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ANFIS controller based frequency linked availability based tariff mechanism for a restructured power system

This paper proposes the frequency linked availability based tariff (ABT) mechanism along with ANFIS controller for extenuating the issues in load frequency control (LFC) for a four area deregulated/restructured power system. To scrutinize the effective performance of the proposed approach, the dynamic performance of the system is analyzed with single and bilateral contract considering the value of the marginal cost higher and lesser than the Unscheduled interchange (UI). For this analysis, UI rate chart from the Central Electricity Regulation Commission (CERC) is used. The simulation results confirm that the ANFIS controller performance is comparatively better than proportional controller in two different contract cases.


Introduction
Load frequency control (LFC) is one of the prominent issues in a deregulated power system operation and control for maintaining secure electric power to the consumers. Major objectives of the LFC in a restructured power system is (i) to maintain the expected power output and the nominal frequency must be in specified limits. (ii) to maintain the exchange of net power between control areas within its pre specified values [1][2][3]. In a competitive power market several market players like Generation companies (GENCO) distribution companies (DISCO), transmission companies (TRANSCO) and independent system operator (ISO) came into existence and main power transactions are carried out by these entities [3,5,9]. In this environment, DISCOs in one control area may undergo contract with GENCOs with its own control area or other areas. [5][6][7]. For secure and reliable operation of the entire system, ISO provide certain ancillary services [4]. Load following and LFC and are treated as the ancillary services for maintaining the frequency and prolonging the power system reliability and security. The dynamic responses of the power system network are improved with the help of some control strategies such as fuzzy logic, AI based neural network, ant colony algorithm, genetic algorithm, particle swam optimization algorithm are applied for controlling the output of the system [7,8,9]. These methodologies continuously tracking the fluctuations in the load and vary the governor setting points as soon as possible to move the system back to the normal stable operation [10][11][12][13].

Availability Based Tariff (ABT)
In 2002, a new tariff structure ABT is introduced in Indian power system. It replaces the conventional monolithic charge structure into a rational three-part tariff structure [17][18][19][20][21]. First part of the ABT is called the constant portion, which is connected to the available generating units, the second portion is the variable part, connected with the energy charge for schedule interchanges and the third portion is called frequency dependent part which is connected with unscheduled interchange. The UI behaves as a tool for controlling grid frequency and encourages the system to regain back to its normal frequency [20-23]. The three major components of ABT are discussed below.

fixed charge or capacity charge
The major parts of fixed charge are the interest and depreciation of the loan, maintenance cost, insurance, taxes etc. The total cost to be paid to the producing company during the year for the fixed cost will depend on mean availability of the unit in the same particular year.

Variable charge or energy charge
This charge comprises of the fuel cost or variable cost of the plant for the scheduled power generation. Energy charges or variable charges are payable by every consumer based on the scheduled energy and it is irrespective to the actual power generation.

Un scheduled interchange
If the system may violate the scheduled power exchange, the third portion of ABT came into picture. If frequency is higher than 50Hz, then the UI price is low and vice versa. In case of excess drawl, beneficiary unit or company has to remunerate the penalty as per the UI rate. As per CERC, UI rate chart are given in Fig.2. The deviation from the scheduled value is termed as UI pricing scheme in ABT [24-26].

Basic scheme for ABT based LFC
A framework for a ABT based frequency linked scheme for a single area LFC system is given in Fig.  1 [16]. In this scheme, the primary control loop mechanisms with Free Governor Mode of Operation (FGMO) is used for suppressing the area frequency oscillations within few seconds and the control loop in secondary side (LFC or AGC) is also try to move back the system frequency to its nominal value within five to ten minutes. Due to the lack of generation, successful application of a system level The concept of ABT based frequency control loop is given in Fig.2. Here each Generating unit monitoring the UI cost ρ and it is compared with the marginal price γ. The error or deviation signal is derived, from the change in present UI cost and its marginal price. The deviation signal, is called as GCE which is given as input to the controller. Because of ABT, the amount received by generating units for UI value are different from the amount to be paid for Schedule Interchange (SI). For all the cases the generating units will earn the profit.  Figure 2. ABT based load frequency control scheme.

