Grid compliance for the land based electrical power generators

• Main Author: Yang, Shicong
• Other Authors: Xu Yan
• Format: Theses and Dissertations
• Language: English
• Published: Nanyang Technological University 2019
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/106105; http://hdl.handle.net/10220/47907
• Institution: Nanyang Technological University

The fault ride-through (FRT) or low voltage ride-through (LVRT) is one of the critical requirements in the grid codes or distribution codes. The codes are normally defined by the grid operators or the local authority, and generators connecting to a grid are required to comply with all the relevant requirements stated in the code. The LVRT requirement was initially established and applied to wind power generators connecting to power grids. But it is now applied to all generators in many recent revised codes regardless of the technologies used. Complying with the LVRT requirement is very challenging for distributed synchronous generators due to the rotor angle stability during fault transients. Many ways and means for LVRT capability enhancement have been researched for wind generators, but only limited researches have been conducted for LVRT capability enhancement for distributed synchronous generators. This work is focused on developing topologies and control strategies for LVRT capability enhancements for engine synchronous generators, an example of distribution synchronous generators (DSG). A topology with the switched-in series-controlled dynamic braking resistors (SSCDBR) is proposed for the LVRT capability enhancement. The SSC-DBR is connected in series with the neutral ends of the star-configured windings of the unit transformer. In steady state, the DBR is bypassed by a mechanical circuit breaker and no power loss is expected. When a severe grid fault is detected, the DBR is switched-in to consume the excessive energy from the generators. Along with the proposed configuration of the SSC-DBR, a control strategy is developed to make the topology effective. The controller is designed so that the SSC-DBR is activated only when the fault voltage is below the predetermined value and the fault duration is longer than the preset time for two-phase and three-phase faults. The proposed SSC-DBR configuration largely reduces the fault current flowing through the power semiconductor switches resulting a reduction on the rating and cost and an enhancement on the reliability of the approach. The proposed control strategy reduces the unnecessary activation of the DBR thus further increases the life-time and reliability of the proposed solution. Furthermore, the inherent dynamic performer of the synchronous generator is maintained for none critical grid faults. An alternative configuration of the SSC-DBR with the delta-configured winding of the unit transformer is proposed as well. Simulation studies are carried out to evaluate the performances of the proposed approach with various scenarios. The simulation results show that the proposed method is able to enhance the LVRT capability of the modeled engine generator to comply one of the most stringent LVRT capability requirement. Finally the effectiveness and performance are validated by a down-scaled prototype in the laboratory. In Addition, two approaches for rotor angle estimation and measurement are proposed in this work. Rotor angle measurement and out-of-synchronism (OOS) protection are normally not required and equipped for DSG. Under a grid disturbance, when the LVRT capability requirement is implemented, an OOS may occurs in the event of LVRT solution failure. Thus it is a need to protect and trip DSG when the DSG is in OOS condition. Other than the conventional OOS protection relay, the estimated angle between the rotor and the grid voltage can also be used to determine the OOS condition. A holistic approach, which compute the rotor angle in steady state and transient differently, is proposed. In steady state, the rotor angle is computer using the static machine parameters and the measured quantities. While the change of the rotor angle and the pre-fault steady state rotor angle are combined to obtained the rotor angle in a fault transient. Simulation study is conducted and the results show that the proposed method is able to the estimate the rotor angle in steady state and transient periods. Another proposed method is using a permanent magnet synchronous signaling generator (PMSSG) for rotor angle estimation and measurement. A PMSSG is attached to the shaft of the rotor and a voltage signal that reflects the internal rotor position is produced at the terminal of the PMSSG. The rotor angle of the power generator is then estimated based on voltage at the PCC and from the PMSSG terminal. A digital signal processor is used for computing the rotor angle in transient conditions.