control strategy for a distributed dc power system with renewable energy

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Control strategy for a distributed DC power system with renewable energy

Kyohei Kurohane a,*, Akie Uehara a,1, Tomonobu Senjyu a,1, Atsushi Yona a,1, Naomitsu Urasaki a,1,Toshihisa Funabashi b, Chul-Hwan Kim c

a University of the Ryukyus, 1 Senbaru, Nishihara-cho, Nakagami, Okinawa 903-0213, Japanb Meidensha Corporation, 36-2 Nihonbashi-Hakozakicho, Chuo-ku, Tokyo 103-8513, Japanc Sungkyunkwan University and NPT Center, Suwon City 440-746, South Korea

a r t i c l e i n f o

Article history:

Received 31 March 2009

Accepted 25 May 2010

Available online 18 June 2010

Keywords:

DC distribution

Micro-grid

Permanent magnet synchronous generator

Pitch angle control

Fault ride-through (FRT)

a b s t r a c t

This paper deals with a DC-micro-grid with renewable energy. The proposed method is composed of

a gearless wind power generation system, a battery, and DC loads in a DC distribution system. The battery

helps to avoid the DC over-voltages by absorbing the power of the permanent magnet synchronous

generator (PMSG) during line-fault. In addition, the control schemes presented in this paper including

the maximum power point tracking (MPPT) control and a pitch angle control for the gearless wind

turbine generator. By means of the proposed method, high-reliable power can be supplied to the DC

distribution system during the line-fault and stable power supply from the PMSG can be achieved after

line-fault clearing. The effectiveness of the proposed method is examined in a MATLAB/SimulinkÒ

environment.

Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Currently, for the greenhouse gas reduction, introduction of the

renewable energies such as photovoltaics and wind energy is

gaining popularity [1e3]. When these renewable Distributed

Generators (DGs) are connected to the power system, there is

a concern about their harmful effects as most of their power uc-

tuate with weather conditions. Therefore, researchers have been

working on practical application of micro-grids. Micro-grid is

a small-scale power grid that has sources in certain areas and can

supply power to a particular consumer. In the future, not only

hospitals, banks and semiconductor factories [4], but also of ces,

shops and residential houses need high quality power. So, DC

distribution systems are attracting attention in the world.

The DC distribution system has the following advantages over

the AC distribution system: (1) Each power generator connected tothe DC distribution system can easily be operated in coordination

because it controls only the DC bus voltage. (2) When the AC-grid

system has fault conditions, the DC distribution system is discon-

nected from the AC grid, and then it is switched to the stand-alone

operation in which the generated power is supplied to the loads

connected to the DC distribution system. (3) The system cost and

loss can be reduced because only a single AC grid-side inverter unit

is needed.

Usually, a permanent magnet synchronous generator (PMSG) is

used as the wind turbine generator (WTG) in the DC distribution

system. Because PMSG has simple structure and high ef ciency.

For a particular wind speed, there is a specic turbine rotational

speed which generates the maximum power. The maximum power

point tracking (MPPT) for each wind speed increases the energy

generation [5e8]. However, the MPPT control for each wind speed

generates the output power uctuations. The voltage and

frequency uctuations of the power system caused by the output

uctuations of the WTG affect the quality of the supplied electrical

power. Therefore, the introduction of fuel cell and ywheel has

been increasing. They have been proposed to reduce the frequency

and voltage uctuations. However, fuel cells are expensive andywheels have noise and vibration problems.

With the rapid growth of the WTG systems, it is dif cult to

stabilize the operation of the powersystem by disconnecting WTGs

when line-faults. Under the line-fault the dispersed power sources

are disconnected from the power system and it needs much more

time and energy compensating for the power supplyedemand

balance. In addition, with the recovery of the power system,

disconnected WTGs need restart. Thus, the frequency of the power

system rises as many WTGs return to the system. As a counter-

measure, in Europe, the large WTGs are required to remain

connected to the power system under line-fault and supply power

* Corresponding author. Tel.: þ81 98 895 8686; fax: þ81 98 895 8708.

