T 2 S 2 G: A Novel Two-Tier Secure Smart Grid Architecture to Protect Network Measurements

: False data injection (FDI) attacks are a major security threat to smart grid (SG) communication systems. In FDI attacks, the attacker has the ability of modifying the measurements transmitted by smart grid entities such as smart meters, buses, etc. Many solutions have been proposed to mitigate FDI attacks in the SG. However, most of these solutions rely on centralized state estimation (SE), which is computationally expensive. To engulf this problem in FDI attack detection and to improve security of SGs, this paper proposes novel two-tier secure smart grid (T 2 S 2 G) architecture with distributed SE. In T 2 S 2 G, measurement collection and SE are involved in ﬁrst tier while compromised meter detection is involved in second tier. Initially the overall SG system is divided into four sections by the weighted quad tree (WQT) method. Each section is provided with an aggregator, which is responsible to perform SE. Measurements from power grids are collected by remote terminal units (RTUs) and encrypted using a parallel enhanced elliptic curve cryptography (PEECC) algorithm. Then the measurements are transmitted to the corresponding aggregator. Upon collected measurements, aggregator estimates state using the amended particle swarm optimization (APSO) algorithm in a distributed manner. To verify authenticity of aggregators, each aggregator is authenticated by a logical schedule based authentication (LSA) scheme at the control server (CS). In the CS, fuzzy with a neural network (FNN) algorithm is employed for measurements classiﬁcation. Our proposed T 2 S 2 G shows promising results in the following performance metrics: Estimation error, number of protected measurements, detection probability, successful detection rate, and detection delay


Introduction
In recent times, smart grids (SGs) have become emerging technology in the electricity market since it offers remote monitoring of distributed energy generation [1][2][3].Typically, SGs are autonomous and self-sufficient systems that resolve the problems that existed in traditional electrical distribution systems.An SG provides the following advantages over traditional electrical systems: Reliability, flexibility in network topology, efficiency, load adjustment/load balancing, peak curtailment, sustainability, market-enabling, demand response support, enables platform for advanced services, and requires the minimum amount of data.However, the integration electrical systems and cyber infrastructure brings many security threats in SGs [4,5].In SGs, meter measurements, system parameters, and price information are subjected to vulnerabilities since this information plays a vital role in critical control processes such as state estimation (SE), economic dispatch, load aggregation, demand response, and so on.Attackers inject the false data in the above mentioned information and attempted to compromise smart meters.It is necessary to improve SG security in the following aspects: Data generation security, data acquisition security, data storage security, and data processing security.
A false data injection (FDI) attack is considered as the most harmful attack since it modifies the measurements from grid sensors (or) remote terminal units (RTUs) in order to introduce undetected errors in the estimation of significant state variables [6].Many research works have been conducted on SG in order to mitigate FDI attacks.The principal component analysis (PCA) approximation method is involved in bad data detection (BDD) in SGs [7].In the PCA based method, the Jacobian matrix and distribution of state variables are secured from attackers.Game theoretic approaches are also utilized for false data detection [8].Here the Stackelberg game model with hybrid satisfaction equilibrium-Nash equilibrium algorithm is designed.The effect of FDI attack is evaluated by bi-level modeling method [9].This evaluation shows that securing the minimal set of sensors (or RTUs) is sufficient to secure an entire SG system since the minimal set of sensors are required to launch the attack.Machine learning approaches such as perceptron, K-nearest neighbor approach, support vector machine (SVM) algorithm, sparse logistic regression method, ensemble learning method, and multiple kernel learning method are also adapted for false data detection [10].In graph-based cyber security analysis system, FDI attack detection is carried out by maximum matching algorithm, commodity flow maximization algorithm, tree pruning algorithm, and minimum S-cut algorithm [11].
Utmost of the FDI detection methods rely on the SE process.In an SG, SE is a process of estimating state variables that defines the operating conditions of power system such as bus voltage, branch current, and power [12].Upon estimating operating conditions of the power system, the state of the power system can be predicted.Typically SE is performed in a centralized manner as well as in a distributed manner.Extended Kalman filter is combined with the particle swarm optimization (PSO) algorithm for dynamic SE [13].The combined SE method is able to tolerate practical issues of missing measurements.An interval SE (ISE) method is presented by utilizing the deep learning algorithm [14].The stacked auto-encoder (SAE) is designed for ISE to support feature extraction from an electric load.The data driven approach that uses the K-nearest neighbor algorithm and kernel trick is exploited for SE [15].In the data driven approach, historical data has a major role for SE.Cryptographic algorithms are contributed in the provision of data security in SG [16,17].In both works, the elliptic curve cryptography (ECC) algorithm is utilized for authentication as well as data security.Involvement of cryptography techniques protects measurement data from intruders during transmission.
The major contributions of this paper in smart grid security are listed as follows, • A novel two-tier secure smart grid (T 2 S 2 G) architecture is designed with RTUs, aggregators in first tier and control server in second tier.T 2 S 2 G protects SG system from FDI attacks and detects compromised meters.

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Initially the entire T 2 S 2 G system is segregated into four sections by the weighted quad tree (WQT) method in order to support the distributed SE process.In each section, an aggregator is deployed in the static position.

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All measurements are secured by RTUs using the parallel enhanced elliptic curve cryptography (PEECC) algorithm, which ensures high level data security with minimum time consumption.

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After receiving all measurements from RTUs in the section, aggregator performs SE.For SE, we proposed an amended PSO (APSO) algorithm.The proposed SE method minimizes computational overhead as well as estimation error.

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For verifying authenticity of aggregators, each aggregator is authenticated at the control server by using the logical schedule based authentication (LSA) scheme.Involvement of the LSA scheme prevents processing of unnecessary measurements from intruders in the control server.
• Compromised meters in the system are identified effectually by analyzing received SE information of each meter.For compromised meter detection, the fuzzy with neural network (FNN) algorithm is employed in the control server.
The rest of this paper is organized as follows: Section 2 discusses the background overview of SGs, SE, and FDI in SG.In Section 3, we survey significant previous works held on SG in the perspective of FDI detection.In Section 4, problems that existed in previous research works and are resolved by the T 2 S 2 G system are highlighted.Section 5 explains the T 2 S 2 G system with novel algorithms.In Section 6, the performance of the proposed T 2 S 2 G system is evaluated in terms of performance metrics.In Section 7, we conclude our achieved contributions with future work.

