This section of the manual covers the modelling of the following aspects of turbine operation: • Start-up
• Shut-down (normal and emergency stops)
• Non-operational situations (rotor parked or idling) • Operation of the shaft brake
• Teeter restraints • Yaw control
The standard implementation of these features in the simulation model is described. As in the case of Closed Loop Control, alternative supervisory control logic can be incorporated in a user-defined controller - see Section 5.9.
6.1 Start-up
Simulation of a wind turbine start-up begins with the rotor at a specified speed (usually but not necessarily zero) and the generator off-line. The brake is assumed to be released at the start of the simulation (i.e. at time zero).
If blade pitch or aileron control is available, the initial pitch or aileron angle is specified, along with a constant rate of change which continues until either a specified angle is reached or the closed loop controller takes over.
When a specified rotational speed is reached, the generator comes on line, and the closed loop controller begins to operate. The simulation continues until the specified simulation end time.
In the case of a variable speed turbine, there may be a transition period after cut-in of the closed loop controller before the turbine is fully in the normal running state. There are two different cases:
Variable speed pitch regulation: in the case when the pitch angle has not yet reached the normal operating value (‘fine pitch’) at the moment when the closed loop controller cuts in, then the pitch change rate for start-up continues to apply until either fine pitch is reached, or until the conditions of Section 5.8 for starting the closed loop pitch controller are satisfied. Variable speed stall regulation: when the closed loop controller cuts in, the above-rated control mode is assumed to apply initially. In practice this assumption does not affect the start-up since in low winds the operating point would be constrained by the quadratic optimum-Cpcharacteristic in any case.
6.2 Normal stops
A normal stop is initiated at a specified time after the start of the simulation. Normal operation in power production mode is assumed prior to this point, with full structural and control dynamics in effect if desired. The structural dynamics continue in effect during the entire simulation.
The standard logic for a normal stop is to start pitching the blades (or moving the ailerons) at a specified rate from the moment that the stop is initiated, continuing until a final pitch angle is reached. The generator is taken off-line when the electrical power reaches zero in the case of a fixed-speed turbine, or when the minimum generator speed is reached in the case of a variable speed turbine.
Once the rotational speed drops below a specified value, the shaft brake is applied to bring the rotor to rest.
The simulation continues until the rotor comes to rest, or for a certain time longer if so desired in order that the transient loads can be simulated as the brake disk stops. However, the simulation end time overrides this, so it must be set long enough for the stop event to be completed.
If there is no pitch control, the brake trip speed may be set high so that the shaft brake is applied immediately at the initiation of the stop.
Section 6.4 describes the dynamic characteristics of the shaft brake itself.
6.3 Emergency stops
An emergency stop is initiated at a specified time after the start of the simulation. Normal operation in power production mode is assumed prior to this point, with full structural and control dynamics in effect if so desired. The structural dynamics continue in effect during the entire simulation.
Several options are available for simulating emergency stops. In all cases it is assumed that the generator load is lost at the initiation of the emergency stop, whether because of grid failure or some electrical or mechanical failure of the turbine.
Pitch (or aileron) action is initiated either immediately or when the rotational speed exceeds a specified value. A fixed pitch rate then applies until a final pitch angle is reached. Provision is made for the pitch of one or more of the blades to ‘stick’ at a specified angle to simulate failure of a pitch bearing or actuator.
The shaft brake can also be applied either at the initiation of the stop or when a specified overspeed is reached. Section 6.4 describes the dynamic characteristics of the shaft brake itself. There is also a rotational speed below which the shaft brake is applied for parking, in the event that it has not already been applied because of load loss or overspeed.
The simulation continues until the rotor comes to rest, or for a certain time longer if so desired in order that the transient loads can be simulated as the brake disk stops. However,
the simulation end time overrides this, so it must be set long enough for the stop event to be completed.
6.4 Brake dynamics
When the shaft brake is applied, either during a normal or an emergency stop, the full braking torque is not available instantly. Instead, the torque builds up to the full value over a short period of time. This torque build-up may be modelled as either a linear torque ramp, or by specifying a look-up table giving achieved braking torque as a function of time.
6.5 Idling and parked simulations
For simulations in the idling and parked states, a fixed pitch angle is specified, the generator is off line, and there is no pitch control action. In the case of a parked rotor the shaft brake is applied, and the rotor azimuth must be specified. The azimuth is measured from zero with blade 1 at top dead centre.
All specified structural dynamics will be in effect during these simulations. This also allows for the possibility of the shaft brake slipping during a parked simulation if the shaft torque exceeds the specified brake torque.
6.6 Yaw control
6.6.1 Active yaw
Active yaw movement may be specified in one of two ways:
1. One fixed-rate yaw manoeuvre may be specified, starting at a given point in any simulation. This represents a change in the nominal nacelle position through a given angle at a specified angular speed.
2. A user-defined controller (Section 5.9) may be used to specify either the yaw rate or the yaw actuator torque at any time.
If active yaw is used to control the yaw rate, the effect of this is to change the ‘demanded nacelle angle’ in a specified way. The actual nacelle angle depends on the yaw dynamics - see next section.
6.6.2 Yaw dynamics
Three options are available to define the yaw dynamics:
1. Rigid yaw: the actual nacelle angle exactly follows the ‘demanded nacelle angle’ 0. 2. Flexible yaw: a certain amount of flexibility is present, usually in the yaw actuation
system, such that the actual nacelle angle may not follow the ‘demanded nacelle angle’ 0exactly. The extreme case is free yaw, when the demanded nacelle angle does not have any effect.
3. Controlled yaw torque: this is available only with an external controller to define the yaw actuator torque demand
Yaw control type Demanded yaw rate
Yaw spring and
damper Friction
Controlled torque
None No No No No
Rigid Yes No No No
Flexible Yes Yes Yes No
Controlled torque No No Yes Yes
In the case of flexible or free yaw, the yaw damping Dy may be specified. This specifies a torque Qd which opposes the yaw motion, given by
Q
d=D
y(&
0&)
In the case of flexible yaw, a yaw spring may be specified either as a linear spring or as a hydraulic accumulator system such as is often used to provide flexibility in hydraulic yaw drives. The hydraulic system is assumed to be double-acting, with one accumulator (or set of accumulators) on either side of the yaw motor. The torque opposing the motion is provided by compression of the gas in the accumulators. If the nominal gas volume is V0 and the instantaneous gas volumes either side of the yaw motor are v1and v2then the opposing torque Qkis given by Q KP V v V v k = 0 0 1 0 2 # # Tower
Demanded yaw rate
Aerodynamic and inertial yaw torque Yaw spring
Damper Friction
Controlled torque
where v1 = V0 + F( - 0) and v2 = V0 - F( - 0) and P0 is the equilibrium pressure in the hydraulic system. The constant K defines the relationship between the torque developed at the yaw bearing and the pressure difference across the yaw motor, while F the relationship between the volume of oil flowing through the yaw motor and the resulting angular movement at the yaw bearing. # is the gas law constant: PV# = RT. Putting # = 1 specifies isothermal conditions in the accumulators.
6.7 Teeter restraint
Although not strictly a supervisory control function, the teeter restraint model available in
Bladed for teetered rotors is described here. The model allows a linear variation of restoring
torque with teeter angle, but also allows a free teeter range and an initial pre-load. Figure 6.1 defines the relevant parameters. Linear damping is also allowed, giving an additional torque contribution proportional to teeter rate.
Figure 6.1: Teeter restraint model
Teeter angle Restoring torque Pre-load free teeter angle Spring constant