Mt2521912199

Y.C. Rama Krishna, M.Lokesh / International Journal of Engineering Research and
Applications (IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue 5, September- October 2012, pp.2191-2199
Coordination Control of a Hybrid AC/DC Microgrid
Y.C. Rama Krishna*, M.Lokesh**
*(P.G., Research Scholar, Department of EEE, MITS College, Madanapalle, Andhra Pradesh,India)
** (Assistant Professor, Department of EEE, MITS College, Madanapalle,Andhra Pradesh,India)

Abstract
This project proposes a hybrid ac/dc are commonly used as drives in order to control the
micro grid to reduce the processes of multiple dc– speed of ac motors in industrial plants.
ac–dc or ac–dc–ac conversions in an individual ac Recently, dc grids are resurging due to the
or dc grid. The hybrid grid consists of both ac development and deployment of renewable dc power
and dc networks connected together by multi- sources and their inherent advantage for dc loads in
bidirectional converters. AC sources and loads commercial, industrial and residential applications.
are connected to the ac network whereas dc The dc microgrid has been proposed [6]–[10] to
sources and loads are tied to the dc network. integrate various distributed generators. However, ac
Energy storage systems can be connected to dc or sources have to be converted into dc before
ac links. The proposed hybrid grid can operate in connected to a dc grid and dc/ac inverters are
a grid-tied or autonomous mode. The required for conventional ac loads. Multiple reverse
coordination control algorithms are proposed for conversions required in individual ac or dc grids
smooth power transfer between ac and dc links may add additional loss to the system operation and
and for stable system operation under various will make the current home and office appliances
generation and load conditions. The more complicated. The smart grid concept is
characteristics of wind speed, solar irradiation currently prevailing in the electric power industry.
level, ambient temperature, and load are also The objective of constructing a smart grid is to
considered in system control and operation. A provide reliable, high quality electric power to
small hybrid grid has been modelled and digital societies in an environmentally friendly and
simulated using the Simulink in the MATLAB. sustainable way. One of most important futures of a
smart grid is the advanced structure which can
Keywords- dc-ac-dc converters, microgrid, facilitate the connections of various ac and dc
Energy management, grid control, grid generation systems, energy storage options, and
operation, PV system, wind power generation. various ac and dc loads with the optimal asset
utilization and operation efficiency. To achieve
1. Introduction those goals, power electronics technology plays a
Three Phase ac power systems have existed most important role to interface different sources
for over 100 years due to their efficient and loads to a smart grid.
transformation of ac power at different voltage A hybrid ac/dc microgrid is proposed in
levels and over long distance as well as the inherent this paper to reduce processes of multiple reverse
characteristic from fossil energy driven rotating conversions in an individual ac or dc grid and to
machines. Recently more renewable power facilitate the connection of various renewable ac and
conversion systems are connected in low voltage ac dc sources and loads to power system. Since energy
distribution systems as distributed generators or ac management, control, and operation of a hybrid grid
micro grids due to environmental issues caused by are more complicated than those of an individual ac
conventional fossil fueled power plants. On other or dc grid, different operating modes of a hybrid
hand, more and more dc loads such as light-emitting ac/dc grid have been investigated.
diode (LED) lights and electric vehicles (EVs) are The coordination control schemes among
connected to ac power systems to save energy and various converters have been proposed to harness
reduce CO emission. When power can be fully maximum power from renewable power sources, to
supplied by local renewable power sources, long minimize power transfer between ac and dc
distance high voltage transmission is no longer networks, and to maintain the stable operation of
necessary [1]. AC micro grids [2]–[5] have been both ac and dc grids under variable supply and
proposed to facilitate the connection of renewable demand conditions when the hybrid grid operates in
power sources to conventional ac systems. However, both grid-tied and islanding modes. The advanced
dc power from photovoltaic (PV) panels or fuel cells power electronics and control technologies used in
has to be converted into ac using dc/dc boosters and this paper will make a future power grid much
dc/ac inverters in order to connect to an ac grid. In smarter.
an ac grid, embedded ac/dc and dc/dc converters are
required for various home and office facilities to
supply different dc voltages. AC/DC/AC converters

