dc microgrid control

Development of Control Strategy for Hybrid Energy Storage System in a DC Microgrid

Mahesh Kumar, Student Member, IEEE S.N. Singh, Senior Member, IEEE S.C. Srivastava, Senior Member, IEEE [email protected] [email protected] [email protected]

Department of Electrical Engineering Indian Institute of Technology Kanpur

Kanpur, India

Abstract—This paper presents a control strategy for Hybrid

Energy Storage System (HESS), integrated to a “dc microgrid” operating in an islanded mode under various operating scenarios. The DC Microgrid (DCMG) consists of wind turbine, solar photo-voltaic, solid oxide fuel cell, HESS (battery and hydrogen energy storage system), and varying dc and ac (single-phase and three-phase) loads. During the power mismatch in the DCMG power pool, the proposed control strategy of HESS allows to control the charging and discharging of battery up to its full capacity, and thereafter store the generated surplus power into hydrogen storage system. The purpose of the proposed control scheme for HESS is to ensure the power balance and dc voltage stability of the DCMG.

Keywords—DC microgrid, hybrid energy storage system, distributed generators, renewable energy sources, dc-dc converters, voltage source converters.

I. INTRODUCTION

In the developing and under-developed countries, remote rural areas may not have access to the electric supply from the grid. The Renewable Energy Sources (RESs), such as Wind Turbine (WT), Solar Photo-Voltaic (SPV), fuel cell generations, constitute the Distributed Generators (DGs) with capacity ranging from 1kW-10MW [1], have an important role, and become attractive solution for such areas. During past few years, the RESs have gone through fast development and attracted increased interests due to various environmental, economical, and technical factors [2]. A microgrid, which can be ac or dc, facilitates the connections of any type of DGs, energy storage systems, and various ac and dc loads by using power-electronic converters. This also provides an opportunity of electrification in the remote rural areas. The DC Micro-Grid (DCMG) is preferred over an ac microgrid due to several advantages such as: 1) higher quality of power supply, 2) better reliability and uninterruptible supply, 3) ease of operation of each DG, as only dc voltage is required to be controlled, 4) higher efficiency due to absence of reactive power, and 5) no need of synchronization for multiple DGs [3], [4].

Nowadays, the Pulse Width Modulation (PWM) based Voltage Source Converters (VSCs) are being widely used to provide effective voltage and power flow control, high quality power conversion, system balancing, fault protection, and Maximum Power Point Tracking (MPPT) of various DGs [4]. Due to uncertainty and intermittent nature of wind speed, Solar Irradiation (SI), and temperature, the output of WT and SPV DGs varies. The loads connected to the DCMG are also

variable in nature. Due to the variations in both power generation and load demand of the DCMG under islanded mode, the controllable DGs such as Solid Oxide Fuel Cell (SOFC) and Energy Storage Systems (ESS) are required to control the power balance and voltage stabilization of the DCMG [5]. Various ESS such as battery, supercapacitor, and flywheel, are being used. Generally, battery is preferred amongst various ESS for long–term power output [6].

This paper demonstrates a dc microgrid based on dc energy pool consisting of WT, SPV, SOFC DGs, and various loads. Due to power mismatch, the DCMG needs the storage system. But, the Battery Energy Storage System (BESS) has limited storage capacity. When, surplus power is being continuously generated for long time by the DGs, integrated to the DCMG, and the BESS is already fully charged, one needs additional storage system for the DCMG in the remote rural areas, where utility grid may not be available. This paper has considered a Hybrid Energy Storage System (HESS), which consists of BESS and Hydrogen Storage System (H2SS). The BESS and H2SS have been integrated to the DCMG through the Bidirectional DC-DC Converter (BDC) and dc-dc buck converter, respectively.

A control strategy for HESS, integrated to the DCMG in islanded mode, under constant as well as variable power generation and loads for both normal and fault conditions, has been proposed. During the power mismatch in the DCMG, the proposed control strategy allows to control the charging and discharging of the BESS up to its full capacity, and thereafter store the generated surplus power into the H2SS, while maintaining constant DCMG voltage. The stored hydrogen can be used for transportation purpose by converting back into electricity through the fuel cell vehicle. When the power generation from the RESs is not sufficient and battery state of charge is also low, the stored hydrogen is converted back to the electricity via the SOFC DG system to fulfill the deficit power in the dc microgrid.

