Excitation Limiters Design for a Micro-Alternator Buck Converter Based Static Excitation System

This paper presents the design of excitation limiters such as an under-excitation limiter, over-excitation limiter and V/Hz limiter as per IEEE std 421.5 and implemented on a DC-DC buck converter-based static excitation system to protect the generator during abnormal conditions such as overloading, overheating and over-fluxing of the machine. A digital time constant regulator (TCR) is incorporated to modify the field’s time constant of micro-alternator, to mimic the dynamics of large synchronous machines. The detailed tuning procedure of limiters and TCR is discussed to comply with IEEE STD 421.2 and IEEE STD 421.5. The performance of the developed excitation system with limiters is tested under various small and large disturbances.

I. Introduction

It is always challenging to study the dynamic behaviour of complex power systems in a laboratory. The micro-alternator is a three-phase synchronous generator which possesses the same electrical constants (in per unit on machine rating) as those generally found in alternators in modern power stations. It is an electrical scaled-down model of machines up to 1000 MW rating and is rated between 1 to 10 kVA [1], [2]. Early experimental platforms used micro-alternators to emulate the behaviour of steam, and hydro turbine generators [2]. In [3], a5kW micro-machine (doubly fed induction machine) capable of mimicking a 2.5 MW wind turbine is developed. This kind of experimental test-beds using micro-machines are necessary to study the interactions of fast power converter controls in renewable sources and fast excitation systems of conventional generators. However, it is very crucial to enable the micro-machines with standards-based realistic control structures for studying these interactions [2].