2019, 43(15):2-9. DOI: 10.7500/AEPS20180730003
Abstract:As the large-scale renewable energy with high volatility accessing to the distribution network, the tie-line power fluctuation of main network increases, which endangers its operation safety. In order to reduce the adverse impact of the renewable energy utilization on the distribution network, a coordinated optimization model is established for the integrated electricity-gas energy system, which takes into account the uncertainty of renewable energy output. In the model, the dynamic scenarios method is adopted to describe the uncertainty of renewable energy output, and the objective is to minimize the whole system operation costs and the tie-line power fluctuation. The model considers the regulation capability of thermostatically controlled load and AC power flow of distribution network and gas flow of gas distribution network. The piecewise linearization and second-order cone relaxation methods are used to transform the model into a mixed-integer second-order cone programming problem. Finally, the summer and winter simulation is conducted on an IEEE 33-node distribution network and a 23-node gas network. It is verified that the gas network with large inertia can be used to smooth the tie-line power fluctuation. Meanwhile, the simulation results show that the thermostatically controlled load can reduce system costs and ensure system operation safety.
2019, 43(15):71-79. DOI: 10.7500/AEPS20190113004
Abstract:With the ever-increasing of unconventional generators in power system, the electrical network resonance instability problems gradually emerge, which is different from the conventional power-angle oscillation. For investigating and suppressing those problems, this paper proposes a resonance analysis method based on the s-domain nodal admittance matrix. Firstly, to describe the unconventional generator, two approaches to establish its s-domain impedance model are introduced, i. e. , the small-disturbance linearization method and the test-signal method. Specially, the inner-loop controller and the phase-locked loop(PLL)are considered in the modelling. Then, based on the s-domain nodal admittance matrix, the resonance structure analysis method of electrical network is proposed. Besides, two indices to determine the main influence area and the sensitive component parameters of the resonant mode are provided. Finally, the above details are illustrated in a wind farm integration system, whose resonance structure is analyzed and the corresponding suppression strategy for the unstable resonant mode is proposed. The analysis indicates that the power system with unconventional generators do have the risk of unstable resonance and need to be suppressed, due to the negative resistance effect of power electronic devices.
2019, 43(15):80-86. DOI: 10.7500/AEPS20190108007
Abstract:Aiming at the problem of low inertia and weak damping existing in power grid with high proportion of new energy, the synchronous motor-generator pair(MGP)system is proposed as a new grid connection method. The modeling method of small signal stability for MGP system is described. On the basis, the damping characteristics of MGP are studied, and quantitative relationship between the damping ratio of MGP and single generator with the same mass block is derived. To verify this quantitative relationship, two working modes of MGP and single generator are designed in 5 kW experimental platform. By fitting the frequency response curves of small disturbance, the damping ratios of MGP and single generator are determined respectively. The comparison results of the relationship between two ratios verify the theoretical deduction, and show that MGP can provide greater damping than single generator with the same mass block. MGP provides a new solution to improve the stability of power grid with high proportion new energy.
2019, 43(15):87-92. DOI: 10.7500/AEPS20181213002
Abstract:In recent years, power grid oscillation problems frequently occur in the wind generation bases of China, such as Guyuan in Hebei Province and Hami in Xinjiang Uygur Autonomous Region. These new oscillation problems often contain two closely coupled components at sub-synchronous and super-synchronous frequencies. Frequency-domain small-signal impedance is proven as an effective method for modeling and analyzing this kind of unstable and oscillation problems caused by the control of power electronic devices. Given that the frequency coupling impedance models for single inverter or wind turbines have been developed, an equivalent method for the sequence impedance model of wind farms is proposed considering both the frequency coupling and the collecting network. The definition and form of the developed equivalent impedance are consistent with the models of single inverter or wind turbines, and can be used to analyze the sub-synchronous and super-synchronous oscillations. Finally, case studies are presented with the PMSG-based wind farm integrated into weak power grid. The accuracy of the proposed model to analyze system stability and predict oscillation frequency is verified by simulation.
2019, 43(15):93-100. DOI: 10.7500/AEPS20190128008
Abstract:One of the major problems of large-scale wind power integration is the frequency stability of the power system at present. This paper puts forward an engineering practical strategy for the time-sequence optimization of wind-farm-level primary frequency modulation, and a cascaded frequency modulation scheme for the order of wind, photovoltaic, hydraulic, thermal power participating in the system frequency modulation. Firstly, by taking the Yunnan power grid as an example, the cascaded frequency modulation scheme is discussed when different power sources of wind, photovoltaic, hydraulic, thermal power are connected to the power grid, the order of different power sources participating in the frequency modulation is studied with the frequency disturbance of the power grid, and the requirement of wind and photovoltaic power units participating in the frequency modulation is put forward. Then the active power variation of wind farm within the time scale of frequency modulation is analyzed, and the active power control strategy of wind turbines during the frequency modulation is discussed. Based on this, the input and exit strategies of primary frequency modulation of wind turbines at the wind farm level are given, and the minimum number of wind turbines that the wind farm needs to input during the frequency modulation is obtained by reading the real-time status of each wind turbine in the wind farm and computing the frequency modulation requirement of the power system. In order to reduce the probability of secondary frequency drop when the wind farm withdraws the primary frequency modulation, the wind turbines withdraw the primary frequency modulation orderly by the time sequence at the end of frequency modulation of the wind farm. Finally, the proposed strategy is verified by the simulation.
