CPSS TPEA Vol.5 No.4 (December 30, 2020)
Qiao Peng, Yongheng Yang, Tianqi Liu, and Frede Blaabjerg
The photovoltaic (PV) systems are being required to support the grid more flexibly than ever before. One of the emerging demands is the frequency regulation, including the virtual inertia control (VIC) and the frequency damping control (FDC). To achieve the full-range frequency regulation, i.e., to deal with under-frequency and over-frequency issues, power reserve is necessary for PV systems, which should be properly utilized to realize optimal frequency support. In this context, a novel coordination strategy for the VIC and the FDC in PV systems is proposed in this paper. According to relevant grid codes, the key indices to evaluate the frequency quality are identified at first, i.e., the rate of change of frequency (RoCoF), the frequency nadir, and the recovery frequency. Then, the impact of the inertia constant and damping gain on the frequency quality is explored. Accordingly, the coordination strategy of the VIC and the FDC is designed to achieve optimal frequency support with a certain amount of power reserve. Simulations are performed to validate the performance of the proposed control strategy.
Peng Yao and Zhiwei Zhu
The newly constructed railway on the plateau (remote and depopulated area) were facing a very practical problem that the railway electric distribution supply system (EDSS) cannot find the public power supply. The conventional solutions suffer from the problems of the huge cost and construction diffi culties. Therefore, the topology, industrial design and control of railway electric special power supply (RESPS) are proposed in this paper for cost-effectively supporting power to EDSS. Regarding to potential over-current driver fault caused by EDSS load, a piecewise linear PI control strategy is proposed to improve the continuous operation capability of the inverter once inrush current occurred . To verify the proposed RESPS, the simulation results and 27.5/10 kV, 1000 kVA RESPS fi eld application tests are carried out to prove the feasibility of the RESPS. Finally, some depth thoughts that maybe helpful to the future development are summarized.
Dingyi Lin, Xingshuo Li, Shuye Ding, and Yang Du
The high variability rate of solar irradiance can lead to fluctuations in the photovoltaic (PV) power generation. Consequently, it will bring severe challenges to the stable operation of the power grid. In order to mitigate those problems, the power ramp rate control (PRRC) is required by some utilities. Generally, the PRRC can be achieved by using two methods: energy storage systems (ESS) or active power curtailment. However, there is a lack of comparison of those methods on economic performance. Usually, the levelized cost of energy (LCOE) index is adopted to quantify and compare the cost for different methods. In previous work, an ideal value of ESS lifetime is mostly set to analyze the LCOE. However, the different ESS methods may require the battery to operate at different regimes (i.e., number of cycles and depth of discharger), which directly affects the lifetime. Thus, the battery aging is necessarily introduced to compare the LCOE. In order to fill this gap, this work demonstrates an evaluation of aging through the annual simulations using actual irradiance data. This is done for the LCOE of different PRRCs to determine which is the most cost-effective method.
Oluleke Babayomi, Zhen Li, and Zhenbin Zhang
The need for converter-based synthetic inertia has become more important due to the increasing level of renewable generation penetration in power systems. In this paper, the distributed secondary regulation of frequency and voltage is implemented for model predictive-controlled (MPC) voltage source converters (VSCs) in an AC microgrid (MG). A virtual synchronous generator (VSG) provides inertia-emulation to reduce the rate of change of frequency (ROCOF) that arises from sudden load changes. First, a small-signal stability analysis for parallel-connected VSG-based inverters in a MG is analyzed. Next, the secondary control of voltage and frequency in a distributed AC MG (with parallel-connected inertia-emulating VSCs) is realized. In addition, for the applied load changes in this study it is shown that the proposed control scheme effectively reduces the load change-induced ROCOF by up to 89% and also has very fast and accurate dynamic response that supports robust and rapid recovery from perturbations to MG stability.
Jingrong Yu, Gang Zhang, Mingkai Peng, Dongran Song, and Maoyun Liu
A power-matching based balancing method is pro-posed to solve the unbalanced state of charge (SOC) of the DC-side batteries in cascaded H-bridge multilevel inverter (CHBMI). First, the essential reason for SOC imbalance in CHBMI with phase disposition (PD) pluse width modulation (PWM) is analyzed, and the disadvantages of the voltage-matching based SOC balancing method are pointed out. Then, from the perspective of power- matching, an SOC balancing method is proposed to achieve fast SOC balance under arbitrary load characteristics. In addition, a power-matching algorithm avoiding zero-crossing detection is presented, which brings about accuracy and simplicity of the proposed method. Finally, the parameter of the proposed method is tuned by taking the switching loss and balancing speed into consideration. The simulation and experimental results show that the proposed method is of lower switching loss and faster balancing rate under arbitrary load characteristics.
Yongle Ai, Xing Li, Qunfeng Liu, and Haijun Tao
Due to the problem of the low output voltage of photovoltaic power generation system and the high demand for DC bus voltage in grid connection, a non-isolated high-gain DC/DC converter with a voltage multiplier cell (VMC) structure is proposed. The converter provides equal current between each phase of VMC, the voltage ripple of each stage can be effectively suppressed, and achieve high gain at low duty cycle. The steady-state analysis of the converter in continuous conduction mode is given, and an experimental prototype with an output power of 200 W is developed. The experimental results show these advantages which are the output voltage level of converter can be adjusted by increasing or decreasing the number of VMC unit according to the demand, and each stage of VMC has equal current, and the voltage ripple can be effectively suppressed, as well as the use of staggered parallel input form suppresses the input current ripple and is controlled easily.
Zhijia Wang and Udaya Kumara Madawala
Behaviour of 3-phase cage induction machines, operated in two series-connected and one isolated (TSCAOI) phase winding configuration at variable speeds to generate single-phase electricity, has been investigated in detail for standalone applications. However, integration of TSCAOI configured 3-phase cage induction machines into single-phase distributed power systems is yet to be investigated. This paper therefore presents a model to investigate the behaviour of grid-connected TSCAOI generator systems and proposes a control strategy to regulate the amount of electricity generation at unity power factor under different operating conditions. Additionally, for further simplifying the overall system, a slip estimation method is proposed to enable the speed-sensorless operation of the generator. In order to validate the system feasibility, a prototype grid-connected TSCAOI generator is to be regulated, with balanced operation, to generate electricity to the grid at unity power factor. Good agreement between theoretical and experimental results under various operating conditions indicates that the generator can be successfully regulated by the proposed control scheme and are expected to be suitable for applications, such as small-scale wind and micro-hydro distributed generation systems.
Milan Utvić and Dražen Dujić
Modular multilevel converters have become standard solution in many power demanding high voltage and medium voltage application. Scalability and modularity come with the price of increased control complexity, compared to the other multilevel converter topologies, in a sense that the submodules within the converter have to maintain their voltage level around some predefined value. Intra-arm voltage balancing methods aim to equally distribute charge among the submodules within a single arm, whereas total energy content within each arm has to correspond to the reference value. While most of the literature explores control methods aiming to balance energies among the converter arms, this paper sets the theoretical foundation for the independent control of arm-energies, and presents generalized control concepts that can be applied for the purpose of arm energy control. Proposed control schemes aim to minimize the current stress imposed on the converter, offer easy implementation and be general in a sense of their application to different types of modular multilevel converters. Moreover, they are valid for both balanced and unbalanced grid conditions, as well as in case of failure of one or several submodules within an arm. Theoretical considerations are verified through an extensive set of simulations.