3.1 EMS of the MG
EMS uses simplified control-oriented model necessary for the effective operation of
a stand-alone MG. The dynamics of the load and DG are very fast compared to the characteristic
sampling time, so we have to ignore them and consider the SOC of the ESS by including
the power balance equation on each bus of the MG.
ESS composed of battery and power conversion system(PCS) is a device that stores and
discharges electric energy when necessary and the SOC dynamic model of ESS and upper
and lower limit conditions are as follows (15).
Where, $T_{s}$ is the sampling time ($T_{s}= 1 s$), $C_{\max}$ is the rated capacity
of the battery, $\eta_{ch}$ and $\eta_{dis}$ are charging and discharging efficiency,
and $P_{esst}(t)$ denotes the ESS power to be charged and discharged.
On the other hand, DG is mainly used as the main power source to supply power to the
load in island areas and isolated areas. In this paper, DG is base power generation,
and output is rated in all modes regardless of the increase or decrease of the load.
If the load that consumes power during power generation is small as the DG is the
base, the ESS operates in the charging mode. If the SOC of the ESS reaches the set
value, the dumpload is operated as shown in Equation (3) to adjust the balance of the total power of the MG.
where $P_{dp,\:i}(t)$ is the nominal output of the dump load, $\delta_{1}(t)$ is 1
when the SOC reaches the set value for operating the dump load, and $\delta_{2}(t)$
becomes 0 when the SOC reaches the set value for stopping the dump load.
On the other hand, when the EV is parked for a certain period of time, the EMS calculates
the optimal charging interval during the parking period in consideration of the SOC
of the ESS and charges the EV with a constant power. When the voltage of the line
is out of the set range due to PV power generation, load and EV charging consumption
in stand-alone MG, EMS controls the voltage drop of the line by reducing the EV charging
power.
Where, $\epsilon_{i}(t)$ is 1 when the EV is being charged, and $P_{ev,\:ch}$ and
$P_{ev,\:curr}$ denote the EV rated charging output and the power reduced by the EMS
for charging the EV, respectively.
The EMS of the stand-alone MG operates based on the data collected at each point.
Charging/discharging operation by bi-directional power flow of ESS stabilizes all
bus voltages within the allowable value and minimizes each feeder current to improve
the acceptability of load and renewable energy. In addition, when the SOC of the ESS
reaches the upper/lower limit specified by the EMS, the stability of the stand-alone
MG is secured by adjusting the controllable load or operating a dump load. The charge/discharge
power of the ESS to increase the hosting capacity is determined according to Equation (5).
where, $P_{ess}(t)$ is the charging/discharging power of ESS, $P_{load}(t)$ is the
load demand, $P_{pv}(t)$ is the PV generation and $P_{dg}$ is DG output.
In this case, if the ESS is connected to the end of the feeder and many PVs are generated
at one time, the grid voltage will deviate from the allowable range. EMS must maintain
the grid voltage to satisfy the following condition.
where $v_{MG,\:\min}$ and $v_{MG,\:\max}$ denote the lower limit and the upper limit
of the voltage, respectively.
3.2 Operation algorithm of MG
In the ESS control algorithm to use the bi-directional power flow, the charge/discharge
mode, operation time, and operating capacity are determined based on the data measured
from the bus to which each load is connected, and the ESS is operated stably within
the set SOC range. For this, three alarm signals are included in the EMS for each
mode, and the operation procedure is as follows.
[Step 1] Determination of charge/discharge of ESS
In order to determine the mode of the ESS, the direction of the power flow at the
measurement point must first be determined. In MG, If the sum of the active power
measured in each bus in MG is positive, it means that the consumed power is greater
than the generated power. At this time, as in Equation (7), a becomes 1, and when the sum of active power is negative, b becomes 0. That is,
when $\alpha(t)$ is activated, the grid voltage may drop below the lower limit, so
the ESS operates in the discharge mode. Conversely, if $\alpha(t)$ is deactivated,
the grid voltage may deviate from the upper limit, so the ESS operates in the charging
mode.
where, $P_{i}(t)$ is the sum of the measurement active power in each bus.
[Step 2] Determining the operation mode of dump load
When the ESS is charging, the dump load is operated when the SOC reaches the set value
for the safety of the ESS and charging operation can no longer be performed. In this
paper, as shown in Equation (8), $\beta(t)$ is activated according to the SOC of the ESS, and the dump load operates
at the nominal output until the SOC falls to a certain extent. When the SOC of the
ESS is below the set value, $\beta(t)$ is deactivated and the dump load stops operating.
[Step 3] Determining the operation mode of the controllable load
The controllable load operates when the ESS is discharged, and the SOC reaches the
set value for the safety of the ESS and the discharge operation can no longer be performed.
In this paper, as shown in Equation (9), $\gamma(t)$ is activated according to the SOC of the ESS, and the controllable load
reduces power consumption until the SOC increases to a certain extent. When the SOC
of the ESS exceeds the set value, $\gamma(t)$ is deactivated and the controllable
load restores power consumption to its original state.
Table 1. Classification of ESS operation signal
|
Classification
|
$\alpha$
|
$\beta$
|
$\gamma$
|
|
ESS charging Operation
|
1
|
-
|
-
|
|
ESS discharging Operation
|
0
|
-
|
-
|
|
SOC ≥ 70%,
|
1
|
1
|
-
|
|
SOC < 68%
|
1
|
0
|
-
|
|
SOC ≤ 20%
|
0
|
-
|
1
|
|
SOC > 23%
|
0
|
-
|
0
|
Fig. 5 shows the ESS control algorithm of the stand-alone MG based on the above-mentioned
procedures. In a situation where the power generated by controllable load such as
EV and PV generation exceeds the capacity that can be accommodated by DG, ESS maximizes
the hosting capacity through charging and discharging operation.
Fig. 5. MG ESS Operation Algorithm
Table 2. Model parameter of MG
|
Bus number
[sending-outgoing]
|
length[km]
|
impedance
[ohm/km]
|
DG
(kVA/V)
|
Load+EV
|
Dump load
[kW]
|
PV
[kWp]
|
ESS
[kW/kWh]
|
PF
[PU]
|
|
DG-1
|
0.01
|
Z=0.730+j0.085
|
20kVA/230V
|
-
|
-
|
|
-
|
1
|
|
1-2
|
0.1
|
Z=0.730+j0.085
|
-
|
12kW
|
|
16kWp
|
-
|
1
|
|
2-3
|
0.1
|
Z=0.730+j0.085
|
-
|
18kW
|
|
16kWp
|
-
|
1
|
|
3-4
|
0.1
|
Z=0.730+j0.085
|
-
|
18kW
|
|
16kWp
|
-
|
1
|
|
4-5
|
0.1
|
Z=0.730+j0.085
|
-
|
|
8kW
|
|
30kW/100kWh
|
1
|