System modeling
The schematic representation of a four area restructured power system with ABT based frequency control loop system is given in Fig. 4. It consists of four different control areas consisting GENCO and DISCO of one in each area. GENCO having Hydro plant is considered in area-1 and thermal GENCOs are considered in other three areas. In a restructured power system GENCOs can contract with DISCOs in the same or other area. This type of contract is termed as "Bilateral contract" and it is implemented through Independent System Operator (ISO) [3,4]. Various schemes of contract are explained in Distribution type Participation Matrix -(DPM) given as follows.
The total number of columns and rows in a DPM shows the number of DISCOs and GENCOs respectively and each factor in the DPM is represented as contract participation factor (cpf), which represents the fraction of total power contracted between DISCOs and GENCOs [4][5][6]. The addition of all the values in a column must be equal to one. ie., It is assumed that all generating units in each individual area are generating power at scheduled value and frequency of the grid frequency is considered as 50Hz. If a sudden load disturbance occurs in any area, the area frequency and tie-line power exchange must be altered, which indicates in the supply frequency deviation Δf.
In addition, with this, the incremental cost of the thermal units and hydro units are expressed as Where a and b are called as incremental cost co-efficient and Gi P is power output of hydro GENCO in ith area.
Similarly, IC in the case of hydro GENCO is expressed as Where c and d are called as co-efficient of incremental cost and Gi P is called power output of ith  The UI based price signal S2 (ρ) is now comparing with the other factor incremental cost based signal S4 (γ) and produce the generation control error (GCE) signal. UI price at 50.0Hz is denoted as ρ 0 and it is calculated from the UI chart issued by CERC, India. Modified GCE control scheme flow chart is shown in Fig.3 [16]. The control scheme ensures that whenever a sudden variation in the load demand occurs each generating unit responds to change their generation based on the error signals received from GCE block for smoothening the frequency. To reduce the Generation Control Error (GCE) of generating units after a sudden load disturbance, ANFIS controller is used for minimizing the area frequency oscillations and tie-line power oscillations for enhancing the stability of the system. Detailed analysis of ANFIS controller is discussed below.

Concept of suggested ANFIS controller
ANFIS controller is the combination of artificial neural network (ANN) and fuzzy logic based adaptive type network having zero synaptic weight. Fig.5 shows the ANFIS structure including outputs and inputs. The concept is taken from the references [12][13]. It is assumed that the FIS with Takagi -Sugeno's controller having inputs x and y and an output z is considered here [27].

Simulated responses -Results and discussions
The suggested four area deregulated system model with ABT based ANFIS control scheme is shown in Fig.4. The parameter values of the test system are obtained from the regional Indian system which is shown in Appendix A. simulation procedures are performed under deregulated environment having three possible contract scenarios which are explained below      Frequency deviations and GENCOs power output deviations of the proposed ABT based multi area deregulated power system with Proportional and ANFIS controllers under unilateral contract are shown in Figs. 6 and 10. UI rate and frequency of area-1 under unilateral contract scenario is shown in Fig.8. Detailed time domain analysis based on the performance indices peak undershoot (PUS), peak overshoot (POS) and settling time (Ts) of frequencies and GENCOs output deviations for the Proportional and ANFIS controllers are given in Table 1 and 2. From all these results, it should be clear that ABT system with ANFIS controller reduces the frequency and GENCOs power output deviations better than the P controller.

Case 2-Bilateral contract
In bilateral contract case, system DISCOs are having the chance to collaborate with any one of the GENCOs in the same control area or other areas. The disturbance on DISCO is considered as     Frequency deviations and GENCOs power output deviations of the proposed ABT based multi area deregulated power system with Proportional and ANFIS controllers under bilateral contract are shown in Figs. 9 and 10. UI rate and frequency of area-1 under bilateral contract scenario is shown in Fig.11. Detailed time domain analysis based on performance indices such as peak undershoot (PUS), peak overshoot (POS) and frequency settling time (Ts) and tie-line power variations for Proportional and ANFIS controllers are given in Table 3. Time domain analysis of each GENCOs are also given in Table 4. From all these results, it should be clear that ABT system with ANFIS controller reduces the frequency and GENCOs power output deviations than other controllers.

Conclusion
This paper reveals that the frequency linked ABT mechanism with ANFIS controller can improve grid frequency and GENCOs power output deviations as compared to the existing manual UI based control structure. The ANFIS controller technique can effectively control all the GENCOs in each area and improves the performance of frequency. This helps in reducing the cost of unneeded exchange of power between generation companies and utilities. Time domain outputs shows that frequency linked ABT mechanism with ANFIS based controller provides better performance in view of performance indices such as undershoot, overshoot, settling time of frequency deviations and GENCO power deviations than conventional proportional controller.