Enhanced Frequency Droop Method for load sharing in LVDC power systems

The modern power system is experiencing transformation from the rotational-generation-equipment-dominated system to a power-electronics-converter-dominated system, with the increasing penetration of renewable energy resources such as wind and photovoltaic. The power-electronics-based renewable generation, as well as energy storage system, can lead to the reduction of system inertia. As dc systems such as dc microgrids are attracting more attention, the low-inertia issues will challenge their stability. In this paper, a comprehensive review of inertia-enhancement methods in dc power systems is presented. The concept and significance of the inertia in dc systems is firstly introduced, and then the types of inertia-providing sources in dc systems are discussed. After that, the different virtual inertia control strategies applied in power electronics converters are classified and investigated. These virtual inertia control methods are proven to have a great ability to enhance the inertia of a dc system. The challenges and future research direction are discussed at the end of the article. In this paper, the previous research work on the inertia of dc power systems is summarized in detail, the inertia-enhancement methods of DC systems are comprehensively introduced, and the future research directions are prospected.

The evolution of the use of electricity in marine vessels is presented and discussed in this paper in an historical perspective. The historical account starts with its first commercial use in the form of light bulbs on the SS Columbia in 1880 for illumination, going forward through use in hybrid propulsion systems with steam turbines and diesel engines and then transitioning to the present with the first fully electric marine vessel based entirely on the use of batteries in 2015. Electricity use is discussed not only in the light of its many benefits but also of the challenges introduced after the emergence of the marine vessel electrical power system. The impact of new conversion technologies like power electronics, battery energy storage, and the dc power system on overall energy efficiency, power quality, and emission level is discussed thoroughly. This paper guides the reader through this development, the present and future challenges by calling attention to the future research needs, and the need to revisit standards that relate to power quality, safety, integrity, and stability of the marine vessel power system, which are strongly impacted by the way electricity is used in the marine vessel.

SUMMARY A significant section of the future electrical power systems, that is, smart grids, will be dc microgrids and distribution systems. Obviously, the voltage control is one of the important issues of these dc power systems. DC-bus signaling (DBS) control methods, as distributed, autonomous, simple, highly reliable, inexpensive, modular and practical control methods, are one of the most important categories of the voltage control methods in dc microgrids and distribution systems. In the dc power systems with DBS control methods, the determination of the generated power shares of the distributed power sources, as vital entries for the conventional load flow methods, before the execution of a load flow method, is not possible. Also, in these dc power systems, none of the power sources can be considered as a constant voltage source, that is, the slack source. Consequently, the conventional load flow methods are not applicable to dc microgrids and distribution systems with DBS control methods. In this paper, a novel, practical, robust, fast, precise load flow method is presented to overcome the mentioned serious complexities in the dc power systems. The proposed load flow method necessitates the least input data. It determines the generated power shares of all the sources during its execution. Besides, the load flow method necessitates no constant voltage source or slack source. Furthermore, the proposed load flow method benefits from low computational time and the storage requirements and the removal of the ill-conditioned problem. The algorithm validation and the numerical results derived in this paper confirm the accuracy and the remarkable performance of the load flow method. Copyright © 2015 John Wiley & Sons, Ltd.

A new stability criterion for bidirectional DC-DC converters in a dc power system is presented in this paper. This proposed criterion gives an overall indicator for the stability at each node in the dc power system. It based on perturbing the system at a certain node by injecting a small ac current at this node, and accordingly monitoring the variation of the voltage at this node. Then, the node impedance can be calculated. This node impedance implies the stability status of the system at that node. The concept of the node impedance stability criterion is introduced, and the application of this criterion to a dc power system is investigated, as well.