II. SYSTEM CONFIGURATION AND MODELING

A. Configuration of DC Microgrid

The proposed architecture of the “dc microgrid”, based on dc energy pool for integration of various DGs, is shown in Fig. 1. Each DG, connected to the DCMG, has been controlled autonomously by using only DCMG voltage as common reference signal for all the DGs without communicating with each other. The need of transformer has also been eliminated

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on both sides’ converters of DGs and loads by selecting proper dc voltage of the DCMG (750V in the present study).

In the proposed DCMG, a 200kW (4 units of 50kW), 415V WT Generator (WTG) comprising of doubly fed induction generator has been integrated to the DCMG through a bidirectional three-phase VSC. A part of WT generated power is consumed by three-phase load, connected directly to the output terminals of the WTG. The remaining WT generated power is converted into dc power through the bidirectional VSC, and transmitted to the DCMG through dc cable. The control strategy of bidirectional three-phase VSC, as proposed in [4], has been implemented in this paper. This control strategy has been used to establish constant DCMG voltage under various operating scenarios. The parameters used for the WTG system are taken from [7], [8]. A 100kW SPV system with MPPT controller has been considered. The parameters used for SPV system are taken from [9]. A 50kW, 340V, SOFC controllable DG (with 380 cells in series), with peak power capacity of 1.8 times the rated power capacity, has been considered, which generates dc power and its voltage varies when load changes. The SPV and SOFC DGs are integrated to the DCMG through dc-dc boost converters to boost the generated low dc voltage equal to the DCMG voltage. The control strategy for dc-dc boost converters for SPV and SOFC, as proposed in [4], has been implemented in this paper. The parameters used for the SOFC DG are taken from [10].

The HESS consisting of the BESS and H2SS has been considered. The BESS is connected to the DCMG through the BDC for controlling its charging and discharging during the power mismatch at the DCMG. During maximum generation and minimum load demand in the DCMG, and considering BESS fully charged, the continuously generated maximum surplus power may be 216kW. Thus, the power rating of dc-dc buck converter, used for integrating H2SS including electrolyzer and storage tank to the DCMG, has been considered as 270kW. The power generated from various RESs can be used to produce hydrogen through electrolyzer. The parameters used for an electrolyzer (40kW rating) of the H2SS are taken from [11]. Hydrogen produced from electrolysis water using the electricity is 20-25US$/GJ [12]. The parameters of the BESS using Valve-Regulated Lead-Acid (VRLA) battery are 100 kW, 375V, 550Ah for 2 hrs.

The DCMG is able to supply both dc and ac loads simultaneously. The rated loads connected to the DCMG are: 1) 100kW single-phase ac load operating at 240V, 50Hz, fed from a single-phase Voltage Controlled Voltage Source Inverter (VCVSI); 2) 40kW dc load operating at 220V dc, through a dc-dc buck converter; 3) 10kW telecommunication load operating at 48V dc, through a dc-dc buck converter; 4) 50kW three-phase load operating at 415V, 50Hz, connected directly to the terminals of WTG. The total rated load connected to the DCMG is 200 kW. The control strategy of single-phase VCVSI, as proposed in [1], has been implemented in this paper.

B. Modeling of DC Microgrid

1) Modeling of the Distributed Generators: Equations for modeling of the WT, SPV, and SOFC DGs have been taken from [4], [7], [10], [11], [13], [14].

2) Modeling of Battery Energy Storage System: The charging and discharging voltage expressions for dynamic

Fig. 1. Proposed architecture of dc microgrid for integration of a mix of DGs.

model of VRLA battery [15] are given by (1) and (2), respectively.

