Suresh Singh

Constant power loads: A solution using sliding mode control

Rapidly increasing penetration of renewable energy sources into conventional power distribution systems has led to the rise in power electronic converter dominated power distribution systems. However, it has been a well established that the presence of tightly regulated point-of-load (POL) converters in the power distribution system, which act as Constant Power Loads (CPLs), cause serious stability challenge in spite of ensuring stability of individual converters. In this paper, we propose a sliding-mode controller with a non-linear sliding surface to mitigate negative impedance instabilities caused by the CPLs in dc distribution systems (dc Micro-grids). The stability of proposed surface and existence of sliding modes are proved. A simplified structure of a dc micro-grid system with dc/dc boost converter and CPL is used for the implementation of the proposed controller. Proposed controller is able to mitigate negative impedance instabilities and ensures stable operation of dc micro-grid system under various disturbances. The performance of the proposed controller, under steady state, line and load variations are validated through simulations in MATLAB Simulink environment.

Sliding mode control of a bidirectional DC/DC converter with constant power load

DC microgrid consists of cascaded power converters in which the constant power load behaviour of point-of-load converters poses challenging stability issues even if individual converters are stable. In this paper, a robust sliding mode controller has been proposed for DC/DC bidirectional buck-boost converter interfacing storage unit in an islanded DC microgrid environment. The controller is designed to ensure DC bus voltage regulation in the presence of renewable energy source and predominantly constant power loads. The performance of the proposed controller is validated through real-time simulation conducted using OPAL-RT® Digital Simulator. The controller ensures DC bus voltage regulation within tight limits and is robust with respect to wide variations in the renewable energy source power and the load.

On design of a robust controller to mitigate CPL effect — A DC micro-grid application

It has been an established fact that the presence of Constant Power Load (CPL) in DC micro-grid causes instability as it has negative impedance characteristic. This poses a serious challenge for the stability of the DC micro-grid in spite of ensuring stability of individual converters. Some efforts are made by the community to address the issue. In this paper, we have proposed a sliding-mode controller with a non-linear sliding surface to ensure large signal stability. The proposed non-linear surface ensures that constant power is maintained by the converter. The proposed controller is able to mitigate CPL effect of tightly regulated POLs and ensures stable operation of DC micro-grid under various disturbances. Proposed controller is implemented on a typical DC micro-grid feeding a CPL. The theoretical development is verified through simulations in MATLAB Simulink environment.

Voltage regulation and stabilization of DC/DC buck converter under constant power loading

The low and medium voltage renewable energy based dc distribution systems are catching increased attention world-wide due to their inherent benefits over ac systems. Higher efficiency, simple control, more suitable for integration of the sources producing dc power, are some of the main benefits of the dc systems. However, these systems are prone to serious stability issues caused by the constant power load (CPL) nature of the point-of-load converters (POLs). This paper deals with the stabilization and voltage regulation of dc/dc buck converter feeding a combination of the constant voltage resistive load (CVL) and CPL in a dc micro-grid application. A sliding mode controller is proposed which ensures supply of constant power demanded by the CPL and tight voltage regulation (error less than 0.1 V) of the output voltage. The effectiveness of the controller is validated through simulation results under different operating conditions. It is shown that the controller is robust to the large changes in supply voltage and the load.

DC bus voltage regulation in the presence of constant power load using sliding mode controlled dc-dc Bi-directional converter interfaced storage unit

The paper presents a robust sliding mode controller for a dc/dc, bidirectional converter interfacing storage unit to ensure dc bus voltage regulation in a stand-alone dc microgrid. The dc bus supplies a combination of a constant power load and conventional resistive load. In order to enhance the battery life, a state-of-charge based charging algorithm is also implemented. Depending on the measured dc bus voltage, the controller selects the appropriate operating mode of the converter. The effectiveness of the proposed controller has been validated through simulation results.

