Studies On Assessment And Control Of Power System Transient Frequency Stability

Frequency is an important index of alternate-current power system and reflects balance between active power generation and load. Frequency stability is an important part of power system stability and refers to the ability of a power system to maintain steady frequency following a severe upset resulting in a significant imbalance between generation and load. It can be classified into short term transient frequency stability (TFS) and long term frequency stability in respect of time span of interest. In the early days, power system is confined to a local area with small reserve and is not connected with other systems. Sever upset such as unit tripping may lead to large frequeny deviation and frequency stability is a key concern of these systems. With the widely use of underfrequency load shedding (UFLS) and inter-connection between different systems, performance of system under disturbance is improved and frequency stability is not an important problem of modern inter-connected power systems. However, with the increase of system complexity, some frequency instability accidents are recognized with the evolution of hidden failues and cascading failures in recent years. Those accidents indicate that modern power systems are still subjected to the threat of frequency instability. Meanwhile, with larger wind power penetration, researches on the effect of intermittency and ramp event of wind power on frequency dynamics and frequency stability is still need to be conducted.Power system frequency is closely related to active power imbalance and frequency stability is essentially the problem of active power balancing. Frequency stability is difficult to evaluate because active power-frequency dynamics is affected by reactive power-voltage dynamics and frequency stability is coupled with rotor angle stability and voltage stability. However, in some cases with abundant reactive power reserve and strong excitation regulation capability, active power imbalance may lead to small voltage variation, and reactive power-voltage dynamics shows little influence on active power-frequency dynamics. To simplify frequency dynamics analysis, attentions can be paid on active power-frequency dynamics with reactive power-voltage dynamics neglected. In this thesis, reactive power-voltage dynamics is neglected to reduce complexity of transient frequency stability evaluation and control for better grasp of relationship between frequency and active power.In this thesis, transient frequency stability evaluation and control is deeply discussed with theoretical analysis and numerical simulation. The main research work and innovative fruits of this thesis are as follows. (1) A simplified frequency response model base on direct current loadflow is proposed for fast active power-frequency dynamics analysis.Frequency response of different place is characterized with dispersibility and unitarity. Frequency stability shows coupling and independence with rotor angle stability and voltage stability. Currently, frequency response analysis methods include full time-domain simulation, long term dynamic model and average system frequency model. Full time-domain simulation is used for detail analysis of frequency dynamics with frequency response dispersibility and coupling of frequency stability. Long term dynamic model is used for frequency response unitarity and coupling of frequency stability. Average system frequency model is simplified for frequency response unitarity and independence of frequency stability. On count of close relationship between frequency dynamics and active power imbalance, a direct current loadflow based frequency response model (DFR) is proposed to analyze frequency response dispersiblity and independence of frequency stability. Neglecting the impact of reactive power-voltage dynamics on active power-frequency dynamics, loads, generating units, high voltage direct current transmissions and doubly-fed induction generators are simplified. Network is preserved with direct current loadflow to imitate frequency response dispersibility. Active power dynamics of units and branches can be analyzed along with frequency dynamics. The DFR model can be represented with differential-algebraic equations, and can be solved with sophisticated methods. It can be expaned with underfrequency load shedding, unit underfrequency relays, overspeed protection, boiler and automatic generation control models.A program of DFR model is composed with modified Euler method. Simulations are performed on IEEE3-machine9-bus system and Fujian power system. Simulations show that the DFR model can simulate disturbances of unit tripping, load variation and branch tripping. It gives accurate analysis of active power-frequency dynamics while reducing computation load. It provides a new method for active power-frequency dynamics analysis.(2) A margin index and evaluation method of transient frequency stability considering unit underfrequency relays is proposed for transient frequency stability assessment.Frequency stability is closely related to active power balancing. Sever frequency decline may threat the safety of many system equipments. To protect units, underfrequency relays are equipped to cut off units during large frequency deviation. However, the action of underfrequency relays may worsen system active power imbalance and cause more frequency decline. For the worst case, it may lead to cascading tripping of other units and frequency collapse. In consideration of the effect of underfrequency relays on frequency stability, unit tripping due to uint underfrequency relays is considered as a necessary condition of transient frequency instability. A margin index and evaluation method is proposed to assess system transient frequency stability with unit underfrequency relays. With quantitative assessment method of transient frequency deviation security (TFDS), the critical disturbance of transient frequency instability is calculated and is used to define the margin index of TFS with comparison between the critical disturbance and real disturbance. To improve the practicability of online assessment of TFS, an analytic model of frequency response is derived with simplification of reheat steam unit. Its fast speed makes it useful in calculating frequency response and the critical disturbance of TFS.Simulations are performed on New England10-machine39-bus system. TFS margin index is assessed under normal operation condition and a splitting disturbance. Simulations show that the proposed index has good linearity and can reflect the close tie between frequency stability and active power imbalance. It can effectively assess system TFS and provide important information for system operation.(3) With the previous TFS assessment, an optimal decision method is proposed for underfrequency load shedding setting based on transient frequency deviation security.Underfrequency load shedding is an important controlling measure of transient frequency stability and is an important part of the third defense-line. Traditional UFLS scheme based on starting frequency and time delay is widely used in China. UFLS scheme desinged with traditional method under assumption of no spinning reserve is rough and is not adaptive to different operation conditions. To improve the adaptability of UFLS, activiating criteria of basic stages are treated as two-element table frequency security and an optimal decision method is proposed based on transient frequency deviation security. Critical disturbance and upper limit of shedding load of each basic stage is calculated with TFDS. Shedding load of each basic stage is optimally decided with a a heuristic method to satisfy the requirement of the severest disturbance. Upper and lower limit of shedding loads of special stages are determined with static frequency characteristics based on the scheme of basic stages. Number of special stages and shedding load of each stage is reasonably set. To overcome the drawback of neglecting reactive power-voltage dynamics and improve the effectiveness of UFLS scheme, the effect of voltage deviation on UFLS is analyzed. A synthesized frequency-voltage criterion is proposed for shedding loads with dynamic frequency and voltage information.Simulations are performed on Shandong power system. Simulations show that the proposed optimal decision method can well design UFLS scheme. Since restraints of system operation condition and transient and steady frequency are considered during optimization, UFLS scheme designed with the proposed method can satisfy frequency control requiements and avoid under-shedding or over-shedding. The synthesized frequency-voltage criteria can improve the performance and adaptivity of UFLS.