Impact of Superconducting Magnetic Energy Storage on Frequency Stability of an Isolated Hybrid

Power System Shailendra Singh1, Harshita Joshi1,Saurabh Chanana2, Rohit kumar verma1

Electrical and Electronics Engineering department1, Electrical Engineering department2 Graphic Era University1 Dehradun, India, National Institute of Technology2 Kurukshetra India.

Corresponding Author email id: singh.shail1986@gmail.com

Abstract— Due to the fast response of superconducting energy storage system, it may improve the stability of system frequency. This paper proposed the modeling and control of a hybrid Wind Power, Diesel-Engine Generator (DEG) – Superconducting Magnetic Energy Storage system connected to an Isolated power system. A case study on the impact of SMES operations on the frequency stability of Hybrid System has been carried out. The limits on the inductor (coil) current due to the possibilities of discontinuous conduction in the presence of large disturbances and limited storage capacity have also been incorporated. The modeling of wind power is also presented and analyzed for a time period of 24-hours with varying wind velocity. Hybrid system has analyzed in the Matlab Simulink environment. Simulation results indicate that SMES system can contribute to frequency stability in both cases when the load increases and also when the load drops. This feature of SMES system will be helpful if it is operated along with a hybrid Wind-PV System.

Keywords— Distributed Generation (DG); Load Frequency Control; Superconducting Magnetic Energy Storage (SMES); Wind Power.

I. INTRODUCTION For sustainable development of any nation, energy plays an essential role. Therefore, there is a need to actively look for renewable energy technology innovations, review for optimization of resource inputs and plan to proceed with effective energy strategic planning. Currently, worldwide renewable energy technologies have been undergoing rapidly development [1]. Distributed power generation technologies such as micro hydro and gas turbines, photovoltaic, biomass and wind power are gradually overtaking traditional generation technologies. Due to rapid development in DG technology, they have many advantages. [2], [3]. When by the available means generation exceeds demand, energy is stored and it is returned when demand exceeds generation. The intermittent nature of power output from renewable source favours the use of energy storage for power provision. To overcome these difficulties renewable power generating systems are integrated along with different energy storage systems like battery, ultra capacitor, and SMES etc[4],[18]. In hazards etc. [4-6], SMES can feed the power(MWs) in few seconds. Due to this fast response, it allows a storage unit to make available spinning reserve and enhance stability of the system. Harmonics presents in the AC bus and in the coil are generally produced by converter. These harmonics can be eliminated by using higher pulse converters. Due to salient features of SMES technology, it has obtained much interest in power system applications, such as frequency control, automatic generation control,

uninterruptible power supplies, etc. The real power as well as the reactive power can be absorbed or released from the SMES coil according to system power requirements. Therefore it can increase the control ability, and enhance reliability and accessibility. Accordingly, SMES has naturally high storage efficiency around 90% [5]. Effect of SMES for minimizing the variation in system frequency has been discussed by various authors [6], [8], [10]. In [7] authors have presented effect of SMES in power distributed network along with PV system. Power flow management and control of micro grid had proposed by [9],[13]. Dynamic modeling of intergradations of SMES with wind and diesel generator has been done by [11], [12], [14]. Due to fast growing technology and development, in future its cost may be in considerable range. In this paper, the impact of SMES operations on the frequency stability under isolated condition has been discussed. Operation and control strategy unit of SMES has also been discussed in this work. Limit of deviation of inductor current and modeling of wind speed to wind power is also incorporated. Organization of the paper is as follows: Operation and control unit of SMES is discussed in Section II. The wind generation model is discussed in Section III. The proposed hybrid system model is explained in Section IV. Operation strategy has been described in Section V. Section VI presents Matlab Simulink based simulation results, case study. Section VII concludes the paper.