Active superconducting DC fault current limiter based on flux compensation

The electrical power system is becoming more and more complex with the increasing demand of the electricity across the world, as the world is becoming the global village. The electrical power system must be reliable and safe enough to fulfil the energy requirements with the continuous supply of energy. The chances of power system's blackouts and outages are increasing with its increased complexity. So, there is a need of an efficient control system to make the power system more safe, efficient and reliable. Wide Area Monitoring, Protection and Control system (WAMPAC) is becoming an emerging technology for an efficient monitoring, protection and control of power system. Phasor measurement unit (PMU) is an integrated part of wide area monitoring, protection and control (WAMPAC) system, which can be used for monitoring, protection and control of complex electrical power systems. PMU gives the synchronized phasor measurements of voltage and current and can easily monitor and control even small disturbances in power system to protect the power system from any blackouts and outages before any fault occurs. In the proposed paper a PMU model is designed in MATLAB/Simulink and then PMU is used for power system protection and the comparative analysis are done with the conventional protection technique for an interconnected two-area network power system.

Guest Editorial: Situational awareness of integrated energy systems

At present, in the typical design of China’s nuclear power plant, main generation system is connected to the power grid by 500 kV system. 500 kV system as a priority power source, 220kV system as an auxiliary power source. Independent operation of 500kV and 220kV system, improved the reliability of power supply of nuclear power plant. However, the DC 220V power system used to control the 500kV and 220kV system in the switch station of partial nuclear power project is not independently configured, and the design form of one set of DC system is used in the transformer station. In recent years, there are many accidents that AC power enters into the DC power system, resulted in the loss of power source in the transformer station. The loss of external power source in the whole plant is very significant. In this paper, the influence of AC power entering into DC power system on relay protection device is analyzed, the measures to prevent the AC power into DC power system are discussed, the necessity of independent configuration of DC control power system for the 500kV priority power system and 220kV auxiliary power supply system is analyzed.

Analyzed some serious issues, including power energy quality and large area power interruption, require complex method to overcome in 3-phase power system, so, suggested to go other way, proposed independent full DC power system to become one of main power systems in the future. The authors emphasized the necessity and capability of full DC power systems in the significance of technology and energy saving, so that safer, more reliable, stable, flexible and controllable DC power grids could serve the people better than that of the present AC power systems. Presented brief structure of full DC power system where DC power would be produced from multi-phase generator-rectifier group in power station, consumers take power from DC bus, which benefits motor control and energy flow between loads connected the same DC bus without 3 phase balance problems, and DC-DC converters based Duty-Cycle control implement commutations at two terminals of HVDC transmission lines, which reduce the technical difficulties than that in 3-phase AC commutation station. Non-crystal material with about 15 kHz of frequency could be selected for advanced isolators, so as to get smallest power consumption. Some new technology lifting DC voltage in geometrical progression was suggested to be available at the sent terminal of HVDC line. With the development of renewable power systems and their energy storages, DC interconnection grids could be a better choice than AC interconnection grids because no frequency and phase shift problems need to be considered. Bulky 3-phase transformer with industrial frequency and vary kinds of compensators for AC power qualities could be saved. The authors suggested scientific policy for economically implementing full DC power systems step by step. It could be predicted that people would see 3 kinds of power systems, full AC, full DC and hybrid AC-DC systems operating simultaneously by the middle of 21 century.

DC power system of the nuclear power plant is used to supply DC loads for engineered safety featured instrumentation and control such as inverter, power system control and indication, and station annunciation [1]. Batteries of DC power system of the nuclear power plant uses lead-acid batteries in present, but new secondary batteries would be installed in nuclear power plant. This paper compares capacity design of lead-acid battery bank and super capacitor battery hybrid energy storage bank, when the battery was connected directly in parallel with the super capacitor. In addition, duty cycle diagram of class 1E DC power system and characteristic of super capacitor battery hybrid energy storage device are analyzed.

It's significant to consider building ac and dc transmission lines in the same corridor or even on the same tower to solve the difficulties of getting the new right of way and improve the power transfer capability of an existing power system. Due to the proximity between ac and dc overhead lines, the problem of electromagnetic coupling effect between those circuits may be of great concern. This paper firstly presents a theoretical analysis for the coupling effect and calculate the induced components in dc lines. Moreover, factors such as tower configuration, the location and length of the parallel part of the hybrid system and conditions including DC power transfer capacity, system strength, fault type and fault location are discussed. The impacts of these factors are considered and the solution for seeking the boundary value is presented. Simulation results demonstrated the correctness of the method. At last some constructive proposals on protection scheme from the aspect of safe operation of ac and dc system were recommended.