Mitigation of Negative Impedance Instabilities in a DC/DC Buck-Boost Converter with Composite Load

A controller to mitigate the destabilizing effect of constant power load (CPL) is proposed for a DC/DC buck–boost converter. The load profile has been considered to be predominantly of CPL type. The negative incremental resistance of the CPL tends to destabilize the feeder system, which may be an input filter or another DC/DC converter. The proposed sliding mode controller aims to ensure system stability under the dominance of CPL. The effectiveness of the controller has been validated through real-time simulation studies and experiments under various operating conditions. The controller has been demonstrated to be robust with respect to variations in supply voltage and load and capable of mitigating instabilities induced by CPL. Furthermore, the controller has been validated using all possible load profiles, which may arise in modern-day DC-distributed power systems.

Performance monitoring of 43 kW thin-film grid-connected roof-top solar PV system

In the emerging photo-voltaic markets worldwide the use of appropriate estimation of the operating performance of the grid-connected PV systems is becoming more and more crucial. The widely used performance indices that are used to analyze the system performance with respect to energy output, solar resource and the effect of system losses in its various components are total yield, final yield/specific yield, performance ratio, PVUSA rating, and performance indicator based on ratio of ac power at PTC to dc power at STC. This paper presents performance analysis of a 43 kW amorphous-silicon thin-film grid-connected PV system. Recorded plant monitoring data during July 2011 to July 2014 is used for the analysis. Major operation and maintenance issues encountered in last three years are also reported.

Stabilization of a Buck Converter Feeding a Mixed Load Using SMC

A buck converter is usually used in a dc distribution system to step-down the dc voltage, either to extend the primary distribution for low voltage applications or to meet a specific low voltage requirement of a load. This chapter presents stabilization of CPL induced destabilizing effects in DC/DC buck converter with CPL and a resistive load using Sliding Mode Control (SMC) approach. Nonlinear switching function based discontinuous and PWM based SMCs are proposed. The existence of sliding mode and stability of switching surface are established. The proposed theory is validated through simulation studies and experimentations. The proposed controllers are robust with respect to the sufficiently large variations in the input voltage and load. However, it was found from experimental results that PWM based SMC is sensitive to the slow variations in the supply.

Emulating DC constant power load: a robust sliding mode control approach

ABSTRACT This article presents emulation of a programmable power electronic, constant power load (CPL) using a dc/dc step-up (boost) converter. The converter is controlled by a robust sliding mode controller (SMC). A novel switching surface is proposed to ensure a required power sunk by the converter. The proposed dc CPL is simple in design, has fast dynamic response and high accuracy, and offers an inexpensive alternative to study converters for cascaded dc distribution power system applications. Furthermore, the proposed CPL is sufficiently robust against the input voltage variations. A laboratory prototype of the proposed dc CPL has been developed and validated with SMC realised through OPAL-RT platform. The capability of the proposed dc CPL is confirmed via experimentations in varied scenarios.

Robust Control of an Islanded DC Microgrid in Presence of CPL

This chapter presents robust control of an islanded DC microgrid in the presence of CPL using sliding mode control. A robust sliding mode control scheme is proposed to ensure system stability in the presence of CPL and the desired DC bus voltage regulation. Furthermore, a charging/discharging algorithm is implemented in the control loops of BDC to facilitate three mode charging of the battery bank. The test system consists of solar PV arrays interfaced to dc bus through dc/dc boost converters, energy storage system (lead-acid batteries) interfaced through a BDC, and a mixed load (Resistive, voltage regulator and a speed controlled drive). Tightly regulated voltage regulator and a speed controlled dc/dc drive, both represent CPLs in the system. The proposed theory is validated through simulation studies and experimental results. It is shown through simulation studies and experimental results that the proposed control scheme ensures stabilized dc bus voltage, i.e. it does not show any destabilizing effect of CPLs, under different operating conditions. It has been found that the proposed control ensures voltage regulation of less than 5 % and is robust to changes in the load.