3.3 Design of MCS for dynamic load management
3.3.1 Individual battery control and its algorithm for charging and discharging 60
3. Multi Charging Station for dynamic load management
3.3 Design of MCS for dynamic load management
this is taken into account in the proposed model as shown in the flow chart in Figure 3.5.
Current Ampere Hour is calculated as:
Ahrj =Ahrrating(SOCrem/100) (3.2)
where, Ahrrating is the rated AHR of the battery. In the case of discharging, SOCrem is the difference of the current SOC (SOCcr) and SOC limit (SOClt) and it is calculated as below.
SOCrem =SOCcr−SOClt (3.3)
In the case of charging, SOCrem is calculated as below.
SOCrem= 100−SOCcr (3.4)
Using Eq. (3.1) and Eq. (3.2), the current of the battery is calculated based on the required Crate and Ahrj of the battery and is given below.
Ahrbj =AhrjCrate (3.5)
where, Ahrbj is the current which flows between the battery and the grid. Ahrbj is positive, if power is flowing to the battery during charging and negative if power is flowing to the grid from the battery during discharging.
The detail flow chart for charging and discharging of a battery is shown in Figure 3.5. In this figure, Cratelimit is the preferred maximum Crate set by the vehicle’s owner. Vbat is the voltage of the battery. All the other symbols have same description as mentioned in Eq. (3.1) to Eq. (3.5).
For example, if AHRrating, Vbat, SOCcr and SOClt of a battery (1st battery of CS1) is 32 AHR, 250 V, 90 and 25 respectively then SOCrem can be found from Eq. (3.3) to be 65 and Ahrj can be found from Eq. (3.2) to be 20.8 AHR. If P b11= 1.16 kW, which is the power required for discharging to the grid by the EV’s battery as decided by Eq. (3.12) during peak hours, Crate at 90% of SOC can be calculated using Eq. (3.1) as 0.223.
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3. Multi Charging Station for dynamic load management
Crate
SOClt SOCcr
Vp.u.
Ahrrating
Yes No
Calculate
To individual battery of Charging station
Calculate Vbat
a
If
Ahrj
Calculate
Crate Cratelt
Ahrj P bij
Eq.(2) Ahrbj
Crate< Cratelt Cratelt
Figure 3.5: Flow chart for controlling the flow of power in individual battery.
3.3.2 Distribution of power among CSs and batteries
The division of power among the CSs is based on the capacity of the CS to handle the power. This is restricted by the power rating of the device. Power can not be injected into the grid beyond the rating of the transformer. Different rating of the transformers implies different power handling capacity of CSs. If transformers’ rating for the three CSs are assumed to be 150 kVA, 100 kVA and 150 kVA, then the maximum power that can be injected or drawn by the MCS will be 400 kVA. Therefore, the FLC will be set to deliver a maximum of 400 kVA of power to/from the MCS. Power required by each CS can be calculated on the basis of the transformer ratings as follows.
The distribution of power among the CSs can be achieved based on the transformer ratings
3.3 Design of MCS for dynamic load management
of the CSs or the energy available/ required by the CSs. Power can not be injected into the grid or drawn by the CS beyond the rating of the CS’s transformer and it is calculated using the following equations.
Pi = ( P ti n
P
i=1
P ti
P)η (3.6)
where,Pi is the power available/required by the ith CS,P ti is theith transformer power rating in kVA,P is the power injected/drawn from the grid which is decided by the FLC and ηis the efficiency of the charging/discharging system. Power division based on the transformer rating is effective if the CS has energy in accordance with the rating of the transformer.
The division of power among the CSs can be made on the basis of transformer’s rating.
However, there can also be a situation in which one of the CS has the highest available energy with a lower transformer rating. In this chapter, each CS has different energies at different time slots and hence, the division of the power is made according to Eq. (3.7) and Eq. (3.8). In the case of charging and discharging, the distribution of power among the CSs are as follows.
Pi = (Ei
EP)η (Charging) (3.7)
Pi = (Ei EP)1
η (Discharging) (3.8)
whereP is the power which should be drawn/supplied from/to the node,Pi is the net power exchanged at the ith CS, Ei is the available/required batteries energy of the ith CS and E is the total energy available/required by the MCS. η is the charging/discharging efficiency of the system.
Available/required energy Ei can be calculated as follows.
Ei =
m
X
j=1
Ebij (3.9)
where Ebij is the energy available/required by the jth battery of the ith CS and is calculated
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3. Multi Charging Station for dynamic load management
as follows:
Ebij = SOCrem
100 VbatAhrrating (3.10)
where SOCrem is same as of Eq. (3.3) or Eq. (3.4) depending on whether Ebij is the energy available in the battery during discharging or energy required by the battery during charging.
Total energy of the MCS can be calculated as follows.
E =
n
X
i=1
Ei (3.11)
Similarly, Pi can be redistributed to each battery of theith CS and this is as follows.
P bij = Ebij m
P
j=1
Ebij
Pi (3.12)
In Eq. (3.12),P bij is the power available/required by thejthbattery ofith CS andPi is same as defined in Eq. (3.7). Power division among the batteries is based on the available/required energy of the battery.
For example, suppose the power required to be discharged to the grid is found to be 246 kW as decided by the FLC. If energy available atCS1,CS2 and CS3 are found to be 255 kWh, 700 kWh and 200 kWh respectively. By using Eq. (3.7), P1 can be calculated to be 54.31 kW.
Similarly, if available energy of each battery is known, the power to be discharged to the grid by an individual battery can be calculated using Eq. (3.12).