Generators used in the electric power industry are fundamentally designed to feed linear loads. However, when the type of load is predominantly nonlinear, generation systems must comply with certain requirements that allow them to operate in stable conditions and without being exposed to excessive heating and torsional torque vibrations, which can make them exceed their permissible operating limits. Essentially, a nonlinear load produces a voltage waveform distortion at the generator terminals; this imposes the following consequences in the operation of a generator:
- Production of positive and negative sequence current contributions that generate torsional torques and vibration mode shapes on the motor axis. The thermodynamic forces created in the rotor can prematurely wear out shaft bearings.
- Voltage waveform distortion on the supply circuit to the excitation system; this can produce voltage regulation problems.
- Excessive negative sequence currents; these can contribute to increased voltage unbalance.
As a reference, the following list describes some of the characteristics of synchronous generators found to perform adequately in land and offshore oil well ESP applications. This relates to cases where VFDs make the largest portion of the load and where isolated DGs source all or most of the demanded power:
Independent static excitation system is present.
Rotor is furnished with ammortisseur (damper) copper bars.
Transient reactance, X'd, is between 16 and 18%.
Direct axis subtransient reactance X"d, is between 13 and 15%. Notice that these values are considerably lower than typical synchronous generators for linear load applications, which are designed with subtransient impedances around 25%.
Insulation (rotor and stator) is according to American Petroleum Standard-546.
Power factor 0.85 is lagging. Here it is important to stress that industrial systems that involve large VFDs or significant numbers of small units may run at power factors close to unitity and in some cases even on leading power factors.
Operation with nonlinear loads is satisfactory. This means that a generator must withstand a maximum THD of 8% across its terminals. Notice that this THDV is in excess of the 5% recommended by IEEE for general distribution systems, 120-69000 V.
An open-circuit voltage shows waveform distortion inferior to 2%.Generators are provided with an oversized damper winding consisting of copper bars to properly handle the additional heating caused by harmonic currents. Static brushless type excitation systems with (around 10%) oversized rectifying diodes are present. PMG (permanent magnet generator) types of excitation systems are present.
The regulator must be able to handle harmonic distortion typical of 12-pulse types of converters.