Distributed Charging Management of Electric Vehicles in Smart Microgrids

Abstract

The uncoordinated integration of a great number of Electric Vehicles (EVs) possibly leads to undervoltages and overcurrents in a Distribution Network (DN). It may also increase the power losses in the DN, and in severe cases, it is probable that the DN collapses. Therefore, control and management systems are necessary for avoiding the negative consequences of EV charging. In addition, by the advent of bidirectional chargers and sophisticated EV Supply Equipment (EVSE), objectives beyond saving the DN from becoming upset can be defined for EVs as auxiliary services (also referred to as ancillary services).

This chapter after addressing and comparing different methods for EV charging and discharging management, concludes that the distributed control mechanisms respond to the particular needs in Smart Microgrids (SMGs) as they make the systems largely-scalable and plug-&-playable, while at low computational and communication costs. Then, the specifications of EV batteries and chargers are presented. As modern communications are the backbone of SMGs, the communication-assisted distributed control system can be set up. Therefore, a wide range of auxiliary services are addressed in this chapter and distributed control systems based on the cooperative control are introduced to accomplish them. The control objectives in this chapter (i) facilitate the integration of variable renewable energies (VREs) to SMGs, (ii) respond to the technical challenges toward EV interconnection, and (iii) increase the EV owners’ economic benefits.

Appendix A

The nomenclature is shown below.

Pi	i-th EVSE’s preferable active output power
Pj	j-th EVSE’s preferable active output power
${P}_i^{Max}$	Maximum active output power of the i-th EVSE
Qi	i-th EVSE’s preferable reactive output power
Qj	j-th EVSE’s preferable reactive output power
${Q}_i^{Max}$	Maximum reactive output power of the i-th EVSE
m	Number of the EVSEs existing in a DN
ωi	i-th EVSE’s output frequency
ωMainGrid	Main grid frequency
VMin	Minimum acceptable voltage in a DN
VMax	Maximum acceptable voltage in a DN
Vi	Output voltage of the i-th EVSE
Ik	Measured current passing through the k-th location
${I}_k^{Max}$	Maximum allowable current at the k-th location
n	Number of current sensors in a DN
SoCi	i-th EV’s state of charge
$So{C}_i^{Min}$	Minimum state of charge for the i-th EV
EC	Emergency charging
IESS	Output current of the energy storage system
PESS	Output power of the energy storage system
ACCi	Average cost of charge
${\omega}_i^{\ast }$	Frequency command from the i-th local controller to the i-th charger
${\omega}_{re{f}_i}$	Frequency reference for the i-th droop controller
${\kappa}_{p_i}$	Droop gain for active power-frequency
${P}_{re{f}_i}$	Active power reference for the i-th droop controller
${P}_{O_i}$	i-th EVSE measured active output power
${V}_i^{\ast }$	Voltage command from the i-th local controller to the i-th charger
${V}_{re{f}_i}$	Voltage reference for the i-th droop controller
${\kappa}_{Q_i}$	Droop gain for reactive power-voltage
${Q}_{re{f}_i}$	Reactive power reference for the i-th droop controller
${Q}_{O_i}$	i-th EVSE measured reactive output power
L	Laplacian matrix
x	State vector
B	Control input matrix
u	Control input vector
Ni	i-th EVSE’s neighbors
|Ni|	Indegree of the i-th EVSE
PPCC	Active power flow at PCC
k	Integrator gain for updating controller’s states
${P}_{PCC}^{Max}$	Maximum active power flow at PCC
LD	Boolean variable defined for reducing the SMG dependency on the main grid
xP	State vector for active power
xQ	State vector for reactive power
xω	State vector for frequency
xV	State vector for voltage
Lp	Laplacian matrix for active power
LQ	Laplacian matrix for reactive power
Lω	Laplacian matrix for frequency
LV	Laplacian matrix for voltage
kp	Integrator gain for active power
kQ	Integrator gain for reactive power
kω	Integrator gain for frequency
kV	Integrator gain for voltage
ωc	Cut frequency of the derivative of the power flow at PCC
ωMainGrid	Frequency reference for the grid in islanded mode
EP	Electricity price
Pref	Control output vector composed of active power references for the local droop controller
SoC	Vector consisting of all the EVs’ SoCs
SoCMin	Vector consisting of all the minimum acceptable EVs’ SoCs
EC	Vector comprising the EVs’ emergency charging choice
PMax	Vector of the maximum active power output of EVSEs
Qref	Control output vector of reactive power references for the local droop controller
QMax	Vector of the maximum reactive power output of EVSEs
Vref	Control output vector consisting of voltage references for the local droop controller
ωref	Control output vector containing frequency references for the local droop controller