Pag-aaral sa Frequency Converter para sa Variable Speed Constant Frequency Wind Turbines

1 Pagpakilala​
Ang enerhiya ng hangin ay isang renewable na pinagkukunan ng enerhiya na may malaking potensyal sa pag-unlad. Sa mga nakaraang taon, ang teknolohiya ng enerhiya ng hangin ay nakuha ang malawakang interes mula sa mga siyentipiko sa buong mundo. Bilang isang pangunahing direksyon para sa pag-unlad ng enerhiya ng hangin, ang teknolohiya ng variable-speed constant-frequency (VSCF) ay gumagamit ng doubly-fed wind power system bilang isang optimisadong solusyon. Sa sistema na ito, ang stator windings ng generator ay direktang konektado sa grid, habang ang kontrol ng VSCF ay natutugunan sa pamamagitan ng pag-ayos ng frequency, amplitude, phase, at phase sequence ng rotor winding power supply. Dahil ang converter lamang ang nagpapadala ng slip power, maaring mabawasan nang malaki ang kapasidad nito.

Kasalukuyan, ang mga doubly-fed wind power systems ay karaniwang gumagamit ng AC/AC o AC/DC/AC converters. Ang mga AC/AC converters ay halos napalitan ng voltage-source AC/DC/AC converters dahil sa kanilang mataas na output harmonics, mababang input power factor, at maraming power devices. Bagama't ang matrix converters ay sinubukan para sa mga doubly-fed systems, ang kanilang komplikadong estruktura, mataas na voltage endurance requirements, at hindi decoupled input/output control ay limita ang kanilang paggamit sa aplikasyon ng enerhiya ng hangin.

Ang pag-aaral na ito ay nagbuo ng voltage-source AC/DC/AC doubly-fed wind power system na kontrolado ng dual DSPs. Ang grid-side converter ay gumagamit ng voltage-oriented vector control, at ang rotor-side converter ay gumagamit ng stator-flux-oriented vector control. Ang mga eksperimento ay nagpapatunay na ang sistema ay sumusuporta sa bidirectional power flow, independent input/output power factor regulation, mababang harmonic distortion, stable wide-range operation, at high-quality power generation mula sa unstable energy sources tulad ng hangin.

​2 Konfigurasyon ng Sistema​
Tulad ng ipinapakita sa Figure 1, ang sistema ay binubuo ng limang bahagi:

• Doubly-fed generator (wound-rotor induction generator)
• Voltage-source AC/DC/AC bidirectional PWM converter (back-to-back three-phase rectifier/inverter gamit ang IPM modules)
• Dual-DSP controller (fixed-point DSP TMS320LF2407A + floating-point DSP TMS320VC33)
• Grid-connection protection device (rotor/stator contactors)
• Virtual variable-speed wind turbine (DC motor + SIEMENS SIVOREG thyristor speed control system)

​Mga Pundamental na Detalye​

• Converter connection: Grid-side via three-phase inductors; rotor-side via slip rings/brushes to generator rotor windings.
• Dual-DSP roles: LF2407A handles data exchange, PWM generation, and grid signals; VC33 executes core algorithms; dual-port RAM enables real-time data sharing; CPLD processes address decoding.
• Grid protection: Upon faults, disconnect stator contactor and block PWM first; delay before opening rotor contactor.

​3 Vector Control para sa Doubly-Fed Generator​
​3.1 Mga Prinsipyong Kontrol​
Sa synchronous rotating frame (d-axis aligned with stator flux), ang modelo ng doubly-fed generator ay:
usd=Rsisd+dψsddt−ωsψsq{u_{sd} = R_s i_{sd} + \frac{d\psi_{sd}}{dt} - \omega_s \psi_{sq}}usd​=Rs​isd​+dtdψsd​​−ωs​ψsq​
usq=Rsisq+dψsqdt+ωsψsd{u_{sq} = R_s i_{sq} + \frac{d\psi_{sq}}{dt} + \omega_s \psi_{sd}}usq​=Rs​isq​+dtdψsq​​+ωs​ψsd​
urd=Rrird+dψrddt−ωslipψrq{u_{rd} = R_r i_{rd} + \frac{d\psi_{rd}}{dt} - \omega_{\text{slip}} \psi_{rq}}urd​=Rr​ird​+dtdψrd​​−ωslip​ψrq​
urq=Rrirq+dψrqdt+ωslipψrd{u_{rq} = R_r i_{rq} + \frac{d\psi_{rq}}{dt} + \omega_{\text{slip}} \psi_{rd}}urq​=Rr​irq​+dtdψrq​​+ωslip​ψrd​

