Younggi Lee

Control of hybrid HVDC transmission system with LCC and FB-MMC

Nowadays, a Line Commutated Converter (LCC)-HVDC is a mature technology with decades industry experience. Even though its with several advantages such as higher reliability, lower cost, and higher efficiency, it presents some drawbacks such as large converter station size and lack of black starting capability. So the LCC-HVDC is not suitable for some applications in which black starting capability is necessary or high power density is highly emphasized. A LCC-Voltage Source Converter(VSC) hybrid HVDC transmission system combines the advantages of both technologies. In this paper, a comprehensive control strategy of a LCC and Full Bridge Modular Multi-level Converter(FB-MMC) hybrid HVDC transmission system, including converter control schemes, power flow control strategy, black starting strategy, and DC short circuit fault ride through strategy is proposed. Also, DC short circuit fault characteristics depending on AC grid types of the FB-MMC are analyzed and appropriate ride through strategies for each case are established. Validity of the proposed control strategies are verified by computer simulation and tested at reduced scale prototype setup.

A novel control strategy of a modular multilevel converter (MMC) based VSC-HVDC transmission system

In the conventional control strategy of the VSC-HVDC system based on the MMC, direct modulation was employed and the terminal behavior of the MMC was similar to that of the two-level converter. The DC bus voltage of the power dispatcher side was regulated indirectly by controlling voltage regulator side DC bus voltage, and the transmission line current was determined passively by the power flow. Fluctuation of the transmission line voltage would occur during rapid power flow variation due to the inherent capacitor-inductor coupling in the DC transmission line. In this paper, a new concept of the control strategy is proposed. At first, by the proposed control strategy AC grid current control, DC bus current control, and arm capacitor voltage balancing control of the MMC are fully decoupled. Secondly, the DC bus of the MMC operates as a controlled voltage source and the power flow is controlled by regulating the transmission current directly and actively, and the transmission line voltage fluctuation is fully suppressed during power flow variation. Validity of the proposed control strategy is verified by both full scale simulation and down scale experiment.

Suppression of Circulating Current in parallel operation of three-level converters

The Zero Sequence Circulating Current (ZSCC) flows inevitably in parallel converters sharing common DC and AC sources. The currents flowing commonly in all converters increase losses and decrease the overall capacity of parallel converters. This paper proposes a simple and effective ZSCC suppression method based on the Space Vector PWM (SVPWM) method with the ZSCC controller. The zero sequence voltage for the proposed SVPWM is calculated based not on the phase voltage references, but on the grid voltage. The limit of linear modulation index of the converters with the proposed method is analyzed compared to other methods, and it is proven that the limit of linear modulation index can be maximized with the proposed method. The effectiveness of the proposed method has been verified through the experimental set-up consisted of the four parallel three-level converters. It has been confirmed that the ZSCC is well suppressed and larger linear modulation index is achieved at the same time with the proposed method. Moreover, the proposed control method does not require any communication between converters to suppress the ZSCC unlike the conventional methods.

Comparison of rotor position estimation performance in fundamental-model-based sensorless control of PMSM

This paper analyzes and compares position estimation performances of fundamental-model-based sensorless control methods considering the effects of inverter nonlinearities and parameter estimation errors. Based on the investigation of four conventional sensorless control algorithms, estimated position error from each algorithm is represented in a quantitative manner. This representation is helpful in understanding how the error influences the position estimation performance. And, it is revealed that steady state position error can be expressed as a unified formula regardless of position estimation methods. The validity of the analysis is verified by computer simulations and experimental results.

A comprehensive AC side single line to ground fault ride through strategy of a modular multilevel converter for HVDC system

The AC side Single Line to Ground (SLG) fault is one of the most frequent faults in power systems. And, in an HVDC system based on modular multilevel converter it calls for the fault ride through strategy to transmit maximum possible electricity during the fault to secure power system stability. It presents different characteristics of SLG faults at the voltage regulator side and the power dispatcher side. In this paper a comprehensive fault ride through strategy for AC side SLG fault occurred at both converters of the HVDC system is proposed. The proposed method presents fast dynamics and promises maximum possible electricity transmission during the faults. Voltage fluctuation and current overshoot in transmission line during SLG fault can be fully suppressed by the proposed method. Moreover, the proposed control strategy is free of inter-station communication and secures the reliability of HVDC transmission system. Validity of the proposed method is verified by simulation of a ±200kV, 400MW point-to-point HVDC system (216 sub-modules per arm).