2019, 43(15):101-108. DOI: 10.7500/AEPS20190109006
Abstract:Aiming at the demand of power system for power quality, economy, load support and rapid response when large-scale wind power is connected to the grid, a multi-time scale coordinated optimization strategy with participation of variable-speed wind turbines in frequency regulation for the power system with high wind power penetration is proposed. The strategy considers the flexibility of load fluctuation and active output of wind power. According to operation characteristics of the variable-speed wind turbines, the flexible load reduction control of wind turbines under different wind speed conditions is established. Meanwhile, the frequency regulation output of wind turbines is coordinated at different time scales, which makes the inertia combined with primary frequency regulation to achieve frequency adjustment optimization. The results show that variable-speed wind turbines could effectively provide inertial support for the system and has flexible and controllable static frequency response characteristics.
2019, 43(15):109-115. DOI: 10.7500/AEPS20181128006
Abstract:The frequency characteristics of power grid will be notably impacted by large-scale wind power participating in inertial control and primary frequency regulation(PFR). These factors, which may cause frequency trajectory distortion and unreasonable load shedding, have not been considered in the existing under-frequency load shedding(UFLS)methods. Therefore, taking doubly-fed induction generator(DFIG)wind turbines as an example, the method for integrating the inertial response time-varying characteristic parameters and the PFR response system model into the process of UFLS is studied. Meanwhile, an improved UFLS strategy and load resection decision are proposed. Firstly, by solving the grid equivalent inertia(contain the virtual inertial response of wind farm)with time-varying characteristics and detecting the rate of change of frequency, the real-time power shortage is calculated at the first-time starting moment of UFLS. Then, some shortages of real-time power are offset by quantitatively characterizing the PFR of wind power and the effect of loads regulation, and the remaining power deficit is removed by an adaptive UFLS strategy. Finally, it can be observed from the theoretical research and simulations that the frequency characteristics of the grid can be reflected more objectively by using the proposed UFLS improvement strategy, and the amount of load removal is much smaller. This phenomenon reflects the beneficial effects of large-scale wind power participating in frequency regulation on UFLS.
2019, 43(15):64-70. DOI: 10.7500/AEPS20190103005
Abstract:The high level of wind power penetration has reduced the system equivalent moment of inertia and challenged the frequency stability of power grid. Firstly, this paper reviews and analyzes the frequency regulation process of power system and conventional virtual inertia control of wind turbine. Then, a dynamic frequency response model of power system with wind power is established. The impact and variation of wind power and its frequency regulation parameters on dynamic characteristics of system frequency are studied. Furthermore, a novel combined virtual inertia control scheme for the wind turbine based on the selection function is proposed. The limited rotor kinetic energy of the wind turbine is utilized to increase the system equivalent moment of inertia effectively, and the secondary power drop caused by the conventional control is avoided. Finally, a simulation model is established in MATLAB/Simulink, and the simulation results verify the effectiveness of proposed control strategy and the improvement on the dynamic characteristics of power system frequency.
2019, 43(15):49-55. DOI: 10.7500/AEPS20180703005
Abstract:To mitigate subsynchronous control interactions(SSCIs)in doubly fed induction generator(DFIG)based wind farm, which is connected to power grid via series compensated transmission line, a static synchronous compensator(STATCOM)based supplementary damping controller(SDC)is proposed. The admittance model of STATCOM with SDC is derived and its equivalent admittance is analyzed. Equivalent admittance reveals that SDC can increase the admittance of STATCOM at the designed frequency. Thus, oscillation energy could be dissipated by STATCOM and the damping of power system is improved. The influence of SDC parameters on the damping of SSCI is analyzed, which includes phase shift angle and proportional gain. Besides, the optimal selection of the input signal of SDC and location of STATCOM are studied to achieve the best mitigation performance. Finally, the proposed scheme is validated by theoretical analysis and simulations with PSCAD/EMTDC. It is proved that the proposed SDC can suppress SSCI effectively and increase the stability margin of the power system.
2019, 43(15):42-48. DOI: 10.7500/AEPS20180731011
Abstract:Ramping events, during which the wind power greatly changes over a short period of time, seriously threaten the safe and stable economic operation of wind power systems. It is extremely urgent to study an effective ramping control strategy. Based on the field group complementarity evaluation index, and aiming at minimizing the field group deviation and increasing the profitability of each wind farm, a cooperative game based ramping control strategy for wind power is proposed considering auxiliary service compensation cost. The payment function is constructed based on the wind farm output and output adjustment and the deviation between the actual output and planned output, which encourages the wind farm to adopt a cooperative game mode. Cooperation helps the wind farm and its group better complete the field group plan, reduce the compensation fees for auxiliary services, and improve the interests of both the individual farm and the overall group. The simulation results show that the proposed cooperative game based ramping control strategy can improve the interests of each wind farm and the whole group when ramping events occur, and also can improve the completion degree of the field group plan.
