Heejin Kim

Dynamic interactions among multiple FACTS controllers — A survey

Increasing number of Flexible AC Transmission System (FACTS) devices have been be installed to reinforce the existing grid and build the envisioned “Smartness” into the grid through controls and optimization. However, it has been noticed that adverse interactions among multiple FACTS controllers may occur when they are not properly coordinated with each other and other slowly acting system equipment. These interactions can amplify oscillations and even destabilize the system by influencing the damping properties of individual FACTS controllers or increasing voltage deviations. This paper presents an extensive survey on the existing cases, system studies and assessment techniques to help system planners understand the underlying mechanism of diverse interactions among multiple FACTS controllers and develop coordinated control schemes for preventing or mitigating any harmful interactions. Control interactions are categorized and discussed in terms of their root causes and resulting frequency ranges. Unfriendly interactions involving shunt FACTS devices are detailed in which Korean Electric Power Corporation (KEPCO) is particularly interested.

Expanded Adoption of HTS Cables in a Metropolitan Area and its Potential Impact on the Neighboring Electric Power Grid

There is an increasing interest in installing high-temperature superconductor (HTS) cables to an highly populated metropolitan area to significantly improve power transmission capacity with minimal electrical loss. Regardless of the attractive advantage, the HTS cable could unpleasantly change power flows in the neighboring grid because of reduced series inductance. Reduced damping of the HTS cables due to lower resistance, but with higher capacitance, may have an adverse impact on the power system stability. Thus, inexperienced oscillatory, voltage instability or both issues may occur in the power system. This paper first characterizes the HTS cable with reference to the existing overhead line and cross-linked polyethylene (XLPE) cable. It addresses a few potential challenges in the future power grid due to the expanded adoption of the HTS cables through analytical and simulation studies. The installation of HTS cables using data gleaned from careful studies and relevant reactive compensation schemes should yield a reliable and sustainable electric power infrastructure.

Operating Region of Modular Multilevel Converter for HVDC With Controlled Second-Order Harmonic Circulating Current: Elaborating P-Q Capability

The operating region of the modular multilevel converter (MMC) is limited by the maximum-allowable voltage ripple of cells, which is associated with the arm energy variation. Injecting the controlled second-order harmonic component into the circulating current can reduce energy variation of the arm and, thus, extend the operating region of MMC. Care must be taken, however, that the second-order circulating current affects the arm current, and inappropriately injected harmonic current may cause an undesirable arm current polarity change. Therefore, this paper first investigates and proposes the maximum-allowable second-order circulating current for extending the operating region reliably and efficiently. The active and reactive power capability of MMC is further elaborated and illustrated in line with the maximum excess energy capability for two cases, running with: 1) the original operating region and 2) the extended operating region. Finally, the P- Q diagrams for contingent operating conditions with faulty or disabled submodules are presented. Understanding the unique shape and change of P- Q capability with credible submodule failure contingency should be crucial for planning MMC-HVDC lines, and determining the energy requirements and the level of redundancy in submodules properly in order to achieve the envisioned benefits of the new HVDC. The efficacy and accuracy of the research findings are validated for the MMC-HVDC system using PSCAD/EMTDC.

Improved PD-PWM for minimizing harmonics of multilevel converter using gradient optimization

This paper proposes the improved phase-disposition PWM (PD-PWM) method, which optimizes the voltage harmonic performance of multilevel converter. The proposed PD-PWM method adjusts the position and magnitude of the carriers to find out the optimal values, which minimize the THD of output voltages. A gradient optimization method is applied as a form of an optimization-enabled transient simulation. A neutral-point clamped converter (NPC) demonstrates that the proposed method improves the THD of output voltages using PSCAD/EMTDC and MATLAB.

Validation for compatible modular multilevel converter models using PSCAD/EMTDC

This paper validates the modular multilevel converter (MMC) models, which are developed by two different institutes using PSCAD/EMTDC. The MMC uses a large number of power-electronic switching devices that operate independently. Modeling such converters with individual switches in an electromagnetic transient (EMT) simulation program leads to extremely slow runs, and hence seems to be impractical. To overcome this issue, MMC can be modeled as a detailed equivalent circuit. In this method, each MMC arm is represented by a Thévenin equivalent branch whose voltage and resistance are calculated at each time step, thus reducing the computational burden. The models have been developed by Yonsei University in South Korea and the Manitoba HVDC Research Centre in Canada based on this technique. The presented models include the detailed equivalent representation of each converter arm, corresponding controls, waveform generation, and cell capacitor balancing strategies. To validate overall performance and transient response of the model, a 7-level MMC-HVDC is modeled with individual switches and through the detailed equivalent circuit. As an example for a realistic system, a 201-level MMC-HVDC system is also simulated. The results demonstrate validity and compatibility of two MMC models.