A cell capacitor energy balancing control of MMC-HVDC under the AC grid faults

This paper presents the techniques for regulation of system balancing in modular multilevel converter (MMC) based HVDC system under the AC grid double line fault condition. Due to the special structure of MMC, the balancing of the cell capacitor voltage is crucial for any operating circumstance of the system. Also, the responses against the grid faults of HVDC system based on MMC are different to those based on the conventional voltage sourced converters. Therefore, even under the nonpermanent fault in AC grid, the balancing of the cell capacitor energy of MMC has to be guaranteed continuously. Among AC grid fault modes, the double line fault causes one of the most severe transients in MMC. In this paper, a balancing control method of MMC based on an offset voltage injection strategy in the output reference voltages is proposed. By the virtue of the proposed method, the HVDC system based on MMC can have the enhanced fault ride-through capability against the double line fault in AC grid. The validity of the proposed method is verified by simulation results of a ±200kV, 400MVA point-to-point HVDC system.

Sensorless control of IPMSM for last 10 years and next 5 years

This paper presents the developments of sensorless control of interior permanent magnet synchronous machine (IPMSM) for last 10 years, which could be divided into the past and the present for each 5 years. Several popular methods developed for last 10 years would be described and evaluated, and the limitations with the methods are discussed. In the past a concept extended EMF (EEMF) was introduced and it can model IPMSM as a non-salient motor, meaning that the representation of IPMSM in the estimated rotor reference frame would be much simpler and accurate. However, because it still relied on back EMF, standstill operation was impossible. And, the position control of IPMSM could not be achieved with EEMF concept. For sensorless drive, the high-frequency signal injection method exploiting inherent saliency of IPMSM has been continuously developed for last 10 years and applied to various industry fields, where torque control at standstill is essential. Its performance has been improved but there are still many problems to be solved. In the present, the square-wave signal injection at estimated d-axis has improved the control performance conspicuously. However, there are strong demands to improve the control performance of sensorless drive, yet. Based on the problems in the present, in this paper the possible developments of sensorless drive of IPMSM in next 5 years are enlightened as the future of sensorless control.

Analysis on the position estimation error in position-sensorless operation using pulsating square wave signal injection

This paper presents the theoretical analysis on the errors in voltage synthesis and position estimation due to the voltage distortion in position-sensorless operation. Especially, the pulsating square wave signal injection method into the estimated d-axis would be focused on, revealing the nature that the position estimation performance depends on the frequency of the injected signal. From the analysis, it would be shown that, in no load condition, the voltage distortion of one phase mainly has a 2nd order harmonic component in negative sequence by the signal of the frequency to the same switching frequency of the inverter and it leads to position estimation error with 3rd order harmonic component. In the case of a half frequency of the switching frequency or in case of heavy load conditions, the effect of inverter nonlinearity does not affect the position estimation error. Therefore, only the effect of cross-coupling inductances is reflected in these cases, and the position estimation error appears in the form of 6th order harmonics. Simulation and experimental results verify the validity of the analysis.

Model-based sensorless control of IPMSM enhancing robustness based on the estimation of speed error

In this paper, new model-based sensorless control methods are proposed, including estimation methods for the position and speed errors and compatible position and speed estimators. In proposed methods, the speed error is estimated as well as the position error in the conventional model-based sensorless methods. By the proposed methods, unity transfer function from actual position to estimated position can be achieved eliminating the effects of load disturbances. It means that the position error would be ideally zero even in transient situations. Experimental results verify the effectiveness of the proposed methods under severe speed transient (20,000 r/min/s) and load torque transient (20 p.u./s). With the proposed methods, position error has been conspicuously reduced by 60% at speed transient and by 70% at load torque transient in high speed region.

Active damping method equivalent to series resistor effect for LCL filters in a grid-connected PWM converters

This paper proposes an active damping method for LCL filters of grid-connected converters. LCL filters are mainly utilized for the interface of distributed generation systems to the grid due to their small size and effectiveness of current harmonics attenuation. However, owing to the resonance of LCL filters, bandwidth of the current controller of the converters is usually limited far to the resonant frequency of the LCL filter. Otherwise, the system stability would be threatened, especially in high power systems where switching frequency and internal loss are relatively low. In this paper, an active damping method equivalent to the implementation of resistors in series to the filter capacitors is suggested and it is shown that the bandwidth of the LCL filter with the proposed damping method can be extended further than that with a conventional capacitor current feedback type damping method. The effectiveness of the proposed method is experimentally confirmed with a 5kW battery energy storage system connected to the grid.