E-mail addresses: [email protected] (K. Kurohane), [email protected]

ryukyu.ac.jp (T. Senjyu), [email protected] (C.-H. Funabashi),

[email protected] (C.-H. Kim).1 Tel.: þ81 98 895 8686; fax: þ81 98 895 8708.

Contents lists available at ScienceDirect

Renewable Energy

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / r e n e n e

0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.renene.2010.05.017

Renewable Energy 36 (2011) 42e49

mailto:[email protected]





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to the power system after fault clearing. This requirement is called

fault ride-through (FRT). Moreover, under the line-fault, DC bus

voltage in the DC distribution system experiences the over-voltage.

Therefore, unstable power supply from DC distribution system and

an over-voltage problem of semiconductor devices of the power

converter occur. It is important to solve these problems of the stable

operation for DC distribution system. However, no paper or article

has been presented or published on a micro-grid (or DC-micro-

grid) which solves these problems simultaneously [9e12].

In this paper, stable power supply strategies for DC distribution

system and stable control strategies for PMSG under the line-fault

are proposed. The proposed method uses a battery for the DC

distribution system. Under the line-fault, a chopper circuit is used

to avoid DC bus over-voltage by absorbing energy from the PMSG

and by supplying to the battery. By means of the proposed method,

stable operation of the DC power system under the line-fault

becomes possible, and high-reliable power supply from the grid-

side inverter to the AC grid after the line-fault clearing can

be achieved. The effectiveness of the proposed method is veried

by simulation results under three-line to ground-fault using

MATLAB/SimulinkÒ.

2. DC distribution system conguration

2.1. System conguration

The DC power system used in this paper is shown in Fig. 1. The

wind power generator is a gearless 2 MW permanent magnet

synchronous generator (PMSG). The PMSG has a simple structure

and high ef ciency. In addition, the DC distribution system consists

of a gearless 2 MW PMSG, a grid-side inverter, a 576 Ah battery and

100 kW DC loads. The DC system is connected to a 10 MVA diesel

generator and 5 MW AC loads through the grid-side inverter. Wind

power energy obtained from the windmill is sent to the PMSG. In

order to generate maximum power, rotational speed of the PMSG is

controlled by the PWM converter and the generated power is

leveled by a pitch angle control. Then these power is supplied to the

DC load. The rest of the power is supplied to the AC load through

the grid-side inverter.

Fig. 1. DC power system.

Fig. 2. Windmill output power characteristics. Fig. 3. Pitch angle control system for PMSG.

K. Kurohane et al. / Renewable Energy 36 (2011) 42e49 43



2.2. Windmill model

The windmill output power P w and the windmill torque T w are

given by the following equations:

P w ¼1

2C pðl; bÞrpR2V 3w; (1)

T w ¼

1

2lC pðl; bÞrpR3

V 2w; (2)

where V w is thewind speed, r is the air density, R is the radius of the

windmill, C p is the windmill powercoef cient, l¼uwR/V w isthe tip

speed ratio, uw is the angular rotor speed for the windmill and b is

the pitch angle. C p is given by the following equation [13]:

C p ¼ 0:22

116

GÀ 0:4b À 5

expÀ12:5

G ; (3)

where

G ¼1

1lþ0:08b

À 0:035b

3þ1

: (4)

From (1) and (3), the windmill output power characteristics are

depicted in Fig. 2, from which it can be seen that, for any particular

wind speed, there is a rotational speed uopt, which is called the

optimum rotational speed, which generates the maximum power

P max. In this way, the MPPT for each wind speed increases the

energy generation. uopt is calculated by differentiating C p with

respect to uw. Therefore, uopt is approximated by

uopt ¼ 0:2V w À 0:2: (5)

MPPTcontrol is applied when the wind speed V w is less than the

rated wind speed V w, ref ¼ 12 m/s, and then the output uctuations

of PMSG are smoothed by the pitch angle system. The pitch angle is

operated in the following cases. When the wind speed is within

0 V w < 5 m/s, the pitch angle is xed at b¼ 90. When the wind

speed is within 5 V w < 12 m/s (rated wind speed), the pitch angle

is operated so that the power uctuations are reduced. When thewind speed is within 12 V w < 24 m/s, the pitch angle is operated

so that the output power of PMSG becomes the rated power 2 MW.

