Control of Master-Slave Microgrid Based on CAN Bus

Control of Master-Slave Microgrid Based on CAN Bus Asma Alfergani, Ashraf Khalil, Zakariya Rajab, Mohammad Zuheir Electrical and Electronics Engineering Department University of Benghazi Benghazi, Libya [email protected]

Sheroz Khan, Ezzidin Hassan Elmabrouk Aboadla, Khairil Azhar Bin Azna, Majdee Tohtayong Department of Electrical and Computer Engineering, KOE International Islamic University of Malaysia (IIUM) Kuala Lumpur, Malaysia

Outlines • Microgrids. • Control Strategies in Microgrid. • Master-Slave Control Strategy of Microgrid over CAN Bus • Modeling of the Inverter Based Microgrid with Communication Delay. – The Mathematical Model of Parallel Inverters. – Master-Slave control Strategy.

• Stability Analysis with Communication Delay. • Simulation Results. • Concluding Remarks. 2

Microgrid • Most of the renewable energy sources are usually equipped with DC/AC inverters which form a system of parallel inverters. Microgrid can be viewed as a system of parallel inverters. Fig. 1 renewable technologies: PV, wind and wave.

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Microgrid

A Microgrid can be defined as a cluster of microsources, storage systems and loads which may be isolated (stand-alone) or connected to the grid as a single entity. 4

Fig. 2 Elementary Microgrid Architecture

Control Strategies In Microgrid

Communication Based

Master/Slave Control

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Distributed control

Non-communication Based

Central control

Droop Control

Time Delay

Load 10Mbps

Fig. 3 The system of two parallel inverters with networked based control 6

The Mathematical Model of Parallel Inverters  

The average model of the phase leg is derived based on the switching averaging. After transformation of the variables in the stationary coordinates Xabc into the rotating coordinates Xdqz, the average model can be simplified based on iz=iz1=-iz2≈ 0: d vd  1  id 1  id 2   1 / RC    vd          1 / RC   vq  dt  vq  2C  iq1  iq 2    

d id 1  1 d d 1  1 vd   0    id 1   V        dc1   dt iq1  L1 d q1  L1 vq   0  iq1  d id 2  1 d d 2  1 vd   0    id 2    i    d Vdc2   v    dt  q 2  L2  q 2  L2  q   0  iq 2  7

(1)

Fig.4 The two parallel inverters Microgrid

The Mathematical Model of Parallel Inverters 



The model is state-space using dq transformation:

x  Ax(t )  Bu(t ) (2) x  [vd vq id 1 iq1 id 2 iq 2 ]T

The matrices A, B can be obtained as  1 / RC      1 / L1 A  0   1 / L2   0

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u  [d d 1 d q1 d d 2

0 0 B 0  0



1 /(2C ) 0 1 /(2C ) 0   1 / RC 0 1 /(2C ) 0 1 /(2C ) 0 0  0 0    1 / L1  0 0 0  0 0 0 0     1 / L2 0 0  0 

0 Vdc1 / L1 0 0 0  0 0 Vdc1 / L1 0 0  0 0 0 Vdc1 / L1 0   0 0 0 0 Vdc1 / L1 

(3)

T

(4)

d q 2 ]T

Master-Slave Control Strategy • With the advances in network technology, a communication network can be used for the control signals exchange. The use of the communication network for control purposes reduces the cost and the complexity of the system, however, the induced time delay has a strong effect on the system stability. • The master-slave control strategy is used where the first inverter has two control loops and the second inverter has only current control loop. • The inner control loop of the master controller independently regulates the inverter output current in the rotating reference frame, id and iq. The outer loop of the master controller in the voltage control mode are used to produce the dq axis current references id-ref and iq-ref, then these control signals are sent through the CAN Bus to the slave controller 9

Master-Slave Control Strategy

Fig. 5 Control loops of two parallel invereters 10

Master-Slave Control Strategy A proportional-Integral (PI) control scheme is used in both the master and slave controllers, The controller model is given as:



dd / dt  (vd -ref - vd ) dq / dt  (vq - ref - vq )

d 1 / dt  ( Kvdp (vd -ref - vd )  K vdid - id 1 )

d 2 / dt  ( Kvqp (vq- ref - vq )  Kvqiq - iq1 ) d 3 / dt  ( Kvdp (vd -ref - vd (t   ))  Kvdid (t   ) - id 2 ) d 4 / dt  ( Kvqp (vq-ref - vq (t   ))  Kvqiq (t   ) - iq2 ) 

