Investigation of the FRT capability of a hydro generator based on the TOR directive

Motivation
Due to the new TOR-Erzeuger guideline, which are based on the EU regulation "Requirements for Generators", short "RfG", new and major revised power generation plants are obliged to provide a proof of conformity. An important requirement is the Fault-Ride-Through capability, which considers the transient stability during voltage dips.

Content of the investigation
Due to the currently non-existing test or simulation rules, various simulation methods were analysed, which can generate a voltage dip. Subsequently, relevant variables were analysed by varying certain parameters within the simulation programme DIgSILENT - PowerFactory.

The following simulation methods were investigated:
  •  Method with fault impedance (purely resistive, purely inductive, mix)
  •  Method with grid-side voltage source and impedance (equivalent circuit of the grid)

Figure 1 shows an exemplary curve of the rotor angle during a voltage dip to 30 % of the pre-fault value for a fault duration of 150 ms. Of great importance for the fulfilment of the FRT capability is the dynamic stability or the course of the rotor angle. If the rotor angle reaches a limit value, no stable operating point can be reached after the fault clearing and the power generation system gets out of step.

Figure 1: Exemplary curve of the rotor angle during a voltage dip to 30 % of the pre-fault voltage for a fault duration of 150 ms

The operating point of the synchronous generator before the fault occurred proved to be an important influencing parameter. Results showed that the underexcited operating mode (drawing reactive power from the grid) represents the critical range. The "most critical" operating point proves to be at maximum power and underexcited operation.

Figure 2 shows the capability diagram of a salient-pole synchronous generator. The coloured scaling represents the critical fault clearing time. Additionally, isolines with constant critical fault clearing time values are deposited.

Figure 2: PQ diagram with voltage-dependent limitation and critical fault clearing time values

In addition, a study was done in cooperation with Energie AG Oberösterreich using a real run-of-river power plant and investigating the FRT capability of the synchronous power generation system.

Publications
Brankovic, Darko
Untersuchung der FRT-Fähigkeit eines Hydrogenerators aufgrund der TOR-Richtlinie
Master thesis, Graz, Austria, 2021