# Variable speed

The Variable speed generator corresponds to a look-up table that gives the torque of the generator depending on the RPM of the generator. The default variable speed generator implemented in Ashes is equivalent to the NREL 5 MW controller.

## Torque vs. RPM curve

### Maximum generator torque

Maximum generator torque increase as a percentage of rated torque. The actual maximum generator torque is calculated by multiplying the rated torque with this percentage plus 100%. This is the maximum value that the generator torque is allowed to reach (i.e. the generator torque can be larger than the rated torque with this percentage).

• Default value: 10
• Unit: $\text{%}$
• Range: 0 — 100

### Minimum time to rated torque

This is a limit to how fast the generator torque can be changed. It represents the shortest time the generator torque can be increased from 0 to rated torque. Thus, the maximum torque rate is the rated torque divided by this duration.

• Default value: 2.873
• Unit: $\text{s}$
• Range: 1e-06 — 1e+08

### Proportionality factor region 2

The factor of proportionality for the parabolic line of the region 2 as a percentage of the maximum factor defined by rated rpm and rated torque: rated torque / (rated rpm)^2.

• Default value: 81.76
• Unit: $\text{%}$
• Range: 0 — 100

### Transition RPM region 1 to 1.5

The generator RPM for transition between region 1 and region 1.5 as a percentage of the rated rpm

• Default value: 57.0844
• Unit: $\text{%}$
• Range: 0 — 100

### Transition RPM region 1.5 to 2

The generator RPM for transition between region 1.5 and region 2 as a percentage of the rated rpm

• Default value: 74.209
• Unit: $\text{%}$
• Range: 0 — 100

### Transition RPM region 2.5 to 3

The generator RPM for transition between region 2.5 and region 3 as a percentage of the rated rpm. Must be greater than the previous points.

• Default value: 99
• Unit: $\text{%}$
• Range: 0 — 100

### Slip

The 'virtual' slip of the generator in percent. Since this is not a real induction generator it does not have real slip. This value is used to define region 2.5.

• Default value: 10
• Unit: $\text{%}$
• Range: 0 — 90

### Torque override due to pitch

If the demanded pitch angle is greater than this limit then the generator torque is computed as if the generator is in region 3, even if the RPM lies outside of region 3.

• Default value: 1
• Unit: $°$
• Range: 0 — 90

## Rating and efficiency

### Rated power scheme

Decides how the rated electrical power of the generator is found.

Options:

From rotor area:

The rated electrical power is calculated based on the rotor area and the target (rated) wind speed.

User defined (default):

The rated electrical power is set by you.

### Rated electrical power

The rated electrical power of the generator (i.e. power 'delivered to the grid'). The corresponding rated mechanical power is higher than this because the generator's efficiency is less than 100%.

• Default value: 5e+06
• Unit: $\text{W}$
• Range: 0 — 1e+08

### Target wind speed

The target wind speed to be used to calculate the target (or rated) mechanical and electrical power of the generator.

This parameter is only visible (and relevant) if Rated power scheme is set to From rotor area. In addition to this wind speed, the rotor area and the air density is used to calculate the target (rated) mechanical power

• Default value: 12
• Unit: ${\text{m}} \over {\text{s}}$
• Range: 1 — 100

### Rated RPM

The generator RPM when the rated power is reached. For higher RPM the blades will typically pitch to decrease the efficency and thus keep the power from exceeding the rated power.

• Default value: 1173.7
• Unit: $\text{rpm}$
• Range: 0 — 1e+08

### Efficiency factor

The efficiency of a generator is the ratio of input mechanical power to output electrical power. The efficiency factor is used to calculate the efficiency of the generator. The default behavior is that the efficiency is equal to the efficiency factor (and thus constant). However, some generators calculate an efficiency explicitly (e.g. the PMSG generators), and for these generators the calculated efficiency is reduced with the efficiency factor. If no such reduction is desired, the factor should be set to 1. The value of the efficiency factor ranges from 0.01 to 1.00 (1% to 100%).

• Default value: 0.944
• Unit:
• Range: 0.01 — 1

### Drivetrain efficiency factor

The drivetrain efficiency is used to take losses in the drivetrain incl. the gearbox (if any) into account. The generator torque is multiplied with the factor before it is applied (i.e. the generator torque is decreased to take the mechanical losses into account). The value of the efficiency factor ranges from 0.01 to 1.00 (1% to 100% efficiency). A value of 1 (i.e. 100% efficiency) means no losses.

• Default value: 1
• Unit:
• Range: 0.01 — 1

### Filtering

The rotational speed (RPM) of the generator can be filtered before use. Filtering is used to mitigate high-frequency excitation of the control systems, so filtering is recommended. The rotational speed is used to calculate the generator torque etc. and in the pitch control system to caclulate pitch.

Options:

None:

The real, physical RPM is used directly.

Recursive, single-pole (default):

Recursive, single-pole low-pass filter with exponential smoothing, see (Jonkman, 2009).

### Corner frequency

The corner frequency. Used in a low pass-filter to calculate the filtered RPM. Suggested value is one-quarter of the blade's first edgewise natural frequency.

• Default value: 0.25
• Unit: $\text{Hz}$
• Range: 0 — 1e+12