Grid connection testing for asynchronous (induction) wind turbine generators is a mandatory conformity assessment that verifies power quality, fault ride-through capability, and control performance before a wind turbine or wind farm is permitted to connect to the utility grid. Conducted by accredited third-party laboratories in accordance with IEC 61400-21-1 and GB/T 19963.1, these tests confirm that squirrel-cage and doubly-fed induction generators satisfy the electrical requirements of modern grid codes. This article explains the applicable standards, test sample requirements, testing items, measurement methods, and practical applications of grid-connection testing for induction-based wind generators.
Technical Overview
An asynchronous wind generator is an induction machine that produces electrical power when its rotor speed exceeds the synchronous speed of the connected grid. Three principal architectures are used in wind energy conversion: the squirrel-cage induction generator (SCIG) coupled directly to the grid through a soft starter, the wound-rotor induction generator with variable rotor resistance for a limited speed range, and the doubly-fed induction generator (DFIG), in which a partial-scale power converter connected to the rotor enables variable-speed operation of approximately plus or minus 30 percent around synchronous speed.
The electrical behavior of induction generators differs fundamentally from that of synchronous machines. They draw magnetizing reactive power from the grid, their torque-speed characteristics produce specific transient responses during voltage dips, and converter-coupled DFIG systems introduce harmonic emissions and crowbar events. Transmission and distribution system operators therefore require documented evidence that asynchronous wind generators will not degrade power quality, will remain connected during grid faults, and will actively support grid voltage and frequency. Grid-connection testing provides this evidence and forms an integral part of wind turbine type certification under the IEC-based certification scheme for renewable energy.
The principal technical framework for grid-connection testing consists of the following standards and grid codes:
- IEC 61400-21-1: Measurement and assessment of electrical characteristics of wind turbines
- IEC 61400-21-3: Measurement and assessment of electrical characteristics of wind power plants
- GB/T 20320: Chinese adoption of IEC 61400-21 covering measurement and assessment of electrical characteristics of wind turbine generator systems
- GB/T 19963.1: Technical rule for connection of wind farms to the power system, defining fault ride-through, reactive power, and frequency support requirements
- IEC 61000-4-7, IEC 61000-4-15, and IEC 61000-4-30: Measurement methods for harmonics, flicker, and general power quality parameters
- IEC 60034-1: Rating and performance of rotating electrical machines, including insulation and dielectric requirements
- UL 1741 and EN 50549: Interconnection requirements applied to converter-based generation units in North American and European distribution networks
Testing Samples
Grid-connection testing is performed on representative samples of the wind turbine electrical system. For type testing, a fully instrumented prototype turbine is installed at a test station with adequate wind statistics and grid characteristics. For routine verification, production units or serialized generator-converter packages are sampled from the manufacturing line. The sample scope must include every subsystem that influences grid interaction, including the generator, power converter, soft starter, reactive compensation equipment, and the turbine control software version that will be deployed commercially.
Squirrel-Cage Induction Generators (SCIG)
The sample comprises the induction generator together with its soft starter and power-factor correction capacitor banks. Because the machine is directly coupled to the grid, the switching devices and compensation stages are integral to the test object and must be operated in their commercial control sequence during measurement.
Doubly-Fed Induction Generators (DFIG)
The sample includes the wound-rotor machine with its slip-ring assembly, the rotor-side and grid-side converters, the crowbar and chopper protection circuits, and the master controller. The exact converter firmware version is recorded, because harmonic emissions, dip response, and grid-support functions are determined by converter control parameters.
Wound-Rotors Induction Generators with Variable Slip Control
The sample covers the generator with its externally controlled rotor resistance circuit. Testing verifies both the steady-state slip operating envelope and the transient behavior when slip control steps occur during switching or grid disturbances.
