Rasmus Pedersen

Direct control implementation of a refrigeration system in smart grid

The thermal capacity of the content in a cold storage room renders it possible to shift the governing refrigeration systems power consumption in time, without compromising temperature constraints. In this paper we introduce a method of implementing such a cold storage room into a directly controlled smart grid, by use of a predictive control strategy. In this application the shift in consumption is used to stabilize a small grid by utilizing excess renewable energy to minimize the need for fossil fueled production sources. In order for the centralized grid controller to handle such a node, its flexibility is communicated in form of a simple generic bucket model. Finally, the provided experiments verify the effectiveness of the proposed method.

DiSC: A simulation framework for distribution system voltage control

This paper presents the MATLAB simulation framework, DiSC, for verifying voltage control approaches in power distribution systems. It consists of real consumption data, stochastic models of renewable resources, flexible assets, electrical grid, and models of the underlying communication channels. The simulation framework makes it possible to validate control approaches, and thus advance realistic and robust control algorithms for distribution system voltage control. Two examples demonstrate the potential voltage issues from penetration of renewables in the distribution grid, along with simple control solutions to alleviate them.

Aggregation and Control of Supermarket Refrigeration Systems in a Smart Grid

Abstract In this work, control strategies for aggregation of a portfolio of supermarkets towards the electricity balancing market, is investigated. The supermarkets are able to shift the power consumption in time by pre-cooling the contained foodstuff. It is shown how the flexibility of an individual supermarket can be modeled and how this model can be used by an aggregator to manage the portfolio to deliver upward and downward regulation. Two control strategies for managing the portfolio to follow a power reference are presented and compared. The first strategy is a non-convex predictive control strategy while the second strategy consists of a PI controller and a dispatch algorithm. The predictive controller has a high performance but is computationally heavy. In contrast the PI/dispatch strategy has lower performance, but requires little computational effort and scales well with the number of supermarkets. Two simulations are conducted based on high-fidelity supermarket models: a small-scale simulation with 20 supermarkets where the performance of the two strategies are compared and a large-scale simulation with 400 supermarkets which only the PI/dispatch controller is able to handle. The large-scale simulation shows how a portfolio of 400 supermarkets successfully can be used for upward regulation of 900 kW for a two hour period.

Test-bed assessment of communication technologies for a power-balancing controller

Due to growing need for sustainable energy, increasing number of different renewable energy resources are being connected into distribution grids. In order to efficiently manage a decentralized power generation units, the smart grid will rely on communication networks for information exchange and control. In this paper, we present a Smart Grid test-bed that integrates various communication technologies and deploys a power balancing controller for LV grids. Control performance of the introduced power balancing controller is subsequently investigated and its robustness to communication network cross-traffic is evaluated. Various scenarios are demonstrated, assessing impact of communication network performance on quality of control.

Improving Demand Response Potential of a Supermarket Refrigeration System: A Food Temperature Estimation Approach

In a smart grid, the load shifting capabilities of demand-side devices such as supermarkets are of high interest. In supermarkets, this potential is represented by the ability to store energy in the thermal mass of refrigerated foodstuff. To harness the full load shifting potential, we propose a method for estimating food temperature based on measurements of evaporator expansion valve opening degree. This method requires no additional hardware or system modeling. We demonstrate the estimation method on a real supermarket display case, and the applicability of knowing food temperature is shown through tests on a full-scale supermarket refrigeration system made available by Danfoss A/S. The conducted application test shows that feedback based on food temperature can increase the demand flexibility during a step by approximately 60% the first 70 min and up to 100% over the first 150 min, thereby strengthening the demand response potential of supermarket refrigeration systems.

Evaluation of communication network state estimators for adaptive power-balancing

Smart grid applications are going to reach the low voltage grid assets and households in order to efficiently use the resources in electrical distribution grids. A cost effective way to connect these devices is to utilize the existing network infrastructure or Power-Line Communication (PLC). In this work, we illustrate the impact of changing communication properties on a power balancing controller used to support frequency control in the setting of a microgrid. More specifically, we focus on PLC communication and show how time-varying delays can affect the control algorithm performance. Further, we propose and compare two different delay estimation techniques and demonstrate how the control algorithm can use this information to adapt its gains - yielding significantly better control performance, compared to the controller using static gains.

Implementing force-feedback in a telesurgery environment, using parameter estimation

During minimal invasive telesurgery with surgical robots, surgeons rely on their vision to determine the forces applied to tissue. Using an end-effector from da Vinci Surgical Systems a force-feedback control system has been developed, in order to reduce the unnecessary forces applied by the surgeon. To avoid adding any additional hardware, the forces in the system have been estimated on the basis of the existing actuators, using parameter estimation techniques. The inevitable time-delays in the network, which imposes challenges in control design, are also estimated and compensated for within the control design. During tests, it has been shown that it is possible to implement a distributed network controller, which is stable over a range of typical time-delays. This shows, that the applied parameter estimation technique is indeed a viable solution for implementing force-feedback in telesurgery.

Coordination of electrical distribution grid voltage control - a fairness approach

In this paper, we present a method for distributing controller gains to assets offering local volt/var control. Gains are distributed based on a fairness principle, where all assets contribute equally to the voltage control, eliminating effects such as physical location. The method relays on a limited need for communication, and is inherently robust against connection loss, since only gains are updated. Through simulation examples it is shown that applying local volt/var control can reduce the amount of tap-changes with up to 84%, and updating local controller gains with the proposed algorithm can decrease the difference in reactive output between assets with 60%, while maintaining the same voltage levels. Thus the proposed control, provides significant economic benefit for distribution system operators.

Active power management in power distribution grids: Disturbance modeling and rejection

This paper presents a control strategy for enabling a distribution system operator (DSO) to manage a power distribution grid towards an active power reference. This is accomplished by allowing the DSO to utilize flexibility in production and consumption. The control system consists of a feedforward based on estimates of inflexible consumption profiles, a feedback based on flexible asset dynamics, and a dispatch algorithm for minimizing active power loss. The estimation approach is validated on real consumption data and illustrates the methods applicability on both a step-ahead and day-ahead scale, which makes it suited for a variety of control scenarios. Simulations demonstrate the control systems ability to track an active power reference and show a 3% reduction in active power loss, compared to a strategy of uniformly dispatching the power between assets.

Supermarket defrost cycles as flexible reserve

This work analyses how supermarket defrost cycles can be used as flexible reserve in a smart grid context. The consumption flexibility originates from being able to shift defrost cycles in time, while adhering to the underlying refrigeration systems constraints. It is shown how this time constrained behavior can be modeled and used by a defrost planner to optimally schedule defrost start times for several defrost cycles, according to electricity price. Because the defrost planning problem is a boolean programming problem a method of relaxing it is proposed. While the relaxed problem requires less computational effort, convexity is dependent on properties of the price signal. Simulation study show that the optimal planning strategy can reduce cost of defrost cycles with 31.7%, compared to a current strategy of executing all defrost cycles at the same time each day.