II. SUPERCONDUCTING ENERGY STORAGE SYSTEM (SMES)

SMES stores energy in the form of magnetic field by converting electrical energy into magnetic energy. It stores power, flows a circulating DC electric current into the coil magnetically without any mechanical linkage. SMES pump out power through a reversal of the process. The rated power (W) and energy stored (Joules) are generally the given specification for SMES devices and can be represented as follows:

(1) Where Lsm is the coil inductance, Icoil is the DC current flowing through the coil and Vcoil is the voltage across the coil [2-6]. A. Operation of SMES The magnetic coil must be remained in the super conducting states, throughout the operation of SMES. The cryogenic system with

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refrigerator is used to maintain the desired temsuperconducting action. Power electronic intepower systems and superconducting coil is pconditioning system (PCS) [2], [13]. PCS permitto respond for maximum charging rate to the marate of power. Generally a current source inverterinverter with a DC–DC chopper interface is function of charging/discharging of a supercondirected by varying the average voltage across theither positive/negative values is controlled througIn standby mode of operation, the coil currenwhich does not affect the storage level. To obchopper duty cycle has to be fixed at 50% whacross the superconducting coil is zero [10]. The generated DC voltage across the inductor is cin nature under the specific range of positive andcan be controlled by varying the firing angle ( ) oinductor is primarily charged to its rated currenttiny positive voltage. When the current reaches voltage across the inductor has to be reduced to current constant [11]. The transformer and the neglected, so the DC voltage is given by:

Ed = 2Vd cos – 2Id RC Where Ed = DC voltage applied to the inductor in = firing angle in degrees, Id = current flowing through the inductor in kA, RC = equivalent commutating resistance in k Vdo = maximum circuit bridge voltage in kV. Controlling of charging and discharging of the SMthe altering of commutation angle . The convecharging mode (converter mode), when is ledischarging mode (inverter mode) when gr[12]. When there is drop in frequency, power is supsystem. Direction of current flowing through indoes not change suddenly, so the control volnature. The incremental change in the voltage appis expressed as

Ed = [ ] F

Figure 1: Circuit diagram of SMES [

mperature for proper erface between AC provided by power ts the SMES system aximum discharging r or a voltage source used in PCS. The

ducting coil can be he coil. Coil voltage gh DC–DC chopper.

nt remains constant, btain this condition hich results voltage

constantly regulating d negative values. It of the converter. The t Ido by supplying a its rated value, the

zero to maintain the converter losses are

(2)

KV,

MES unit is through erter operates in the ess than 90° and in reater than 90° [11]

pplied back into the ductor and thyristor ltage is negative in plied to the inductor

(3)

[10]

A. Block diagram and control uni

Figure 2: SMES block diagram withfeedba

where, Ed is the incremental chaconverter time delay, KSMES the gainput signal to the SMES condeviation ( Id) is given by

Id = Due to change in system frequencSMES power flow PSMES is given PSMES = Ed

Id = Ido + Id Where Id is net inductor current Assuming the power flow from Dis negative [11], [14].

III. WIND SPEED TO POWIn this paper, a unique method forconversion has considered. By usireproduce sequences of wind speeunderstood that wind speed is components: average speed (vwa),(vwg) and wind turbulence (vwt). Th This wind speed model is called projected by Wasynczuk et al. [15significant equations for the escomponents may be study in ref. also proposed by Chanana and Kusimulated real time analysis for a tthe daily average wind speed in average speeds taken in other apmultiple wind gusts and wind ramand ending times has also been incDue to fluctuations present in higlocally, will generate uneven flucttype of fluctuations in rotor part isTo approximate this effect, low paconversion model as shown in affects the time constant are mspeed and turbulence intensity of wThe power produced by wind turbfollowing equation:

it of SMES

h negative inductor current deviation ack [14] ange in converter voltage, Tdc is the ain of the control loop and F is the ntrol logic. The inductor current

(4) cy F net resulting the deviation in n as:

. Id (5)

(6)

