A Decentralized Optimal Load Current Sharing Method for Power Line Loss Minimization in MT-HVDC Systems

Yiqi Liu,Wenlong Song,Ningning Li,Linquan Bai,Yanchao Ji 전력전자학회 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.6

This paper discusses the elimination of DC voltage deviation and the enhancement of load current sharing accuracy in multi-terminal high voltage direct current (MT-HVDC) systems. In order to minimize the power line losses in different parallel network topologies and to insure the stable operation of systems, a decentralized control method based on a modified droop control is presented in this paper. Averaging the DC output voltage and averaging the output current of two neighboring converters are employed to reduce the congestion of the communication network in a control system, and the decentralized control method is implemented. By minimizing the power loss of the cable, the optimal load current sharing proportion is derived in order to achieve rational current sharing among different converters. The validity of the proposed method using a low bandwidth communication (LBC) network for different topologies is verified. The influence of the parameters of the power cable on the control system stability is analyzed in detail. Finally, transient response simulations and experiments are performed to demonstrate the feasibility of the proposed control strategy for a MT-HVDC system.

Development of a novel self-validating soft sensor

Yiqi Liu,Daoping Huang,Yan Li,Xuefeng Zhu 한국화학공학회 2012 Korean Journal of Chemical Engineering Vol.29 No.9

A self-validating soft sensor is proposed that not only can perform self-diagnostics and self-reconstruction,but also generate a variety of output data types, including the prediction values, input sensors status of soft sensor and the uncertainty values which represent the credibility of soft sensor’s output. The input sensors are validated before performing a prediction by principal components analysis (PCA) model. These validated data are then employed for subsequent recursive partial least square (RPLS) prediction. Other than input sensor validation and modeling for prediction,a t-statistic confidence interval is created and the status of input sensors is offered. By using this self-validating soft sensor, we can determine the work condition of the soft sensor and take proper actions in real time. The usefulness of the proposed method is demonstrated through a case study of a wastewater treatment process.

On DC-Side Impedance Frequency Characteristics Analysis and DC Voltage Ripple Prediction under Unbalanced Conditions for MMC-HVDC System Based on Maximum Modulation Index

Yiqi Liu,Qichao Chen,Ningning Li,Bing Xie,Jianze Wang,Yanchao Ji 전력전자학회 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.1

In this study, we first briefly introduce the effect of circulating current control on the modulation signal of a modular multilevel converter (MMC). The maximum modulation index is also theoretically derived. According to the optimal modulation index analysis and the model in the continuous domain, different DC-side output impedance equivalent models of MMC with/without compensating component are derived. The DC-side impedance of MMC inverter station can be regarded as a series xR + yL + zC branch in both cases. The compensating component of the maximum modulation index is also related to the DC equivalent impedance with circulating current control. The frequency characteristic of impedance for MMC, which is observed from its DC side, is analyzed. Finally, this study investigates the prediction of the DC voltage ripple transfer between two-terminal MMC high-voltage direct current systems under unbalanced conditions. The rationality and accuracy of the impedance model are verified through MATLAB/Simulink simulations and experimental results.

Hybrid DC circuit breaker based on oscillation circuit

Liu, Yiqi,Xia, Tian,Li, Deqing The Korean Institute of Power Electronics 2021 JOURNAL OF POWER ELECTRONICS Vol.21 No.1

With the rapid development of DC transmission technology, DC circuit breakers, the key equipment for DC fault handling, have become a research hotspot of scholars. This paper proposes a novel hybrid DC circuit breaker topology to improve the shortcomings of traditional hybrid DC circuit breakers. Traditional hybrid DC circuit breakers use too many IGBTs, which results in a large volume and a high cost. The main innovation of this topology is the use of a mechanical switch in series in a solid-state branch, which withstands most of the voltage. An oscillating circuit is in parallel on the mechanical switch, which can generate reverse current to reduce the current flowing through the mechanical switch to zero, to achieve arc free breaking. The feasibility of the proposed topology is verified based on a PSCAD simulation platform. It should serve as a valuable reference for the design of DC distribution systems and DC breakers.

Novel submodule voltage balancing topology for hybrid modular multilevel converters

Liu, Yiqi,Liu, Yanchao,Jin, Yonglin,Chen, Jianlong The Korean Institute of Power Electronics 2021 JOURNAL OF POWER ELECTRONICS Vol.21 No.10

A hybrid modular multilevel converter (HMMC) topology based on a bidirectional diode clamp circuit is proposed in this paper. The proposed topology ensures that the voltages at the ends of the capacitance between the two leads of each submodule in the same phase are clamped to be equal to each other through the diode clamp circuit. The balancing process through the diode clamp circuit is bidirectional, which is only achieved by the diode clamp circuit. This topology has two advantages when compared with the traditional voltage balance control method. First, the framework used for controlling the entire structure is simplified while the link controlling the voltage balance is eliminated. Second, the control schemes used for the voltage balance are simplified, and the number of the corresponding high-frequency voltage sensors is reduced. Hence, the complexity of the system control is decreased, and communication is realized more easily. When compared with other voltage balancing circuits, the proposed circuit achieves bidirectional equalization and reduces the usage of the inductors. Then, the hardware cost is reduced. Finally, the feasibility of the proposed converter is verified by simulation results given in MATLAB/Simulink.

