How Full-Converter Wind Turbine Generators Satisfy Interconnection Requirements

Transcription

1 How Full-Converter Wind Turbine Generators Satisfy Interconnection Requirements Robert Nelson Senior Expert Engineering Manager and Manager of Codes, Standards, and Regulations Siemens Wind Turbines - Americas Copyright Siemens Energy, Inc All rights reserved.

2 How does the Full-Converter (FC) system work? Full AC-AC converter Rotor AC/DC (Generator Side Converter) DC/AC (Line Side Converter) Circuit breaker Step-up transformer Generator ~ = = ~ Gearbox (not in DD) DC BUS 50 or 60 Hz Collector system (>30kV typ) Rotor drives gearbox in geared systems increases generator shaft speed Gearbox eliminated in DD (direct drive); rotor directly drives low-speed, multi-pole generator Generator converts mechanical power to AC electric power. Generator can be asynchronous, permanent magnet or synchronous for geared system, pm or synchronous for DD. Generator-side converter converts AC electric power to DC Line-side converter converts DC to system-frequency AC (50 Hz or 60 Hz, as appropriate) and provides voltage regulation capability Converter decouples machine from grid, so no winding time constants quick response Page 2 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

3 What are the advantages of the Full Converter system? Full AC-AC converter Rotor AC/DC (Generator Side Converter) DC/AC (Line Side Converter) Circuit breaker Step-up transformer Generator ~ = = ~ Gearbox (not in DD) DC BUS 50 or 60 Hz Collector system (>30 kv typ) Variable Speed: During abnormal conditions, can increase or decrease shaft speed/kinetic energy to satisfy system needs Optimal energy extraction by optimizing tip speed ratio Increase shaft speed during low-voltage ride-through extra kinetic energy stored in shaft when Pgen 0. Shaft can absorb energy from gusts without changing output Full Converter: Maximum flexibility and fast response; decouples machine: Rapid response short time delays compared to directly connected magnetic machines, with winding time constants Full control of short circuit current from >100% of nominal output current to zero (standby); useful for voltage regulation during low-voltage ride-through and response to faults Precise control of output and rate of change of output as required (subject to availability of wind power) Turbine can be used for frequency response (for regulation down) or, with standby reserve, for spinning reserve/regulation up Machine decoupled from power system no SSTI, negative sequence heating concerns, minimal short circuit torques. Page 3 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

4 Modeling Full Converter Machines Full AC-AC converter Rotor AC/DC (Generator Side Converter) DC/AC (Line Side Converter) Circuit breaker Step-up transformer Generator ~ = = ~ Gearbox (not in DD) DC BUS 50 or 60 Hz Collector system (>30 kv typ) Load Flow Approximated by synchronous machine; reactive capability curve. Short circuit Produces short circuit current per algorithm (e.g., 2% reactive current for every 1% reduction in voltage below nominal when voltage drops below 90%, up to 110% of rated current; positive sequence currents only.). Dynamics Can be approximated quite accurately by line-side converter model (basis of most generic models); consideration of second order effects, like rotor inertia, dc bus voltage, etc., included in proprietary models. Transient Full model of line-side converter; detailed model of upstream components. Page 4 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

5 What Are Common Reactive Control Requirements in the Americas (including Latin America)? Existing: Voltage Regulation Transmission Voltage Medium Voltage Reactive Power Control Transmission Voltage Medium Voltage Power Factor Control Transmission Voltage Medium Voltage Reactive Control without Active Power Production Voltage Regulation Reactive Power Control Transmission Voltage Medium Voltage Page 5 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

6 Reactive Power and Voltage Control SCADA system Wind Turbine WF Voltage set- point + Voltage error VR regulator Voltage set-point Checking Wind Farm limitations Voltage set-point Wind Turbine Control system WT Q set-point PCC measured values Voltage & Reactive Power Wind Turbine limitations PQTV Distribution of voltage set-points Wind turbine limitations secured by the embedded WT control system PQTV (Power, Reactive Power, Temperature, Voltage) Can operate in voltage regulation, reactive power control (constant Q, or power factor control (constant ratio of P to Q) at high side or low side of Park Transformer Page 6 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