DC to AC is Positive and AC to DC

WER CONVERSION MODEL r modeling of wind speeds to power ing this technique, it is realizable to eds with desired characteristics. It is the mixture of following four wind speed ramp (vwr), wind gust he resultant wind speed is given by:

(7)

as composite wind model, initially ] and Anderson and Bose [16]. The stimation of various wind speed [17]. Hourly wind speed model is

umar [2],[18]. In this paper, we have typical day. Therefore, in this study,

hours to contrast in minutes and pplications. The accommodation of mps happening at different starting corporated in this model. gh-speed wind, which may vary in tuations over the rotor surface. This s generally occurs in bulky turbine. ass filter is used in the wind power fig. 3 (a). The parameters which

mainly rotor diameter, average wind wind. bine can be represented by using the

142 2014 International Conference on Computing for Sustainable Global Development (INDIACom)

where cp is the power coefficient of wind turbispeed ratio, which is defined by:

Power coefficient is determined by taerodynamics of wind turbine which is generally

Fig. 3(a). Block diagram representation of Wind powe

Fig. 3(b). A typical 200 kW wind turbine power cu

IV. HYBID SYSTEM MOD

A diesel generator, Wind power and SMES facilitan isolated hybrid power system. Wind, PV and supplying power to the system or remote area utilbeen consuming and supplying power from the system is connected to a variable load. Hybrimainly an energy source for electricity. Assumhave incorporated in the system model. PCS conDC power and vice versa. Converter, convertsoutput to AC power that has supplied with the sythe duty cycle of the chopper and inductor cuimposed. This plant model is also applicable fordistributed power generation system. If the elhave connected to the utility point, the hybrid sysboth import and export power.

Fig. 4. Schematic diagram of an isolated hybrid wind

(8)

ine and is the tip

(9)

the knowledge of

er generation model [2]

urve

DEL

ty have connected to diesel generator are

lity plant. SMES has system. The hybrid

id system output is ming that, converters

nverts AC power to s SMES DC power ystem. The limit of

urrent also has been r grid connected and lectricity consumers stem can be used for

d - DEG with SMES

A. Mathematical Model In this section, a mathematical generation system has presented. A block diagram shown in Fig. hybrid power system. Power convappropriate locations. The mathedone separately and parameters uTable I.

Fig. 5. Block diagram of isolated

To maintain a stable operation ofgeneration must be effectively conmeet the total power demand of thControl strategy has determined demand PD and change in total ge where PS is net power deviation. The system frequency variation

Since an inherent time delay existstransfer function for system freqdeviation can be expressed by

Where M and D are, respectivelydamping constant in per unit of thMW power generations as base va

V. OPERATI

Figure 6 has shown a flow chstrategy of hybrid system. Whenevchange in frequency of power sysupon change in load demand sosystem frequency decreases and visystem stability frequency of systemake change in frequency to be zethat means whenever there is increincreased and when load demand dOn this basis in hybrid system probecomes negative SMES is supplbefore operating of SMES, it muinductor current (Id) of the superlower limit of inductor current (Id

L

model of proposed hybrid power

5 describes the modeling of the verters are properly operated at their ematical analysis of each block is used in the analysis have given in

d hybrid Wind, DEG-SMES system

f a hybrid system, the total power ntrolled and properly dispatched to e connected loads. by the difference between power

eneration (10)

(11) F is calculated by-

(12)

s between system frequencies so the uency variation to per unit power

(13)

y, the equivalent inertia constant a he hybrid power system. Consider 2 alue of the hybrid system.

ING STRATEGY

hart, which describes the operating ver load demand is change there is stem. Change in frequency depends whenever load demand increases ce-versa. From this point of view of em must be constant. So, for this to ro, power supply should be adjusted

ease in load demand power supply is decreases power should be reduced.

oceeds as when change in frequency lying the power to the system but

ust undergo that process where the rconducting coil is compared to its L). If Id of the SMES coil is greater

2014 International Conference on Computing for Sustainable Global Development (INDIACom) 143

than IdL, SMES supplies the power to the grid or system and getting

discharged otherwise SMES does not produce power. In another case when the change in frequency is positive SMES does not supply the power and will reduce the total power. Some of the excess power supply to the SMES before charging the SMES, its inductor coil current has compared to its upper limit of inductor current (Id

U). If inductor current is less than Id

U then SMES is in charging mode and consumes power from system otherwise SMES does not charge. When change in frequency is zero SMES remains in standby mode i.e. it is neither in charging mode nor in discharging mode.

