The Use of the Hybrid Active Power Filter in LCC-HVDC Considering the Delay-Dependent Stability
Abstract : The line commutated converter (LCC) based HVDC projects generally use passive AC filters for harmonic cancellation, which has disadvantages of large footprint, poor filtering effect due to the offset of the resonance point, etc. This paper introduces a high-power LC hybrid active power filter (HAPF), which can meet the requirements of filtering and dynamic reactive power compensation. First, the passive part of the HAPF adopts two parallel single-tuned LC filters, which has low impedance at 12th and 24th. Second, considering the effects of the time delay of the high-voltage sensors, current controller and valve controllers, the possible negative impedance of the HAPF may bring a great risk of resonance and thus endanger the stability of the system. Third, an impedance model of the HAPF is established and the impact of the time delay on system stability is analyzed. Further, an adaptive resonance damping strategy based on least mean square (LMS) algorithm is proposed. Finally, the overall performances of the HAPF are verified by simulations on PSCAD/EMTDC.
? It is necessary to conduct a study on the operation and control of the hybrid LCC-VSC HVDC transmission system.
? Besides, the conception of hybrid LCC-VSC HVDC can be applied to upgrade the existing LCC-HVDC connection by replacing one of the terminals with VSC. Also, the LCC-HVDC and VSC-HVDC system can be interconnected to build a new hybrid network.
? However, since the control strategy and operation method of the LCC and VSC is totally different.
? Voltage source converter (VSC) based HVDC is a more recent technology that provides higher operational flexibility including independent control of active and reactive power as well as black start capability. However, VSCs have higher switching losses and construction costs compared to LCCs.
? Commutation failures at the inverter side of an LCC station, caused by ac side disturbances, lead to interruption of power transmission.
? Voltage source converter (VSC)-based HVDC can independently control active and reactive power at each terminal with reduced filtering requirements.
? Also, VSC-HVDC does not suffer from commutation failures and has black-start and ancillary service capabilities, while the construction cost is higher than LCC-based HVDC system.
? The dc-link capacitor and smoothing reactor are essential in AAC terminal due to the six-pulse current ripple caused by the alternate operation of the AAC arms.
• The power factor and reactive power consumption of the LCC can be calculated based on its working status with the method proposed in the previous section.
• Here, the AC filter is designed for rated DC power transmission in order to balance the reactive power under the nominal operation.
• It is more common to use the rectifier to regulate the DC current and the inverter to keep the DC voltage constant. The V-I curve with the proposed control scheme.
• Four different configurations of hybrid LCC-VSC HVDC transmission system has been compared and analyzed. The operation strategies have been proposed for different configurations.
? The SM capacitor voltages are regulated to their nominal references and the arm currents remain balanced demonstrating the performance of low-level energy balancing and circulating current control functions.
? The results based on a 3000 MVA HVDC system demonstrates the operation and performance of LCC and predominantly the AAC in hybrid HVDC systems, including internal converter operation and limitations for such applications.
? The proposed hybrid HVDC system lays the first steps of developing complex mixedconverter dc grids.
? In addition, VDCOL is adopted for fast recovery of dc current. In general, the developed system under ac faults at LCC station presents the same performance of other LCC-VSC topologies demonstrating the accuracy of developed LCC-AAC hybrid system.
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