IEEE Electrification - December 2020 - 61

Recovery and Resiliency

Data

Forecast
Results
Data

Load
Data

PV Generation
Interface

Optimization
Module

Prediction Module

Management Module
Optimal
Results

Data

Energy Storage

Weather
Others ...

Historical Data Module
Control

Data Calculation Module

State Monitoring Module

Figure 8. The architecture of the EMS.

Objective

Energy Cost

Physical Constraints

Balance

restrictions. The specific content
of the model is shown in Figure 9.
The objective function is to
minimize the total costs over the
scheduling periods, which include
the energy and environmental
costs. The energy-consumption
cost is equal to the purchasing electric energy cost plus the distributed
generation operation cost minus
the income from selling electric
energy. The environmental cost is
measured by CO 2 emission quantities and the corresponding emission price.
The physical constraints should
include the power balance and output constraints of the PV and energystorage equipment, among others.
The power balance constraint requires
that the " energy generated in the
microgrid + the purchased energy
from the grid + the energy from the
storage " must be equal to " the energy
demand in the microgrid + the energy into storage + the energy sold to
the grid " at each time period.
Economic constraints restrict
the maximum allowable cost,
allowed payback period, and
maximum allowable savings. Policy constraints describe the utility
tariff, electricity price mechanism
(capacity price and time-of-use
electricity price), CO 2 emission
limits, and so on.

Energy Generated
+ Purchased Energy
+ Energy From Storage
= Energy Demand
+ Energy Into Storage
+ Energy Sold to Grid

Output Limits of PV and
Energy-Storage Equipment

Environmental Cost
Economic Constraints
* The Maximum Allowable Cost
* Allowed Payback Period
* Maximum Allowable Savings
Policy Constraints
* The Utility Tariff
* Electricity Price Mechanism
* CO2 Emission Limits

Figure 9. The economic dispatch model of the Xi'an Industrial Park microgrid.

The DGs and distributed storages
in the microgrid have significantly changed the characteristics of a power system fault. Traditional power system protection principles and fault-detection methods
may not be able to accurately identify fault positions
and, thus, cannot play their protective roles. There are
two differences between the microgrid protection system and traditional power system protection system.
The first difference is that the power flow may have
two directions. This is because many DGs in the
microgrid and distribution network together form a
multigeneration power system, and the microgrid operator can choose to buy or transmit the electrical power
from the distribution network, resulting in a change of
power flow direction. When the microgrid is running in
off-grid mode, there may be bidirectional power flow
due to the differences in the power balance between
the DGs and distributed storage.

The second difference is that, in the two microgrid
operation modes, i.e., the on-grid and off-grid modes,
the short circuit current performs different values. In
the off-grid mode, due to the protection of inverters,
the short-circuit current is small. These changes make
the working principle and operation logic of the
microgrid protection system extremely complicated.
Traditional protection methods cannot meet the
requirements of microgrids, and it is necessary to de--
sign a fault-treatment method suitable for the mi--
crogrid. The following protection principles need to be
observed separately:
xx
In a microgrid that operates in on-grid mode, when
the electrical equipment in the microgrid goes wrong,
it should be ensured that the microgrid can continue
operating safely and steadily after the fault device is
cut off.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 2 0

IEEE Electrification - December 2020

Table of Contents for the Digital Edition of IEEE Electrification - December 2020