fuzzy based coordinated voltage controller

Abstract—The increase in power demand is stressing the transmission and generation system capabilities, leading to frequent power outages. This led to increased research aiming to meet the growing energy demand without adding the transmission system capabilities. The use of distributed generation at the distribution system seems to be a viable solution. Therefore, a new peer-to-peer network architecture for distribution system, namely micro-grid was proposed. Sufficient amount of dynamic reactive capabilities are needed to avoid voltage collapse in micro-grid as well as in power systems. In principle, a coordinated effort among the reactive power sources could result in better effectiveness of these resources. In microgrids, the electrical distances between the sources of the reactive power and the loads that need the reactive compensation is not long; thus a coordinated compensation of reactive sources for dynamic voltage stability should be desirable.

This paper proposes the use of a fuzzy based central controller for the improvement of the dynamic voltage profile in a Micro Grid. The associated controller is termed as a Fuzzy Based Coordinated Voltage Controller (FCVC). The FCVC is a secondary level voltage controller implemented in a Micro Grid test system. A dynamic simulation of the test system is carried out for the cases of with and without the FCVC for various disturbances in both grid-connected and islanded modes of operation.

I. INTRODUCTION ISTRIBUTED Energy Resources (DERs) such as Fuel Cells (FCs), Diesel Engine Generators, Photo- Voltaic

systems (PVs), Wind Turbines (WTs) and Distributed Energy Storage systems (DESs) offer many advantages for power system. They can effectively reduce CO2 emissions in the electricity generation field, mitigate peak demand, increase reliability against power system faults and improve power system quality via sophisticated control schemes [1], [2], [3]. The concept of Micro Grid has been proposed to solve the common interconnection problems of individual DERs in various power systems. A Micro Grid is defined as an independent Low Voltage (LV) or Medium Voltage (MV) distribution network comprising various Distributed Generations (DGs), Distributed Energy Storages (DESs), and controllable loads that can be operated in three distinct modes: 1) grid-connected, 2) islanded (autonomous), and 3) transition mode [4]. During power outage or disturbance, Micro Grids

Amalu Philip, M.Tech Power System, Saintgits College of .Engineering,

Kottayam. Rishi Menon, Assistant Professor, Saintgits College Of Engineering,

Kottayam

can island themselves and retain power availability, avoiding blackouts and lost productivity [5].

Sufficient amount of dynamic reactive power capabilities are needed to avoid a fast voltage collapse. In principle, a coordinated effort among the reactive sources could result in better effectiveness of these resources. However, in typical power systems, where the electrical distances between the reactive sources and where these reactive powers are needed are long, a coordinated effort may not be suitable due to the excessive voltage drop resulting from the transfer of reactive power within long distances. That is why, in practice, reactive power compensation is usually coming from local sources. In micro-grids, the electrical distances between the sources of the reactive power and the loads, which need the reactive power compensation, is not much; thus a coordinated compensation of reactive power sources for dynamic voltage stability should be desirable.

In this paper, the multi-level control structure of a typical Micro Grid and modelling of microgrid is described and a fuzzy based centralized control method to improve the Micro Grid dynamic voltage profile is proposed. The associated controller is termed as a Fuzzy Based Coordinated Voltage Controller (FCVC). It can be used to coordinate the consumption of reactive power in a micro-grid system. The FCVC based on the inputs from load buses, runs a subroutine and gives a signal to the DGs to kick in the reactive power, to support the new reactive load that switches into the micro-grid system. A 21-Bus microgrid test system is used to verify the performance of the proposed controller. The dynamic modelling of the microgrid and the proposed controller has been done using SIMULINK. Simulations are run for various dynamic events and the voltages of the load Buses are compared with and without the presence of FCVC in both grid-connected and islanded modes of microgrid.

The following section discusses modelling of microgrid and proposed FCVC. Simulation results and conclusion are explained in the section III and section IV.

II. MODELING OF MICROGRID AND FCVC In this paper, the modelling of the microgrid includes the

modelling of the Diesel Engine Generators, system loads and the transmission system.

A. Modeling of Diesel Engine Generator

A diesel engine generator (DEG) is widely used in remote locations, household, commercial and industrial applications. The prime-mover is an internal combustion engine which is coupled to a synchronous generator with exciter and a governor. The generator and the prime-mover are mechanically coupled; the dynamics of the SG are not

Fuzzy Based Coordinated Voltage Controller Amalu Philip and Rishi Menon

D

Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications – ICMF 2013 433

ISBN 978-93-82338-83-3 © 2013 Bonfring

electrically decoupled from dynamics of the output of the generator [4].

Diesel generating set is made up of diesel prime mover, speed governor, generator and excitation system. Block scheme of system structure shown as figure2. From figure 2, we can see this system has two closed-loop feedback control system: speed feedback control system (figure 3) and excitation feedback control system (figure 4).