Background Overview
This section provides an overview on the SG system and cyber security.This section comprises two subsections as follows: (i) Security in SG, and (ii) state estimation in SG.

Security in SG
SG is an electrical grid that uses analog or digital information and communication technology in electrical systems [18,19].The SG system is able to supply electricity to consumers through two-way digital communication.It enables monitoring, analysis, control, and communication within the supply chain, which improves efficiency with reduced cost.Smart meter and the supervisory control and data acquisition (SCADA) system are significant entities of the SG system.Here the SCADA system is responsible of having the overall control of the system and is often called the control server.A modern SG system has the following capabilities: Self-repairing ability, encouraging participation of consumer in grid operations, ensuring minimum power leakages, and support growth of electricity markets.Typically, SGs are involved with the following benefits:

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Provides more efficient transmission of electricity, • Supports quick restoration of electricity after power disturbances,

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Efficient in terms of operational and management costs for utilities, • SG helps to lower the power costs for consumers,

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Provides reduced peak amount as well as electricity rates, • Supports large-scale renewable energy systems,

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Integrates consumer-owner power generation systems.
Even though SG is involved with significant benefits; security and privacy is still a challenging issue [20,21].Involvement of numerous devices such as smart meters, intelligent appliances, distributed generation, and storage equipment located in a physically insecure environment in SG increases the vulnerability of the system.In addition, the wireless technologies used for communication in SG are also vulnerable to several security threats.FDI is a significant security threat in SGs [22].An FDI attack is launched by attackers within the power system as well as in the wireless technology.In an FDI attack, an attacker modifies the measured value during transmission between smart meters and the control server.Illustration of an FDI attack held on an SG system is depicted in Figure 1.

State Estimation in SG
SE is a significant function in modern SGs that contributes to the management and control of operations of electrical transmissions [23,24].Typically, the system state is defined as the minimum set of state variables that are used to define the entire power system through network topology, impedance parameters, etc.The prime objectives of state estimator are listed as follows:

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Bad data detection,

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Minimizing small errors found in measurements, • Topology errors and wrong switch statuses detection,

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Managing missing and delayed measurements through providing estimation for unmonitored parts of the system, • Analyzing measurements redundancy for estimating network parameters.

Bad data detection, •
Minimizing small errors found in measurements,

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Topology errors and wrong switch statuses detection,

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Managing missing and delayed measurements through providing estimation for unmonitored parts of the system, • Analyzing measurements redundancy for estimating network parameters.
The SE process is carried out using two systems as follows: (i) Centralized SE system, and (ii) distributed SE system.The major problems involved in the centralized SE system are high computational overhead and high complexity burdens for the control center [25].To resolve all problems existing in the centralized SE system, the distributed SE system was introduced.In the distributed SE system, state estimator performs SE in a distributed manner (i.e., on each branch, bus, etc.).The distributed SE system supports high R/X ratios, low real-measurements availability, better scalability, and minimized complexity.Distributed SE achieves better performance than centralized SE in SGs when applied on a high voltage direct current (HVDC ) system [26].

Related Works
In this section, significant previous research works were analyzed in order to identify the problems raised in SG security.The critical survey opens up the way for innovative research work on SG security.

Related Works on FDI Attack Detection
Covert cyber assault is a type of FDI attack held on SGs, which was detected by SE [27].For attack detection, a supervised machine learning algorithm based bad data detector was designed.Initially, the attack was launched by altering the measurements aggregated by sensors.Then the measurements collected over a period were used to train the classifier.In the bad data detector, an SVM classifier was utilized.In SVM, optimal features were selected by the genetic algorithm (GA) in order to improve classification accuracy.However, this method detects modified data and is not able The SE process is carried out using two systems as follows: (i) Centralized SE system, and (ii) distributed SE system.The major problems involved in the centralized SE system are high computational overhead and high complexity burdens for the control center [25].To resolve all problems existing in the centralized SE system, the distributed SE system was introduced.In the distributed SE system, state estimator performs SE in a distributed manner (i.e., on each branch, bus, etc.).The distributed SE system supports high R/X ratios, low real-measurements availability, better scalability, and minimized complexity.Distributed SE achieves better performance than centralized SE in SGs when applied on a high voltage direct current (HVDC) system [26].

Related Works
In this section, significant previous research works were analyzed in order to identify the problems raised in SG security.The critical survey opens up the way for innovative research work on SG security.

Related Works on FDI Attack Detection
Covert cyber assault is a type of FDI attack held on SGs, which was detected by SE [27].For attack detection, a supervised machine learning algorithm based bad data detector was designed.Initially, the attack was launched by altering the measurements aggregated by sensors.Then the measurements collected over a period were used to train the classifier.In the bad data detector, an SVM classifier was utilized.In SVM, optimal features were selected by the genetic algorithm (GA) in order to improve classification accuracy.However, this method detects modified data and is not able to detect compromised meters.Furthermore, centralized SE introduces high complexity and overhead in the system.
The data injection attack was detected based on the chi-square method and cosine similarity measurement [28].In this method, measurements were acquired from SCADA using sensors.Upon collected measurements, Kalman filter based SE was performed.Further, a chi-square test and cosine similarity measure were carried out on the estimated value and original measurement.Based on the similarity level (by cosine similarity measure) and deviation level (by chi-square test) the FDI attack was detected.Recursive systematic convolutional (RSC) was combined with the Kalman filter in order to detect FDI attacks in SGs [29].In such a system, the RSC code was employed to handle impairments in the system states and the Kalman filter was used for SE.Based on semi-definite programming, a feedback control strategy was employed for voltage regulation.In both methods, involvement of the Kalman filter based SE limits the performance of FDI detection in a non-linear system and in uncertainties.However, in general SGs are non-linear.These methods are not suitable for effectual attack detection.
For real-time FDI detection, an attack detection model based on Markov chain based analytical model and the cumulative sum (CUSUM) method was designed [30].In addition, the CUSUM method was modified as the adaptive CUSUM method in order to improve delay and accuracy.This detection model was recursive in nature and each recursion was comprised with the following tests: (i) Unknown variable solver based on the Rao test, and (ii) multithread CUSUM test.This method detects modified data only and is not able to detect compromised meters.Thus, the possibility of attackers in the system is increased.A collaborative intrusion detection mechanism was presented for FDI attack detection in SGs [31].In order to protect the measurements from smart meters, a spying domain concept was utilized.The approach was relying on the following constraints: Secret information, event log, and spying domain.Then the attackers were classified into innocent attackers, skilled attackers, and powerful attackers.In this method, secret information is required to be shared in advance.However, if the secret is known to the attacker then the attacker is able to compromise the system easily.
Therefore, most of the FDI detection methods rely on the centralized SE method, which increases the system overhead.In addition, the performance of detectors is limited to linear systems only.