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Vol. 2, Issue 5, September- October 2012, pp.
II. SYSTEM CONFIGURATION AND is less than the total load at the dc side, the converter
MODELING injects power from the ac to dc side. When the total
A. Grid Configuration power generation is greater than the total load in the
Fig. 1 shows a conceptual hybrid system hybrid grid, it will inject power to the utility grid.
configuration where various ac and dc sources and Otherwise, the hybrid grid will receive power from
loads are connected to the corresponding dc and ac the utility grid. In the grid tied mode, the battery
networks. The ac and dc links are connected together converter is not very important in system operation
through two transformers and two four-quadrant because power is balanced by the utility grid.
operating three phase converters. The ac bus of the In autonomous mode, the battery plays a
hybrid grid is tied to the utility grid. A compact very important role for both power balance and
hybrid grid as shown in Fig.2 is modeled using the voltage stability. Control objectives for various
Simulink in the MATLAB to simulate system converters are dispatched by energy management
operations and controls. 40 kW PV arrays are system. DC bus voltage is maintained stable by a
connected to dc bus through a dc/dc boost converter battery converter or boost converter according to
to simulate dc sources. A capacitor is to suppress different operating conditions. The main converter is
high frequency ripples of the PV output voltage. A controlled to provide a stable and high quality ac bus
50kW wind turbine generator (WTG) with doubly voltage. Both PV and WTG can operate on
fed induction generator (DFIG) is connected to an ac maximum power point tracking (MPPT) or off-
bus to simulate ac sources. MPPT mode based on system operating
requirements. Variable wind speed and solar
irradiation are applied to the WTG and PV arrays
respectively to simulate variation of power of ac and
dc sources and test the MPPT control algorithm.

Fig. 1. A compact representation of the proposed
hybrid grid.

A 65 Ah battery as energy storage is
connected to dc bus through a bidirectional dc/dc
converter. Variable dc load (20 kW–40 kW) and ac C. Modeling of PV Panel
load (20 kW–40 kW) are connected to dc and ac The equivalent circuit of a PV panel with a
buses respectively. The rated voltages for dc and ac load.The current output of the PV panel is modelled
buses are 400 V and 400 V rms respectively. A three by the following three equations [11], [12]. All the
phase bidirectional dc/ac main converter with R-L-C parameters are shown in Table I:
filter connects the dc bus to the ac bus through an 𝑰 𝒑𝒗 = 𝒏 𝒑 𝑰 𝒑𝒉 − 𝒏 𝒑 𝑰 𝒔𝒂𝒕
isolation transformer. 𝒒 𝑽 𝒑𝒗
× 𝒆𝒙𝒑 + 𝑰 𝒑𝒗 𝑹 𝒔
B. Grid Operation 𝑨𝒌𝑻 𝒏𝒔
The hybrid grid can operate in two modes.
In grid-tied mode, the main converter is to provide − 𝟏
stable dc bus voltage and required reactive power
and to exchange power between the ac and dc buses.
The boost converter and WTG are controlled to (1)
𝑺
provide the maximum power. When the output 𝑰 𝒑𝒉 = 𝑰 𝒔𝒔𝒐 + 𝒌 𝒊 𝑻 − 𝑻 𝒓 . 𝟏𝟎𝟎𝟎
(2)
power of the dc sources is greater than the dc loads,
the converter acts as an inverter and injects power 𝟑
𝒒𝑬 𝒈𝒂𝒑 𝟏
−
𝟏
𝑻 𝒌𝑨 𝑻𝒓 𝑻
from dc to ac side. When the total power generation 𝑰 𝒔𝒂𝒕 = 𝑰 𝒓𝒓 𝒆 (3)
𝑻𝒓