Hybrid DC circuit breaker with current‑limiting capability

Yiqi Liu,Bingkun Li,Laicheng Yin,Junyuan Zheng,Zhaoyu Duan,Zhenjie Li 전력전자학회 2023 JOURNAL OF POWER ELECTRONICS Vol.23 No.4

Nowadays, traditional DC circuit breakers (DCCBs) are always expensive and lack current-limiting capabilities. Hence, this paper proposes a current limiting and low-cost hybrid DC circuit breaker (HCB). When a fault occurs, the paralleled inductors in the proposed HCB are converted to a series connection due to the cutoff of the converter module, effectively limiting the increase of fault current. Then the cascaded IGBTs undertake the transient interruption voltage. Energy dissipation circuits (EDCs) reduce the fault isolation time (FIT) by bypassing the current-limiting inductor during energy dissipation based on the metal oxide varistors (MOVs). Therefore, the proposed HCB can limit the rate of the fault current increase, reducing the FIT and cost. Finally, the feasibility of the proposed HCB is verified by a single-ended equivalent system and DC grid test systems built in PSCAD/EMTDC. In addition, the proposed HCB FIT is 33.7% quicker, its energy consumption is 87%, and its cost is lower than that of the traditional ABB HCB.

A Decentralized Optimal Load Current Sharing Method for Power Line Loss Minimization in MT-HVDC Systems

Liu, Yiqi,Song, Wenlong,Li, Ningning,Bai, Linquan,Ji, Yanchao The Korean Institute of Power Electronics 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.6

This paper discusses the elimination of DC voltage deviation and the enhancement of load current sharing accuracy in multi-terminal high voltage direct current (MT-HVDC) systems. In order to minimize the power line losses in different parallel network topologies and to insure the stable operation of systems, a decentralized control method based on a modified droop control is presented in this paper. Averaging the DC output voltage and averaging the output current of two neighboring converters are employed to reduce the congestion of the communication network in a control system, and the decentralized control method is implemented. By minimizing the power loss of the cable, the optimal load current sharing proportion is derived in order to achieve rational current sharing among different converters. The validity of the proposed method using a low bandwidth communication (LBC) network for different topologies is verified. The influence of the parameters of the power cable on the control system stability is analyzed in detail. Finally, transient response simulations and experiments are performed to demonstrate the feasibility of the proposed control strategy for a MT-HVDC system.

On DC-Side Impedance Frequency Characteristics Analysis and DC Voltage Ripple Prediction under Unbalanced Conditions for MMC-HVDC System Based on Maximum Modulation Index

Liu, Yiqi,Chen, Qichao,Li, Ningning,Xie, Bing,Wang, Jianze,Ji, Yanchao The Korean Institute of Power Electronics 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.1

In this study, we first briefly introduce the effect of circulating current control on the modulation signal of a modular multilevel converter (MMC). The maximum modulation index is also theoretically derived. According to the optimal modulation index analysis and the model in the continuous domain, different DC-side output impedance equivalent models of MMC with/without compensating component are derived. The DC-side impedance of MMC inverter station can be regarded as a series xR + yL + zC branch in both cases. The compensating component of the maximum modulation index is also related to the DC equivalent impedance with circulating current control. The frequency characteristic of impedance for MMC, which is observed from its DC side, is analyzed. Finally, this study investigates the prediction of the DC voltage ripple transfer between two-terminal MMC high-voltage direct current systems under unbalanced conditions. The rationality and accuracy of the impedance model are verified through MATLAB/Simulink simulations and experimental results.

MMC‑modified sub‑module structure with double reverse blocking IGBTs

Yiqi Liu,Zhaoyu Duan,Qichao Chen,Bingkun Li,Mingfei Ban,Zhenjie Li 전력전자학회 2023 JOURNAL OF POWER ELECTRONICS Vol.23 No.3

In high-voltage direct current (HVDC) systems, among the voltage source converters (VSCs), the modular multilevel converter (MMC) is a popular choice for power transmission. Unfortunately, the conventional half-bridge sub-module (SM) cannot deal with DC faults by itself. Thus, improved SM topologies enabling DC fault ride-through are significant. With this in mind, an MMC-modified SM structure is presented with double reverse blocking IGBTs (DRBSM). When all of the IGBTs in an MMC are blocked, the DRBSM can insert two capacitors in series to the fault circuit to rapidly overcome the DC fault. First, the DRBSM topology and working principle are analyzed. Second, the DRBSM control strategy is illustrated. Third, the device withstand voltage, DC fault ride-through, cost, and loss performance of the DRBSM are presented in detail. Finally, according to simulation results, the DRBSM fault ride-through speed is shown to be as fast as the full-bridge SM, and faster than the clamp double SM. In addition, experimental results validate the feasibility of the proposed DRBSM structure.

대용량 자료에 대한 밀도 적응 격자 기반의 k-NN 회귀 모형

유의기 ( Yiqi Liu ),정욱 ( Jung Uk ) 한국품질경영학회 2021 품질경영학회지 Vol.49 No.2

Purpose: This paper proposes a density adaptive grid algorithm for the k-NN regression model to reduce the computation time for large datasets without significant prediction accuracy loss. Methods: The proposed method utilizes the concept of the grid with centroid to reduce the number of reference data points so that the required computation time is much reduced. Since the grid generation process in this paper is based on quantiles of original variables, the proposed method can fully reflect the density information of the original reference data set. Results: Using five real-life datasets, the proposed k-NN regression model is compared with the original k-NN regression model. The results show that the proposed density adaptive grid-based k-NN regression model is superior to the original k-NN regression in terms of data reduction ratio and time efficiency ratio, and provides a similar prediction error if the appropriate number of grids is selected. Conclusion: The proposed density adaptive grid algorithm for the k-NN regression model is a simple and effective model which can help avoid a large loss of prediction accuracy with faster execution speed and fewer memory requirements during the testing phase.