Related Works on SE and Security in SG
The hybrid PSO (HPSO) algorithm was introduced for SE in a distributed manner [32].Here the PSO algorithm was improved by incorporating the tournament selection process, which was inspired from GA.In this method, the HPSO algorithm was employed in each sensor, smart meters, phasor measurement unit (PMU), etc., which increases overhead and complexity in the sensors.A PMU based robust SE method (PRSEM) was introduced for real-time monitoring in SGs [33].In this method the robustness of the SE process was improved by an adaptive weight assignment function.Here the weight assignment function was involved to adjust the measurement weight based on unwanted disturbances from the PMU measurements.This method increases computational complexity due to involvement of multiple computations.Furthermore, SE in this method is not optimal since this method relies on mathematical computations.Hamiltonian cycle theory was adapted for SE in the power system [34].In this method, the network states were obtained quickly through a calculation scheme in a distributed manner.For tolerating high computation cost, the search space was reduced in this work.However, minimizing search space only was not able to reduce computational cost since Hamiltonian cycle requires high computational cost.In addition, this method is not able minimize the estimation errors.
A mathematical morphology (MM) method was proposed to defend against major security attacks in the SG system [35].In this method, intrinsic components were obtained by decomposing power system measurements.Based on intrinsic components, the time-frequency sparsity mapping was established.With this information, the measurement source was authenticated by the server.Perhaps this method authenticates the source; this method is not able to secure the measurements.To provide security for measurements, a random-noise-disturbed triple data encryption standard (3DES) algorithm was introduced [36].Here random computation was introduced by considering power consumption characteristics.Here a 3DES algorithm was involved for wireless links protection whereas an authentication scheme was proposed for terminal protection.However, 3DES algorithm is inefficient in terms of security and time consumption (i.e., it consumes more time to provide low level security).To improve the security level, a state estimation based dynamic encryption and authentication (SEDEA) approach was presented to protect communication between the control center and RTUs [37].All measurements were encrypted by the SEDEA approach and SE was performed by the control server.In SEDEA, magnitude and phase voltage were considered as a common secret to generate dynamic encryption.Generating the encryption key for each communication in a dynamic manner increases the complexity and overhead in the system.In addition, involvement of the centralized SE also leads to large computational overhead.
Therefore, security provision against FDI attacks in SGs still requires improvements.In addition, SE, which plays a significant role in FDI detection, also needs to be improved with reduced computational complexity.

Problem Definition
In FDI attack detection, compromised PMU was identified based on suggestion from host monitors [38].In this method, SE was performed by utilizing four rules in order to detect anomaly measurements.FDI was detected based on a corresponding rule deployed whereas compromised PMU was detected by enabling a majority voting method.Here the accuracy of FDI detection scheme depends upon the number of rules specified.Increasing the number of rules results in high detection accuracy and also in higher time consumption and complexity.In compromised PMU detection, involvement of the majority rule limits the detection accuracy since it is not ensured that all host monitors are trustworthy.For FDI detection, parallel dynamic SE with the Markov chain model and Euclidean distance was adapted in SGs [39].To place PMUs optimally, a set of smart meters was considered as critical meters.The states estimated by the Markov chain model were verified to detect an FDI attack with the support of trusted buses.Here attack detection accuracy is improved by measuring the Euclidean distance between trusted bus measurements with all historical data.The Euclidean distance measurement requires large computational time and results in high overhead.Furthermore, requirement of additional trusted buses limits the performance of the detection system.
A greedy subset searching algorithm and a minimal subset selection algorithm were presented to improve the accuracy of the FDI attack detection [40].Attack detection was carried out by robust principal component analysis (RPCA) algorithm with entry wise constraints.Involvement of centralized SE increases time consumption, complexity, and estimation errors.In addition, this method is only suitable for an attack that alters small number of measurements and fails to detect large attacks in the system.The nodes involved in the SG system were clustered into the most vulnerable nodes, moderately vulnerable nodes, and least vulnerable nodes to defend against an FDI attack [41].Cluster formation was performed by the constriction factor-PSO (CS-PSO) algorithm.Then authentication and intrusion detection system were enabled to the nodes presented in the most vulnerable cluster.It is worth mentioning here that providing security for the most vulnerability node is not an intelligent decision since moderately vulnerable nodes are also affected by attackers.In other words, this method is not able to detect an attack launched on measurements from the moderately and least vulnerable nodes.To minimize attack detection time in wide area SGs, sequential detector based on generalized likelihood ratio was designed [42].This method was specially designed to handle a variety of attacking strategies and load situations in power systems.Employing the attack detector and SE in a centralized manner leads to the introduction of huge complexity.Measurement transmission without security encourages the presence of attackers in the system.
Therefore, many of the existing FDI detection methods achieve high detection accuracy in the cost of large computational complexity.In addition, SE, which supports FDI detection, also is involved with high computational complexity.The problems highlighted in this section were resolved by our proposed T 2 S 2 G system.

System Overview
In this paper, we designed a novel T 2 S 2 G system that consisted of power generators (G), buses (B), load (L), RTUs, aggregators (A), and control server (CS).The overall system architecture is depicted in Figure 2.

System Overview
In this paper, we designed a novel T 2 S 2 G system that consisted of power generators (), buses (), load (), RTUs, aggregators (), and control server ().The overall system architecture is depicted in Figure 2. Here, the power generated by power generators is distributed among loads through transmission line.The control server is involved with SCADA, and the energy management system.All measurements generated by smart meters (  ), and PMUs are collected by RTUs and transmitted to the control server (CS) through buses.The attackers attempted to inject false data during measurement transmission.To defend against such FDI attacks, distributed SE is performed in T 2 S 2 G.To support distributed SE, the entire T 2 S 2 G system is segmented into four sections initially.For system partitioning, the WQT method is proposed.In each section, an aggregator node is deployed in static position.All measurements collected by RTUs are encrypted using the PEECC algorithm to protect measurements from adversaries.Then the aggregator performs the SE upon aggregated measurements using the APSO algorithm in a distributed manner.Each aggregator is authenticated at the CS through the LSA scheme.Finally, the CS detects compromised meters and PMUs by analyzing measurements collected from aggregators.For compromised meter detection, the FNN algorithm is employed in the CS.Each significant process is explained in the following sections.Here, the power generated by power generators is distributed among loads through transmission line.The control server is involved with SCADA, and the energy management system.All measurements generated by smart meters (SMs), and PMUs are collected by RTUs and transmitted to the control server (CS) through buses.The attackers attempted to inject false data during measurement transmission.