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D. Modelling of Battery 𝝀 𝒅𝒔 −𝑳 𝒔 𝟎 𝑳 𝒎 𝟎 𝒊 𝒅𝒔
Two important parameters to represent state 𝝀 𝒒𝒔 𝟎 −𝑳 𝒔 𝟎 𝑳 𝒎
𝒊 𝒒𝒔
of a battery are terminal voltage 𝑽 𝒃 and state of = (8)
𝝀 𝒅𝒓 −𝑳 𝒎 𝟎 𝑳𝒓 𝟎 𝒊 𝒅𝒓
charge (SOC) as follows [13]: 𝝀 𝒒𝒓 𝟎 −𝑳 𝒎 𝟎 𝑳𝒓 𝒊 𝒒𝒓
𝑸
𝑽𝒃 = 𝑽𝒐 + 𝑹𝒃𝒊𝒃− 𝑲 𝑸+ 𝒊 𝒃 𝒅𝒕
+ 𝑨. 𝒆 𝑩 𝒊 𝒃 𝒅𝒕
(4) The dynamic equation of the DFIG
𝑱 𝒅𝒘 𝒓
𝒏𝒑 𝒅𝒕
= 𝑻 𝒎 − 𝑻 𝒆𝒎 (9)
𝒊 𝒃 𝒅𝒕
𝑺𝑶𝑪 = 𝟏𝟎𝟎 𝟏 + 𝑸
(5)
𝑻 𝒆𝒎 = 𝒏 𝒑 𝑳 𝒎 𝒊 𝒒𝒔 𝒊 𝒅𝒓 − 𝒊 𝒅𝒔 𝒊 𝒒𝒓 (10)
Where 𝑹 𝒃 is internal resistance of the
battery, Vo is the open circuit voltage of the where the subscripts 𝒅, 𝒒, 𝒔 and 𝒓 , and
battery,𝒊 𝒃 is battery charging current, K is denote 𝒅-axis,𝒒-axis, stator, and rotor respectively,
polarization voltage, Q is battery capacity, A is L represents the inductance, λ is the flux linkage, u
exponential voltage, and B is exponential capacity. and i represent voltage and current respectively, and
are the angular synchronous speed and slip speed
TABLE II respectively, w2=w1-wr, Tm is the mechanical
Parameters of DFIG torque, Tem is the electromagnetic torque and other
parameters of DIFG are listed in Table II.
If the synchronous rotating d-q reference is
oriented by the stator voltage vector, the d-axis is
aligned with the stator voltage vector while the q-
axis is aligned with the stator flux reference frame.
Therefore , λds=0 and λqs=λs. The following
equations can be obtained in the stator voltage
oriented reference frame as [14]:

𝑳𝒎 𝑳𝒎
𝒊 𝒅𝒔 = − 𝒊 𝑻 𝒆𝒎 = 𝒏 𝒑 𝝀 𝒊
𝑳 𝒔 𝒅𝒓 𝑳 𝒔 𝒔 𝒅𝒓
𝑳 𝒔 𝑳 𝒓 −𝑳 𝒎 𝟐
𝝈= (11)
𝑳𝒔 𝑳𝒓

𝒅𝒊 𝒅𝒓
𝒖 𝒅𝒓 = 𝑹 𝒓 𝒊 𝒅𝒓 + 𝝈𝑳 𝒓 − 𝒘𝟏 − 𝒘𝒓 𝑳 𝒎 𝒊 𝒒𝒔 + 𝑳 𝒓 𝒊 𝒒𝒓 (12)
𝒅𝒕

𝒅𝒊 𝒒𝒓
𝒖 𝒒𝒓 = 𝑹 𝒓 𝒊 𝒒𝒓 + 𝝈𝑳 𝒓 − 𝒘𝟏 − 𝒘𝒓 𝑳 𝒎 𝒊 𝒅𝒔 + 𝑳 𝒓 𝒊 𝒅𝒓 (13)
𝒅𝒕
E. Modeling of Wind Turbine Generator
Power output 𝑷 𝒎 from a WTG is determined by (6) III. COORDINATION CONTROL OF THE
CONVERTERS
𝑷 𝒎 = 𝟎. 𝟓ƴ𝒂𝑪 𝒑 𝝀, 𝜷 𝑽 𝒘 𝟑 (6) There are five types of converters in the
hybrid grid. Those converters have to be
Where ƴ is air density a is rotor swept area, Vw is coordinately controlled with the utility grid to supply
wind speed, an uninterrupted, high efficiency, and high quality
and 𝑪 𝒑 𝝀, 𝜷 is the power coefficient, which is the power to variable dc and ac loads under variable
function of tip speed ratio𝝀 and pitch angle 𝜷. solar irradiation and wind speed when the hybrid
grid operates in both isolated and grid tied modes.
The mathematical models of a DFIG are essential The control algorithms for those converters are
requirements for its control system. The voltage presented in this section.
equations of an induction motor in a rotating d-q
coordinate are as follows: A. Grid-Connected Mode
When the hybrid grid operates in this mode,
𝒖 𝒅𝒔 −𝑹 𝒔 𝟎 𝟎 𝟎 𝒊 𝒅𝒔 𝝀 𝒅𝒔 −𝒘 𝟏 𝝀 𝒒𝒔 the control objective of the boost converter is to
𝒖 𝒒𝒔 𝟎 −𝑹 𝒔 𝟎 𝟎 𝒊 𝒒𝒔 𝝀 𝒒𝒔 𝒘 𝟏 𝝀 𝒅𝒔 track the MPPT of the PV array by regulating its
𝒖 𝒅𝒓 = 𝟎 𝟎 𝑹𝒓 𝟎 𝒊 𝒅𝒓
+p +
𝝀 𝒅𝒓 −𝒘 𝟐 𝝀 𝒒𝒓
(7) terminal voltage. The back-to-back ac/dc/ac
𝒖 𝒒𝒔 𝟎 𝟎 𝟎 𝑹𝒓 𝒊 𝒒𝒓 𝝀 𝒒𝒓 𝒘 𝟐 𝝀 𝒅𝒓 converter of the DFIG is controlled to regulate rotor
side current to achieve MPPT and to synchronize
with ac grid. The energy surplus of the hybrid grid
can be sent to the utility system. The role of the

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battery as the energy storage becomes less important 𝒊𝑨 𝒊𝑨 𝒗 𝑨𝑪 𝒗 𝑺𝑨
𝒅
because the power is balanced by the utility grid. In 𝑳𝟐 𝒊 𝑩 + 𝑹 𝒊 𝑩 = 𝒗 𝑩𝑪 − 𝒗 𝑺𝑩 (20)
𝒅𝒕
this case, the only function of the battery is to 𝒊𝑪 𝒊𝑪 𝒗 𝑪𝑪 𝒗 𝑺𝑪
eliminate frequent power transfer between the dc
and ac link. The dc/dc converter of the battery can 𝒅 𝒊𝒅 −𝑹 𝟐 𝒘𝑳 𝟐 𝒊𝒅 𝒗 𝒄𝒅 𝒗 𝒔𝒅
𝑳𝟐 𝒊 𝒒 = −𝒘𝑳 𝟐 𝒊 𝒒 + 𝒗 𝒄𝒒 − 𝒗 𝒔𝒒 (21)
be controlled as the energy buffer using the 𝒅𝒕 −𝑹 𝟐
technique [15]. The main converter is designed to
operate bidirectional to incorporate complementary Where(VCA,VCB,VCC) are ac side voltages
characteristic of wind and solar sources [16], [17]. of the main converter,(VSA,VSB,VSC) are voltages
The control objectives of the main converter are to across𝑪 𝟐 in Fig. 1, and (id,iq), (vsd,vsq) and (vcd,vcq)
maintain a stable dc-link voltage for variable dc load are the corresponding d-q coordinate variables. In
and to synchronize with the ac link and utility order to maintain stable operation of the hybrid grid
system. under various supply and demand conditions, a
The combined time average equivalent coordination control algorithm for booster and main
circuit model of the booster and main converter is converter is proposed based on basic control
shown in Fig. 4 based on the basic principles and algorithms of the grid interactive inverter in [19].
descriptions in [18] and [19] for booster and inverter The control block diagram is shown in Fig. 2.The
respectively. reference value of the solar panel terminal voltage
𝒗∗𝒑𝒗 is determined by the basic perturbation and
Power flow equations at the dc and ac links are as observation (P&O) algorithm based on solar
follows: irradiation and temperature to harness the maximum
power [21], [22]. Dual-loop control for the dc/dc
𝑷 𝒑𝒗 + 𝑷 𝒂𝒄 = 𝑷 𝒅𝒄𝑳 + 𝑷 𝒃 (14) boost converter is described in [23], where the
control objective is to provide a high quality dc
𝑷 𝒔 = 𝑷 𝒘 − 𝑷 𝒂𝒄𝑳 − 𝑷 𝒂𝒄 (15) voltage with good dynamic response. This control
scheme is applied for the PV system to track optimal
solar panel terminal voltage using the MPPT
algorithm with minor modifications. The outer
voltage loop can guarantee voltage reference
tracking with zero steady-state error and the inner
current loop can improve dynamic response.

