Research On Key Theoretical Issues Of Dynamic Frequency Response Operation

With the development of ultra high voltage direct current(UHVDC)transmission and renewable energy integration,the low-frequency risk of infeed power system is increased steeply due to the major power deficit caused by the DC-pole blocking fault,and the system frequency control ability is continued weakend due to the substituted generation of conventional units by the new energy units.Following which,the frequency stability posture is increasingly grave and the frequency response contradiction is also increasingly prominent.If the static point of view is stilled adopted to analyze the frequency response operation,then the result is not only too simple and rough,but also easy to cause misjudgment.This paper carried out a research towards key theoretical issues of dynamic frequency response operation,in which the modeling,monitoring,control and assessment are mainly focused and dynamic characteristic is fully considered,aiming to tap the potential of frequency response for the conventional units and achieve refined control and management eventually.(1)In frequency response modeling,given that the frequency response ability manifested by the static parameter is significantly inaccurate since the intense disparity of heat storage in extreme conditions,a dynamic frequency response model considering heat storage change is constructed.Adoption of the heat storage section in boiler and linearization of the valve function can degrade the order of dynamic frequency response model,thus the relation between frequency response ability and thermal parameters can be depicted in frequency domain.With the analytic description of the low-order model,the quantized expression of dynamic frequency response ability can be derived,based on which,the numerical uncertainty that stem from the thermal parameters can be mapped by point estimate and cumulate methods,thereby getting access to the probabilistic statistical distribution of dynamic frequency response ability for thermal unit in various operational conditions.Case studies show that,compared with the static manifestation,the dynamic frequency response ability can be assessed accurately and effectively with the construction of dynamic model and utilization of assessment methods.(2)In frequency response monitoring,given that the joint high efficiency and accuracy appears unattainable with the current dynamic frequency analysis methods whose scope are limited,the methods for frequency nadir analysis following a major power deficit are proposed.In simulated method,the response of step disturbance induced by the valve regulation is fitted to a polynomial in time domain,then the derived polynomial expression is transferred into frequency domain to obtain the equivalent governor model.Since the order of this model is independent with the number of generators in system,the efficiency for solving the frequency nadir is clearly improved.In analytic method,the frequency feedback is broken and a parabolic frequency deviation is input to decouple the power-frequency calculation of the frequency response,by which the accuracy of the solution can be improved significantly.Based on the analysis of the frequency response characteristics,a numerical iterative algorithm is proposed to solve the analytic model efficiently.Case studies show that,compared with the methods of the same kind,the proposed analysis methods can achieve data-less,rapider and more precise computation of the frequency nadir.(3)In frequency response control,given that the static fixed deadband-based frequency response control is difficult to coordinate the operational economy and security flexibly,then a dynamic deadband-based frequency response control strategy is proposed.The equivalent single-machine aggregation of the frequency response models and their parameters for multimachines can reduce the state variables substantially,therefore mitigate the "curse of the dimensionality”.To optimize and promote the frequency response control performance with the derived dynamic deadband threshold strategy,a sequential decision theory-based frequency response control Markov Decision Process(MDP)model is established by taking into consideration of cost-benefit and state transferred probability in multiple stages,afterwards,a value iteration algorithm is introduced to solve the Bellman optimized equation.An implemented plan with the "offline calculation and real-time match" principle is chosen to avoid the online execution with time-consuming iteration.Case studies show that,compared with the static strategy,the proposed dynamic strategy can maximize the safety headroom while ensuring the operational economy.(4)In frequency response assessment,given that the result of assessment may not objective.reasonable or accurate if the static experiential judgement is applied for evaluation,an acquisition method of the dynamic parameters for frequency response control performance assessment is proposed.In the theoretical benchmark acquisition,the convolution method is introduced to ensure parameters' calculation accurately.In the actual performance acquisition,the identification technology is applied to guarantee parameters' measurement reasonably.The selection and application of the frequency response model with multi-sections of turbine,boiler and generator towards control performance assessment,will ensure accurate acquisition of the assessment parameters in the whole process of frequency response.Case studies show that,the shift of static experiential judgement to dynamic theoretical derivation in parameters'acquisition,will contribute to the objective,reasonable or accurate frequency response control performance assessment.