*1,0

0.1( )batt batt

Q QV V R i k i k it Exp t

it Q Q it

(1)

*1,0

Pol. voltage Pol. resistance

( )batt battQ Q

V V R i k it k i Exp tQ it Q it

(2)

where, Vbatt is the battery voltage (V), Vbatt,0 is the battery constant voltage (V), k is the polarization constant (V/Ah), Q is the battery capacity (Ah), it (=∫idt) is the actual battery charge (Ah), i is the battery current (A), i* is the filtered current (A), R1 is the battery resistance, Exp(t) is exponential zone voltage (V).

3) Modeling of Hydrogen Storage System: The modeling of a 240kW alkaline type electrolyzer has been described below. The reversible voltage (Vrev) or thermodynamic cell voltage (V0) solely depends on Gibb’s energy (ΔG), is expressed as following [16].

0 0,0 ,andrev Elz cell ElzG

V V V V NzF

(3)

where, VElz,0 is the no load voltage of electrolyzer (V), Ncell,Elz is the number of series cells in a stack of electrolyzer, z (=2) is the number of electrons transferred in each reaction, and F is the Faraday constant (C/mol). The Faraday (or current) efficiency (ηI) is expressed as [11], [16].

2

221 ,0

1 2

where,

and, 50 2.5 , 1 0.00075

,

Elz Elz ElzI Elz Elz

Elz Elz Elz

Elz Elz

J I PK J I

K J A V

K T K T

(4)

where, JElz is the current density of electrolyzer (A/m2), IElz is the total rated current of electrolyzer (A), AElz is the area of a stack of electrolyzer (m2), PElz is the rated power of electrolyzer (W), K1 and K2 are the parameters related to Faraday efficiency.

The volumetric flow rate of hydrogen (ŮH), hydrogen molar production rate (MH,Prod) as a function of current, and V-I relationship of electrolyzer, are expressed by (5), (6), and (7), respectively [11], [16].

, (Ltr / hr)3600 tH H prodU M v (5)

, , ,, and ElzIH prod Elz Elz P Stack Elz Stack

NM I I N I

zF (6)


Fig. 2. DC-DC converters (a) buck (b) bidirectional.

1

20, ,

21 2

1 ,3

+

log 1

Elz

Elzcell Elz Elz stack

Elz ElzElz stack

Elz

r r TV V IA

t t T t TV I

A

(7)

where, vt is the molar volume of ideal gas (Ltr/mole), Vcell,Elz is the voltage across one cell of electrolyzer (V), IElz,,stack, is the current in one stack of electrolyzer, Np,stack, is the number of parallel stacks of electrolyzer, TElz is the electrolyzer temperature, r1 and r2 are the parameters of Ohmic resistances, V1 and tis are the parameters for electrolyzer over voltage and over voltage coefficients, respectively.

4) Modeling of DC-DC Converters: The dc-dc buck and bidirectional converters are shown in Fig. 2. The small signal state-space models of buck converter and BDC (in boost mode) are given by (8) and (9), respectively.

10

1 100

inL L

inout out

o o o

VDLd i i

v dLLv vdt

C R C

(8)

22 2

00 11

0 0

01 0and = + 00 1

0

dc

L Lobatt

Ldc dc

LL

dcdc

L

D Vd i iL L

d iLD Idt v v CC C

ii

v dv

IDi

v

(9)

where, D is the duty ratio, d is the small perturbation in duty ratio, Vdc is the DCMG voltage, and Ro is the load resistance.

III. PROPOSED CONTROL STRATEGY FOR HESS

The Proposed control strategy for the HESS, integrated to the DCMG in islanded mode, is shown in Fig. 3. During the

the power mismatch (Pgap) in the DCMG, the proposed control strategy allows to control the charging and discharging of the BESS up to its full capacity first, and then store the continuously generated surplus power into the H2SS, under constant as well as variable power generation and loads scenario.