Fig.2. Time average model for the booster and main
converter.

where real power Ppv and Pw are
produced by PV and WTG respectively, PacL and
PdcL are real power loads connected to ac and dc
buses respectively, Pac is the power exchange
between ac and dc links, Pb is power injection to
battery, and Ps is power injection from the hybrid g
rid to the utility.
The current and voltage equations at dc bus are as
follows:

𝑽 𝒑𝒗 − 𝑽 𝑻 = 𝑳 𝟏 .
𝒅𝒊 𝟏
+ 𝑹𝟏𝒊𝟏 (16) Fig. 3. The control block diagram for boost
𝒅𝒕 converter and main converter.
𝒅𝑽 𝒑𝒗
𝑰 𝒑𝒗 − 𝒊 𝟏 = 𝑪 𝒑𝒗 . (17)
𝒅𝒕
𝑽 𝑻 = 𝑽 𝒅 (𝟏 − 𝒅 𝟏 ) (18) The one-cycle delay and saturation limiter
𝒅𝑽
𝒊 𝟏 𝟏 − 𝒅 𝟏 − 𝑪 𝒅 𝒅𝒕 𝒅 −
𝑽𝒅
− 𝒊 𝒃 − 𝒊 𝒂𝒄 = 𝟎 (19) in Fig. 2 can assist Controller to track 𝒗∗𝒑𝒗 faster. In
𝑹𝑳 steady state, 𝒊∗𝟏−𝒑𝒓𝒆 resides in the linear region of the
saturation limiter and is equal to𝒊∗𝟏 . It can be seen
Where d1 is the duty ratio of switch ST.
that a step increase of𝒗∗𝒑𝒗 makes 𝒊∗𝟏−𝒑𝒓𝒆 becomes
Equations (20) and (21) show the ac side voltage
equations of the main converter in ABC and d- negative, which in turn makes 𝒊∗𝟏 to be zero during
coordinates respectively the first switching period of the transient process.
This leads to a lower d1 for driving the average

