System and method for operating a renewable energy source in grid-forming mode (GFM) as a virtual synchronous machine (VSM) with damper winding emulation
A method and associated system provide grid-forming mode (GFM) control of an inverter-based renewable energy source having an asynchronous machine connected to a power grid. The method includes deriving a power error signal (P ER ) between a real power output (P B ) from the renewable energy source and a power reference (P REF ) representing a desired power output of the renewable energy source. With an inertial power regulator, a phase shift angle (δ IT ) is generated from the power error signal (P ER ) to provide virtual synchronous machine (VSM) control functionality to the GFM control of the renewable energy source. A power angle compensation (Δδ) is applied to the phase shift angle (δ IT ) from the inertial power regulator to dampen power oscillations and load fluctuations during transient power events.
1. A method for providing grid-forming mode (GFM) control of an inverter-based renewable energy source having an asynchronous machine connected to a power grid, the method comprising:
deriving a power error signal (P ER ) between an actual real power output (P B ) from the renewable energy source and a power reference (P REF ) representing a desired power output of the renewable energy source;
with an inertial power regulator, generating a phase shift angle (Δδ) from the power error signal (P ER ) to provide virtual synchronous machine (VSM) control functionality to the GFM control of the renewable energy source; and
applying a power angle compensation (Δδ) to the phase shift angle (δ IT ) from the inertial power regulator to dampen power oscillations and load fluctuations during transient power events.
2. The method according to claim 1 , wherein the inverter-based renewable energy source is a wind turbine power system and the asynchronous machine is a doubly-fed induction generator (DFIG).
3. The method according to claim 2 , wherein the transient power events include a low voltage ride through (LVRT) event.
4. The method according to claim 3 , further comprising activating the power angle compensation after the LVRT event and slider gain is below a threshold value.
5. The method according to claim 2 , wherein the phase shift signal (δ IT ) is used to adjust a value of a phase angle (θ 1 ) provided to gating logic circuitry of the wind turbine power system, the power angle compensation (Δδ) used to adjust the phase shift signal (δ IT ).
6. The method according to claim 5 , wherein the power angle compensation (Δδ) is derived as a function of the power error signal (P ER ) input to the inertial power regulator.
7. The method according to claim 6 , wherein the function is according to:
Power angle compensation (Δδ)=f(ΔPe0, B)
B=E1*Vg/X1
Pe0=(E1*Vg/X1)*sin(δ0)
(ΔPe0)=Kp*(Pref−Pfbk)
Kp=control gain constant
(Δδ)=Sin −1 (Pe0+Pe0/B)−δ0
(Δδ)˜ΔPe0/B
E1: terminal voltage of machine operating as a VSM
δ: Power angle
X1: effective impedance of machine (stator leakage+virtual impedance)
Pe0: steady state power of machine
Vg: grid voltage at steady state power.
8. The method according to claim 6 , wherein the DFIG is subsequently operated in torque control to further reduce drive trail loads, wherein a torque demand (T DEM ) is generated and modified by a damping torque demand (T DTD ) to provide a generator torque demand (T GEN ) made on the DFIG, and wherein the power angle compensation (Δδ) is a function of torsion induced in drivetrain components of the wind turbine power system and serves as an additional damping torque demand (T DTDadd ).
9. The method according to claim 8 , wherein the torsion is calculated based on a speed difference between a high speed shaft (HSS) and a low speed shaft (LSS) in the drivetrain components, or is estimated based on the HSS speed and torque acting on the HSS.
10. The method according to claim 9 , wherein the power angle compensation (Δδ) is derived from the torsion, grid voltage, effective impedance of the wind turbine power system, and terminal voltage of the DFIG according to:
(Δδ)=f(ΔPe0,B)
B=E1*Vg/X1
Pe0=(E1*Vg/X1)*sin(60)
Additional DTD torque (T DTDadd )=Kp*torsion
Kp=control gain constant
(ΔΔPe0)=Speed*(T DTDadd )
(Δδ)=Sin −1 (Pe0+Pe0/B)−δ0
(Δδ)˜ΔPe0/B
E1: terminal voltage of machine operating as a VSM
δ: Power angle
X1: effective impedance of machine (stator leakage+virtual impedance)
Pe0: steady state power of machine
Vg: grid voltage at steady state power.
