Design and development of small wind turbines - Wind Power Generation and Wind Turbine Design - page 271

The generator is disconnected from the grid and then connected to a series-parallel
combination of resistors and capacitors. The capacitors provide the excitation and
the resistors dissipate the energy. It provides very smooth braking torque until
excitation is lost at a low rpm, and a small disc brake is engaged.
A disc brake is commonly used on large wind turbines, and is sometimes used
on small machines. The Enertech 1500 (Fig. 5) had a gearbox, induction generator,
and a disc brake on the high speed shaft. The drawback of this arrangement was
that the brake had to be sized for the maximum anticipated rotor torque, and so
every time the rotor was halted the drivetrain (and particularly the gearbox) expe-
rienced maximum torque. Often a disc brake produces torque spikes as the discs
come together, rebound and then fi nally settle. It is also possible to put the disc
brake on the low-speed shaft, thereby eliminating strain on the gearbox, but a
much bigger brake is required and so this is seldom used.
Other creative ideas have been tried, such as the so-called “hydraulic brake”. In
this case a hydraulic pump is coupled onto the high speed shaft rather than a disc
brake. Hydraulic fl uid is pumped through an open solenoid valve during normal
operation, and when braking action is required the solenoid valve is closed and the
fl ow diverts through a pressure relief valve. This produces very smooth braking
torque, however there are pumping losses during normal turbine operation.
1.3.4 Power cabling
Typically on small wind turbines power cabling comes from the generator to slip
rings, and then a separate power cable goes down the tower to a disconnect switch
at the base of the tower. However the “twist cable” concept was introduced in the
early 1980s, and is now typically used on large machines, and sometimes used on
small machines.
The power cable is suspended from beneath the turbine, supported by a strain
relief connection at the turbine. Depending on the cable length and fl exibility, it is
generally able to withstand many yaw revolutions in one direction before it is in
mechanical stress. The cable is physically disconnected at the bottom of the tower
and allowed to unwind during service visits to the turbine. In the case of large wind
turbines, yaw revolutions are counted, and when a certain number have accumu-
lated in one direction the turbine is stopped and the cable is “unwound” via the
yaw mechanism.
1.3.5 Control system design
The control system depends very much on the application, and in general there
are two applications: grid connected and battery charging (though there are other
applications such as direct heating and direct pumping).
1.3.5.1 Grid-connected control systems
This is discussed to some extent in Section 1.3.2, and the controls depend on
whether it is a grid-tie inverter system or an induction generator-based system.
In the case of the former, the inverter effectively is the control system, load-
ing the generator according to the DC voltage that it sees as discussed above.