Assessment of Improved Power Quality Due to Fault Current Limiting HTS Cable

Fault current limiting high-temperature superconductor (FCL-HTS) cable not only provides effective power transmission, but also reduces the negative impact of fault current. As the demand grows rapidly, distributed generation (DG) is being implemented into the power grids. The integration of DG into the existing power grid causes an increase in short circuit current. In order to facilitate reliable planning and operation of future power systems, power system planners and operators will need to avoid the potential threat of increased fault current. The superconductor cable is considered as an enabling technology because it can raise power transmission capability and effectiveness with minimal loss. However, high fault current through the superconductor cable may seriously damage the power system. However, FCL-HTS cables can reduce the impact from unexpected high fault current. The FCL-HTS cable helps avoid interruption and voltage sag without harming existing coordination of protection devices. This study addresses the power quality issues for increased fault current that will be caused by the DG. We also analyze the response of FCL-HTS cable in perspective of power quality, when the fault occurs with DG.

Modeling and analysis of bipolar HVDC interlink for Tanzania power grid

There are rising challenges in installing High Voltage Direct Current (HVDC) to Tanzania power grid to significantly improve network controllability and system security. In addition, to expand generation capacity and to increase bulk power transmission capacity over long distances, feasibility study for installing HVDC in Tanzania power grid needs to be performed. Therefore, this paper presents the modeling and analysis of bipolar HVDC proposed for new Iringa-Shinyanga interconnection of 1,000 MW/±400 kV. This paper first describes the needs of HVDC link in Tanzania power grid. Then, we propose the modeling of bipolar HVDC for 667 km between Iringa and Shinyanga. We analyze the proposed HVDC model under steady state and transient conditions. Simulation results show that the basic characteristics of HVDC model.

A Study on synchronizing two separate RTDS simulation instances

This paper presents a method to interconnect and synchronize two instances of real time simulation. A Bergeron transmission line model is utilized as interface between two simulation instances. Communication to transfer the necessary transmission line model quantities is implemented by utilizing TCP/IP socket communication. Part of the real time simulator includes the hardware communication facility. In regards to the necessary synchronization, an external precision clock source is used at the both ends of the communication. Each end would represent the separate instance of the real time simulation in synchronism. Again, part of the real time simulation hardware includes the necessary mechanism for the synchronization. The proposed concept is proved with a simple power simulation, separated into two instances of real time simulation, which are interconnected by communication and synchronized by external precision clock source.

Operating region of modular multilevel converter for HVDC with controlled second-order harmonic circulating current: Elaborating P-Q capability

The operating region of modular multilevel converter (MMC) is limited by the maximum allowable voltage ripple of cells, which is associated with the arm energy variation. Injecting the controlled second-order harmonic component into the circulating current can reduce energy variation of arm and thus extend the operating region of MMC. Care must be taken, however that the second-order circulating current affects the arm current and inappropriately injected harmonic current may cause the arm current polarity change undesirably. Therefore, this paper first investigates and proposes the maximum allowable second-order circulating current for extending the operating region reliably and efficiently. The active and reactive power capability of MMC is further elaborated and illustrated in line with the maximum excess energy capability for two cases, running with (1) the original operating region and (2) the extended operating region. Finally, the P-Q diagrams for contingent operating conditions with faulty or disabled submodules are presented. Understanding unique shape and change of P-Q capability with credible submodule failure contingency should be crucial for planning MMC-HVDC lines, and determining the energy requirements and the level of redundancy in submodules properly in order to achieve the envisioned benefits of the new HVDC. The efficacy and accuracy of the research findings are validated for MMC-HVDC system using PSCAD/EMTDC.

Trade-Off Strategies in Designing Capacitor Voltage Balancing Schemes for Modular Multilevel Converter HVDC

This paper focuses on the engineering trade-offs in designing capacitor voltage balancing schemes for modular multilevel converters (MMC) HVDC: regulation performance and switching loss. MMC is driven by the on/off switch operation of numerous submodules and the key design concern is balancing submodule capacitor voltages minimizing switching transition among submodules because it represents the voltage regulation performance and system loss. This paper first introduces the state-ofthe-art MMC-HVDC submodule capacitor voltage balancing methods reported in the literatures and discusses the trade-offs in designing these methods for HVDC application. This paper further proposes a submodule capacitor balancing scheme exploiting a control signal to flexibly interchange between the on-state and the off-state submodules. The proposed scheme enables desired performance-based voltage regulation and avoids unnecessary switching transitions among submodules, consequently reducing the switching loss. The flexibility and controllability particularly fit in high-level MMC HVDC applications where the aforementioned design trade-offs become more crucial. Simulation studies for MMC HVDC are performed to demonstrate the validity and effectiveness of the proposed capacitor voltage balancing algorithm.