Method and system for controlling non-inertial generators, in particular wind generators, by inertia emulation

A method of controlling, by inertia emulation, electric power generators provided with rotational inertia and connected to an electricity grid by electric power converters, the method comprising:

• measuring grid frequency and/or frequency over-time variation with respect to a rated frequency value of the electricity grid, to which an output of a generator power signal is connected;
• providing a threshold value for said grid frequency or said frequency over-time variation;
• comparing the measured grid frequency or the frequency over-time variation of the grid frequency with corresponding threshold values;
• and
• converting a part of a kinetic energy of a generator rotor into electric power, by decreasing rotation velocity of the generator rotor so as to provide additional supporting power to a variation of the grid frequency, whereby a total power supplied by a generator is greater than a maximum actual power corresponding to wind conditions,
• wherein:
• said additional supporting power is calculated as a function of the grid frequency and/or the frequency over-time variation and of a predetermined variable coefficient of inertia of the generator,
• said additional supporting power is supplied until said grid frequency falls within a range of preset tolerances, until the frequency over-time variation has been reduced within preset limits or said grid frequency variation is removed, or until injected power has fallen below a preset limit value,
• a coefficient of inertia is calculated by emulating an inertial behavior of generators consisting of synchronous machines,
• the coefficient of inertia of the generator is calculated as a function of rotation velocity of the generator rotor, and
• wherein a total power supplied by the generator during the step of converting a part of a kinetic energy is provided as a sum of maximum actual power corresponding to wind conditions, determined based on a MPPT (maximum power point tracking) control, and of the additional supporting power calculated based on the following equation:
• P
• F
• ⁢
• S
• =
• 2
• ⁢
• H
• s
• ⁢
• y
• ⁢
• n
• ⁢
• t
• ⁡
• (
• ω
• r
• )
• ⁢
• f
• e
• ⁢
• d
• ⁢
• f
• e
• d
• ⁢
• t
• (
• 1
• )
• wherein:
• H
• s
• ⁢
• y
• ⁢
• n
• ⁢
• t
• ⁡
• (
• ω
• r
• )
• =
• H
• max
• ⁡
• [
• 1
• +
• ω
• r
• -
• ω
• r
• ⁢
• ⁢
• 0
• ω
• r
• ⁢
• ⁢
• 0
• -
• ω
• r
• ,
• min
• ]
• (
• 2
• )
• and
• fe is system frequency expressed in p.u.;
• t is the time;
• ωr,min is a minimum value of rotor velocity in p.u.;
• ωr0 is a value of the rotation velocity in p.u. of the generator rotor at a beginning of the step of converting;
• ωr is the value of the rotation velocity of the rotor in p.u.;
• Hsynt is a coefficient of synthetic inertia in seconds;
• and
• Hmax is a maximum value of the coefficient of inertia calculated for the rotation velocity of the maximum actual power corresponding to the wind conditions at the beginning of the converting step.