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voltage 𝒗 𝒅(𝟏−𝒅 𝟏) and𝒗 𝒑𝒗 upward to follow the𝒗∗𝒑𝒗 characteristic of the wind turbine [26]. The rotational
command. speed 𝒘 𝒓 and mechanical power𝑷 𝒎 are used to
To smoothly exchange power between dc calculate the electromagnetic torque𝑻∗𝒆𝒎 .The d –axis
and ac grids 𝒊∗𝟏 andsupply a given reactive power to rotor side current reference is determined based
the ac link, PQ control is implemented using a on𝑻∗𝒆𝒎 through stator flux estimation. The rotor
current controlled voltage source for the main side𝒅 − 𝒒 voltages are maintained through
converter. Two PI controllers are used for real and controlling the corresponding current with
reactive power control respectively. When resource appropriate feed forward voltage compensation.
conditions or load capacities change, the dc bus
voltage is adjusted to constant through PI regulation. B. Isolated Mode
The PI controller is set as instantaneous active When the hybrid grid operates in the
current 𝒊 𝒅 reference whereas the instantaneous islanding mode, the boost converter and the back-to-
reactive current𝒊 𝒒 reference is determined by reactive back ac/dc/ac converter of the DFIG may operate in
power compensation command. the on-MPPT or off-MPPT based on system power
When a sudden dc load drop causes power balance and energy constraints. The main converter
surplus at dc side, the main converter is controlled to acts as a voltage source to provide a stable voltage
transfer power from the dc to the ac side. The active and frequency for the ac grid and operates either in
power absorbed by capacitor𝒄 𝒅 leads to the rising inverter or converter mode for the smooth power
of dc-link voltage 𝒗 𝒅 . The negative error (𝒗∗𝒅 − exchange between ac and dc links. The battery
𝒗 𝒅 )caused by the increase of𝒗 𝒅 produces a higher converter operates either in charging or discharging
active current reference𝒊∗𝒅 through the PI control. mode based on power balance in the system. The dc-
The active current𝒊 𝒅 and𝒊∗𝒅 its reference are both link voltage is maintained by either the battery or the
positive. A higher positive reference 𝒊∗𝒅 will force boost converter based on system operating
condition. Powers under various load and supply
active current𝒊 𝒅 to increase through the inner current
conditions should be balanced as follows:
control loop. Therefore, the power surplus of the dc
grid can be transferred to the ac side. Similarly, a
sudden increase of dc load causes the power 𝑷 𝒑𝒗 + 𝑷 𝒘 = 𝑷 𝒂𝒄𝑳 + 𝑷 𝒅𝒄𝑳 + 𝑷 𝒍𝒐𝒔𝒔 + 𝑷 𝒃 (22)
shortage and drop 𝒗 𝒅 at the dc grid. The main
converter is controlled to supply power from the ac where 𝒑 𝒍𝒐𝒔𝒔 is the total grid loss.
to the dc side. Thepositive voltage error(𝒗∗𝒅 − 𝒗 𝒅 ) Two level coordination controls are used to
caused by𝒗 𝒅 drop makes the magnitude of𝒊∗𝒅 maintain system stable operation. At the system
increase through the PI control. Because 𝒊 𝒅 and𝒊 𝒅 level, operation modes of the individual converters
are both negative, the magnitude of 𝒊 𝒅 is increased are determined by the energy management system
through the inner current control loop. Therefore, (EMS) based on the system net power 𝒑 𝒏𝒆𝒕 and the
power is transferred from the ac grid to the dc side. energy constraints and the charging/discharging rate
The DFIG is controlled to maintain a stable dc- of battery. The system control logic diagram is
link voltage of the back-to-back ac/dc/ac converter. shown in 𝒑 𝒏𝒆𝒕 .is defined as the total maximum
The objectives of the rotor side converter are to track power generation minus the total load and
MPPT of the WTG and to manage the stator side minus𝒑 𝒍𝒐𝒔𝒔 . The energy constraints of the battery
reactive power. Different control schemes such as are determined based on the state of charge (SOC)
limits using SOCmin < SOC< SOCmax . It should be
noted that SOC cannot be measured directly, but can
be attained through some estimation methods as
described in [27], [28]. The constraint of charging
and discharging rate is Pb ≤Pbmax. At local level, the
individual converters operate based on mode
commends from the EMS. Either the PV system or
WTG or both have to operate in the off-MPPT mode
for Case 1 and Case 2 and in the on-MPPT mode for
Fig. 4. The DTC control scheme for the rotor side
converter.

the direct torque control (DTC) and direct
power control (DPC) have been proposed for a
DFIG in literature [24] and [14], [25].The DTC
scheme as shown in Fig. 4 is selected as the control
method for the rotor side converter in this paper. The
rotor rotational speed is obtained through the MPPT Fig. 5. Time average equivalent circuit model for the
algorithm, which is based on the power and speed three converters.

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other cases. The battery converter may operate in the 𝒊 𝒂𝒄 − 𝒊 𝒅𝒄 . It should be noted that the output of the
idle, charging, or discharging mode for different outer voltage loop is multiplied by -1 before it is set
cases. The main converter will operate in the as the inner loop current reference. Current 𝒊 𝒃 is
inverter mode if 𝒑 𝒘 − 𝒑 𝒂𝒄𝑳 is negative or in the defined positive when flowing into the battery,
converter mode with positive 𝒑 𝒘 − 𝒑 𝒂𝒄𝑳 . Load where the preset dc-link voltage𝒗∗𝒅𝒄 is set to constant
shedding is required to maintain power balance if 400 V. Considering a decrease of 𝒗 𝒅𝒄 caused by
power supply is less than demand and the battery is sudden load increase or decrease of solar irradiation,
at the minimum SOC. the positive voltage error
The time average equivalent circuit model
of the booster, main converter, and battery converter
for the isolated operation is shown in Fig. 8. The
inverter part of the circuit model in Fig. 5 is based
on the basic principles and descriptions in [29].
The current and voltage equations for the battery
converter and dc link are as follows:

𝒅𝒊
𝑽 𝑫 − 𝑽 𝒃 = 𝑳 𝟑 𝒅𝒕𝒃 + 𝑹 𝟑 𝒊 𝒃 (23)
𝑽𝑫 = 𝑽𝒅 𝒅𝟑 (24)
𝒅𝑽 𝒅
𝒊𝟏 𝟏 − 𝒅 𝟏 − 𝒊 𝒂𝒄 − 𝒊 𝒅𝒄 − 𝒊 𝒃 𝒅 𝟑 = 𝒊 𝒄 = 𝑪 𝒅 (25)
𝒅𝒕

Where 𝒅 𝟑 and(1-𝒅 𝟑 ) are the duty ratio of the
switchesST7 and ST8 respectively.
The ac side current equations of the main converter
in 𝒅 − 𝒒 coordinate are as follows:

𝒅 𝒗 𝒔𝒅 𝒊𝒅 𝟎 𝒘 𝒗 𝒔𝒅 𝒊 𝒐𝒅
𝑪𝟐 𝒗 𝒔𝒒 = 𝒊 𝒒 + −𝒘 𝒗 𝒔𝒒 − 𝒊 𝒐𝒒 (26)
𝒅𝒕 𝟎

Where 𝒊 𝒐𝒅 and𝒊 𝒐𝒒 are 𝒅 − 𝒒 currents at the
converter side of the transformer respectively.
Multi-loop voltage control for a dc/ac Fig.7. Block diagram of the booster and battery
inverter is described in [30], where the control converter for Case 1.
objective is to provide a high quality ac voltage with
good dynamic response at different load conditions. 𝒗∗𝒅𝒄 − 𝒗 𝒅𝒄 multiplied by-1 through the PI produces
This control scheme can also be applied for main a negative 𝒊∗𝒃 for the inner current loop, which makes
converter control to provide high quality ac voltage the battery to transfer from charging into discharging
in stand-alone mode with minor modifications. the mode and to rise 𝒗 𝒅𝒄 back to its present value 𝒗∗𝒅𝒄 .
coordinated control block diagram for the normal The battery converter is transferred from discharging
case is shown in fig9. into charging mode in the similar control method.
The main converter provides a stable ac bus voltage
for the DFIG converter.
The control objectives for the converters
change when the system transfers from one
operating scenario to another. For example, the role
of the boost converter is changed to provide a stable
dc-link voltage rather than the MPPT for cases 1 and
2, while the battery converter is controlled to absorb
the maximum power in case 1 and is switched off in
case 2. The coordinated control block diagram for
these two converters in Case 1.The boost converter
provides a stable dc-link voltage. The main
converter is controlled to provide a stable ac bus

Fig.6 Block diagram of the battery and main
converters for the normal case.
To provide a stable dc-link voltage, the dual
loop control scheme is applied for the battery
converter. The injection current 𝑰 𝒊𝒏 = 𝒊 𝟏 𝟏 − 𝒅 𝟏 −

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fast tracing speed should be used in the PV sites with
fast variation of solar irradiation.
The curves of the solar radiation (radiation
level times 30 for comparison) and the output power
of the PV panel. The output power varies from 13.5
kW to 37.5 kW, which closely follows the solar
irradiation when the ambient temperature is fixed.
Fig.8 shows the voltage (voltage times 0.2 for
comparison) and current responses at the ac side of
the main converter when the solar irradiation level
decreases from1000 W/m2 at 0.3 s to 400W/m2 at 0.4
s with a fixed dc load 20 kW. It can be seen from the
current directions that the power

Many anti-islanding detection and control
schemes [31], [32] have been developed for
conventional and power-converter-based distributed
generators and various microgrids. Those techniques
can be modified and implemented in the proposed Fig.8. AC side voltage and current of the main
hybrid grid to make the system transfer smoothly converter with variable solar irradiation level and
from the grid tied to isolated mode. constant dc load.

IV. SIMULATION RESULTS
The operations of the hybrid grid under
various source and load conditions are simulated to
verify the proposed control algorithms. The
parameters of components for the hybrid grid are
listed in Table III.