11. An inverter-based renewable energy source connected to a power grid, the inverter-based resource comprising:
an asynchronous machine;
a controller for controlling the inverter-based resource to provide grid-forming mode (GFM) control thereof, the controller comprising a processor configured to perform a plurality of operations, the plurality of operations comprising:
deriving a power error signal (P ER ) between an actual real power output (P B ) from the renewable energy source and a power reference (P REF ) representing a desired power output of the renewable energy source;
with an inertial power regulator, generating a phase shift angle (δ IT ) from the power error signal (P ER ) to provide virtual synchronous machine (VSM) control functionality to the GFM control of the renewable energy source; and
applying a power angle compensation (Δδ) to the phase shift angle (δ IT ) from the inertial power regulator to dampen power oscillations and load fluctuations during transient power events.
12. The inverter-based renewable energy source according to claim 11 , wherein the inverter-based renewable energy source is a wind turbine power system and the asynchronous machine comprises a doubly-fed induction generator (DFIG).
13. The inverter-based renewable energy source according to claim 12 , wherein the plurality of operations further comprises activating the power angle compensation after an LVRT event and slider gain is below a threshold value.
14. The inverter-based renewable energy source according to claim 12 , wherein the phase shift signal (δ IT ) used to adjust a value of a phase angle (θ 1 ) provided to gating logic circuitry, the power angle compensation (Δδ) used to adjust the phase shift signal (δ IT ).
15. The inverter-based renewable energy source according to claim 14 , wherein the power angle compensation (Δδ) is derived as a function of the power error signal (P ER ) input to the inertial power regulator according to:
Power angle compensation (Δδ)=f(ΔPe0, B)
B=E1*Vg/X1
Pe0=(E1*Vg/X1)*sin(δ0)
(ΔPe0)=Kp*(Pref−Pfbk)
(Δδ)=Sin −1 (Pe0+Pe0/B)−δ0
(Δδ)˜ΔPe0/B
E1: terminal voltage of machine operating as a VSM
δ: Power angle
X1: effective impedance of machine (stator leakage+virtual impedance)
Pe0: steady state power of machine
Vg: grid voltage at steady state power.
16. The inverter-based renewable energy source according to claim 12 , the plurality of operations includes:
subsequently operating the DFIG in torque control to reduce drivetrain loads wherein a torque demand (T DEM ) is generated and modified by a damping torque demand (T DTD ) to provide a generator torque demand (T GEN ) made on the DFIG, the power angle compensation (Δδ) functioning as an additional damping torque demand (DTD);
deriving the power angle compensation (Δδ) as a function of torsion induced in drivetrain components of the wind turbine power system according to:
(Δδ)=f(ΔPe0, B)
B=E1*Vg/X1
Pe0=(E1*Vg/X1)*sin(δ0)
Additional DTD torque (T DTDadd )=Kp*torsion
Kp=control gain constant
(ΔPe0)=Speed*(T DTDadd )
(Δδ)=Sin −1 (Pe0+Pe0/B)−δ0
(Δδ)˜ΔPe0/B
E1: terminal voltage of DFIG machine operating as a VSM
δ: Power angle
X1: effective impedance of machine (stator leakage+virtual impedance)
Pe0: steady state power of machine
Vg: grid voltage at steady state power.
17. The inverter-based renewable energy source according to claim 16 , wherein the torsion is calculated based on a speed difference between a high speed shaft (HSS) and a low speed shaft (LSS) in the drivetrain components, or is estimated based on the HSS speed and torque acting on the HSS.