7 Voltage Control Reactive Droop User Interface Droop of 4 % Recommended Droop of 2% to 7% New feature: Voltage Deadband Page 7 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

8 Reactive capability characteristic of full-converter wind turbine Voltage-limited; linear with shallow slope Voltage + current-limited; linear with steep slope Current-limited; arcs Page 8 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

9 How does a full-converter wind turbine compare with a synchronous generator for voltage control? Representative parameters: Synchronous generator (gas or steam turbine) Full-converter wind turbine Terminal voltage range 0.95 pu 1.05 pu 0.90 pu 1.10 pu Max lag/lead rated P 0.5 pu / pu 0.5 pu / -0.5 pu Reactive capability varies with Vt? No Yes Impedance between gen and trans sys 12% 16% 35% 45%* Synchronous condenser mode (continuous Difficult, Yes, voltage available? expensive control option Typical max lag/lead P=0 0.7 pu / -0.5 pu 1.0 pu / -1.0 pu * includes converter reactor Page 9 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

10 How does a full-converter wind park compare with a synchronous generator for steady-state voltage control? Compare reactive capability of 100 MW (rated pf of 0.9 lag to 0.95 lead) synch generator with Xt = 13% with 100 MW wind park with equivalent reactance (inc. turbine transformer, park transformer and collector system) of 22% and RC curve developed in example. Assume both are connected to a 230kV transmission system and the collector system is 34.5kV; ignore resistance and collector charging. 1) Determine how much lagging reactive power can be delivered to transmission system, varying Vsys from 0.85 to 1.0 pu, with P=1.0 pu 2) Determine how much reactive power can be absorbed from the transmission system, varying Vsys from 1.0 to 1.15 pu, with P=1.0 pu Xs P E Gen step-up transformer Q Vsys Page 10 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

11 MVAr How does a full-converter park compare with a synchronous generator for voltage control? Solution 1): WTs terminal voltage drops below 0.90 pu limit; go into low-voltage ride-through. 40 Sync Gen term voltage drops below 0.95 pu limit Continuous Lagging Reactive Capability, Full Real Power Output Synch Gen Full-converter WP Note: neglects WP charging Vsys, pu Similar max capabilities, but The synchronous generator has a narrower operating voltage range and significantly greater capability near nominal voltage The WP has a wider control voltage range and is superior for very low transmission voltages (significantly below 0.90 pu, where the synchronous generator cannot provide voltage support ) Page 11 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

12 MVAr How does a full-converter park compare with a synchronous generator for voltage control? Solution 2): Continuous Leading Reactive Capability, Full Real Power Output Synch Gen Full-converter WP Synch gen terminal voltage goes above 1.05 pu limit Vsys, pu Note: neglects WP charging. Similar capabilities near rated voltage, but The synchronous generator has a narrower operating voltage range and less capability above 100% system voltage. The WP has a wider control voltage range and is superior for very high transmission voltages (above 1.0 pu, and particularly above 1.1 pu, where the synchronous generator is incapable of providing voltage support). Page 12 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

13 MVAr How does a full-converter park compare with a synchronous generator for voltage control? Solution 1a): Continuous Lagging Reactive Capability, Full Real Power Output Voltage Support at 0.8 pu system voltage Full-converter WP Full-converter WP Vsys, pu Include collector system charging (2 MVA at nominal voltage) and adjust fixed transformer taps to accommodate lower voltage system operation (0.975 taps on turbine and park transformers). Allows reactive support for system to voltages to below 0.8 pu. Page 13 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

14 Voltage [pu] Reactive Power [pu] Voltage control: Fast response to change in reference Park controllers update turbines 6 times/s; turbines respond within 0.1s Wind Farm Voltage Reference Test :50:00 10:50:01 10:50:02 10:50:03 10:50:04 10:50:05 10:50:06 10:50:07 10:50:08 10:50:09 10:50: Time [h] Voltage VoltageReference ReactivePow er Page 14 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

15 Voltage control test (V in blue, Q in red) Page 15 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

16 New option: Voltage Regulation without Active Power Production ( STATCOM Mode ) V-mode Option: Voltage Regulation (reactive droop), or Reactive Power Control (constant MVAr) Converters operate in STATCOM mode to regulate voltage or reactive power under control of Park Pilot Appropriate for: Sites that receive compensation for reactive control or where voltage regulation is required at all times. Sites where reactive control is required for very low output levels Page 16 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

