Seul-Ki Kim

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.

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.

Unified compensation control of a hybrid energy storage system for enhancing power quality and operation efficiency in a diesel and wind-turbine based stand-alone microgrid

An intermittent power of renewable sources such as wind turbine and photovoltaic system can damage power quality and operation efficiency of generator. Specially, in case of standalone microgrid such as island power system, this intermittent characteristic of renewable resource can lead to severe problems, such as frequency oscillation and power fluctuation. This frequency oscillation and power fluctuation can be a cause of system stability problem and operation efficiency drop. This paper presents unified compensation control strategy of a hybrid electric energy storage system in order to improve power quality and operating efficiency in a diesel and wind-turbine based stand-alone microgrid. This study addressed an AC hybrid energy storage system which was composed of lead acid battery storage and electric double layered capacitor, and presented a compensation control strategy with modified droop and wind power compensation for damping system frequency and power fluctuation. Usefulness of the proposed control algorithm is verified by experimental test results for system frequency, voltage, power fluctuation of the generator, and operation efficiency. Comparison tests with conventional compensation methods were performed to validate the effectiveness of the proposed control method.

Power control of a grid-connected hybrid generation system with photovoltaic/wind turbine/battery sources

A grid-connected hybrid distributed generation system, composed of PV(photovoltaic) array, wind turbine and BESS (battery energy storage system), is proposed for various power transfer functions to the distribution network. The proposed system has several operation modes which are normal operation, power dispatching, and power averaging, according to coordinate control of the BESS and grid inverter. PV array and wind turbine are individually controlled to operate at the maximum power point for the most use of renewable energy. The BESS operates as an energy buffer to flexibly shift the generation from the renewable energy sources without excessively frequent shifts between battery charging and discharging. A grid interface inverter regulates the generated power injection into the grid. Developed prototype system design and experimental results are presented to demonstrate the performance and to validate the proposed system during its operation modes.

Optimal Operation Control for Multiple BESSs of a Large-Scale Customer Under Time-Based Pricing

This paper presents an online optimal operation framework for multiple battery energy storage systems (BESSs) of a large-scale customer under time-based pricing. Many publications have been reported on optimal battery operation techniques but most of them were analyzed in a simulation environment or a specifically designed test bed. However, this paper focuses on implementing the proposed scheme into actual multiple battery storage units and investigating the performance during long-term field operation. The operation framework consists of two levels: optimal scheduling and real-time dispatch. The optimal scheduling is calculated every hour, using a model predictive control based nonlinear optimization model, to minimize the daily electricity usage cost while regulating the peak. The real-time dispatch determines final commands to multiple battery systems by monitoring the system state and checking for any violations of the operation constraints. The two-level control scheme was designed to handle uncertainty in forecast load and estimated state-of-charge levels of batteries. The operation method was applied into the energy management system supervising one lithium-polymer BESS and two lead-acid BESSs of an industrial site. Comprehensive field operation results prove the reliability and effectiveness of the optimal operation framework.

Cooperative ESS management strategy for stable islanded operations in a bipolar-type LVDC system

In this paper, a new energy storage system (ESS) management strategy for a bipolar-type low voltage direct current (LVDC) distribution system is being proposed. The dc distribution system is considered as an alternative to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and ESSs. The dc distribution system has several advantages such as conversion losses reduction, easy connection scheme for dc based DER, simple energy management strategies (no reactive power issues). In a bipolar-type, if it has each grid-interfaced converter, both sides (upper, lower-side) can be operated individually or collectively. A cooperative ESS strategy using interchange and transfer algorithm in both sides for effective and stable operation is proposed in this paper. Detailed energy management strategies of the main controller and local controllers are described. Simulation analysis using PSCAD/EMTDC is being performed to verify the performances of the proposed control strategy. In recent, the authors are trying to implement this strategy in real hardware pilot plant.