Tuning of Multi-Band Power System Stabilizers in Multi-Machine Power Systems

Abstract

Intermittent nature of renewables acts as a frequent trigger for small signal oscillations in power grid. These oscillatory modes correspond to either the rotor modes associated with the synchronous machines of conventional generators or control interactions due to the power electronic interfaces in renewable sources. Frequent tuning of power system stabilizers (PSS) of synchronous machines becomes inevitable to maintain small signal stability of the grid over a wide range of operating and system conditions. Multi-band power system stabilizers (MB-PSS or IEEE-PSS4B) play a very important role in such scenarios as they provide separate compensators for different frequency bands covering a wide frequency range. However, tuning the compensators of MB-PSS becomes very challenging due its complex structure.

The MB-PSS uses three separate compensator blocks for low frequencies (0.01Hz to 0.1 Hz), intermediate frequencies (0.1Hz to 0.8 Hz) and high frequencies (0.8Hz to 4 Hz). Electrical power and speed are used as inputs to the MB-PSS. In this thesis each band of an MB-PSS is viewed as a conventional speed input PSS. A systematic approach for tuning MB-PSS is proposed in this thesis based on conventional tuning approach using phase compensation of GEP(s) transfer function. The gain of each band is selected to compensate the GEP(s) in the respective frequency band so that pure damping torque is achieved. A new gain, common for all bands, is introduced for achieving desired damping of the rotor modes. The effectiveness of the tuning PSS under weak and strong system conditions is evaluated using a single machine infinite bus test system (SMIB). It is found that phase compensation under strong system and gain selection to produce 10% to 15% damping under weak system conditions is found to provide better damping performance over a wide range of operating and system conditions. The tuning methodology is extended to multi machine power systems by representing each machine as an equivalent single machine infinite bus. Widely used test systems 4 Generator 9 bus system, 5 generator 10 bus system and 10 generator 39 bus systems are used to evaluate the proposed tuning approach.

This thesis also focuses on the development of a generalized FPGA platform for implementing different IEEE PSS types defined in IEEE STD 421.5. A low-cost open source credit card sized supercomputer called Parallella has been used in the development of PSS. It contains a dual-core ARM-A9 + FPGA Zynq SoC and a 16-core Epiphany co-processor. PSS types which use speed and electric power as input are considered for realization in FPGA as they cover majority of the PSS types. Implementation of PSS is broken down to smaller independent structural blocks which are common across different PSS types. Development of the structural blocks and a process controller, which assembles a desired PSS type functionality, is described. A separate module is built to interface the PSS-FPGA controller and any external micro-controller of an excitation system. All the developments are optimized to ensure minimum resource utilization. The developed framework in FPGA is utilized to implement a speed PSS for a laboratory synchronous machine with a buck converter excitation system.