A. Grid-Connected Mode
In this mode, the main converter operates in
the PQ mode.
Power is balanced by the utility grid. The battery is Fig. 9. AC side voltage and current of the main
fully charged and operates in the rest mode in the converter with constant solar irradiation level and
simulation. AC bus voltage is maintained by the variable dc load.
utility grid and dc bus voltage is maintained by the
main converter. is injected from the dc to the ac grid before 0.3 s and
The optimal terminal voltage is determined reversed after 0.4 s. Fig. 14 shows the voltage
using the basic P&O algorithm based on the (voltage times 0.2 for comparison) and current
corresponding solar irradiation. The voltages for responses at the ac side of the main converter when
different solar irradiations.The solar irradiation level the dc load increases from 20 kW to 40 kW at 0.25 s
is set as400 W/m2 from 0.0 s to 0.1s, increases with a fixed irradiation level 750 W/m2 .
linearly to1000 W/m2 from 0.1 s to 0.2 s, keeps
constant until 0.3 s, decreases to400 W/m2 from 0.3
s to 0.4 s and keeps that value until the final time 0.5
s. The initial voltage for the P&O is set at 250 V. It
can be seen that the P&O is continuously tracing the
optimal voltage from 0 to 0.2 s. The algorithm only
finds the optimal voltage at 0.2 s due to the slow
tracing speed. The algorithm is searching the new
optimal voltage from 0.3 s and finds the optimal
voltage at 0.48 s. It can be seen that the basic
algorithm can correctly follow the change of solar
irradiation but needs some time to search the
optimal voltage. The improved P&O methods with

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Fig. 9. Upper: output power of the DFIG; Lower: 0.2 s, which means that the operating modes are
AC side voltage versus current (Voltage times 1/3 changed from MPPT to off-MPPT mode. The PV
for comparison). output power changes from 35 kW to 25 kW after
0.2 s.
B.Isolated Mode
The control strategies for the normal case V. CONCLUSION
and Case 1 are verified. In the normal case, dc bus A hybrid ac/dc microgrid is proposed and
voltage is maintained stable by the battery converter comprehensively studied in this paper. The models
and ac bus voltage is provided by the main and coordination control schemes are proposed for
converter. The reference of dc-link voltage is set as the all the converters to maintain stable system
400 V.The dynamic responses at the ac side of the operation under various load and resource
main converter when the ac load increases from 20 conditions. The coordinated control strategies are
kW to 40 kW at 0.3 s with a fixed wind speed 12 verified by Matlab/Simulink. Various control
m/s. It is shown clearly that the ac grid injects power methods have been incorporated to harness the
to the dc grid before 0.3 s and receives power from maximum power from dc and ac sources and to
the dc grid after 0.3 s. The voltage at the ac bus is coordinate the power exchange between dc and ac
kept 326.5 V constant regardless of load conditions. grid. Different resource conditions and load
The nominal voltage and rated capacity of the capacities are tested to validate the control methods.
battery are selected as 200 V and 65 Ah respectively. The simulation results show that the hybrid grid can
Fig. 16 also shows the transient process of the DFIG operate stably in the grid-tied or isolated mode.
power output, which becomes stable after 0.45 s due Stable ac and dc bus voltage can be guaranteed when
to the mechanical inertia. the operating conditions or load capacities change in
Fig. 10 shows the current and SOC of the the two modes. The power is smoothly transferred
battery. The total power generated is greater than the when load condition changes.
total load before 0.3 s and less than the total load Although the hybrid grid can reduce the
after 0.3 s that the battery operates in charging mode processes of dc/ac and ac/dc conversions in an
before 0.3 s because of the positive current and individual ac or dc grid, there are many practical
discharging mode after 0.3 s due to the negative problems for implementing the hybrid grid based on
current. The SOC increases and decreases before and the current ac dominated infrastructure. The total
after 0.3s respectively. The voltage drops at 0.3s system efficiency depends on the reduction of
and recovers to 400V quickly. When the system is at conversion losses and the increase for an extra dc
off-MPPT mode in Case 1, the dc bus voltage is link. It is also difficult for companies to redesign
maintained stable by the boost converter and ac bus their home and office products without the
voltage is provided by the main converters the dc embedded ac/dc rectifiers although it is theoretically
bus voltage, PV output power, and battery charging possible. Therefore, the hybrid grids may be
current respectively when the dc load decreases from implemented when some small customers
20 kW to 10 kW at 0.2 s with a constant solar want to install their own PV systems on the roofs
irradiation level 1000 W/m2 and are willing to use LED lighting systems and EV
charging systems. The hybrid grid may also be
feasible for some small isolated industrial plants
with both PV system and wind turbine generator as
the major power supply.

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