Fig. 6. Illustration of the operation strategy

TABLE I. RATING AND PARAMETERS OF HYBRID MODEL

M = 0.12, and D = 0.1 System power rating – 2 MW

KDEG = 1, and TDEG = 0.05s, R =. 5 DEG. Power = 250KW, Peak Wind Power = 200 KW,

Capacity of SMES 100KW Upper limit of Inductor current(Id

U) 150% of Intial inductor current Lower limit of Inductor current (Id

L) 30% of Intial inductor current Initial Inductor current(Ido) 2 KA Gain constant of SMES (KSMES) 6000KV/Hz Converter Time constant (TDC) . 08 s Inductance (L) 10 H, KId = 5kV/kA

VI. SIMULATION RESULTS AND ANALYSIS

Simulation results are analyzed in real time load demand variation throughout a day in Matlab/simulink environment. For the purpose of calculation, all input and output quantities in the plots have considered in per unit (p.u.), responses of the hybrid system, shown by time simulation are in per unit value except fig.11. Variation in frequency is in Hz. Change in wind power generation has shown fig. 7. Real-time demand load has taken from a substation 11kV DS feeder Kaniana urban Haryana Bijli Vitran Nigam Haryana India [19]. Change in load demand with respect to real-time load demand has shown in figure 8. Time domain simulated responses of the system under various operating points are observed and considered. mainly two cases of disturbances in demand load are discussed.

Fig. 7. Variation of Wind Power

Fig. 8. Variation of load demand

Case I: Increase in load demand As change in load demand increases, change in frequency decreases so production of power supply should be increase to balance the frequency variation. Wind power is continuously supplying power to the hybrid system. Due to decrease in system frequency, DEG power can be increase to supply power to the system, up to its rated capacity and increase the production power .To compensate the load SMES supplying power to the power system as shown in figure 9. SMES is operating in discharging mode. Case II: Decrease in load Demand As change in load decreases, change in frequency increases so production of supply power should be decreased to maintain the frequency variation. Due to increase in frequencies, power is decreased to reduce the generation power. Due to variation in frequency, net change in DEG power shown in fig.10. As in fig. 8 when the load demand is decreasing, power supplied by SMES should be decreasing response reached to zero as shown in Fig. 9 but the power supply is still more than load demand so for balancing power SMES starts consuming power from the system. SMES is in charging mode.

Figure 9: Variation of SMES Power in both charging & discharging mode

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Figure 10: Variation in DEG. Power output.

Figure 11: Variation in Frequency of the system.

From Fig.11 it can be conclude that fluctuation in frequency is decreases by adding SMES with wind- DEG power system i.e., frequency decreases when demand increases and increases as load demand decreases.

VII. CONCLUSION

This paper emphasizes on the role of Superconducting magnetic energy storage system connected to hybrid system along with wind and DEG system to control the system frequency. Modelling of SMES has been discussed. Impact of SMES with wind , DEG on isolated hybrid system has been presented. Result of simulation indicates that a combind SMES and diesel generator gives better response in controlling frequency deviation in comparison to only diesel generator. However, although it appears that SMES systems are costly, due to its salient properties such as very fast response, high efficiency, and capability of control of real power and reactive power etc., SMES is getting interest in the field of power and energy system. It is hoped that its potential advantages and environmental benefits will make SMES units a viable alternative for energy storage and management. Continues research and development is likely to bring the price down and make the technology appear even more attractive devices in the future.

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