WQT Method Based System Partitioning
To defend against such FDI attacks, distributed SE is performed in T 2 S 2 G.To support distributed SE, the entire T 2 S 2 G system is segmented into four sections initially.For system partitioning, the WQT method is proposed.In each section, an aggregator node is deployed in static position.All measurements collected by RTUs are encrypted using the PEECC algorithm to protect measurements from adversaries.Then the aggregator performs the SE upon aggregated measurements using the APSO algorithm in a distributed manner.Each aggregator is authenticated at the CS through the LSA scheme.Finally, the CS detects compromised meters and PMUs by analyzing measurements collected from aggregators.For compromised meter detection, the FNN algorithm is employed in the CS.Each significant process is explained in the following sections.

WQT Method Based System Partitioning
System partitioning is the foremost process and it is performed by using the WQT method.The first tier in the system is partitioned into four sections as S = {S 1 , S 2 , S 3 , S 4 } based on the weight value.Here each generator and load involved in the system is provided as the weight value.Typically, quad tree divides the space into adaptable cells based on the capacity value.In quad tree, each cell is provided with a maximum capacity value.When the cell reaches its maximum capacity value, then the cell is divided into four cells.This process is iterated until the necessary conditions are met.However, in the T 2 S 2 G system, quad tree is adapted for dividing the first tier in the system space into four sections.In quad tree the entire system space (Sys S ) is initiated as the root (R) node of the tree.Then the system space is divided into four sections such that the root node has exactly four children.Here each section has the maximum capacity value, which is computed from weight values.The major entities of the SG system are G, B, and L. Each entity has different specification in the different SG system.We utilize these specification values as weight values for the entities.Based on weight value, the capacity of each section is assigned.The system is partitioned until each section reaches its maximum capacity.For example, in the IEEE-9 bus system, the G and L has relative power specification as depicted in Table 1.
Table 1.Weight values for G and L.

Generator
Power Load Power Based on the related power value, maximum capacity for each section is assigned.For IEEE-9 bus system, the capacity of each section is assigned as follows: The above equations are suitable only for the IEEE-9 bus system and have different values for different bus systems.Upon capacity of each section, the system is partitioned into four sections.The capacity of four sections is determined by Table 1 as follows: The first tier of the system is partitioned into four sections based on weight values of power generators and load.System partitioning is performed to enable distributed SE in the system.The network partitioning by the WQT method is shown in Figure 3.
After completion of system partitioning, the aggregator is deployed in each section as follows: Energies

PEECC Algorithm for Measurements Protection
To protect the measurements from adversaries during transmission, all measurements collected by RTUs are encrypted before transmitting to aggregator.For measurement protection, we proposed the PEECC algorithm based encryption scheme, which is similar in terms of performance to [43].In the PEECC algorithm, measurement security is ensured with minimum time consumption.Typically, the ECC algorithm is a public key algorithm in which two separate keys such as the private key ( ) and public key ( ).In the PEECC algorithm, the ECC algorithm is adapted for key generation.In the PEECC algorithm, at first the measurements are converted into American standard code for information interchange (ASCII) codes.The generated ASCII codes are divided into '' number of data and encrypted in a parallel manner using ' ' of the aggregator.Here key generation is performed based on the elliptic curve, which is represented as follows: where, 4 + 27 ≠ 0. A prime number '' is a point on elliptic curve if it satisfies the following equation: When the condition is satisfied then the key generation process is performed as follows: 1. Select a random number 'ℛ' between the range of [1,  − 1] where '' is a prime number that satisfies Equation (11).2. Generate the public key ' ' as follows:

PEECC Algorithm for Measurements Protection
To protect the measurements from adversaries during transmission, all measurements collected by RTUs are encrypted before transmitting to aggregator.For measurement protection, we proposed the PEECC algorithm based encryption scheme, which is similar in terms of performance to [43].In the PEECC algorithm, measurement security is ensured with minimum time consumption.Typically, the ECC algorithm is a public key algorithm in which two separate keys such as the private key (K Priv ) and public key (K Pub ).In the PEECC algorithm, the ECC algorithm is adapted for key generation.In the PEECC algorithm, at first the measurements are converted into American standard code for information interchange (ASCII) codes.The generated ASCII codes are divided into 'n' number of data and encrypted in a parallel manner using 'K Pub ' of the aggregator.Here key generation is performed based on the elliptic curve, which is represented as follows: where, 4a 3 + 27b 2 0. A prime number 'P' is a point on elliptic curve if it satisfies the following equation: When the condition is satisfied then the key generation process is performed as follows: 1.
Select a random number 'R' between the range of [1, P − 1] where 'P' is a prime number that satisfies Equation (11).

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Generate the public key 'K Pub ' as follows: where 'G' is a base point in ECC curve.In this manner the public key is generated in the ECC algorithm.

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Generate the private key 'K Priv ' using the public key.
After generation of 'K Pub ' and 'K Priv ', the next process is to secure the measurements obtained from meters and buses.Initially the measurement (Z i ) from buses/meters is divided into 'n' sub-measurements by RTU.The measurement from 'i th ' bus/meter is divided as follows: Then each sub-measurement is encrypted using 'K Pub ' of the corresponding aggregator.Encryption is performed in a parallel manner in order to minimize the time consumption.The overall process of the PEECC algorithm is depicted in Figure 4.The PEECC algorithm involves of two steps as follows: In the first step, each sub-measurement is converted into an ASCII code, and in the next step each coded measurement is encrypted using 'K Pub '.Using the PEECC algorithm, RTUs convert all measurements into the ciphertext.Then, the measurements are transmitted to aggregators.Algorithm 1 explains the process of the PEECC algorithm.Then, the measurements are transmitted to aggregators.In the aggregator, the collected measurements are decrypted using 'K Priv ' in order to obtain the original measurements.Here the decrypted measurements are obtained in the form of the ASCII code, which is to be converted into plaintext again.Involvement of the PEECC algorithm protects the measurements during transmission from aggregators to the CS.