In the proposed control strategy of HESS, the controlling of the BDC for BESS employs two PI controllers for voltage regulation and one PI controller for battery current controller. When total power generation (PG,Tot) is less than the total load demand (PL,Tot) of the DCMG, this deficit power is met by discharging the BESS. During the discharging of the BESS, the measured DCMG voltage has been compared with its reference value, and the voltage difference is sent to the PI controller, which generates a reference current signal (Idch) for battery current controller. Depending on the minimum value of the battery power (Pbatt_min), this reference signal may be high (Idch) or zero. When power generation is more than the loads connected to the DCMG, the charging of the BESS starts, and measured voltage of the battery (Vbatt) is compared with its high voltage reference setting (Vbatt_H_ref). This difference of battery side dc voltage is sent to the PI controller for controlling the charging of BESS, which provides another reference current signal (Ich) for battery current controller. This signal will be high (Ich) or zero depending on the charging status of the BESS. The combination of these two reference signals (Idch and Ich) provides a reference current signal (Ibatt_ref) for battery current controller, and is compared with the measured current of the battery (Ibatt). This current difference is sent to the PI current controller, which generates a reference signal for PWM generator to control the duty ratio of the BDC for the BESS.

When BESS is fully charged, and still the DGs generate continuously surplus power, the H2SS is turned on to store this surplus power. For this, the measured dc voltage of electrolyzer (Vdc_Elz) has been compared with its reference value (Vdc_Elz_ref), and the voltage difference is sent to the PI controller of buck converter used for H2SS. The output of this PI controller generates a reference signal for PWM generator to control the duty ratio of the dc-dc buck converter for H2SS. A flow chart for operation of HESS, connected to the DCMG, is shown in Fig. 4.

The parameters of the PI controllers, for stable closed loop system, are determined by using Bode plot based technique. The power ratings of the dc-dc converters have been considered

Fig. 3. Proposed control strategy of a hybrid energy storage system integrated to the dc microgrid.


TABLE I. PARAMETERS OF PI CONTROLLERS FOR DC-DC CONVERTERS

PI Controllers Proportional Gain Integral Gain Voltage Controllers for BDC KPv = 0.0002462 KIv = 0.009848 s-1

Current Controller for BDC KPc = 0.000353 KIc = 0.08972 s-1

Voltage Controller for Buck KPv = 0.0025427 KIv = 0.04068 s-1

as 1.25 times the power capacity of the ESS. The parameters of the PI controllers for BDC of BESS with voltage 750V/375V, and for dc-dc buck converter of H2SS with voltage 750V/340V are given in Table I. PG,Tot is the total generation by all three DGs of the DCMG, Pbatt_avail is the available battery power, and Pbatt_max is the maximum storage power capacity of the battery.

IV. SIMULATION RESULTS

The performance analysis of the DCMG with the proposed control strategy of the dc-dc converters used for integrating the HESS to the DCMG in islanded mode, has been evaluated under various operating scenarios including system impact and fault scenario. Two case studies have been conducted to show the effectiveness of the proposed control strategy in MATLAB/Simulink environment.

1) Case 1. Variable Generation by WT and SPV DGs, and Variable Load:In this mode, power generations by the WTG (at wind speeds 12.5, 2.5, 15.5, 1.2, 5, 18.2, 14.5, 6.4, 12, 11.3, 18.5, 16, m/s respectively), the SPV system following the intermittent nature of SI and temperature, and the SOFC DG, are variable as shown in Fig. 5. The variations of total load, three-phase load, and single-phase load connected to the DCMG are considered according to the residential load curve for 24 hours from [17], as given in Table II, and assumed to remain constant for the next two hours. The variations of the dc and telecommunication loads are shown in Fig. 6. The total power generation, total load, and power mismatch are shown in Fig. 7. During 00:00-2:00 hrs, 4:00-6:00 hrs, 10:00-14:00 hrs, 16:00-18:00 hrs, and 22:00-24:00 hrs, the generation is more than the loads (as in Fig. 7). Initially the BESS has been charged and after being fully charged (i.e. when voltage of battery reaches 408.306V), the remaining generated surplus power is stored continuous into the H2SS, as shown in Figs. 8, 9. During 2:00-

4:00 hrs, the generation is less than the total load demand, as shown in Fig. 7. The deficit power has been met by dischrging the BESS upto its capacity and by controllable SOFC DG, as shown in Fig. 8. After BESS becoming fully charged (at 6:00hr) (as shown in Fig. 8), the surplus power generated has been stored into the H2SS, as shown in Fig. 9. The current and voltage of the electrolyzer of H2SS, and stored hydrogen, are shown in Fig. 9. Thus, during this mode, the proposed control strategy of dc-dc converters for HESS ensures proper power balance in the DCMG under islanded mode. The dc voltage of the DCMG is maintained almost constant, as shown in Fig. 10. The output voltages of various PWM converters, used for connecting various ac and dc loads to the DCMG, have been maintained almost constant, as shown in Fig. 11.