Fig.2 Block scheme of diesel generator set system structure

Fig. 3 Diesel engine speed feedback control system

Fig. 4 Generator excitation feedback control system

Diesel engine has no automatic speed control , thus it must furnish governor. The purpose is to ensure that the diesel engine has specified speed at stable operation. Excitation system is the important segment to ensure the voltage accuracy of generation and improve the stable operation of power system. In the power station, excitation control system is the basic and essential component.

The generator model takes into account the dynamics of the stator, field, and damper windings. The equivalent circuit of the model is represented in the rotor reference frame (q d frame). All rotor parameters and electrical quantities are viewed from the stator. The following equations guide the dynamics of synchronous machine.

(1)

. ′ . ′ ′ (2) ′

′

′ ′ (3)

′′

′ ′ (4)

B. Sub Transmission Network : PI Model

Fig 7 Sub Transmission Network Model

(5)

(6)

Where Zc is characteristic impedance and is propagation constant.

C. Transformers : PI Model

Fig 8 Transformer PI Model

D. Automatic Controller (FCVC)

The FCVC works similarly as a power system stabilizer (PSS) in a power system. The FCVC gives the input to the excitation system or reactive power loop of DGs, which acts to kick in more reactive power into the micro grid to prevent any voltage collapse. Any small reactive load can be met by the DGs avoiding the use of expensive dynamic reactive sources, such as STATCOM, SVC or capacitor banks.

The input to the MGVS is a measure of the voltage deficiency of the micro grid. The per unit voltage deviation (ΔVerr i) between the desired voltage (Vdes i) and the dynamic voltage (V act i) is calculated for all the load Buses (i = 1…l ) as shown below in (7).

∆ (7)

Weighting factors for all Buses, based on the importance

of the Bus (i.e. induction motor loads are more sensitive to disturbances than the resistive loads) are defined. A weighted average of ΔVerri is taken, to get an aggregate voltage deficiency (ΔVerr) of the system as shown below in (8). The weighting factors for all the load Buses, i= 1 to l are α1, α2….


ISBN 978-93-82338-83-3 © 2013 Bonfring

Load weighing factor,

∆ ∆ ∆ .... ∆ (8)

ΔVerr is fed to FCVC block. The output of this FCVC controller is VFCVC , as shown in Figure 9.

Fig 9.Fuzzy Based Coordinated Voltage Controller

VFCVC represents the total FCVC correcting signal, which is divided between the DGs depending on the generation reactive reserve, proximity to inductive loads, etc. The weighting factors for the generator Buses (1 to g) are β1, β2, … … . . βg. VFCVCi is the input to the ith generator’s excitation system as shown in Figure 10.

Fig 10. Excitation system of a generator

E. Fuzzy Controller Modeling

There are three principal elements to a fuzzy logic controller:

Fuzzification module (Fuzzifier) Rule base and Inference engine Defuzzification module (defuzzifier)

The general algorithm for a fuzzy system designer can be

synthesized as follows:

F. Fuzzification

Select fuzzy input and output vectors.

Choose number and shape of the membership functions for the fuzzy input and output vectors.

Calculate of the membership grades for every crisp value of the fuzzy inputs.

G. Fuzzy Inference

Complete the rule base. Identify the valid rules stored in the rule base. Calculate membership grades contributed by each

rule.

H. Defuzzification

Calculate the fuzzy output vector, using an adequate defuzzification method.

Simulation results are obtained.

III. SIMULATION RESULTS A 21-Bus microgrid test system is used to verify the

performance of the proposed controller. The system is composed of a 13.8-kV, three-feeder distribution subsystem which is connected to a large network through a 69-kV radial line. The 13.8-kV substation bus bar is radially connected to the main grid through the substation transformer and a 69-kV line. The network at the end of the 69-kV line is represented by a 69-kV, 1000-MVA short-circuit capacity bus. Several loads are supplied through four radial feeders of the subsystem. The loads are of constant power type and/or dynamically varying loads.

In this section, simulation results are presented for dynamic voltage analysis for various dynamic events under both, grid-connected and islanded micro grid modes of operation and also during the islanding process. The dynamic events include three-phase short circuit fault, and load switching. The results are compared with and without the presence of the proposed FCVC in each case. This will show the effectiveness of the FCVC to use reactive power compensation to improve the voltage profile of the micro grid in case of such different disturbances.

Simulation results are presented for dynamic voltage analysis for various dynamic events under both, grid-connected and islanded micro grid modes of operation. The dynamic events include line outage, three phase short circuit fault, and load switching. The results are compared with and without the presence of the proposed FCVC in each case.


ISBN 978-93-82338-83-3 © 2013 Bonfring

Fig 11. 21 Bus Test System

I. Voltage Stability Analysis In Grid Connected Mode

In the grid connected mode, the microgrid test system is connected to the main grid. The load at Bus 15 is the largest load in the microgrid. So, the load Bus weighing factor is highest for Bus 15. The load Bus weighting factors are given as follows.

Table III. Load weighting factor data

0.108 0 0.134 0.42 0.22 0.105

A three phase short circuit fault is applied at Bus 15. The disturbance starts at 5% of the total time (i.e. 10 sec), and ends after 2 secs. The results show that the load bus voltages improve by more than 10 % in the presence of a FCVC as shown in Figure 12 at Bus 19.