Sample and Documentation Requirements
- Complete nameplate data, generator drawings, and converter schematic documentation
- Declared electrical ratings, including rated voltage, rated current, rated speed range, and converter apparent power
- Control parameter set and protection relay settings corresponding to the certified configuration
- Installation at a test site with a meteorological mast and a defined short-circuit ratio at the point of common coupling
- Calibration certificates for all measurement transducers within their validity period
Testing Items
The test program is structured to characterize every electrical interaction between the asynchronous generator and the grid, from continuous power quality emissions to dynamic behavior during network disturbances. The principal test items are summarized below.
Power Quality Assessment
- Flicker emission during continuous operation, expressed as flicker coefficients used to calculate Pst and Plt values
- Flicker step factor and voltage change factor during turbine startup, shutdown, and generator switching operations
- Harmonic and interharmonic current emissions up to the 50th order (2 kHz), including supraharmonic components between 2 kHz and 9 kHz for converter-coupled machines
- Rapid voltage changes at the point of common coupling
- Phase current unbalance during normal operation
Fault Ride-Through Capability
- Low-voltage ride-through (LVRT) for symmetrical and asymmetrical voltage dips, typically down to 20 percent residual voltage for 625 milliseconds with recovery to 90 percent of nominal voltage within 2 seconds
- High-voltage ride-through (HVRT) under temporary overvoltage conditions, typically between 110 and 130 percent of nominal voltage
- Dynamic reactive current injection during voltage dips as a function of voltage depression
- Active power behavior during faults and the post-fault recovery ramp rate
Active Power Control and Generation Characteristics
- Maximum measured power expressed as 10-minute and 60-second mean values
- Active power set-point control accuracy and steady-state deviation
- Upward and downward power ramp-rate limitation
- Power performance correlation with wind speed at the test site
Reactive Power and Voltage Control
- Reactive power capability envelope (P-Q chart) across the entire active power range
- Power factor range, typically 0.95 leading to 0.95 lagging at the point of connection
- Reactive power set-point control, voltage control mode, and automatic power factor control mode
- Dynamic response times of the reactive power control loop
Frequency Response and Grid Support
- Continuous and time-limited operating frequency ranges, for example 48 Hz to 50.5 Hz under GB/T 19963.1
- Active power versus frequency droop characteristics and scheduled operating zones
- Rate-of-change-of-frequency (RoCoF) withstand capability
- Synthetic or natural inertial response evaluation where applicable
Protection and Reconnection Behavior
- Verification of over-voltage, under-voltage, over-frequency, and under-frequency protection settings and clearing times
- Reconnection sequences after grid outages, including voltage and frequency check functions
- Limitation of switching operations within defined time windows per the applicable grid code
Testing Methods
On-Site Power Quality Measurement
Power quality testing is conducted as an on-site measurement campaign on the operating turbine. Class A measurement instruments compliant with IEC 61000-4-30 record three-phase voltages and currents, a flickermeter compliant with IEC 61000-4-15 evaluates flicker severity, and harmonic analysis follows the grouping and aggregation rules of IEC 61000-4-7. Calibrated current and voltage transducers with GPS time synchronization ensure traceable, correlated data. A meteorological mast installed according to IEC 61400-12-1 provides the wind data needed to correlate electrical emissions with wind-speed bins. Data are collected continuously across the range from cut-in to rated wind speed and beyond, with statistically valid sample counts per wind-speed bin, until the data completeness requirements of IEC 61400-21-1 are satisfied.
Voltage Dip Testing for LVRT and HVRT
Because natural grid faults cannot be scheduled, a voltage dip generator is connected between the grid and the turbine under test. Depending on the test facility, the dip generator may be impedance-based, transformer-based, or a full power-electronic grid simulator capable of producing precise residual-voltage profiles. Symmetrical three-phase dips and asymmetrical two-phase dips are applied at multiple depth-duration points along the voltage-limit curve of the applicable grid code. High-speed data acquisition records three-phase voltages, currents, active and reactive power, and rotor speed throughout each event, from which fault current contribution, reactive current injection, crowbar activation in DFIG systems, and post-fault recovery behavior are assessed against the acceptance criteria.
