Wenping Qin

Reactive Power Aspects in Reliability Assessment of Power Systems

Reactive power plays a significant role in power system operation. However, in reliability evaluation, attention has seldom been paid to reactive power. In conventional power system reliability evaluations, the fixed maximum and minimum values are applied as the reactive power limits of generators. Failures of reactive power sources are rarely considered. The detailed causes of network violations for a contingency are also seldom studied. Real power load shedding is usually used to alleviate network violations without considering the role of reactive power. There are no corresponding reliability indices defined to represent the reactive power shortage in the existing techniques. Reactive power shortage and the associated voltage violations due to the failures of reactive power sources are considered in this paper. New reliability indices are proposed to represent the effect of reactive power shortage on system reliability. The reliability indices due to reactive power shortages have been defined and are separated with those due to real power shortages. Reactive power limits determined by real power output of a generator using P-Q curve have been studied. A reactive power injection technique is proposed to determine possible reactive power shortage and location. The IEEE 30-bus system has been modified and analyzed to illustrate the proposed technique. The results provide system planners and operators very important information for real and reactive power management.

Reliability assessment of power systems considering reactive power sources

Reactive power plays a significant rule in power system reliability and security. Reactive power is considered as the network constraint in conventional reliability evaluation techniques. The impact of the failures of reactive power sources such as synchronous condensers and compensators on system reliability has not been considered in the existing reliability techniques. This paper presents a technique to evaluate system and load point reliability indices of power systems considering reactive power shortages due to the failures caused by reactive power sources. The reliability indices due to the reactive power shortages are separated from those due to the real power shortages. Two reliability indices related to reactive power shortage are proposed. The IEEE 30-bus system is modified and analyzed to illustrate the proposed technique. The results provide very important information for system planners and operators for reactive power management.

Investigation of a microgrid with vanadium redox flow battery storages as a black start source for power system restoration

Microgrids (MGs) in distribution networks have been dramatically increasing due to integration of distributed renewable sources. With connection of MGs, utility grid and customer supply becomes more reliable. An emergency modes for MGs must be envisaged after a general black out. Considering operating and control flexibility, capability of grid-tied, and autonomous operation, MGs with energy storages can provide cranking power and ensure smooth load pick up during the system restoration process. This paper investigates the technical characteristics of MGs containing vanadium redox flow battery (VRB) as a black start (BS) source. The procedure for power system restoration is developed to minimize the total restoration time by maximizing the total capacity of the BS sources. The proposed restoration process is tested through simulations.

Reactive power impact on reliability of 220kV Taiyuan Power System

Reactive power plays a crucial role in power system operation and voltage management. However reactive power issues have rarely been considered in reliability evaluation of power systems. This paper investigates the effect of reactive power on reliability of the 220kV Taiyuan Power System in Shanxi Province of China. Reactive power shortage and the associated voltage violations due to the failures of reactive power sources have been considered in the studies. The results provide system planners and operators very important practical information for voltage and reactive power management.

Power system risk assessment based on cascading failures and malfunction scene selection

With the development of power system and the emerging of power market, the operation of power network is closer to its limit than before. The security and reliability assessments for cascading failures of the power system are important and necessary. However, the system malfunction scenes are not possible evaluated wholly if multiple failures are considered. This paper presents a new approach to assess the system risk based on cascading failures firstly. This approach associates the occurrence probability of cascading failures with their damage on power system. The load conditions, the change of bus voltage and line flow are considered too. Secondly, malfunction scenes based on cascading failures are screened and selected according to the system risk evaluation results. At last, a practical 220kV Taiyuan power system in Shanxi Province of China has been analyzed to illustrate the proposed technique.

Transient stability studies of doubly-fed induction generator using different drive train models

Three equivalent drive train models of a wind turbine generator (WTG) are presented in this paper. The flexibility of the blades bending and the drive train are considered to analyze its transient stability under a three-phase fault. The simulations show that the fifth order lumped-mass model provides too optimistic estimation while the ninth order three-mass model can observe the motion of shaft system, which is more suitable for transient stability analysis. When a WTG is perturbed, a large torque may be produced in the drive train which may result in torsional oscillations between different parts of the WTG shaft system, and thereby reduce the fatigue life of the shaft due to bearing the large torque repeatedly.