DC power systems connecting to single-phase DC/AC inverters with nonlinear loads will have their DC sources being injected with AC ripple currents containing a low-frequency component at twice the output voltage frequency of the inverter and also other current harmonics. Such a current may create instability in the DC power system, lower its efficiency, and shorten the lifetime of the DC source. This paper presents a general waveform control method that can mitigate the injection of the low-frequency ripple current by the single-phase DC/AC inverter into the DC source. It also discusses the inhibiting ability of the waveform control method on other coexisting harmonics, while the DC source delivers AC power to a nonlinear load. With the application of the waveform control, the average DC output power is supplied by the DC source, while the other harmonics pulsation power can be confined to the AC side (between the capacitors and the AC load). Theoretical analyses and experiment results are provided to explain the operation and the inhibiting ability, and to showcase the performance of the approach in tackling the harmonics. Results validate that the proposed solution can achieve significant mitigation of harmonic component at DC side with nonlinear load as well as a high quality output voltage without the need for extra large capacitive energy storage element implementation.

Conventional control techniques such as proportional integral (PI) control and proportional integral derivative (PID) control are widely used as the load frequency controller in power system control applications; however, the increase in complexity of the power system is degrading the performance of classical control. Therefore, in this paper a new control approach using fractional calculus has been proposed for the load frequency control problem of two-area hydro-thermal power system. The paper presents an interconnected hydro-thermal power system working under different operating conditions introduced in the form of various nonlinearities. Four case studies have been presented in this paper considering transient analysis with (i) different loading conditions, (ii) parameter variations, (iii) different nonlinearities (governor dead band nonlinearity, time delay and generation rate constraints), and (iv) superconducting magnetic energy storage (SMES) device. A tilt integral derivative (TID) control has been presented as a secondary controller to stabilize the frequency deviations occurring in the two-area power system with the above-mentioned different operating conditions. The parameters of TID controller have been optimized using a differential evolution algorithm which solves an optimization problem formulated using the integral of time-weighted absolute error (ITAE) performance index. In addition, a comparison of dynamic system response obtained using TID control and PID control has been presented, which focusses on the better performance of TID control over PID control in an interconnected power system.

Class I surge protective devices (SPDs) - for the diversion of direct lightning currents - are frequently used for the protection of low-voltage AC power systems. Nowadays DC power systems are used frequently in many telecommunication systems (e. g. mobile phone base stations). The standards which describe the requirements and the test conditions for surge protective devices [1, 2] do not address SPDs connected to DC power systems. In this paper special requirements for SPDs, connected to 48 V DC power systems are defined and described. Tests carried out with different high performance Class I lightning current arresters, connected to a newly designed test circuit, show the possibilities and restrictions for the use of these arresters in DC power systems.

Heavy-load multi-wheel electric vehicles are deemed to have significant potential by their superior fuel utilization efficiency and low-carbon environmental benefits. The DC power systems with relatively high bus voltages are adopted for HLMWEVs and inevitable to faults under harsh operating conditions, which threaten the safety and stability of the system, even lead to equipment damage and personnel injuries. Therefore, it is essential to implement comprehensive fault protection for HLMWEV DC power systems, encompassing both local-level and system-level protection. As means of local-level fault protection, fuses play a crucial role in ensuring the reliability and safety of power system operations. In order to minimize impacts of the fault or to avoid other undesirable situations in the system, fuses should totally clear the fault in the designed time while ensuring uninterrupted operation of healthy feeders. However, due to the flexible topology, diverse operating conditions, and complex fault current characteristics, it is a significant challenge to achieve the selectivity and sensitivity of fuses. This paper proposes a novel approach by means of additional branch inductors and bus capacitors to coordinate fuse protection actions and improve fault response performance. The principle of the proposed approach is introduced and the selection criteria for these components are discussed in detail. By modeling and simulating the DC power system of HLMWEVs, the fault clearing time of feeder protection is analyzed to validate the proposed method. The results demonstrate that the proposed approach effectively enhances the selectivity and sensitivity of feeder protection.