TABLE II. RESIDENTIAL LOAD CURVE VARIATION

Time (hrs.) 00:00 2:00 4:00 6:00 8:00 10:00 Load (p.u.) 0.6 0.51 0.42 0.47 0.64 0.75 Time (hrs.) 12:00 14:00 16:00 18:00 20:00 22:00 Load (p.u.) 0.8 0.85 0.95 1.1 1.0 0.8

00:00 4:00 8:00 12:00 16:00 20:00 24:000

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25x 10

5

Time (hrs.)

Po

we

r G

ene

ratio

ns

(W)

WT generation

SPV generation

SOFC generation

Fig. 5. Power generation by WT, SPV, and SOFC DGs.

0

1.5

3

4.5

6x 10

4

DC

Loa

d(W

)

00:00 4:00 8:00 12:00 16:00 20:00 24:000.5

1

1.5

2x 10

4

Time (hrs.)

Te

lec

omLo

ad (

W)

Fig. 6. DC load and telecommunication load connected to the DCMG.

00:00 4:00 8:00 12:00 16:00 20:00 24:00-0.5

0

0.5

1

1.5

2

2.5

3x 10

5

Time (hrs.)

Pow

er (

W)

Total power generation

Total load demand

Power mismatch

Fig. 7. Total power generation, total load demand, and power mismatch at DCMG.

Fig. 4. Flow chart for operation of hybrid energy storage system.


-7.5-5

-2.50

2.55

x 104

Ba

tte

ry O

utpu

tP

ow

er

(W)

00:00 4:00 8:00 12:00 16:00 20:00 24:00360

380

400

420

Time (hrs.)

Bat

tery

Vol

tage

(V

)

Fig. 8. Output power and voltage of the battery.

0

0.5

1

1.5

2x 10

5

Pow

er

toH

2S

S (

W)

0

200

400

600

Ele

ctro

lyz

erC

urre

nt (

A)

340

365

390

415

440

Ele

ctr

oly

zer

Vol

tag

e (V

)

00:00 4:00 8:00 12:00 16:00 20:00 24:000

2.5

5

7.5

10x 10

4

Time (hrs.)

Hyd

orge

n(L

tr /

hr)

Fig. 9. Power to H2SS, current and voltage of electrolyzer, and stored hydrogen.

00:00 4:00 8:00 12:00 16:00 20:00 24:00700

725

750

775

800

Time (hrs.)

DC

MG

V

olta

ge (

V)

Fig. 10. DC voltage of the DCMG.

180

200

220

240

260

Vol

;ta

ge (

V)

00:00 4:00 8:00 12:00 16:00 20:00 24:000

50

100

Time (hrs.)Tel

eco

m L

oad

Vol

;ta

ge (

V)

1Ph VCVSI output voltage

DC load converter voltage

Fig. 11. Output voltages of single-phase VCVSI, dc-dc buck converter for dc load (220V), and dc-dc buck converter for telecommunication load (48V).

2) Case 2. Constant Generation by WT and SPV DGs, and Variable Load with a Fault (L-G) on the DCMG bus: The performance analysis of the DCMG with the proposed control strategy, under a Line-to-Ground (L-G) fault on the DCMG, has been carried out. In this mode, power generations by the WTG (200kW at rated wind speed 11.3 m/s) and by the SPV DG (100kW at STC i.e. 250C and 1kW/m2) are constant, and controlled DG (SOFC) has been turned off. At t=1.2s, a dc fault

with fault resistance (Rf =100mΩ) occurs on the DCMG bus, which reduces its voltage from 750V to 580V, as shown in Fig. 12. The total power generation (surplus power during the whole operation), total load demand, and power mismatch are shown in Fig. 13. The power consumed by the single-phase load, dc load, and telecommunication load reamins the same during the fault and post fault (as before the fault), while three-phase load experiences slight variation due to transients during the fault, as shown in Fig. 14. Therefore, the power consumed by the total load has also slight variation during the fault, as shown in Fig. 13. During t=0 to t=1s, initially the BESS has been charged by a part of generated surplus power, and as BESS gets fully charged (as shown in Fig. 15), the remaining generated surplus power has been stored in the H2SS, as shown in Fig. 16.

