Jong-Yul Kim

Cooperative Control Strategy of Energy Storage System and Microsources for Stabilizing the Microgrid during Islanded Operation

In this paper, the cooperative control strategy of microsources and the energy storage system (ESS) during islanded operation is presented and evaluated by a simulation and experiment. The ESS handles the frequency and the voltage as a primary control. And then, the secondary control in microgrid management system returns the current power output of the ESS into zero. The test results show that the proposed cooperative control strategy can regulate the frequency and the voltage, and the secondary control action can contribute to improve the control capability.

Active Synchronizing Control of a Microgrid

A microgrid is an aggregation of multiple distributed generators (DGs), such as renewable energy sources, conventional generators, and energy storage systems that provide both electric power and thermal energy. Typically, a microgrid operates in parallel with the main grid. However, there are cases in which a microgrid operates in an islanded mode, or in a disconnected state. Islanded microgrid can change its operational mode to grid-connected operation by reconnection to the grid, which is referred to as synchronization. Generally, a single machine simply synchronizes with the grid using a synchronizer. However, the synchronization of microgrids that operate with multiple DGs and loads cannot be controlled by a traditional synchronizer. It is needed to control multiple generators and energy storage systems in a coordinated way for the microgrid synchronization. This is not a simple problem, considering that a microgrid consists of various power electronics-based DGs as well as alternator-based generators that produce power together. This paper proposes an active synchronizing control scheme that adopts the network-based coordinated control of multiple DGs. From the simulation results using Simulink dynamic models, it is shown that the scheme provides the microgrid with a deterministic and reliable reconnection to the grid. The proposed method is verified by using the test cases with the experimental setup of a practical microgrid pilot plant.

Development of Hardware In-the-Loop Simulation System for Testing Operation and Control Functions of Microgrid

This paper proposes a hardware in-the-loop simulation (HILS) system as a new method to develop and test control algorithms and operation strategies for a microgrid. The HILS system is composed of a real-time digital simulator (RTDS) for real-time simulation of the microgrid, a prototype microgrid management system (MMS) under test, and a communication emulator for interface between the prototype MMS and the RTDS. The prototype MMS is designed to operate microsources of microgrid and to control power flow at the point of common coupling (PCC) in the grid-connected mode, and voltages and frequency in the islanded mode of the microgrid. The MMS is tested in the grid-connected mode and in the islanded mode, respectively, to show the validation of the proposed HILS system.

Coordinated State-of-Charge Control Strategy for Microgrid during Islanded Operation

In this paper, a coordinated state-of-charge (SOC) control strategy for the energy storage system operating under microgrid islanded mode to stabilize the frequency and voltage was proposed. The proposed SOC control loop makes up of PI controller, which uses a SOC state of the energy storage system (ESS) as an input and an auxiliary reference value of secondary control as an output. The SOC controller changes the auxiliary reference value of secondary control to charge or discharge the ESS. To verify the proposed control strategy, PSCAD/EMTDC simulation study was performed. The simulation results show that the SOC of the ESS can be regulated at desired operating range without degrading of stabilizing control performance by proposed coordinated SOC control method.

Contribution of an Energy Storage System for Stabilizing a Microgrid during Islanded Operation

In this paper, the cooperative control scheme of micro sources and an ESS (Energy Storage System) during islanded operation is presented and evaluated by a simulation study. The ESS handles the frequency and voltage as a primary control. Then, the secondary regulation control returns the cur- rent power output of ESS into a pre-planned value. The simulations results show that the proposed co- operative control scheme can regulate the frequency and voltage and reduce the consumption of the stored energy of ESS.

Voltage Shift Acceleration Control for Anti-Islanding of Distributed Generation Inverters

This paper proposes voltage-shift acceleration control for islanding detection of distributed-generation (DG) inverters. The proposed method is conceptually based on a traditional dq -based voltage positive feedback control. A main improvement of the proposed control is that the anti-islanding performance and its impact on inverter output control can be consistent regardless of inverter operation points. An analytical approach based on a small-signal model of an inverter controller is used to prove the proposed control. Also, an analytical design method is proposed to obtain proper control gain that satisfies two conflicting issues: successful anti-islanding and limited impact on inverter control. Simulation and experimental tests were conducted to validate the proposed detection algorithm and its design method. In addition, simulation results on a two DGs system are shown to investigate the performance in multiple DGs operation.

Coordinated state-of-charge control strategy for microgrid during islanded operation

In this paper, a coordinated state-of-charge (SOC) control strategy for the energy storage system operating under microgrid islanded mode to stabilize the frequency and voltage was proposed. The proposed SOC control loop makes up of PI controller, which uses a SOC state of the energy storage system (ESS) as an input and an auxiliary reference value of secondary control as an output. The SOC controller changes the auxiliary reference value of secondary control to charge or discharge the ESS. To verify the proposed control strategy, PSCAD/EMTDC simulation study was performed. The simulation results show that the SOC of the ESS can be regulated at desired operating range without degrading of stabilizing control performance by proposed coordinated SOC control method.

Test result of microgrid management function in KERI pilot plant

As usual, the microgrid operates in grid-connect mode, but, when a fault occurs in the upstream grid, it should disconnect and shift into islanded operation mode. In grid-connect mode, the controlling power flow at PCC is necessary management function rather than the frequency and voltage. However, the controlling the frequency and voltage is asked in islanded mode. To achieve these management goals appropriately, the cooperative control strategy among microsources is needed. This supervisory control action is executed at MMS. In this paper, the management function of MMS is presented and evaluated by HILS test system and microgrid pilot plant. The proposed HILS test system was composed of the RTDS and a communication emulator. The RTDS performed real-time simulation of component models of the microgrid and the communication emulator simulated communication functions of components of the microgrid. The pilot plant consists of the PV, PV/Wind Hybrid system, BESS, diesel generators, and RLC loads. The test results show that the proper control strategy can ensure stability of microgrid in islanded mode and maintain the power flow of PCC at desired value successfully.

Complementary Power Control of the Bipolar-type Low Voltage DC Distribution System

In this paper, a new power control strategy for the bipolar-type low voltage direct current (LVDC) distribution system is being proposed. The dc distribution system is considered as an innovative system according to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and energy storage systems (ESS). Since the dc distribution system has many advantages such as feasible connection of DERs, reduction of conversion losses between dc output sources and loads, no reactive power issues, it is very suitable solution for new type buildings and residences interfaced with DERs and ESSs. In the bipolar-type, if it has each grid-interfaced converter, both sides (upper, lower-side) can be operated individually or collectively. A complementary power control strategy using two ESSs in both sides for effective and reliable operation is proposed in this paper. Detailed power control methods of the host controller and local controllers are described. To verify the performances of the proposed control strategy, simulation analysis using PSCAD/EMTDC is being performed where the results show that the proposed strategy provides efficient operations and can be applied to the bipolar-type dc distribution system.

Development of HILS(Hardware In-Loop Simulation) system for MMS(Microgrid Management System) by using RTDS

This paper proposes a structure of an HILS based development system for MMS. This system is composed of RTDS and its hardware interface devices, MMS, communication emulator modules. The RTDS is operated for real-time simulation for both microgrid and microsource models. By using this HILS system, the secondary regulation action of MMS during an islanded operation is evaluated. The secondary regulation action of MMS is designed to control the microsources power output set-point in order to make the BESS power output zero. The test results show that the secondary regulation action of MMS can regulate the frequency and voltage while minimizing the consumption of the stored energy of BESS.