Fast Evaluation Method for Harmonics of Static Frequency Converters under Black-Start Mode of Floating Power Stations

Abstract

The present invention relates to a method for quickly evaluating the harmonics of a static frequency converter under the black start mode of a floating power station. For the first time, a harmonic influence coefficient HIC is proposed to quantitatively characterize the influence of the short-circuit capacity of the black start system power supply and the output power of the SFC on harmonics. A fitting formula is used to describe the relationship between the total harmonic distortion rate THD and the harmonic influence coefficient HIC of the black start system. This fitting formula can quickly calculate the total harmonic distortion rate, with small calculation workload and easy data collection, being fast and simple. A method of optimizing the black start scheme by adjusting the speed-up curve is used. The specific approach is to reduce the angular acceleration, so that the maximum value of the active power output by the SFC is reduced, thereby reducing the harmonic influence coefficient and the total harmonic distortion rate. This is beneficial to the feasibility of the black start scheme. This optimization and adjustment process is carried out according to the calculation formula, which is convenient to apply, and adjusting the speed-up curve has a lower cost compared to configuring harmonic suppression measures.

Description

Technical Field

[0001] The present invention relates to a power quality technology, and particularly to a method for rapidly evaluating the harmonics of a static frequency converter under the black start mode of a floating power station. Background Art

[0002] Different from most regions in China, in some islands and some similar areas, due to the imperfect power grid system and relatively scattered electricity loads, the offshore floating power station has become a power supply method that takes into account both economy and electricity demand in these areas. The first domestic floating power station project with a capacity of 240 MW includes two gas turbines, one steam turbine and the generators supporting them, as well as liquefied natural gas ships for supplying fuel. Since the offshore floating power station belongs to an isolated power system, this means that the floating power station needs to perform black start (start by itself without relying on the help of other networks). The black start of the floating power station is to start the gas turbine through a static frequency converter. Since a large amount of harmonics will be generated during the frequency conversion start process of the static frequency converter (SFC), the harmonic pollution will have an adverse impact on the safe operation of the equipment and system of the floating power station. Therefore, it is necessary to evaluate the harmonic level of the static frequency converter under the black start mode of the floating power station.

[0003] At present, the common method for harmonic evaluation is to establish a model on a simulation platform, simulate and calculate the influence of the frequency conversion start system parameters on harmonics, and evaluate whether the harmonic level meets the requirements. Since the floating power station with a capacity of hundreds of megawatts is a newly developed engineering project, there are few literature reports on the research of its black start harmonics. The short-circuit capacity of the black start system is small, and the influence of harmonics is more serious. At present, there is still a lack of quantitative theoretical research on the relationship between the harmonics generated during the black start process of the floating power station and their influencing factors. Summary of the Invention

[0004] Aiming at the harmonic problems generated during the black start process of the floating power station, a method for rapidly evaluating the harmonics of a static frequency converter under the black start mode of the floating power station is proposed. The harmonic level of the black start system of the floating power station is characterized by a harmonic action coefficient, and a fitting formula is used to quickly estimate the total harmonic distortion rate from the harmonic action coefficient, so as to rapidly evaluate whether the harmonic level of the black start system of the floating power station meets the requirements. At the same time, a high-cost-effective method for reducing the total harmonic distortion rate by optimizing and adjusting the speed-up curve is given.

[0005] The technical solution of the present invention is as follows: A method for rapidly evaluating the harmonics of a static frequency converter under the black start mode of a floating power station, specifically including the following steps:

[0006] 1) For the black start system of the floating power station, according to the rotor moment of inertia of the generator set and the pre-designed rotor speed-up curve, calculate the active power required for the static frequency converter to output during the start process;

[0007] 2) Calculate the harmonic action coefficient based on the short-circuit capacity of the black start system power supply and the maximum active power output of the static frequency converter, so as to characterize the influence of the short-circuit capacity of the black start system power supply and the SFC output power on the harmonic level;

[0008] 3) Interpolate and calculate the total harmonic distortion rate corresponding to the harmonic action coefficient according to the fitting formula between the total harmonic distortion rate at the point of common coupling (PCC) and the harmonic action coefficient;

[0009] 4) Evaluate whether the total harmonic distortion rate meets the requirements: If the total harmonic distortion rate does not exceed its threshold value, it indicates that the harmonic level under the black start mode of the floating power station meets the requirements and the speed-up time is within the allowable range, and the evaluation ends; If the total harmonic distortion rate exceeds the threshold value, it indicates that the harmonic level does not meet the requirements, and go to step 5) to optimize and adjust the speed-up curve;

[0010] 5) Optimize and adjust the speed-up curve, and repeat steps 1) to 4). If the total harmonic distortion rate does not exceed its threshold value, it indicates that the harmonic level under the black start mode of the floating power station meets the requirements. At the same time, if the speed-up time is within the allowable range, the evaluation ends; If the speed-up time is close to the upper limit of the allowable value and still cannot meet the harmonic requirements, it indicates that optimizing and adjusting the speed-up curve cannot completely solve the harmonic problem, and harmonic suppression measures need to be added, and then evaluate steps 1) to 5).

