Performance evaluation of time-delay control schemes for uninterruptible power supplies

Uninterruptible power supplies (UPSs) are commonly used to supply quality power at a well-regulated voltage to all nonlinear and unbalanced loads. To achieve that, a number of control schemes have since been proposed with the traditional double-loop linear control scheme being more popular because of its simple conceptual formulation and physical implementation. Despite of its popularity, the harmonic compensation ability of the traditional double-loop scheme is relatively poor, which although can be improved by using better controller transfer functions, is usually more complex and thus requires a more powerful processor. Avoiding these added complexities, this paper presents and compares a number of time-delay control schemes for UPS control, where the main building blocks needed are readily available memory storages and simple transfer functions formulated with either no or at least one control zero for canceling the plant pole. Unlike other existing schemes, the presented time delay control schemes have high compensating gains only at those harmonic frequencies typically produced by most non-linear and unbalanced loads, with zero gains observed at all other frequencies. With this load-matching characteristic, the presented control schemes are expected to be more robust and less sensitive to implementation noises. In addition, the presented control schemes are deduced to have fast dynamic response, implying that the supplypsilas output voltage is virtually not influenced by any transient load transitions. Even more favorable is that the above-described performance features are attained under all load-operating conditions and at the same level of implementation easiness as for the traditional double-loop control scheme. Lastly, for verifying the practicality of the control schemes, an experimental UPS was assembled for testing with some results presented in a later section of the paper.