These equations can be written in matrix form as:

z  Ex(t )  Fz(t )  Ed x(t   )  Fd z(t   ) 11

(5)

(6)

where,

 d     q  1  z  2  3      4 

Master-Slave Control Strategy 

The duty cycle equations are given as:

d d 1  ( Kidp1 )( Kvdp (vd -ref - vd )  Kvdid - id 1 )  Kidi1 1 d q1  ( Kiqp1 )( Kvqp (vq-ref - vq )  Kvqiq - iq1 )  Kiqi1 2 d d 2  K idp 2 ( K vdp (vd - ref - vd (t   ))  K vdid (t   ) - id 2 )  K idi 2 3 d q 2  K iqp 2 ( K vqp (vq- ref - vq (t   ))  K vqiq (t   ) - iq 2 ) (7)

 K iqi 2 4

These equations can be written in matrix form as: (8) u  Cx(t )  Cd x(t   )  Dz (t )  Dd z(t   )  The Matrix form of the state space model can be written as:



 x (t )  A  BC  z(t )   E   

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BD   x(t )  BCd  F   z (t )  Ed

BDd   x(t   (t ))  Fd   z (t   (t )) 

xcl (t ) = A0 x(t ) + Ad x(t - (t ))

(9)

(10)

Stability Analysis with Communication Delay • The presence of the communication delay makes the Microgrid a time delay dependent control system. To guarantee the stability the maximum allowable delay bound (MADB) must be found. • The method used in this paper is based on LyapunovKrasovskii functional and replacing the delay term by NewtonLeibnitz formula. Then the stability problem is formulated as a set of LMIs.

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Theorem.. The time-delay system is asymptotically stable if there exist: • Symmetric positive-definite matrices: P  PT  0

,

T Q  QT  0 , Z  Z  0

• Symmetric semi-positive-definite matrix: X   XX 11T XX 12   0 22   12 • and any appropriate dimensioned matrices Y and T such that the following LMIs are true:   11  12  T     12  22  ZA ZA d 

AT Z   AdT Z   0  Z  

 X 11 T    X 12  Y T

where; 11  PA  AT P  Y  Y T  Q  X 11 14

12  PAd  Y  T T  Q  X 12  22  T  T T  (1   )Q  X 22

X 12 X 22 TT

Y T   0 Z 

Simulation Results… • The three-phase output voltages of the parallel inverters shows a stable operation of the system. • The power of the second inverter accurately tracks the power of the first inverter.

Fig.6 The output three-phase voltages 15

Fig.7 The active and reactive power of first and second inverters

Simulation Results… • The first inverter current, the second inverter and the load current are shown. • The balancing current and good current sharing are clear from output currents of first and second inverters.

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Fig.8 The output currents of the first and second inverters

Fig.9 The output currents of the load

Simulation Results… • Since the two inverters are synchronized using the PLL, there is no phase difference between the phase current of the first and second inverter as shown below • The id1, iq1, id2 and iq2 are shown below.

Fig.10 Syncronised phase currents of first and second inverters 17

Fig.11 The dqz current of first and second inverters

Simulation Results… • The flow of message among the network nodes is shown below. Which shows that the sending Id_ref and Iq_ref is only active nodes in the network.

Fig.12 The network schedule in the CAN 18

Concluding Remarks. • The parallel inverters implement master-slave control strategy where the master inverter sends the reference signal to the slave inverter through the CAN bus. • The parameters that affect the maximum time delay are investigated; these are the gains of the master and slave controllers, and the time constant of the low-pass filter. Changing the inductance of the filters has no effect on the maximum time delay margin. • The stability criterion is formulated as a set of LMIs which is solved using the LMI Matlab toolbox. • The controller gains are used as the controller parameters. • The controller is tested using the nonlinear models in the Matlab/Simulink. The CAN bus is implemented using True-Time 2.0 beta simulator. • The master-slave control scheme for the Microgrid system based on the CAN bus has shown a good current balance between the two inverters. 19

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