Page 469 - Wind Energy Handbook

MECHANICAL BRAKE 443

used to bring the rotor to a standstill during high wind shut-downs for the majority
of machine designs, and during low-speed shut-downs as well in some cases.
Aerodynamic braking is used to decelerate the rotor initially, so the mechanical
brake torque can be quite low. However, IEC 61400-1 requires that the mechanical
brake be capable of bringing the rotor to a complete stop from a hazardous idling
state in any wind speed less than the 1 year return period 3 s gust (see Table 5.1).
If the mechanical brake is required to arrest the rotor in the event of a complete
failure of the aerodynamic braking system, then there are two deployment options
to consider. Either the mechanical brake can be actuated when an overspeed
resulting from the failure of the aerodynamic system is detected, or actuated
simultaneously with the aerodynamic brake as part of the standard emergency
shut-down procedure. The advantage of the former strategy is that the mechanical
brake will rarely, if ever, have to be deployed in this way, so that some pad or even
disc damage can be tolerated when deployment actually occurs. In addition, fatigue
loading of the gearbox will be reduced if the brake is mounted on the high-speed
shaft. On the other hand, if the mechanical brake is actuated before significant
overspeed has developed, then the aerodynamic torque to be overcome by the
mechanical brake in the event of aerodynamic braking failure will be less.
The most severe emergency braking case will arise following a grid loss during
generation in winds above rated. In the case of pitch-regulated machines, the
maximum overspeed will occur after grid loss at rated wind speed because the rate
of change of aerodynamic torque with rotational speed decreases and soon becomes
negative at higher wind speeds. Conversely, if the pitch mechanism should jam, the
braking duty becomes more severe at wind speeds at or above cut-out, because
much higher aerodynamic torques are developed as the rotor slows down and the
angle of attack increases. For stall-regulated machines the critical wind speed is
generally at an intermediate value between rated and cut-out.

7.6.2 Factors governing brake design

The braking torque provided by callipers gripping a brake disc (Figure 7.34) is
simply the product of twice the calliper force, the coefficient of friction (typically
0.4), the number of callipers and the effective pad radius. Callipers providing
clamping forces of up to 500 KN are available. However, the brake design is also
limited by:

• centrifugal stresses in the disc,
• pad rubbing speed,
• power dissipation per unit area of pad, and
• disc temperature rise.

The nature of these constraints is described below.
The critical stress generated by centrifugal stresses is in the tangential direction at