value for the frequency is exceeded (i.e. by high wind speed, gusts) an electrical dummy-load is added by phase control to regulate the frequency to a stable value which is programmed by the customer. The tolerance range is within some tenth of a Hertz, thus any consuming device can be used in this supply system.
If an additional load is switched onto the grid at the same time the same amount of electrical power in the dummy-load is reduced. It is not possible to add a load which exceeds the actual regulated power because otherwise the frequency of the system would decrease and the wind turbine would have to be shut-down. The maximum power available in the system depends on the actual wind energy input.
Therefore we give the user of a windpower stand-alone system the possibility to define a number of different load-circuits which are programmed on the keyboard. Each circuit has a fixed maximum power and a connection to the grid is only possible when the actual power available exceeds the sum of the required power.
If the frequency (revolutions per minute) falls short of a programmable value the generator is separated from the grid. A reconnection is possible if the wind turbine exceeds its nominal revolutions-per-minute again and is able to deliver a defined quantity of electrical power.
3. Parallel use of wind- and diesel-generators
This configuration is able to deliver an autonomous grid for all possible situations of loads and available windpower. There is no connection to the public grid. The information about the balance of power necessary for the decision if a diesel engine is started can be taken out of one measurement value which is available to each energy supply unit: the frequency of the grid. Therefore we call a system like this ´frequency managed´, that means the frequency of the grid varies depending on it´s actual condition in balance of power in a range of typically *2% of it´s nominal value (50 Hz). If the frequency is higher than the nominal value only the wind turbine is connected to the grid. With a decreasing frequency the energy which can be taken from the wind turbine also decreases and the diesel-engine/s must be started to stabilize the grid.
Concerning the system-management each energy supply unit needs to have a power/frequency-characteristics as shown in figure 2. The wind turbine is stall-regulated with the rotor blades at a fixed angle to the axis. Because of the highest priority given to the use of wind energy in the wind-diesel system the control characteristics for the wind turbine are the same as in a stand-alone system. If a higher power than actually available is demanded, the frequency decreases and the diesel starts working. It´s preset-value for the nominal frequency is slightly beyond the nominal value of the wind turbine. Because of this small difference in the characteristics a certain amount of power flows back into the diesel engine/s until they cut off at a preset-value of reverse power if the demanded power is reduced or the wind energy input rises. The wind turbine then works on it´s own.
Depending upon the demanded availability of the grid other variations of power management are possible. With the ´fuel-save´-mode for instance the diesel keeps running independently of the actually required power and the wind turbine only reduces the fuel-input. This mode is used if a high base-load is connected to the grid. With anoter mode the diesel is only started if a certain load is exceeded.
In this configuration the wind turbine or the wind-diesel system is connected to another electrical network, i.e. the public grid.
The regulation of the wind turbine with dummy-loads is only necessary at the moment of synchronization to the grid because the programmed nominal value is higher than the maximum frequency of the grid.
All the power generated with the wind energy converter is fed into the grid which may be considered as an infinite load breaking the wind turbine exactly at grid-frequency. If the wind energy input is lowered down to zero the generator is driven by the grid like a motor. Therefore the wind turbine is disconnected by the control electronics beyond a programmable amount of reverse power.
The diesel engine also works independently of the number/power of generators/loads giving or taking electrical energy at the moment when it is used in parallel connection to the public mains. For economical reasons the thermal losses of an engine running parallel to the public network is commonly used in addition to conventional oil or gas heating (combined heat and power). The conditions for controlling the diesel are given by the thermal energy flow in this special case. If the preset-value for the frequency is set higher than the maximum frequency of the grid the diesel engine always runs full power.A regulation to a required amount of thermal power is also possible.
In the case of grid-failure the controls are able to automatically change their mode to a stand-alone system. The diesel then works with a speed-regulation at partial load depending on the balance of electrical power. It will rescue a partial grid (in parallel to the wind turbine if possible) and regulate it´s frequency to the preset-value with the above mentioned frequency/power-characteristics.
4.1. Wind turbine parallel to the public mains
Wind energy converters for use in stand-alone systems normally are regulated to a fixed value of voltage. The controls work as follows:
If the voltage of the grid is lowered by an additional load, the controller increases the induction current of the electrical generator to stabilize the voltage again. If the load decreases the magnetic field is also decreased.
If two or more units run parallel, the current-flow (as a vector) in each generator must be taken into consideration because the voltage of the grid is now regulated by more than one controlling system. This performance of the electronic controls is called ´statics´. A setting of the statics-value causes a lower nominal voltage on inductive loads. Therefore two parallel generators also deliver a certain amount of reactive power.
In a grid-parallel system this performance can no longer be used because small changes in the voltage of the grid would desturb the controller. Therefore an additional regulation of the phase-angle between voltage and current in the generator (cos phi) is necessary.
In November 1993 the first wind energy converter with stand-alone and grid-parallel operation, a ´Südwind N/E 1230´ (30 kW) was connected to the electrical system and put into operation in Emden/Northern Germany. Since early 1993 the synchronous generators of the wind turbine and the parallel natural-gas-fired combined heat and power unit have been running with good results. This was the first application for a wind turbine with the possibility of use in a grid-parallel or autonomous system and the whole range in between ´weak´ and ´hard´ networks.
Up to then the use of synchronous generators in small wind turbines (up to 100 kW) had not been carried out worldwide. There is no technical literature available on this particular application. Within the context of a rising importance of an interconnection between small autonomous power supply systems up to large regional networks in many countries the world over, the results of this wind-diesel system carries particular weight.