GB2036881A

GB2036881A - Wind Turbine Driven Generator Plant

Info

Publication number: GB2036881A
Application number: GB7848765A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-12-15
Filing date: 1978-12-15
Publication date: 1980-07-02

Abstract

The plant uses one, two or more standard generators (and or change pole or pole amplitude modulation machines) all linked to a common driving shaft and offering a range of different synchronous speeds such that the wind turbine rotor is always allowed to work at or near its optimum efficiency. Switching from one speed to another either on a single generator or between two generators is initiated by a wind speed sensing unit. Slip couplings, aero and mechanical brakes may be used to aid the switching process. <IMAGE>

Description

SPECIFICATION Stepped Speed Generator Scheme For Wind Driven Turbine Background A wind turbine only works at its best efficiency at a single tip speed ratio, that is blade tip speed divided by the free flow wind velocity (Vo) see Fig.

1. But the wind rotor is expected to work over a wide range of wind speeds and therefore to maintain high efficiency the rotor's angular velocity must be linearly related to Vo. This creates a difficulty for any standard wind turbine driven electrical generator from which constant voltage and frequency is required. To date, all the published electrical engineering solutions to obtain constant voltage and frequency from variable speed wind turbines have involved the use of commutator machines or solid state power processing. The extra cost of these proposals has very often grossly exceeded the improved return arising from the higher efficiency. The majority of grid feeding turbines built to date have, therefore, had fixed rotor speeds (save for small speed increments arising from the slip on induction generators) and the inevitable inefficiencies have been accepted.

An associated probiem arises because the power in the wind is a function of the cube of the Vo so that the power available increases eightfold for any doubling of the windspeed. In practice, this means that once a certain wind speed has been chosen as the rated output from the generator (Vr), then the range of windspeeds over which generation can be expected is from .7 Vr to 1.3 Vr approx. The lower limit is set by poor part load efficiencies of the generator and the upper limit by fears for the safety of the rotor.

The result of these two difficulties is that a fixed speed, grid feeding turbine at a site with a typical U.K. wind frequency distribution only supplies approx. 52% of its annual practical potential and only works for some 48% of the time. These percentages will vary a little according to the wind speed adopted for Vr.

Proposed Scheme Both problems are resolved by the use of one, two or more generators designed to work at different rotational speeds and linked to the rotor via a common driving shaft. Switching between speeds and generators will be initiated by an anemometer which assesses the mean wind speed every 20-30 minutes, though other periods are possible. General arrangements are shown on Fig. 2. Couplings are indicated and these may or may not be included. They would allow the rotor to start up without the attendant drag of the generators and subsequently to allow GEN I to work without the drag of Gen II and vice versa. These clutch couplings will allow a degree of slip to reduce transient shocks to the gearbox and rotor when switching is initiated.

Example The following example relates to a grid feeding turbine on a site where the annual mean wind speed is 6 m/s and the wind frequency distribution is typical of the U.K. During low windspeeds (Vo) of between 5.5 and 7.5 m/s Gen I with an angular velocity of n allows the rotor to work at or very close to its optimum efficiency.

When the anemometer assesses the 20 to 30 minute mean windspeed to have risen to between 7.5 and 9.5 m/s it initiates pole switching or pole amplitude modulation to allow the generator to work at a speed of 1.33 n. The generator may briefly act as a motor while it runs up to this new speed or it may be disconnected while the rotor freely accelerates to its new optimum working speed. This mode of optimised rotor speed/wind speed continues until the anemometer assesses a mean Vo of above 9.5 m/s. At that time Gen I is switched off, the rotor speed is allowed to freely increase to 1.5 n when Gen II is switched in. This mode continues in a rising Vo until shutdown is initiated at 1 6-1 8 m/s.The reverse sequence of switching occurs in a falling Vo when the higher speed generator is switched off and the lower speed generator (if asynchronous type) is switched in thereby pulling the rotor speed down to the new level. Alternatively an aero and/or mechanical and/or electrical brake may be used for the same purpose. At sites with very high annual mean wind speeds of say 9-14 m/s it will be necessary to use 3 or 4 generators. This is shown in Fig. 3.

Results By using this scheme the output per annum of the wind turbine rises to 90% of the practical maximum and the set operates for over 64% of the year.

Claims

One, two, three or more generators linked to a single wind driven rotor via a gearbox, if required, shafting and such slip clutches as may be required where a wind sensor assesses the ambient wind speed and initiates switching on a single generator (pole changing or pole amplitude modulation) or from one generator to another generator or from one or more generators to one or more generators in such a manner that the resultant synchronous speed of the generator(s) or synchronous speed plus slip speed is such that the wind rotor is allowed to rotate at or near its optimum speed for the prevailing ambient wind speed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Stepped Speed Generator Scheme For Wind Driven Turbine Background A wind turbine only works at its best efficiency at a single tip speed ratio, that is blade tip speed divided by the free flow wind velocity (Vo) see Fig.

1.3 Vr approx. The lower limit is set by poor part load efficiencies of the generator and the upper limit by fears for the safety of the rotor.

Proposed Scheme Both problems are resolved by the use of one, two or more generators designed to work at different rotational speeds and linked to the rotor via a common driving shaft. Switching between speeds and generators will be initiated by an anemometer which assesses the mean wind speed every 20-30 minutes, though other periods are possible. General arrangements are shown on Fig.

2. Couplings are indicated and these may or may not be included. They would allow the rotor to start up without the attendant drag of the generators and subsequently to allow GEN I to work without the drag of Gen II and vice versa. These clutch couplings will allow a degree of slip to reduce transient shocks to the gearbox and rotor when switching is initiated.

When the anemometer assesses the 20 to 30 minute mean windspeed to have risen to between 7.5 and 9.5 m/s it initiates pole switching or pole amplitude modulation to allow the generator to work at a speed of 1.33 n. The generator may briefly act as a motor while it runs up to this new speed or it may be disconnected while the rotor freely accelerates to its new optimum working speed. This mode of optimised rotor speed/wind speed continues until the anemometer assesses a mean Vo of above 9.5 m/s. At that time Gen I is switched off, the rotor speed is allowed to freely increase to 1.5 n when Gen II is switched in. This mode continues in a rising Vo until shutdown is initiated at 1 6-1 8 m/s.The reverse sequence of switching occurs in a falling Vo when the higher speed generator is switched off and the lower speed generator (if asynchronous type) is switched in thereby pulling the rotor speed down to the new level. Alternatively an aero and/or mechanical and/or electrical brake may be used for the same purpose. At sites with very high annual mean wind speeds of say 9-14 m/s it will be necessary to use 3 or 4 generators. This is shown in Fig.

3.

Claims

An associated probiem arises because the power in the wind is a function of the cube of the Vo so that the power available increases eightfold for any doubling of the windspeed. In practice, this means that once a certain wind speed has been chosen as the rated output from the generator (Vr), then the range of windspeeds over which generation can be expected is from .7 Vr to