0

4

8

12x 10

4

Fau

ltP

ower

(W

)

0 0.5 1 1.5 2 2.5 3500

750

1000

1250

Time (s)

DC

MG

V

olta

ge (

V)

0

50100150200

Fau

lt C

urre

nt (

A)

Fig. 12. Fault power, fault current, and DCMG voltage, during fault, pre-fault, and post-fault.

0.51

1.52

2.53

3.5x 10

5

Pow

er

(W)

0 0.5 1 1.5 2 2.5 30

1

2

3x 10

5

Time (s)

Pow

er G

ap(W

)

Total power generation

Total load demand

Fig. 13. Total generation, total load demand, and power mismatch at DCMG.

468

1012

x 104

1Ph

Load

(W)

0

2

4

6x 10

4

3Ph

Lo

ad

(W)

12345

x 104

DC

Lo

ad

(W)

0 0.5 1 1.5 2 2.5 36000

80001000012000

Time (s)

Te

leco

m

Loa

d (W

)

Fig. 14. Variations of various loads connected to the DCMG, during fault, pre-fault, and post-fault.


-10

-505

x 104

Bat

tery

Ou

tput

(W

)

0 0.5 1 1.5 2 2.5 3375

390

405

420

Time (s)

Bat

tery

Vol

tage

(V

)

Fig. 15. Output power and voltage of the battery.

0

1

2

3x 10

5

Pow

er t

oH

2SS

(W

)

0

500

1000

Ele

ctro

lyz

erC

urre

nt (

A)

300

350

400

450

Ele

ctr

oly

zer

Vol

tag

e (V

)

0 0.5 1 1.5 2 2.5 30

2.55

7.510

x 104

Time (s)

Hyd

roge

n (L

tr /

hr)

Fig. 16. Power stored in H2SS, current and voltage of electrolyzer, and stored hydrogen during fault, pre-fault, and post-fault.

During t=1s to t=3s, the generated surplus power has been continuously stored into the H2SS, even during the fault (as shown in Fig. 16), since the BESS is already fully charged. The current and voltage of the electrolyzer of H2SS, and stored hydrogen, are also shown in Fig. 16. Some power is also dissipated through the fault resistance during the fault, as shown in Fig.12. As the fault is cleared at t=1.4s, the system operates in the normal mode, as shown in Figs. 12-16. Thus, the proposed control strategy for HESS properly controls the power balance of the DCMG in islanded mode, even during the fault. The DCMG voltage has been maintained constant after clearing the fault (as before the fault), as shown in Fig.12.

V. CONCLUSION

This paper proposes a control strategy for the HESS, integrated to the DCMG in islanded mode under constant as well as variable power generation and loads. The HESS consists of the BESS and H2SS, which are integrated to the DCMG through the BDC and dc-dc buck converter, respectively. The proposed control strategy, during the power mismatch at the DCMG, controls the charging and discharging of the BESS up to its full capacity, and effectively stores the surplus power generated into the H2SS, after the battery being fully charged, while maintaining the desired constant DCMG voltage, under various operating scenarios for both normal as well as fault conditions.

The simulation results also demonstrate the effectiveness of the DCMG with the proposed control strategy of the HESS under different operating scenarios. The system remains under

normal operation even during the fault, except the fault transient period. The proposed DCMG, with the HESS, offers the opportunity of electrification, and will help in meeting the power supply required in the remote rural areas.

ACKNOWLEDGMENT

Authors acknowledge with thanks the partial financial support provided by the Department of Science and Technology (DST), New Delhi, India, under project no. DST/EE/20100258, to carry out the present research work.

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