[0011] Further, the calculation formula of the harmonic action coefficient is: where the maximum active power output P of the static frequency converter max = max(P SFC (kT)), P SFC (kT) is the active power output by the static frequency converter, k = 1, 2, 3,..., represents the sampling point, and T is the sampling period; S sc is the short-circuit capacity of the black start system power supply.

[0012] Further, the fitting formula between the total harmonic distortion rate THD and the harmonic action coefficient HIC is: THD = a·HIC + b, where the coefficients a and b are obtained by fitting the results of a finite number of numerical simulations.

[0013] Further, the method for optimizing and adjusting the speed-up curve is: reduce the angular acceleration, so that the maximum active power output by the static frequency converter decreases, and the total harmonic distortion rate decreases accordingly.

[0014] The beneficial effects of the present invention are as follows: The method for rapidly evaluating the harmonics of the static frequency converter in the black start mode of the floating power station of the present invention first proposes to quantitatively characterize the influence of the short-circuit capacity of the power supply of the black start system and the output power of the SFC on harmonics by using the harmonic influence coefficient HIC. The existing research on the analysis of harmonic influencing factors mainly uses numerical simulation methods and lacks methods for rapid calculation and quantitative characterization. The relationship between the total harmonic distortion rate THD and the harmonic influence coefficient HIC of the black start system is described by a fitting formula, which can quickly calculate the total harmonic distortion rate. The existing methods for calculating harmonics in the black start system of the floating power station often use detailed numerical simulations, which are laborious and time-consuming. The present invention proposes a processing method estimated by using the fitting relationship curve, which has a small amount of calculation work and the required data is easy to collect, fast and simple. The method of optimizing the black start scheme by adjusting the speed-up curve is used. The specific approach is to reduce the angular acceleration, so that the maximum value of the active power output by the SFC is reduced, thereby reducing the harmonic influence coefficient and the total harmonic distortion rate. This is beneficial to the feasibility of the black start scheme. This optimization and adjustment process is carried out according to the calculation formula, which is easy to apply, and adjusting the speed-up curve has a lower cost than configuring harmonic suppression measures. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a flowchart of the method for rapidly evaluating the harmonics of the static frequency converter in the black start mode of the floating power station of the present invention;

[0016] Figure 2 It is a structural diagram of the black start system of the floating power station, which is the object of the embodiment of the method of the present invention;

[0017] Figure 3a It is a pre-designed rotor speed-up curve diagram of the method of the present invention;

[0018] Figure 3b It is the speed-up curve diagram after optimization and adjustment of the method of the present invention;

[0019] Figure 4 It is the relationship curve between the harmonic influence coefficient and the harmonic distortion rate of the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described in detail below with reference to the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and gives the detailed implementation manner and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0021] Taking the black start process of a 240MW floating power station as an example, this embodiment rapidly evaluates the voltage harmonics at the point of common coupling PCC. The flowchart is as Figure 1 shown and is achieved through the following steps:

[0022] Step 1: For the black start system of the floating power station, calculate the active power required to be output by the static frequency converter (SFC) during the start-up process according to the rotor inertia of the generator set and the pre-designed rotor speed-up curve.

[0023] The black start system of the floating power station consists of a start-up power supply, a converter transformer, a static frequency converter, and a gas turbine generator set connected in sequence, as Figure 2 shown. The static frequency converter uses switching tubes to convert the input of industrial frequency alternating current into an alternating voltage with continuously adjustable frequency and amplitude, and then injects it into the stator of the generator, enabling the generator to rotate in the form of a motor during the start-up stage and accelerating to the self-sustaining speed (subsequently, it is only driven to accelerate and generate electric power by the mechanical driving torque of the gas turbine).

[0024] The formula for calculating the active power P SFC (kT) output by the static frequency converter is:

[0025]

[0026] In the formula, ω(kT) is the angular velocity sampled from the rotor speed-up curve of the generator set, k = 1, 2, 3,... represents the sampling points, T is the sampling period, J is the rotor inertia of the generator set, T m (ω) and T f (ω) are the mechanical driving torque characteristic curve and the resistance torque characteristic curve of the gas turbine, both of which are functions of the angular velocity ω.