Figure 12. Three phase fault: Comparison of voltage a Bus 19

with and without FCVC in grid connected mode

Figure 13. Load Switching: Comparison of voltage of Bus 15 with and without FCVC for 30% increase in load at Bus 15 in

grid connected mode. In other case, the load at Bus 15 has been increased by

30%. The disturbance starts at 5% of the total time (i.e. 10 sec), and ends after 5 seconds. The results show that the voltage at Bus 15 improves in the presence of a CVC as shown in Figure 13.

J. Voltage Stability Analysis of the Microgrid System during

Islanding Operation During the islanding operation, the line between Bus 2 and

Bus 3 is removed, effectively disconnecting the microgrid from the main grid. Now the total load is supported by DGs at Buses 5, 9 and 17. The load parameters remain unchanged, but the power generation at the generators changes during the dynamic events. The disturbance starts at 5% of the total time (i.e. 10 sec), and ends after 3 seconds. The results show that the voltage at Bus 19 improves by more than 7 % in the presence of a FCVC as shown in Figure 14.


ISBN 978-93-82338-83-3 © 2013 Bonfring

Figure 14. Islanding operation: Comparison of voltages a bus

15 with and without FCVC

K. Voltage Stability Analysis of the Microgrid System in Islanded Mode

In the islanded mode of operation, the main grid is removed and the total load is supported by DGs.

Figure 15. Three phase fault: Comparison of voltage a bus 19

with and without FCVC in islanded mode A three phase short circuit fault is applied at Bus 15. The

disturbance starts at 5% of the total time (5 sec), and ends after 70 cycles. The results show that the load bus voltages improve by more than 15 % in the presence of a FCVC as shown in Figure 15 at Bus 19.

Figure 16. Line outage: Comparison of voltage a Bus 19 with

and without F CVC in islanded mode

A line outage disturbance is applied at between Bus 3 and Bus 16. The disturbance starts at 5% of the total time (i.e. 6 sec), and ends after 2 seconds. The results show that the load bus voltages improve in the presence of a CVC as shown in Figure 9 at Bus 21.

IV. CONCLUSION This paper investigates the 13.8 kV multiple DG feeder

micro grid regarding voltage stability enhancement by a fuzzy based coordinated voltage controller. It works as secondary loop control for automatic voltage regulator (AVR) same as power system stabilizer. FCVC is modelled in MATLAB simulink and implemented for 21 bus test system.

The load bus voltages and reactive power compensation of DGs are compared with and without the presence of FCVC for various disturbances. The effectiveness of FCVC is studied in both grid connected and islanded modes of micro-grid operation separately. The use of a FCVC in a micro-grid improves its voltage profile during disturbances. Also, the FCVC designers have the ability to choose the load buses to be controlled and the reactive power compensation to be given by each DG. It is also observed that there is considerable decrement in line losses as well as increment in line power flows.

REFERENCES [1] M.U. Zahnd, "Control Strategies for Load-Following Unbalanced

MicroGrids Islanded Operation," Faculté Sciences et Techniques de l'Ingénieur, Lausanne, MS Thesis 2007.

[2] A. Kurita and T. Sakurai, "The power system failure on July 23, 1987 in Tokyo," in Proceedings of the 27th IEEEConference on Decision and Control, Austin, 1988, pp. 2093 - 2097.

[3] US-Canada power system outage task force, "Final report on the august 14,2003 Blackout in the United States and Canada: Causes and Recommendations," Nautral resources Canada and U.S. department of energy, 18 Pages Report.

[4] L. Robertr, A. Akhil, M. Chris, S. John, D. Jeff, G. Ross, A. S. Sakis, Y. Robert and E. Joe, "Integration of Distributed Energy Resources: The CERTS MicroGrid Concept," U.S. Department of Energy, White Paper LBNL-50829, 2002.

[5] P.W. Sauer and M.A. Pai, Power System Dynamics and Stability, 2nd ed. Champaign, USA/Illinois: Stipes Publishing, 2006.

[6] P. L. Jeffrey, "Modeling of Dynamic Loads for Voltage Stability Studies," Tennessee Technological University, Cookeville, MS Thesis 2007.

[7] F. Katiraei, M.R. Iravani and P.W. Lehn, "Micro-grid autonomous operation during and subsequent to islanding process," IEEE Transactions on Power Delivery, vol. 20, no. 1, pp. 248-257, Jan 2005.

[8] P. L. Jeffrey, "Modeling of Dynamic Loads for Voltage Stability Studies," Tennessee Technological University, Cookeville, MS Thesis 2007.

[9] R.H. Lasseter and P. Paigi, "Micro-grid: a conceptual solution," in IEEE 35th Annual Power Electronics Specialists Conference, Madison, 2004, pp. 4285 -4290.


ISBN 978-93-82338-83-3 © 2013 Bonfring

fuzzy based coordinated voltage controller

Documents