Control Strategy and Modes of Operation

The pervasive penetration of wind power into the electric power system indicates that WECSs should also be able to take part in the power-flow control process which, presently, is exclusively undertaken by the conventional power plants; the feature should supplement the MPPT capability that modern WECSs currently possess. As such, two modes of operation are defined for the WECS of Figure 2.1: (1) the MPPT mode, and (2) the

controlled-power (CP) mode. The operating mode is determined by the way that the setpoint Pr

e is stipulated, as explained next.

2.4.1

MPPT Mode of Operation

In the MPPT mode, the objective is to maximize the power that the turbine extracts from wind. Based on (2.1), this can be achieved if Cp is maximized. To maximize Cp, the turbine tip-speed ratio must be kept constant at its optimum value, λopt, regardless of the wind speed; λopt is the tip-speed ratio at which Cp peaks, for the minimum pitch angle. The objective is fulfilled if the PMSG power setpoint is determined based on the following law [89]:

Per =koptω 3

r, (2.18)

in which the constant kopt (in Nms 2

/rad2

) is a parameter (that depends on the turbine construction and characteristic), and can be obtained by evaluating Cp for λ=λopt and β =βmin, and is given by [89]:

kopt = 0.5π R5 ρ λ3 opt Cp(λopt, βmin). (2.19)

It then follows from assuming a fast control that Pe ≈ Per. Thus, (2.18) can be rewritten as

Pe =koptω 3

r. (2.20)

Figure 2.4 illustrates the characteristic curves of a wind turbine, for a wind speed and two different values of pitch angle, that is, βmin (heavy solid line) and β > βmin (light solid line). The figure also plots the PMSG power versus rotor speed, based on (2.20) (dashed line). It is observed that if the WECS is in the MPPT mode and the rotor speed is smaller than ωr−opt = λoptvw/R, then the turbine mechanical power, Ptur, is larger than the PMSG electrical power, Pe, and, therefore, the rotor speed increases towards the valueωr−opt. By contrast, if the rotor speed is larger thanωr−opt,Ptur is smaller than

Pe, and ωr decreases. In a steady state,Ptur equals Pe, andωr settles at the valueωr−opt. Thus, point A on Figure 2.4 is a stable operating point corresponding to the maximum turbine power at the given wind speed. Based on (2.20), the maximum power, Popt, can

P o w er Rotor Speed A min β β = C B min β β>

ω

ω

P

ω

P

P

k

ω

=

Figure 2.4: Characteristic curves of a wind turbine for a wind speed and two different values of pitch angle.

be formulated as

Popt =koptω 3

r−opt. (2.21)

It should also be noted that using koptω 3

r as the power setpoint in the MPPT mode of operation generates a smooth output power. This is because the changes in the rotor speed caused by the wind speed variations is slow due to the high inertia of the WECS rotor.

2.4.2

Controlled-Power Mode of Operation

In the CP mode, the objective is to regulate the WECS output power at the command value Pcmd, regardless of the wind speed. Therefore, Per is given the value of Pcmd. Let us assume that, initially, the WECS is in the MPPT mode, β =βmin, Pe =Ptur =Popt, and ωr =ωr−opt; then the value of P

r

e (and thereforePe) is rapidly changed fromPopt to Pcmd, i.e., subsequent to a switching from the MPPT mode to the CP mode. As Figure 2.4 indicates, this causes the PMSG power to drop below the turbine power and results in an increase in ωr towards the value ωrB. Depending on the wind speed, ωrB can be larger than the maximum permissible rotor speed, ωr−max, as for the example illustrated in Figure 2.4. The situation is circumvented by the pitch-angle control mechanism; thus, once ωr exceeds ωr−max, the pitch-angle control scheme increases β and consequently alters the power-speed characteristic of the wind turbine, to the one shown by light solid

cmd P koptωr3 Limiter r e P

Figure 2.5: Block diagram illustrating the generation of the power setpoint.

line in Figure 2.4, such that Ptur drops to Pcmd and the rotor speed settles at ωr−max (corresponding to the point C in Figure 2.4). To ensure that the PMSG and turbine power-speed curves have at least one crossing point [see Figure 2.4],Pr

e in the CP mode is limited to the valuekoptω

3

r. Therefore, ifPcmd is so large that the turbine power cannot overtake it at the given wind speed, thenPe will be limited tokoptω

3

r and, effectively, the system continues to operate in the MPPT mode until either there will be a rise in the wind speed (thus increasing the corresponding Popt) or the system operator steps down the command Pcmd.

Figure 2.5 illustrates the proposed mechanism for selecting between the MPPT and CP modes of operation. As Figure 2.5 shows, the setpoint Pr

e is obtained from the output of a hard limiter whose input and upper saturation limit are Pcmd and koptω

3 r, respectively (the lower saturation limit is zero). Thus, Pr

e is equal to Pcmd, and the CP mode is exercised, as long as Pcmd is smaller than koptω

3

r; otherwise, Per is equal to koptω

3

r and the energy capture subsystem operates in the MPPT mode. Therefore, to permanently leave the system in the MPPT mode, it is sufficient to assign Pcmd an adequately large value (e.g., larger than the value of Popt that corresponds to the rated wind speed).