APSO Algorithm Based Distributed SE
In the T 2 S 2 G system, SE is performed at each aggregator in a distributed manner.For SE, we employed the APSO algorithm in each aggregator.In general, SE consists of estimating the state vector (v) from a set of measurements (z) in the presence of an error (e).The functional relation between v, z, e is mathematically modeled as follows: Here, i = 1, 2, 3, . . ., m represents 'm' number of measurements.The 'i th ' measurement vector is denoted as 'z i '.The non-linear function relating the state variables is represented as ' f i (v)' and 'v' is the state vector of dimension 'n'.Noise presented in the measurements is represented by noise vector 'e i '.When the errors are minimized, then all available measurements and calculated measurement variables are contained in the function ' f (v)'.The estimated measurement can be rewritten as: where 'Z true ' represents the true measurement and 'RAND' denotes the random number.The standard deviation of 'i th ' measurement error is represented by 'σ i '.The estimated variables are voltage magnitudes, angles, and tap positions (i.e., v i = V k i δ k i t k i ).Based on true measurements and estimated measurements the error values can be computed.Here we have provided true measurement values for the three-phase power system (l = 3).The objective of the proposed SE is to minimize the square of difference between the true measurement and estimated measurement (i.e., estimation error).It can be expressed as follows: In the T 2 S 2 G system, the PSO algorithm is improved as the APSO algorithm for SE.In general, the PSO algorithm is a population based optimization algorithm proposed to find the optimal solution.We adapt the APSO algorithm for SE and it includes the following steps: Energies 2019, 12, 2555 12 of 24

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Step 1: In this step, all particles are initialized in the APSO algorithm.In APSO based SE, the state variables such as V k i , δ k i , t k i are initialized as particles.

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Step 2: The next step is to evaluate the fitness value of each particle initialized.In APSO, the fitness value (FV) of each particle is computed as follows: The penalty function (Pe(v)) is computed as follows: The penalty function is computed in terms of the number of penalized control variables (Q), scalar quadratic penalty weight (ρ), current value of control variable (v 0 ), and control variable penalty offset (v i ).

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Step 3: In this step, the velocity of each particle is updated based on 'FV'.Firstly, the fitness value of the particle in the current iteration is compared with the previous value in order to update the particle best (pbest) value.If the current 'FV' value is higher than the previous 'FV' then the current value is updated as 'pbest', otherwise the previous value is maintained as 'pbest'.Then the current 'FV' of a particle is compared with 'FV' of other particles in order to update the global best (gbest) value.After the update of 'pbest' and 'gbest', the velocity of each particle is updated as follows: Current velocity of the particle (υ[c + 1]) is computed in terms of the previous velocity of the particle (υ[c]), two learning factors ( , C), and random number (rand).The current position of particle is updated as: Current position Pos[c + 1] is computed based on the previous position (Pos[c]) and current velocity (υ[c + 1]).

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Step 4: This step in the APSO algorithm differs from the traditional PSO algorithm.In this step, new solutions are generated by employing Levy f lights in the PSO algorithm.In this step, new solutions are generated as follows: Here 'α' is step size and always 'α > 0 while 'Levy(λ)' is the transition probability.Then the new solutions also are evaluated based on the fitness value as in Step 2 until maximum iteration has been reached.After completion of the maximum iteration, the optimal solution for each state variable is estimated.

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Step 5: In this step, attack detection is carried out based on estimated states.For standard direct current (DC) power flow, the measurements are received by aggregators as follows in the absence of an attacker: The measurements are obtained by (22) where 'H' represents the measurements matrix and 'e' represents the error matrix.In the presence of false data, the measurement vector is defined as follows: Here 'a' is the attack vector injected by the attacker and it is non-zero for the measurement vectors obtained by compromised meters or sensors.This can be defined as follows: a i 0, i f i th meter is controlled by attacker . ( In SG, an attacker is able to control more than one meter.For all compromised meters the attack vector is non-zero in the measurement vector.In addition, the attack vector becomes strong when a = HC.For such an attack, we obtain measurements as: Here 'v' is indistinguishable from 'v + C'.The aggregator decides that 'v + C' as a true measurement.Attack detection is carried out based on the similarity between estimated measurements and obtained measurements.We adapt Cosine similarity measurement for attack detection.Similarity between the estimated measurement ' v' and obtained measurement ' v' is computed as follows: When both estimated and obtained measurements are exactly the same then the similarity will be '1 .Otherwise, the similarity value will be decreased.Here we can see that when the attack vector is injected, then the obtained measurement has a large deviation with estimated measurements, which results in a lower similarity level.If the similarity level is too low, then the FDI attack is detected.In our work, attack detection accuracy is improved since we have estimated states using the APSO algorithm accurately.The affected measurements are detected in this step by the aggregator.
In Algorithm 2, the overall process involved in the APSO algorithm based SE and FDI detection is illustrated.In this manner, the injected false data is identified by aggregators.

LSA Based Authentication and FNN Based Classification
The aggregator is responsible to estimate the state variables and to report to the CS in tier-2.In the CS, each aggregator is authenticated by the LSA based authentication scheme.Authentication is performed in order to verify the authenticity of aggregators presented in tier-1 of T 2 S 2 G system.All four aggregators are registered with the CS and provided with secure keys (K Pub , K Priv ) generated by the ECC algorithm.Here each aggregator has its own ID, and secure keys.In LSA, the logical operator is adapted for authentication in a scheduled manner.Here we have utilized AND operator in which the output will be true if both inputs are true.In the LSA scheme, scheduling is performed on secrets shared between the aggregator and CS.The process is performed in two stages as follows:

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First stage: In LSA, the first stage is initiated by the aggregator.In this stage, the aggregator submits the ID and password (PW) to the CS.The CS verifies the ID and 'PW', if ID and 'PW' is matched then it considers the aggregator request for the second stage.In this stage, AND logic is utilized.Otherwise, the aggregator request is denied.

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Second stage: The aggregator that completes the first stage is considered for the second stage.In this stage, the CS requests for a shared secret value (SS S ) for the current session from the aggregator.Each aggregator is provided with a set of 'SS' including four secret values initially.
Each value is scheduled for different sessions.Upon receiving a 'SS S ' request, the aggregator submits its current session 'SS S '.If the current session 'SS S ' submitted by the aggregator is true, then the aggregator is authenticated at the CS.
Algorithm 2 APSO based SE For all aggregators 3.
Collect Z i from RTUs 4.
For each Z i ∈ Z 5.
For each particle 7.
Update velocity and position 20.
If (Maximum iteration reached)

End if 25.
End for 26.
Data is not false data 31.