​Flux equations:​​
ψsd=Lmims+Lsisd=Lmims{\psi_{sd} = L_m i_{ms} + L_s i_{sd} = L_m i_{ms}}ψsd​=Lm​ims​+Ls​isd​=Lm​ims​
ψsq=−Lmirq{\psi_{sq} = -L_m i_{rq}}ψsq​=−Lm​irq​
ψrd=Lrird+Lmisd{\psi_{rd} = L_r i_{rd} + L_m i_{sd}}ψrd​=Lr​ird​+Lm​isd​
ψrq=Lrirq+Lmisq{\psi_{rq} = L_r i_{rq} + L_m i_{sq}}ψrq​=Lr​irq​+Lm​isq​

​Torque equation:​​
Te=−npLmimsirqLs{T_e = -\frac{n_p L_m i_{ms} i_{rq}}{L_s}}Te​=−Ls​np​Lm​ims​irq​​

Neglecting stator resistance voltage drop, stator flux satisfies:
ψsd≈usq/ωs,ψsq≈0{\psi_{sd} \approx u_{sq}/\omega_s, \quad \psi_{sq} \approx 0}ψsd​≈usq​/ωs​,ψsq​≈0

​Control strategy:​​

• Constant stator generalized excitation current imsi_{ms}ims​ → Electromagnetic torque Te∝irqT_e \propto i_{rq}Te​∝irq​
• For unity power factor, excitation current fully supplied by rotor (ims=irdi_{ms} = i_{rd}ims​=ird​)
• After feedforward decoupling compensation, regulate urdu_{rd}urd​ and urqu_{rq}urq​ to control rotor flux and torque, respectively.

​3.2 Grid Control​

• Soft Grid-Connection:
1. When wind speed reaches cut-in value, turbine drives generator to minimum speed.
2. Activate converter to match stator voltage to grid (amplitude, phase, frequency).
3. Automatic synchronization upon meeting grid-connection conditions.
• Disconnection: Gradually unload to no-load state before disconnecting. Must operate within permitted speed range.

​4 Grid-Side Rectifier Vector Control​
In the two-phase synchronous rotating frame (d-axis aligned with phase-A voltage), the PWM rectifier model is:
ud=Ldiddt+Rid−ωsLiq+sdudc{u_d = L\frac{di_d}{dt} + R i_d - \omega_s L i_q + s_d u_{dc}}ud​=Ldtdid​​+Rid​−ωs​Liq​+sd​udc​
uq=Ldiqdt+Riq+ωsLid+squdc{u_q = L\frac{di_q}{dt} + R i_q + \omega_s L i_d + s_q u_{dc}}uq​=Ldtdiq​​+Riq​+ωs​Lid​+sq​udc​
Cdudcdt=32(sdid+sqiq)−iload{C\frac{du_{dc}}{dt} = \frac{3}{2}(s_d i_d + s_q i_q) - i_{\text{load}}}Cdtdudc​​=23​(sd​id​+sq​iq​)−iload​

​Power equations:​​
P=udid,Q=udiq{P = u_d i_d, \quad Q = u_d i_q}P=ud​id​,Q=ud​iq​

​Control logic:​​

• Constant grid voltage → Regulate idi_did​ to control active power; iqi_qiq​ for reactive power.
• Control equations with voltage compensation:
  ud∗=(R+Lddt)id−ωsLiq+ud{u_d^* = (R + L\frac{d}{dt})i_d - \omega_s L i_q + u_d}ud∗​=(R+Ldtd​)id​−ωs​Liq​+ud​
  uq∗=(R+Lddt)iq+ωsLid{u_q^* = (R + L\frac{d}{dt})i_q + \omega_s L i_d}uq∗​=(R+Ldtd​)iq​+ωs​Lid​

​5 Experimental Results​
​Key Verifications:​​

• Reliable soft grid-connection across a wide speed range;
• Independent power factor regulation (stator/grid side both reach unity);
• Bidirectional power flow capability of AC/DC/AC converter meets generation demands.

​6 Conclusion​
This study develops a dual-DSP-based voltage-source AC/DC/AC doubly-fed wind power system. Combined with grid-side voltage-oriented and rotor-side stator-flux-oriented vector control, experiments demonstrate:

1. The system achieves bidirectional power flow and independent input/output power factor regulation;
2. Low harmonics and high power factor ensure power quality;
3. Soft grid-connection/disconnection reduces mechanical/electrical stress;
4. Applicability to megawatt-class large-scale wind power installations.