Fig. 3 shows the pitch angle control system that determines the

pitch angleb, where the output power error e isused asinputof the

PI controller. Actually, the pitch angle control system includes

a hydraulic servo system. The system has nonlinear characteristics,

but can be modeled as rst-order lag system [14]. Therefore, in this

paper, the rst-order lag system is used where the time constant is

1 s. Moreover, the pitch angleb islimited bya limiter within 2e90

and the maximum rate of change is 10/s.

2.3. PMSG model

The mathematical model of PMSG is same as the permanent

magnet synchronous motor (PMSM). The voltage and torqueequations of PMSM in the synchronous reference frame are given

by the following equations:

vd ¼ ðRa þ PLdÞid À ueLqiq; (6)

vq ¼ ueLdid þÀ

Ra þ PLq

Áiq þ ueK ; (7)

T e ¼ pÈ

Kiq þÀ

Ld À Lq

Áidiq

É; (8)

where vd and vq are the dq axis voltages, id and iq are the dq axis

currents, Ra is the stator resistance, Ld and Lq are the dq axis

inductances, ue is the electrical rotational speed, K is the perma-

nent magnetic ux, P is the differential operator, and p is the

number of pole pairs. Power generation starts when the electro-

magnetic torque T e is negative. In addition, the equation of the

motion of PMSG is given by the following equation:

Fig. 4. Model of drive train for wind generator PMSG.

Fig. 5. Model of PMSG.

Table 1

Parameter of PMSG and windmill.

Rated power P ref 2 MW

Rated wind speed V w, ref 12 m/s

Resistance Ra 0.1U

Inductance L 2.0 mH

Number of pole pairs p 80

Field ux K 10.68 V s/rad

Equivalent inertia J eq 8000 kg m2

Rotational damping D 0

Fig. 6. Wind generator-side converter control system.

Fig. 7. AC grid-side inverter control system.

K. Kurohane et al. / Renewable Energy 36 (2011) 42e4944



T e ¼ J eqdug

dt þ Dug þ T l; (9)

where D is the rotational damping, J eq (J eq ¼ J g þ J w) is the equivalent

inertia, T l is the loadtorque,andug is themechanicalrotational speed.

The models of the drive train and the PMSG are given in Figs. 4

and 5, respectively. The parameters of the PMSG and the windmill

are shown in Table 1 [8,14].

2.4. Diesel generator and battery model

The diesel generator model consists of a synchronous generator

model with AVR control and GOV control, and the battery model is

designed in MATLAB/SimulinkÒ [15]. The battery model is consid-

ered for battery’s discharge and charge characteristics. In this

paper, we consider a Lithiumeion battery. The state of charge (SOC)

is calculated by the integration of the discharge and charge power

of the battery.

3. Conguration of DC distribution control system

3.1. Control system of power converters

The power converter control systems are shown in Figs. 6 and 7.

Generator-side converter achieves variable speed operation by

controlling rotational speed of the PMSG. On the other hand, grid-

side inverter supplies electrical power and its frequency is

synchronized with the frequency of the power system. Each of the

powerconverters is a standard 3-phase two-level unit, is composed

of six IGBTs and is controlled by the triangle-wave PWM law. In

addition, DC distribution system includes a battery, in order toavoid DC bus over-voltages under line-fault. The control systems

are described below.

