High Accuracy Impedance Detection to Improve Transient Stability in Microgrids
Abstract:
The advancements in DC microgrids and the increase in distributed generation systems has led to a new trend towards the co-existence of multiple power converters from different sources (renewable, storage, etc.) supplying a variety of loads of different natures in a weak network. The loads can behave as passive loads (resistances) or be implemented by tightly regulated power converters, leading to Constant Power Load (CPL) behavior. The CPLs present a characteristic negative incremental resistance that can alter the response of the system, even causing instability. In this work, a novel embedded technique based on a Digital Lock-In Amplifier is proposed that enables the real-time detection of the dynamic impedance present in a power converter. The proposed technique uses a very efficient algorithm, along with standard sensors available in the converter, to measure the magnitude and phase of the dynamic load, and uses this information to improve the performance of the converter. A sample application of the proposed technique in an adaptive control system is described. Although the total output power of the converter is independent of the nature of the load, the converter’s dynamic response is not. The interaction of the CPL, passive load and control loop will determine not only the stability but also the transient response. The proposed instrument allows the incremental load of the converter to be accurately measured while reducing the complexity and sensor requirements, and improving the performance of the controller. Simulations of the proposed technique are presented to illustrate its behavior. Experimental results for different kinds of loads are presented to validate the proposed strategy.
Existing System:
Recent techniques have been proposed to measure this equivalent dynamic impedance, such as Recursive Methods Least Squares (RMLS) and Kalman filters [3], discrete sequences [15], and DFT based methods [1]. However, most methods either require large amounts of computational power to determine the equivalent impedance or sacrifice precision. Some issues and opportunities to reduce the perturbation size and computational cost, and to improve accuracy, remain open as DC microgrids evolve and new features are required from power converters. Furthermore, the estimation of the equivalent load has been presented as a candidate solution to detect problems in the grid connection, such as islanding [16] and ground faults.
Proposed System:
In this paper an algorithm will be developed to detect the dynamic load of a power converter connected to a DC distribution system and an application of the technique for an adaptive control system will be presented. A schematic diagram of the proposed system is presented in Fig. 1; the DC system integrates multiple types of loads and sources and its equivalent behavior is not easy to predict, from the point of view of each converter. A digital algorithm will be implemented to detect the load from a small perturbation of known frequency and determine the correct controller coefficients. The detection of the load is done based on a Digital Lock-In Amplifier (D-LIA); this technique has been used in the past to characterize the impedance in transformers [19] and fuel-cells [20], and more recently to determine the Maximum Power Point of a Photovoltaic System [21]. In [22], a technique is proposed to extract the load impedance based on an LIA basic analysis of the system stability in open loop, but only the basic concept is outlined and no application is presented. The novel embedded technique presents three key advantages a) a simple compact digital instrument is implemented at the local level providing real time information to the rest of the system, b) the stability of the system around the operating point is estimated in real time, and c) the instrument provides both the magnitude and phase of the impedance. This instrument is in turn used to implement the adaptive control technique that monitors the load and determines the correction to the controller coefficients to maintain the stable and constant dynamic response, as indicated in Fig. 1. Mathematical analysis of the proposed technique, as well as the selected coefficients are provided. The proposed technique is supported with simulation results and validated with experimental results implemented in an industry standard microcontroller. The results obtained using the proposed algorithm are compared with those obtained using other techniques, in order to identify the advantages and disadvantages of the proposed technique.
Conclusion:
In this paper a novel equivalent dynamic impedance (zeq;o) measurement technique based on a Digital Lock-In Amplifier (D-LIA) was proposed to be embedded in a power converter operating in a DC network. The power converter feeds several loads of different types and magnitudes, and its stability and dynamic behavior depend on Zeq;o. Constant Power Loads (CPLs) present an especially challenging problem, since their dynamic behavior has a characteristic negative resistance that leads to instability. The proposed D-LIA-based technique was used to produce an adaptive control scheme that keeps the transient behavior of the converter predictable under load change conditions. This technique offers several key benefits: a) the D-LIA is a compact and efficient algorithm that can be implemented at the local level in each converter, providing real time information about the system; b) stability information is provided; and c) magnitude and phase are obtained, and can be used in many applications, such as in the adaptive control technique illustrated in this paper. The adaptive control technique introduced in this paper allows the real-time adjustment of the controller coefficients in order to obtain a reliable response under load change conditions. Simulations and experimental results have been provided to validate the proposed technique implemented in a standard microcontroller. The experimental validations show the power of the technique to detect perturbation much smaller than the switching ripple. A comparison of the proposed technique with other impedance estimation methods for stability was introduced, comparing the advantages and disadvantages of the proposed technique.
References:
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