Introduction

Based on the Renewable Energy Policy Network for the 21st Century (REN21) Global Status Report \cite{renewable}, renewable energy generates almost 23.7% of global electricity. Wind energy consumption is 19% of the total renewable energy consumption in 2015 and this percentage is expected to increase \cite{Wind}. However, the wind generation highly depends on local weather conditions. The intermittent and uncertain nature of power generation from wind farm may cause frequent violations of the ampacity ratings of the low voltage transmission lines. Curtailing some of the wind generation can avoid overload in the system.
Remedial Action Scheme (RAS) provides automatic control action with high impact on the system performance. The input data to a response based RAS must be processed to remove sensor noise; and identify a possible malicious attack, if cybersecurity is of concern. Typical time bounds for different RAS operations are several seconds for thermal overload, a few hundred milliseconds for voltage instability and a few tens of milliseconds for transient instability problems. The RAS under consideration is related to thermal overload problems which requires that the time bound is in the range of several seconds.
There has been a renewed interests among both system operators and wind power producers on wind curtailment as a solution to the transmission overload problems caused by the increased wind penetration. Curtailment is a reduction in the output of a generator from what it could otherwise produce given available resources, typically on an involuntary basis \cite{bird2014wind}. Owing to technical and economic constraints \cite{gu2014fast}, excessive portions of the wind generation are curtailed and thus wasted \cite{bird2014wind}. Curtailment of wind can lead to many undesirable consequences. Primarily, it results in useful energy being wasted. Secondly, revenue of the wind industry can be reduced significantly, which puts wind projects at risk of not meeting their debt obligations or reducing the amount of debt the project’s cash flows can support. Hence, there is a need to minimize wind curtailment such that line limits are not violated.
In many cases, curtailments are implemented manually whereby grid operators call wind facilities to issue curtailment directives. Manual curtailment instructions are used by Salt River Project, Tucson Electric, New England Independent System Operator (NE- ISO), PacifiCorp, and Mid Continent Independent System Operator (MISO) \cite{NREL1}. PacifiCorp’s generation center can manually control wind output levels for wind facilities they own.
Majority of the utilities, including the Electric Reliability Council of Texas (ERCOT) have realized the importance of the optimal and automatic curtailment. Manual curtailment processes for wind has generally extended curtailment periods due to the time required to implement curtailment and reluctance to release units from curtailment orders. Automatic communication procedures can speed the implementation of curtailment orders and reduce overall curtailment time. Moreover, automated wind curtailments can also relieve the system stress faster, thereby ensuring better reliability of the power supply.
Recently, more and more information and communication technologies have been deployed in the power system for maintaining the reliability of power grid and increasing the integration of renewable energy. Remedial Action Scheme (RAS), which is designed for detecting the abnormal system conditions and maintaining the system stability, has been widely used in the power system operation and control \cite{van1981two}. There are several RAS, which have already been implemented in industry \cite{14,15,16,baskin2009design,vaughan2010idaho,miller2009pacificorp}.
Based on the literature survey, the following are key issues identified in the existing automated RAS approaches:
Based on the special features, RAS can be used for wind generation curtailment.
In authors’ previous paper \cite{liu2016decentralized}, a RAS is developed to minimize the wind generation curtailment and maintain the transmission line flows at the same time. RAS in \cite{liu2016decentralized} is based on the DC power flow and linear programming. In this paper, we propose a new RAS with AC formulation and resiliency to limited cyber failures. Since the proposed new RAS uses AC formulation, it is more accurate than DC formulation RAS and it can also take care of the voltage limits on the load bus. In addition, due to the large communication and data requirement of RAS, communication failure may cause serious effect on the RAS operation. In order to overcome this problem, communication failure tolerance is also designed for the proposed new RAS. The main contributions of this paper can be summarized as:
This paper is organized as following. Section II presents the high level architecture of the RAS. Section III introduces the mathematic formulation of the proposed RAS. Section IV presents the architecture of the testbed to validate the proposed RAS. Section V provides simulation results. Section VI concludes this paper.