End if 32.
End for 33.End for 34.

End
The scheduling process enabled in the LSA scheme is depicted in Figure 5.For each session, the aggregator must submit the corresponding 'SS S ' value in order to crack the LSA scheme.
After completion of successful authentication, all measurements aggregated by the aggregator are collected by the CS.
Here each aggregator reports the aggregated measurements, estimated state variables, similarity level, and error level.Upon received measurements, the CS classifies measurements into two classes as: (i) Measurements from normal meters, and (ii) measurements from compromised meters.For efficient classification, the FNN algorithm is employed in the CS.In FNN, error level (e i l ) between estimated measurement and true measurement, similarity level (Sim i l ) between estimated measurements and obtained measurements, and trust level (T i l ) for 'i th ' bus/meter are considered as fuzzy metrics.Here the error level and similarity level are provided by the aggregator and the trust level of the meter/bus is estimated by the CS.All measurements are taken as input in the input layer of FNN and fuzzy rules are applied on a hidden layer.The classified result is obtained in the output layer as shown in Figure 6.Here the weight value is computed in each hidden layer in order to detect the malicious measurements.If a measurement is classified as malicious, then the meter from which the measurement is obtained is verified.The trust value of that meter is checked.If the trust value is also too low, then that meter is identified as the compromised meter.In this manner, the compromised meter detection is carried out in the CS by using the FNN algorithm.The fuzzy rules deployed in FNN are depicted in Table 2.Here the trust level is computed based on previous behavior of the bus/meter.Trust value is considered to be 100 to represent that the meter is highly trusted and 0 to represent that the meter is highly untrusted.Error level and similarity level are computed in the rage between [0, 1].By utilizing fuzzy rules, all measurements are classified.
Our proposed T 2 S 2 G architecture improves the security of the SG system by employing effective system partitioning, security for measurements, efficient distributed SE process, authentication process, and compromised meter detection process.

Performance Evaluation
In this section, the performance of the proposed T 2 S 2 G system was evaluated in terms of performance metrics.This section comprises two subsections as follows: (i) The simulation setup, and (ii) comparative analysis.

Simulation Setup
To explore the efficiency of our proposed T 2 S 2 G in FDI detection and security provision, we performed simulations on MATLAB-2017b (MathWorks, Natick, MA, USA).The experiments were conducted on a 2.5 GHz Intel i3-M380 processor, with a Windows 7 operating system of 64-bits, and 4 GB RAM (Lenovo PC HK limited, Hong Kong, China).The proposed T 2 S 2 G system was modeled based on the IEEE-9 bus system.The overall system was partitioned into four sections by the WQT method.The overall simulation setup is shown in Figure 7.
In Table 3, significant simulation parameters considered to implement T 2 S 2 G system are depicted.Here bus9 and bus6 are compromised buses.
To explore the efficiency of our proposed T 2 S 2 G in FDI detection and security provision, we performed simulations on MATLAB-2017b (MathWorks, Natick, MA, USA).The experiments were conducted on a 2.5 GHz Intel i3-M380 processor, with a Windows 7 operating system of 64-bits, and 4 GB RAM (Lenovo PC HK limited, Hong Kong, China).The proposed T 2 S 2 G system was modeled based on the IEEE-9 bus system.The overall system was partitioned into four sections by the WQT method.The overall simulation setup is shown in Figure 7.In Table 3, significant simulation parameters considered to implement T 2 S 2 G system are depicted.Here bus9 and bus6 are compromised buses.

Comparative Analysis
In this sub-section, we compared our proposed T 2 S 2 G system with previous works parallel dynamic SE [39], sparse method [40], and quickest detection method [42].The evaluation was Energies 2019, 12, 2555 18 of 24 performed in terms of the estimation error, number of protected measurements, detection probability, successful detection rate, and detection delay.One of most of the evaluation assumption attack scenarios in SG communication is that the presence of one attacker has knowledge on grid topology and security mechanisms [39].
In Table 4, significant previous works were analyzed with demerits involved in each method.Our proposed work was compared with these methods in order to show betterment achieved by our work.• Effectiveness on estimation error.Estimation error is defined as the difference between the real state and estimated state.The estimation error is too large under the attack condition and minimized in the normal condition.The voltage magnitude estimated in the T 2 S 2 G system is compared with its original state.The proposed APSO algorithm in the T 2 S 2 G system estimates voltage magnitude near to real-state values (i.e., estimated states and real states are nearly the same).
In Figure 8, we compared the estimation error obtained by our proposed APSO based SE with the previous parallel SE method.Here we could see that APSO based distributed SE minimized the estimation error significantly compared with parallel SE even in the presence of an FDI attacker.The reason behind large estimation error in the parallel SE method is that this method requires additional trusted buses for SE and attack detection.However, it is not ensured that the trusted buses always provide original values.In the T 2 S 2 G system, all measurements are protected by the PEECC algorithm before being transmitted for SE.This will prevent the measurements from alterations.In addition, the APSO algorithm has the ability to estimate states optimally by considering the estimation error and penalty function in the fitness function.The proposed work minimized the estimation error significantly.In parallel SE, average estimation error was obtained as 0.02 p.u whereas the APSO algorithm obtained an average estimation error as 0.014 p.u.In our proposed T 2 S 2 G system, 0.6% of the estimation error was minimized compared with parallel SE.
Energies 2019, 12, x FOR PEER REVIEW 18 of 23 algorithm obtained an average estimation error as 0.014 p.u.In our proposed T 2 S 2 G system, 0.6% of the estimation error was minimized compared with parallel SE. • Effectiveness on the number of protected measurements.This metric estimates the number of measurements protected from the adversaries in the system.The number of measurements protected in the proposed work was compared with the sparse method since it protects a subset of measurements from adversaries.This metric estimates the number of measurements protected from the adversaries in the system.The number of measurements protected in the proposed work was compared with the sparse method since it protects a subset of measurements from adversaries.
In Figure 9, the number of measurements protected in the T 2 S 2 G system and in the sparse method was compared.In the sparse method, the minimal subset of measurements was identified and the measurements in that subset were protected from adversaries.The sparse method was only able to protect the minimum number of measurements up to five measurements.However, in the T 2 S 2 G system the PEECC algorithm was proposed to protect the measurements and the FNN algorithm was employed to identify the compromised meters and buses.Each measurement collected by RTU was encrypted using the PEECC algorithm, which protects the measurements from adversaries during transmission.With the support of the PEECC algorithm, it protects 85% of measurements from adversaries.Here 300 measurements out of 350 measurements were protected by the PEECC algorithm.This analysis shows that the proposed T 2 S 2 G system had the ability to protect the measurements in the SG system.