17 Summary How Siemens WTGs provide Reactive Power Control Capability Voltage Regulation with reactive droop Medium Voltage Transmission Voltage Reactive Power Control Power Factor Control Voltage Reg without Active Power Production Now Soon Page 17 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

18 What Are Common Active Power Control Requirements in the Americas (including Latin America)? Existing: Power Output (Curtailment) Control Ramp Rate Control Curtailments Start-up Regulation Up for Underfrequency Adjustable Droop Regulation Down for Overfrequency Adjustable Droop High Wind Shutdown Rate Variation Control Page 18 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

19 Frequency, Hz Output, % Frequency Droop Control - Primary Frequency Response Some ISO/RTOs require the use of frequency droop response from wind parks 61.4 Frequency Droop Response Normally constant (5%) frequency droop (5% change in freq 100% change in output), but variable droop sometimes required (e.g., larger droop for small frequency excursions, smaller droop for larger excursions). Both reg up (underfreq), assuming curtailed state, and reg down (overfrequency) required Freq, Hz Output, % Sometimes conflicts w/ curtailments, Special Protection System operations t, sec Page 19 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

20 Park MW, pu Frequency, Hz Frequency response Simulations calibrated with test in Electric Reliability Council of Texas (ERCOT), USA ERCOT Frequency Response Test representative; simulations calibrated with actual tests, 5% droop, f = 0.2 Hz step P = - f x 10 / (60 x % droop), pu = -/ pu t, sec Park Output, pu Frequency, Hz Page 20 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

21 Transient underfrequency response ( inertial response ) Preliminary simulations for 20% wind case with different transient controls Rapid response required after sudden frequency drop necessary to forestall load shedding, especially on island systems Siemens is developing new controls to address this need Page 21 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

22 V at POI, pu Low-voltage ride-through (LVRT) Chilean requirements similar to those in Brazil. Siemens wind turbine generators conform, given reasonable fault clearing times (<0.5 sec) and Short Circuit Ratio levels of 5 or higher (Short Circuit Ratio = Short Circuit POI / Turbine Aggregate MW) May require adjustment of transformer taps and/or supplemental reactive resources to accommodate continuous system voltage of 0.8 pu in Chile Brazil and Chile - Low Voltage Ride- Through Required (assumes 480 ms clearing time in Chile) time, sec Brazil Chile Page 22 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

23 Voltage [pu] Modeling for power system analysis Dynamics models available in major simulation platforms Generic (library) simple models for interconnection studies, contingency assessments, etc. (PSS/E, PSLF, ANATEM) User-defined more detailed models for optimization, in-house studies, etc. (PSS/E, DigSilent) Transient models PSS/E LVRT Benchmark Simulation LV WTG Voltage PSCAD for protection coordination, insulation coordination, subsynchronous resonance assessments, special protection schemes, etc. now available Time [s] WT4 U LV RMS PSSE 60 Hz [pu] U LV RMS PSSE 60 Hz [pu] Page 23 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

24 Summary How Full Converter WTGs provide Power Control (existing and anticipated) Capability Power Output (Curtailment) Control Ramp-Rate Control (ref change and startup) Frequency Droop Regulation Up Frequency Droop Regulation Down Frequency-Dependent Droop Spinning reserve ( delta control ) capability Transient underfrequency ( inertial ) response High wind shutdown AGC Response (from Park RTU) Rate Variation Control Now Soon Page 24 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

25 Future Developments Weak grid controls for sustained stable operation in systems with SCR <2.5 (SCR = 3-phase short circuit MVA at regulation point / aggregate turbine MW) Power oscillation damping for inter-area modes. High-Voltage Ride-Through ENDLESS POSSIBILITIES BECAUSE OF FLEXIBLE POWER ELECTRONIC DESIGN YOUR SUGGESTIONS MOST WELCOME!! Page 25 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.

26 Thank you for your time Questions? Page 26 Oct, 2012 Copyright Siemens Energy, Inc All rights reserved.