Laboratory Characterization of Generator and Converter
Component-level tests supplement the field campaign. Insulation resistance and dielectric withstand tests are performed on the generator in accordance with IEC 60034-1, winding resistance is measured for thermal model verification, and efficiency is determined according to IEC 60034-2-1. Converter subsystems can be evaluated on laboratory test benches using grid emulators to verify harmonic emissions, control response, and grid-support functions under reproducible network conditions, consistent with the component testing framework of IEC 61400-21-2. These laboratory results are integrated with field measurements into the overall conformity assessment.
Data Analysis, Uncertainty Evaluation, and Model Validation
Flicker coefficients and voltage change factors are calculated as dimensionless parameters so that emissions from a single turbine can be scaled to a complete wind farm at any grid connection point with a known short-circuit capacity. Harmonic currents are reported as maximum individual and total distortion values. A full measurement uncertainty budget covering transducers, acquisition chains, and data reduction is documented for every reported quantity. In addition, the recorded transient waveforms from dip tests are used to validate the simulation models that system operators employ in wind farm interconnection and stability studies, making model validation an essential deliverable of the test campaign.
Applications
- Grid interconnection approval: demonstrating compliance of wind turbines and wind farms with utility connection requirements before energization
- Wind turbine type certification: supplying the electrical characteristic test evidence required under the IEC-based certification scheme for renewable energy systems
- Commissioning and acceptance testing: verifying that installed fleets meet contractual and grid-code performance figures
- Design verification and research: benchmarking generator, converter, and control designs for manufacturers during product development
- Retrofit and repowering verification: confirming continued compliance after control software updates, converter replacements, or generator refurbishment
- Project financing and insurance: providing independent technical documentation that supports bankability assessments for wind projects
Because grid codes continue to evolve toward stricter fault ride-through, frequency support, and voltage control obligations, periodic re-evaluation of asynchronous generator systems against updated requirements is increasingly common for both new installations and operating assets.
FAQ
Why do asynchronous wind generators require dedicated grid-connection testing?
Induction generators consume reactive power for magnetization, produce significant transient currents when energized, and generate harmonic emissions when coupled through power converters. Grid-connection testing quantifies these characteristics and verifies that the protective and control functions of the turbine satisfy grid-code requirements for power quality and fault ride-through.
What are the LVRT requirements under GB/T 19963.1?
Wind turbines must remain connected and continue operation when the voltage at the point of common coupling dips to 20 percent of nominal for 625 milliseconds, and must ride through events in which voltage recovers to 90 percent within 2 seconds. Reactive current support proportional to the voltage depression is required during the dip, and active power must be restored to pre-fault levels at a defined ramp rate after fault clearance.
How does testing differ between SCIG and DFIG systems?
For SCIG systems, testing focuses on switching transients, soft-starter performance, flicker emission, and reactive compensation behavior, because the machine is directly coupled to the grid. For DFIG systems, testing additionally covers converter-induced harmonics, crowbar activation during voltage dips, and the variable-speed control functions executed by the rotor-side and grid-side converters.
Where are the tests performed?
Power quality and fault ride-through tests are performed on prototype turbines installed at dedicated test stations with sufficient wind statistics, a meteorological mast, and suitable grid characteristics. Component-level electrical, insulation, and converter bench tests are carried out in accredited third-party laboratories, and the combined results form the complete conformity assessment.
How long does a complete test campaign take?
The power quality measurement campaign typically requires several weeks to several months of on-site data acquisition, because statistically valid data must be collected across the full wind-speed range from cut-in to rated power. Voltage dip tests and control function verification can usually be completed within a few days once the turbine and dip generator are prepared.
What deliverables are issued after successful testing?
Accredited third-party laboratories issue measurement reports containing flicker coefficients, voltage change factors, harmonic current tables, P-Q capability charts, fault ride-through event records, and control performance results, together with the measurement uncertainty budget. These reports support type certification files, grid interconnection applications, and the validated simulation models required by system operators.