2019, 43(15):10-17. DOI: 10.7500/AEPS20180723010
Abstract:The day-ahead forecasting of renewable energy is significant for guiding preparation of power grid planning, but the current prediction accuracy restricts the full application of prediction results. The idea of scenario generation method for renewable energy prediction and prediction consequence which is added into dispatch plan is proposed. Firstly, by comparing and analyzing marginal distribution of generated power and predictive power for renewable energy, a Copula model is established to reveal the dependence relationship between two distributions. Meanwhile, the multi-prediction scenario simulation method based on the dependence relationship is presented. Secondly, the method which integrates the reliability of prediction into provincial power grid dispatch plan is put forward by analyzing the impact of prediction deviation on power supply balance and the law of prediction deviations in multiple scenarios. The relevant results have been applied in the northwest power control sub-center of State Grid Corporation of China. On the basis of the reserve for renewable energy, the measurements which utilizes optimization of operation for conventional power resources, organizes transaction of power market are taken. In 2017, the renewable energy generating capacity is increased by 4. 7 GW·h and the blocking rate is reduced by 3%, which provides a potent support of incorporating renewable energy into the power grid dispatching plan.
2019, 43(15):34-41. DOI: 10.7500/AEPS20180602005
Abstract:With the growing proportion of wind power in the grid, the higher requirements are put forward for the flexibility of power system operation. Based on this background, the paper aims at improving the flexibility of power system from demand side and power generation side. At demand side, time-of-use price and interruptible load are adopted to guide users actively optimize load curve, which can relieve the peak regulation pressure from the “anti-peak regulation” character of wind power. Simultaneously, based on the current deep peak regulation technology of thermal power units, this paper presents the energy consumption cost model of thermal power units considering fatigue life loss and oil cost in deep peak regulation. Moreover, the optimal dispatch model of power system with high wind power penetration is proposed, which takes demand response and deep peak regulation of thermal power units into account. Finally, taking modified IEEE 118-bus system as an example, a variety of cases are simulated and the results show that the demand-side means and deep peak regulation of thermal power units can effectively improve the capacity of the consumption of wind power, which verifies the validity of the proposed model. This work is supported by National Natural Science Foundation of China(No. 51677076)and National Key R&D Program of China(No. 2016YFB0900100).
2019, 43(15):18-25. DOI: 10.7500/AEPS20181123004
Abstract:Currently, the proportion of renewable power is increasing in power system in China. Different from the conventional power generators, it is needed to consider the uncertainty of renewable power in economic dispatch. A day-ahead economic dispatch method considering conventional power generators, renewable power producers and demand response of load aggregators is proposed by minimizing the social cost. The uncertainty of renewable power is modelled based on the truncated versatile distribution model, which could accurately represent the renewable power uncertainty and improve the algorithm efficiency by its analytical cumulative distribution function. Based on the alternating direction method of multipliers, the day-ahead economic dispatch model is solved in a distributed manner on the premise of ensuring the privacy of load aggregators. Results show that the proposed method can effectively consider the renewable power uncertainty, encouraging renewable power producers to improve their forecasting technology and reducing the social cost at the same time. The proposed algorithm based on the alternating direction method of multipliers has the characteristic of good convergence and can guarantee the high solving efficiency.
2019, 43(15):56-63. DOI: 10.7500/AEPS20180731006
Abstract:When the renewable energy is connected to the weak grid through the converter, the output reactive power from converter to grid is beneficial to improve the voltage quality of connection point. But the dynamic characteristics of reactive power control have huge impacts on the stability of the converter system. To explore the impacts, the impedance model of converters with the fixed reactive power control strategy is proposed. By analyzing the impedance characteristics of each element changed by the dynamic variation of outer reactive power control loop, the influence of the dynamic outer reactive power control loop on system stability is judged, and the generalized Nyquist criterion is used to verify the above analysis conclusion. Moreover, the eigenvalue analysis method is used to explore the influences of the proportional parameter of outer reactive power control loop and the strength of the grid on system stability. The results reveal that the outer reactive power control loop may cause medium/high frequency oscillations when the converters are connected to weak grids. Finally, the hard-ware-in-the-loop(HIL)simulation is conducted to validate the theoretical analysis results based on RT-LAB.
2019, 43(15):26-33. DOI: 10.7500/AEPS20180904006
Abstract:A partition method of three-state for wind power accommodation is proposed, which is modeled based on the operation states of power system under different operation conditions. The combined heat and power system with thermal energy storage, electric energy storage and flexible load are divided into three states that include normal state, alert state and emergency state according to wind power curtailment and its variation trend. A phased optimal scheduling model in different states is established and solved by particle swarm optimization algorithm. In normal state, the participation of flexible load and thermal energy storage is able to reduce the pressure of peak shaving and operation costs of system. In alert state, the energy complementary relationship between electric energy and wind power can be found to achieve the target of alleviating wind power fluctuation, which is helpful for safety and stable operation of the system. In emergency state, the system of thermal energy storage transforms abandoned wind power into heat energy to meet the demand for heat supply of users, stores the excess energy and enhances energy use efficiently.