Voltage and Frequency Control of Embedded Low-Voltage Islanded Grids with Power-Electronic Generation Units
Creators: name: Dietmannsberger, Markus
Publisher: name: Universitätsbibliothek der HSU / UniBwH
Year: 2017
Subjects: subject: Wechselrichter
subject: Erneuerbare Energien
subject: Anti-Islanding Detection
subject: Low-Voltage Ride-Through
subject: 620 Ingenieurwissenschaften

description: This work provides a new controller design for three-phase inverter-based generation in low-voltage grids with linear and nonlinear loads below the inverter nominal power. Both voltage and frequency control loops are based on the simplification of the inner current control loop irrespective of the used inverter hardware. Despite existing design methods for voltage frequency control loops, the new design only requires the knowledge of the inverter nominal power rating and the bandwidth of the inner current control loop, whereas the latter can also be estimated. The transferability and scalability is demonstrated with an analytical and simulation based control stability analysis for inverters from 1 kW to 100 kW. In simulation and experimental tests, the new controller design is able to maintain nominal values for symmetrical resistive, resistive-inductive and resistive-capacitive loads up to the inverter nominal power and also for nonlinear constant power loads. Experimental tests with an 8 kW inverter using a standard LCL filter without transformer and an intentionally sub-optimally designed LC filter including transformer underline the transferability of the design for different hardware setups. A novel multiple inverter control strategy for cooperative voltage and frequency control is devel-oped which incorporates islanding operation for single and multiple inverter setups without the need for communication. The novel voltage and frequency control is based on a Master/Follower and a dead-band concept. Followers have the full potential of the Master but only use it when necessary. This concept allows to maintain nominal voltage without communication and without circulating steady-state currents among the inverter units. A procedure from zero-voltage condi- tion is developed which allows to use the full potential of controllable power in the whole island to conjointly reach nominal grid voltage. With this, islanding operation for loads that are larger than the largest inverter but still smaller than the sum of all connected inverters is possible. In experimental tests of up to 6 kW nominal power, two inverters were able to reach nominal voltage and frequency within 200 milliseconds for a linear symmetrical load that was larger than each of the inverters. Voltage and frequency were controlled at nominal values during several load changes and even in open circuit condition. Despite existing master-slave approaches, the novel control strategy is able to compensate an outage of the Master unit. Experimental tests in the laboratory demonstrated that subsequently to a Master outage the Follower detects voltage and frequency deviations and drives them back to nominal values. Furthermore, a new countermeasure against an excess of active power due to non-controllable inverters is presented. It can be used for all inverters in low-voltage grids that provide active re-duction due to over-frequency in accordance with the grid codes. This allows to operate islanded grids that have non-controllable inverters which provide almost twice as much active power as needed by the linear resistive loads. The presented functionality is validated in simulation and in experimental tests for a 4 kW system. Intentional islanding operation, low-voltage ride-through capability (LVRT) and anti-islanding detection (AID) have conflicting aims during grid faults. For grid integration purposes, a novel concept was proposed which provides a possible solution for those conflicting aims with the help of a time decoupling. A case study initially shows that the effectiveness of AID is not undermined if LVRT is implemented simultaneously. Subsequently to the fault behaviour, the novel islanding control strategies of this thesis can be initiated. In laboratory tests, the novel concept is demonstrated for an 8 kW inverter setup. Finally, open issues and future hallenges of intentional islanding operation in low-voltage grids are provided.