43 CIGRE Session - CIGRE 2010, Paris (France). 22-27 August 2010
Summary:
Ten years ago, wind generators used to disconnect from the grid in case of a voltage dip. Undervoltage
protection systems used to trip wind generators not only to protect both electric machines and power
electronic converters but also to prevent their islanded operation. In the mean-time, the massive
development of wind power generation has required the increase of the voltage ride through capability
of wind generators to prevent that a fault in the network could cause the collapse of the system. The
requirement of voltage ride through capability of wind generators has been incorporated into many
grid codes. The main requirement has been established as a time-voltage curve that must be sustained
by wind generators.
Moreover, the voltage ride through capability requirement has been extended to synchronous
generators in a number of grid codes. Grid codes have added new requirements with respect to what
IEC and IEEE standards had already established. Grid codes require that synchronous generators
should remain connected to the grid in case of transient variations of voltage at the generator step-up
transformer HV terminals defined by a time-voltage curve.
This paper has reviewed the voltage ride through capability required by a number of grid codes. The
comprehensive review has shown that grid code requirements are not uniform. The paper has also
studied the sensitivity of synchronous generator response to the English grid code requirements with
respect to a number of parameters such as the step-up transformer reactance, the turbine-generator
rotor inertia, the synchronous machine short circuit ratio and excitation ceiling factor. The study has
been conducted on a generator taken from the technical literature and which data are provided. The
sensitivity study has shown that both a lower value of the step-up transformer reactance (0.12 pu) and
a higher value of synchronous generator SCR (0.61 pu) would ensure that the Engish grid code
requirements in case of solid and non-solid faults are fulfilled. It should be pointed out however that
such low transformer reactance or high SCR may not be possible due to other design and economic
aspects of the power plant.
This paper has outlined a solution for generating units equipped with bus fed static excitation systems.
The approach consists in connecting a supplementary dc voltage source in the generator excitation
winding when a fault occurs. The paper has shown that such an approach allows compliance with the
English grid code requirements.
Keywords: Synchronous generators, voltage ride through capability, grid codes, excitation control.
Publication date: 2010-08-22.
Citation:
L. Rouco, C. Ginet, K. Chan, K. Major, O. Malcher, L. Díez Maroto, R. Cherkaoui, Improvement of the voltage ride through capability of synchronous generators by excitation control, 43 CIGRE Session - CIGRE 2010, Paris (France). 22-27 August 2010.