Improved Bisection Searching Technique for voltage collapse analysis in reliability evaluation

This paper proposes an Improved Bisection Searching Technique (IBSST) for voltage collapse analysis in reliability evaluation. The loading factor which is the closest to the voltage collapse point for a contingency state can be determined using the proposed technique. The IEEE 30-Bus System has been analyzed using both the IBSST and the Binomial Searching Technique (BNST). The expected system loading factor considering up to first order line contingencies has been defined and calculated to represent the effect of system configurations. Load point reliability in terms of voltage stability is presented by the probability of a bus being the weakest bus considering the first order failures. The computation time of the IBSST is three times faster than that of the BNST. The more accurate loading factors can be obtained using the IBSST compared with the BNST.

Operational Reliability and Economics of Power Systems With Considering Frequency Control Processes

With high renewable power penetration, uncertainty and intermittence of renewable sources become major concerns of power system planning and operation. Large and fast wind speed change may cause great variation of active power generation, which may lead to system stability and reliability problems. The response speeds of the committed conventional generators (CGs) for frequency regulations are critical for system reliable and stable operation. The slow response of the committed CGs may result in power shortage or surplus, which may affect system frequency. This paper proposes a technique to evaluate operational reliability and efficiency problems of power systems with high wind power penetration from frequency aspect. Energy unnecessarily consumed and less supplied during system frequency control processes are modeled in detail. The reliability and economic indexes are formulated with considering system dynamic frequency control processes. The IEEE-RTS79 is used to verify the proposed models and method.

Hybrid AC/DC Micro-Grids: Solution for High Efficient Future Power Systems

This chapter titled “Hybrid AC/DC Micro-grids: Solution for High Efficient Future Power Systems” presents a new configuration for future power systems which is the hybrid AC/DC gird for high efficient connection of the inherent AC and DC sources and loads. Three-phase AC power systems have been in dominant position for over hundred years due to invention of transformer and the inherent characteristic from fossil energy-driven rotating machines. However, the gradual changes of load types and distributed renewable generation (DRG) in AC local distribution systems provide food for consideration of adding DC networks. Renewable sources such as fuel cells and solar photovoltaics are DC inherent and should be connected to AC grid through DC/AC conversion techniques whereas some AC inherent renewable sources like wind generators also need DC links in their conversion systems to increase efficiency and mitigate power variation caused by intermittency and uncertainty. The disadvantage of AC grids for connection of DC inherent sources and loads as well as AC loads with DC links is that additional DC/AC or AC/DC converters are required, which may result in efficiency loss from the reverse conversion. In the other hand DC grids are resurging due to the development and deployment of renewable DC power sources and their inherent advantage for DC loads in commercial, industrial and residential applications. The number of power conversions in a DC microgrid has been significantly reduced to enhance system energy efficiency. A more likely scenario is the coexistence of both AC and DC micro-grids, which is so-called the hybrid AC/DC microgrid in order to reduce processes of multiple reverse conversions in an individual AC or DC microgrid and facilitate the connection of various renewable AC/DC sources and loads to power system. Therefore the concept of hybrid micro-grids, which can harmonize both AC and DC sources and loads, has been proposed for future high efficient power systems. Conventional AC and DC grids are interconnected together through the bidirectional AC/DC converter. The component model has been introduced. The control and operation of individual sources and energy storages are presented. The coordination control and power sharing techniques are also introduced.

Battery Energy Storage Selection Based on a Novel Intermittent Wind Speed Model for Improving Power System Dynamic Reliability

Owing to intermittency of wind power and slow ramp rates of conventional generators, a considerable amount of wind energy cannot be effectively utilized during frequency control processes. This paper proposes a technique for power system planners and operators to select or commit power capacity and energy capacity of battery energy storage system (BESS) for mitigating the effects of intermittent wind speeds on reliability and economics. A novel procedure is proposed to determine intermittent wind speeds based on the chaos theory and intermittency-related parameters. The frequency-related reliability and economic indices of a power system with BESS are formulated based on frequency control processes. The effects of BESS on dynamic reliability and economics of a practical power system are investigated using the proposed techniques and models.