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Effectiveness on the number of protected measurements.This metric estimates the number of measurements protected from the adversaries in the system.The number of measurements protected in the proposed work was compared with the sparse method since it protects a subset of measurements from adversaries.
In Figure 9, the number of measurements protected in the T 2 S 2 G system and in the sparse method was compared.In the sparse method, the minimal subset of measurements was identified and the measurements in that subset were protected from adversaries.The sparse method was only able to protect the minimum number of measurements up to five measurements.However, in the T 2 S 2 G system the PEECC algorithm was proposed to protect the measurements and the FNN algorithm was employed to identify the compromised meters and buses.Each measurement collected by RTU was encrypted using the PEECC algorithm, which protects the measurements from adversaries during transmission.With the support of the PEECC algorithm, it protects 85% of measurements from adversaries.Here 300 measurements out of 350 measurements were protected by the PEECC algorithm.This analysis shows that the proposed T 2 S 2 G system had the ability to protect the measurements in the SG system.

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Effectiveness of detection probability.Detection probability defines the ability of the attack detection method to identify the attacker accurately.In the SG system, this metric evaluates the efficiency of the SE method since attack detection is carried out based on estimated states.
We compared the detection probability attained by the sparse method and proposed T 2 S 2 G system in Figure 10.The graphical analysis shows that the proposed T 2 S 2 G system achieved a • Effectiveness of detection probability.
Detection probability defines the ability of the attack detection method to identify the attacker accurately.In the SG system, this metric evaluates the efficiency of the SE method since attack detection is carried out based on estimated states.
We compared the detection probability attained by the sparse method and proposed T 2 S 2 G system in Figure 10.The graphical analysis shows that the proposed T 2 S 2 G system achieved a detection probability better than the sparse method.Detection probability was minimized in the sparse method since it increases estimation errors due to the centralized SE process.However, SE plays an important role in attack detection in the SG system.The sparse method suffers from lower detection probability.In the T 2 S 2 G system, better detection probability is achieved with the support of an effective APSO algorithm based SE, and FNN based compromised meter detection.Involvement of efficient processes in the T 2 S 2 G system improved the detection probability.However, SE plays an important role in attack detection in the SG system.The sparse method suffers from lower detection probability.In the T 2 S 2 G system, better detection probability is achieved with the support of an effective APSO algorithm based SE, and FNN based compromised meter detection.Involvement of efficient processes in the T 2 S 2 G system improved the detection probability.Averagely, 81% of attackers were detected by the sparse method whereas 90% of attackers were identified accurately in the T 2 S 2 G system.The T 2 S 2 G system improved detection probability up to 9% compared with the sparse method.
• Effectiveness on the successful detection rate.
This metric is also evaluating the efficiency of the proposed T 2 S 2 G system in attack detection.This metric measures the number of compromised meters/attackers identified as attackers accurately.
In Figure 11, we compared the successful detection rate achieved by the proposed T 2 S 2 G system and existing quickest detection method.From the analysis we can understand that the proposed T 2 S 2 G system achieved a detection rate better than the previous quickest method.In our work, 88% of attackers and compromised meters were detected with the help of APSO based SE and FNN based measurement classification processes.However, in the quickest detection method measurement security is not focused and the centralized SE process increases the system complexity.Since the measurements are not protected, there are more chances for measurements to be corrupted by attackers.The SE method results in inaccurate estimation, which leads to minimized detection rate.In the quickest detection method, 68% attackers were detected averagely (i.e., the proposed work achieved a 20% better detection rate than previous work).However, SE plays an important role in attack detection in the SG system.The sparse method suffers from lower detection probability.In the T 2 S 2 G system, better detection probability is achieved with the support of an effective APSO algorithm based SE, and FNN based compromised meter detection.Involvement of efficient processes in the T 2 S 2 G system improved the detection probability.Averagely, 81% of attackers were detected by the sparse method whereas 90% of attackers were identified accurately in the T 2 S 2 G system.The T 2 S 2 G system improved detection probability up to 9% compared with the sparse method.

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Effectiveness on the successful detection rate.This metric is also evaluating the efficiency of the proposed T 2 S 2 G system in attack detection.This metric measures the number of compromised meters/attackers identified as attackers accurately.
In Figure 11, we compared the successful detection rate achieved by the proposed T 2 S 2 G system and existing quickest detection method.From the analysis we can understand that the proposed T 2 S 2 G system achieved a detection rate better than the previous quickest method.In our work, 88% of attackers and compromised meters were detected with the help of APSO based SE and FNN based measurement classification processes.However, in the quickest detection method measurement security is not focused and the centralized SE process increases the system complexity.Since the measurements are not protected, there are more chances for measurements to be corrupted by attackers.The SE method results in inaccurate estimation, which leads to minimized detection rate.In the quickest detection method, 68% attackers were detected averagely (i.e., the proposed work achieved a 20% better detection rate than previous work).

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Effectiveness on detection delay.• Effectiveness on detection delay.Detection probability and detection rate measures the efficiency of the T 2 S 2 G system in attack detection.The attack detection performance is further evaluated based on the detection delay.Detection delay is defined as the time consumed for attack detection by the T 2 S 2 G system.
Comparative analysis on detection delay is illustrated in Figure 12.For an efficient detection system, this metric should be low as possible.The analysis shows that the proposed T 2 S 2 G system required a minimum time for attack detection.The detection delay in the quickest detection method was large since the involvement of centralized SE increases the time consumption for attack detection.However, in the T 2 S 2 G system SE process was performed in a distributed manner that minimizes the time required for attack detection.In addition, the involvement of FNN in CS also helps to minimize the detection delay.In the quickest method, 6.5 ms of detection delay was averagely experienced for attack detection.This delay was minimized to 1.95 ms in the T 2 S 2 G system 4.55 ms lower than the previous quickest method.
was large since the involvement of centralized SE increases the time consumption for attack detection.However, in the T 2 S 2 G system SE process was performed in a distributed manner that minimizes the time required for attack detection.In addition, the involvement of FNN in CS also helps to minimize the detection delay.In the quickest method, 6.5 ms of detection delay was averagely experienced for attack detection.This delay was minimized to 1.95 ms in the T 2 S 2 G system 4.55 ms lower than the previous quickest method.The overall comparative analysis shows that the proposed T 2 S 2 G system achieved a better performance than previous works in security provisioning of SG systems.Involvement of the WQT method supports the distributed SE process.The measurements are protected by the PEECC algorithm and SE is performed by using the APSO algorithm.Each aggregator is authenticated in the CS through the LSA scheme.In the CS, FNN is employed for compromised meter detection by analyzing aggregated measurements.The proposed T 2 S 2 G system improved security of SG systems under an FDI attack.