3.2. Generator-side converter

The generator-side converter controls the rotational speed of

the PMSG in order to achieve the variable speed operation with the

MPPT control. The vector control scheme is used and is shown in

Fig. 6. The speed control of PMSG is realized on a rotating frame,

where the rotational speed error is used as the input of the speed

controller which produces the q-axis stator current command i1q* .

Generally, the cylindrical pole type synchronous machine is

considered to control the d-axis stator current and i1d

is set to zero.

Therefore, in this paper, the d-axis stator current command i1d* , is

set to zero. The errors between the dq axis current commands, i1d*

Fig. 8. Output power reference system of PMSG.

Fig. 10. DC bus constant voltage control system for fault.

Fig. 9. Battery control system.




and i1q* , and the actual dq axis currents are used as the inputs of

current controllers. The current controller outputs produce the dq

axis voltage commands v1d* and v1q

* after decoupling. The rotor

position qe, is used the transformation from abc to dq variables and

is calculated from the rotational speed of the PMSG.

3.3. Grid-side inverter

The grid-side inverter is aimed at the constant control of the

DC bus voltage V dc and unity power factor operation. The control

system for the grid-side inverter is shown in Fig. 7. The d-axis

Fig. 11. Simulation results during three-line to ground-fault. (a) Wind speed; (b) Active power of PMSG; (c) Reactive power of PMSG; (d) DC bus voltage; (e) DC load; (f) Terminal

voltage of DC load; (g) Terminal voltage of battery; (h) Output power of battery; (i) State of charge; (j) Active power of AC grid-side inverter; (k) Reactive power of AC grid-side

inverter; (l) Active power of diesel generator; (m) Reactive power of diesel generator; (n) AC load; (o) Terminal voltage of AC load; (p) Pitch angle.




current can control the DC bus voltage V dc, and the q-axis

current can control the reactive power Q i. The DC bus voltage

reference V dc* is set to 3500 V while the reactive power command

Q i* is set to zero for unity power factor operation. The phase

angle qs, for the transformation between abc and dq frame, is

detected from the three phase voltages at the low voltage side of

the grid-side transformer by using the phase-locked loop (PLL).

The angular position of the dq reference frame is controlled by

a feedback loop which regulates the q-axis component to be

zero, where the d-axis component depicts the voltage vector

amplitude and its phase is determined by the output of the

feedback loop.

4. The proposed power leveling control and DC bus voltage

control system

4.1. PMSG output power reference system

The PMSG output power reference P ref in Fig. 3 is determined by

control system as shown in Fig. 8. The operating point to obtain

maximum power for each wind speed is set (wind speed V w (m/s),

rotational speed um (pu)) and expressed as a rst-order function. In

this paper, the rated PMSG power is 2 MWand is set to 1 pu and the

operating points are set to 11 m/s, 0.9 pu and 12 m/s, 1 pu. Conse-

quently, the

rst-order function determines the reference vary

Fig. 11. (continued).




according to the wind speed. The output of comparator 1 depends

on the comparison of the wind speed V w and the rated wind speed

(12 m/s). The output of comparator 1 is 1 when the wind speed V wis greater than the rated wind speed and is 0 when the wind speed

V w is less than the rated wind speed.Therefore, P ref is obtained from

(10). Here, the rising rate for P ref is restricted by the rate limiter in

order to vary P ref for the wind speed uctuation at the low wind

speed. Consequently, it is possible to reduce the PMSG output

uctuation when wind speed is less than the rated wind speed.

P ref ¼ 0:1848V w À 1:129 ½pu�: (10)

4.2. DC bus constant voltage control system

The proposed constant DC bus voltage control is achieved by

using a bi-directional DC chopper in connection with a battery. The

control systems are shown in Figs. 9 and 10. The bi-directional DC

chopper controls the duty-ratio for normal operation (SWnormal) or

line-fault (SWfault). The switching determinations for SWnormal and

SWfault are performed by considering the AC-grid voltage vt . When

the AC-grid voltage is vt ! 0.8 pu, the bi-directional DC chopper

performs the normal operation. If the AC-grid voltage is vt < 0.8 pu,the bi-directional DC chopper performs under the fault operation.

Under the line-fault, this chopper circuit helps to avoid the DC

bus over-voltage by absorbing energy from PMSG and supplying to

battery, and keeps the DC bus voltage constant. Because the

rapidly rising DC bus voltage is dif cult to keep constant by using

the only PI controller of the AC grid-side inverter. The control

system under the line-fault is shown in Fig. 10. In this system, the

PWM reference signal 2 is determined by the output of PI11

controller. The output of the comparator 4 depends on the

comparison of PWM reference signal 2 and carrier wave signal.

Carrier wave signal is 1 when the carrier wave signal is greater

than the reference signal, and is 0 when the carrier wave is less

than the reference signal. IGBTs are used as switching devices for

the DC chopper circuit.

5. Simulation results

In this paper, the effectiveness of the proposed method is

examined by a switching simulation with the system model shown

in Fig. 1. This simulation considers that the three-line to ground-

fault occursat the middle of transmission lineof Fig.1 and electrical

power supply to the load is shut-down. The sequence of simulation

is described below:

(1) At t ¼ 5.0 s: The three-line to ground-fault occurs at the middle

of transmission line.

(2) When the AC-grid voltage is within vt < 0.8 pu, the gate signals

for grid-side inverter are stopped.(3) At t ¼ 5.1 s: The line-fault is cleared.

(4) When the AC-grid voltage is within vt ! 0.8 pu, the gate signals

for grid-side inverter are re-started.

The simulation results are shown in Fig. 9(a)e(p).

5.1. Normal operation

As can be seen from Fig. 11(b) and (c), when the wind speed

becomes less than the rated wind speed at t ¼ 4.0e6.5 s, the PMSG

output uctuation is smoothed by the pitch angle control system.

Moreover, when the wind speed V w is above the rated speed at

t ¼ 6.5e8.0 s, the pitch angle b, as shown in Fig. 11(p), is well

operated by the pitch angle control system. Besides, the active and

reactive power of PMSG, as shown in Fig. 11(b) and (c), are kept

constant within the rated output power by the pitch angle control

system. Here, the output power does not match with the rated

power, because the rate limiter in Fig. 8 restricts the rising rate of

the PMSG output reference, P ref . As can be seen from Fig. 11(d)e(f)

and (m), stable DC power is supplied to DC loads and rest of the

power is supplied to AC loads with unity power factor through the

grid-side inverter. From Fig. 11(h) and (i), it is found that the battery

does not discharge, because the initialvalue of the SOC is set to50%.

5.2. Line-fault operation

Under the line-fault, the current owing to the battery is

controlled by the switching as shown in Fig. 11(h) and (i). As

a result, it is found that the DC bus voltage V dc follows the command

V dc* . As can be seen in the simulation results, the stable power

supply to DC loads is achieved by the constant control of the DC bus

voltage V dc under the line-fault and stable operation for the PMSG

is possible. In addition, it is also found that the output power

uctuation after the line-fault clearing is reduced and electrical

power supply from the PMSG to the power system can be achieved.

6. Conclusion

This paper presents stable power supply strategies for a DC

distribution system and stable operation strategies for the PMSG

under the line-fault. In normal operation, the proposed system

presents a control strategy based on the MPPT control to generate

the maximum power for the variable wind speed and a pitch angle

control to smooth the output uctuation at low wind speed.

Besides, at high wind speeds, it is possible to control the output

power of the PMSG by the pitch angle control system. In addition to

these control strategies, the DC bus voltage is controlled by using

a bi-directional chopper and battery under the line-fault. From the

simulation results, it is conrmed that the DC distribution system

with the proposed method can stabilize power system operation

under the line-fault and can supply stable power from the grid-side

inverter to the power system after the line-fault clearing.

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