Conclusions
This paper proposed a novel T 2 S 2 G system to provide protection for SG systems against an FDI attack.To minimize overhead and complexity distributed SE process is involved in the T 2 S 2 G system.The distributed SE process is supported by system partition, which is performed by the WQT method.In each partition, an aggregator is deployed to enable the distributed SE process.All measurements collected by RTUs are encrypted using the PEECC algorithm in order to ensure that no adversary can alter the measurements during transmission.Upon aggregated measurements, aggregator estimates state variables.For the SE process, the APSO algorithm is proposed in which the estimation error is minimized.Aggregators are authenticated at the CS through the LSA scheme to verify the authenticity of aggregator.Then the measurements are received by the CS from legitimate aggregators.For measurement classification, FNN is employed in the CS to identify the compromised meter/buses.The proposed T 2 S 2 G system improved security of SGs under an FDI attack through SE, encryption, authentication, and classification processes.Simulation of the T 2 S 2 G system showed better performance was achieved by the T 2 S 2 G system in terms of estimation error, number of protected measurements, detection probability, detection rate, and detection delay.
In the future, we have planned to extend the T 2 S 2 G system with other attack detection mechanisms to defend against attacks held on the SCADA system and under more diverse The overall comparative analysis shows that the proposed T 2 S 2 G system achieved a better performance than previous works in security provisioning of SG systems.Involvement of the WQT method supports the distributed SE process.The measurements are protected by the PEECC algorithm and SE is performed by using the APSO algorithm.Each aggregator is authenticated in the CS through the LSA scheme.In the CS, FNN is employed for compromised meter detection by analyzing aggregated measurements.The proposed T 2 S 2 G system improved security of SG systems under an FDI attack.

Conclusions
This paper proposed a novel T 2 S 2 G system to provide protection for SG systems against an FDI attack.To minimize overhead and complexity distributed SE process is involved in the T 2 S 2 G system.The distributed SE process is supported by system partition, which is performed by the WQT method.In each partition, an aggregator is deployed to enable the distributed SE process.All measurements collected by RTUs are encrypted using the PEECC algorithm in order to ensure that no adversary can alter the measurements during transmission.Upon aggregated measurements, aggregator estimates state variables.For the SE process, the APSO algorithm is proposed in which the estimation error is minimized.Aggregators are authenticated at the CS through the LSA scheme to verify the authenticity of aggregator.Then the measurements are received by the CS from legitimate aggregators.For measurement classification, FNN is employed in the CS to identify the compromised meter/buses.The proposed T 2 S 2 G system improved security of SGs under an FDI attack through SE, encryption, authentication, and classification processes.Simulation of the T 2 S 2 G system showed better performance was achieved by the T 2 S 2 G system in terms of estimation error, number of protected measurements, detection probability, detection rate, and detection delay.
In the future, we have planned to extend the T 2 S 2 G system with other attack detection mechanisms to defend against attacks held on the SCADA system and under more diverse conditions.In addition, the T 2 S 2 G system can be extended for more investigation regarding to energy consumption, measurements computation, and communication overhead.

Figure 1 .
Figure 1.False data injection (FDI) attack on a smart grid (SG) system.

Figure 2 .
Figure 2. Two-tier secure smart grid (T 2 S 2 G) system architecture based on the IEEE-9 bus system.

Figure 2 .
Figure 2. Two-tier secure smart grid (T 2 S 2 G) system architecture based on the IEEE-9 bus system.

Figure 5 .
Figure 5. Scheduling process in the logical schedule based authentication (LSA) scheme.

Figure 7 .
Figure 7.The T 2 S 2 G system based on the IEEE-9 bus system.

Figure 7 .
Figure 7.The T 2 S 2 G system based on the IEEE-9 bus system.

Figure 8 .
Figure 8. Analysis on the estimation error.

Figure 8 .
Figure 8. Analysis on the estimation error.

Energies 2019, 12 , 2555 •
Effectiveness on the number of protected measurements.

Figure 8 .
Figure 8. Analysis on the estimation error.

Figure 9 .
Figure 9. Analysis on the number of protected measurements.

Figure 9 .
Figure 9. Analysis on the number of protected measurements.

Energies 2019 ,
12,  x FOR PEER REVIEW 19 of 23 detection probability better than the sparse method.Detection probability was minimized in the sparse method since it increases estimation errors due to the centralized SE process.

Figure 10 .
Figure 10.Analysis on the detection probability.

Figure 10 .
Figure 10.Analysis on the detection probability.

Figure 10 .
Figure 10.Analysis on the detection probability.

Figure 11 .
Figure 11.Analysis on the successful detection rate.

Figure 11 .
Figure 11.Analysis on the successful detection rate.
2019, 12, 2555 9 of 24Here A 1 is responsible to aggregate measurements from G 1 , B 2 , and B 7 whereas A 2 is responsible for aggregating measurements from L 1 , L 2 , B 1 , and B 8 .Similarly, A 3 aggregates measurements from G 2 , B 1 , and B 4 .Measurements from G 3 , L 3 , B 3 , and B 9 are aggregated by A 4 .In this manner, all measurements are collected by a deployed aggregator in each section.The measurements are collected from buses, generators by RTUs. , and  .Measurements from  ,  ,  , and  are aggregated by  .In this manner, all measurements are collected by a deployed aggregator in each section.The measurements are collected from buses, generators by RTUs.
Plain text of Z i Output: Cipher text of Z i (En[Z i ])

Table 2 .
Fuzzy rules deployed in the FNN.

Table 4 .
Analysis on previous works.