[0027] In this embodiment, the rated capacity of the static frequency converter SFC is 3.0 MW, the rotor inertia of the generator set J = 3265 kg·m 2 , and the pre-designed rotor speed-up curve is as Figure 3a shown, the speed-up time is 110 s, and the mechanical driving torque characteristic curve and the resistance torque characteristic curve of the gas turbine are both known.

[0028] Step 2: According to the short-circuit capacity S sc of the power supply of the black start system and the maximum value P max of the active power output by the static frequency converter SFC, calculate the harmonic interaction coefficient HIC to characterize the influence of the short-circuit capacity of the power supply of the black start system and the output power of the static frequency converter SFC on the harmonic level.

[0029] The formula for calculating the harmonic interaction coefficient is: where P max = max(P SFC (kT)).

[0030] In this embodiment, the short-circuit capacity S sc of the power supply of the black start system is 60 MVA, P max=3.56MW. After calculation, the harmonic action coefficient HIC of the black start process is =0.2436.

[0031] Step 3: Based on the fitting formula between the total harmonic distortion THD at the common connection point PCC and the harmonic action coefficient HIC, interpolate and calculate the total harmonic distortion THD of the floating power station black start system corresponding to the harmonic action coefficient HIC.

[0032] The fitting formula between the total harmonic distortion THD and the harmonic action coefficient HIC is: THD=a·HIC+b, wherein the coefficients a and b are obtained by fitting the results of a limited number of numerical simulation calculations.

[0033] In this embodiment, a relationship curve between the total harmonic distortion rate THD and the harmonic action coefficient HIC is obtained through a limited number of simulation calculations, such as Figure 4 As shown, the fitting formula coefficients a=0.5, b=-0.04, and the total harmonic distortion THD obtained by interpolation calculation is 8.17%.

[0034] Step 4: Evaluate whether the total harmonic distortion (THD) meets the requirements. If the THD does not exceed the threshold, it indicates that the harmonic level in the black start mode of the floating power station meets the requirements and the speed-up time is within the allowable range, and the evaluation ends. If the THD exceeds the threshold, it indicates that the harmonic level does not meet the requirements, and go to Step 5 to optimize and adjust the speed-up curve.

[0035] The total harmonic distortion threshold is 8%.

[0036] In this embodiment, since the total harmonic distortion THD exceeds the threshold (8.17%>8%), it indicates that the harmonic level does not meet the requirements, so the process goes to step 5 to optimize and adjust the speed-up curve.

[0037] Step 5: Optimize and adjust the speed-up curve and repeat steps 1 to 4. If the total harmonic distortion (THD) does not exceed the threshold, the harmonic level of the floating power plant in black start mode meets the requirements. If the speed-up time is within the allowable range, the evaluation ends. If the speed-up time approaches the upper limit of the allowable value and still cannot meet the harmonic requirements, it indicates that the optimized speed-up curve cannot completely solve the harmonic problem. Harmonic suppression measures need to be added, and the evaluation of steps 1 to 5 is repeated.

[0038] The method for optimizing the acceleration curve is to appropriately reduce angular acceleration to lower the maximum active power output of the SFC and, consequently, the total harmonic distortion. Note that this method increases the acceleration time, so it is necessary to determine whether the acceleration time is within the allowable range.

[0039] In this embodiment, the acceleration curve is adjusted by extending the startup process and reducing the angular acceleration. The optimized and adjusted acceleration curve is as follows:Figure 3b As shown, the acceleration time after adjustment increases from 110 seconds to 175 seconds. Repeat Step 1 and Step 2, and calculate the maximum active power P output by the SFC. max = 1.86 MW, the harmonic impact coefficient HIC = 0.1801; repeat Step 3, and the total harmonic distortion rate THD calculated by interpolation is 5.01%; repeat Step 4, the total harmonic distortion rate THD is less than the threshold (5.01% < 8%), indicating that the harmonic level meets the requirements, and the acceleration time is less than the upper limit of the allowable value of 200 seconds, meeting the requirements, and the evaluation is ended.

[0040] In this embodiment, by optimizing and adjusting the acceleration curve, the harmonic level of the floating power station black start system is improved from not meeting the requirements to meeting the requirements, which is both effective and economical.

[0041] The results of the two evaluations before and after adjusting the acceleration curve are summarized in Table 1. The table also gives the total harmonic distortion rate THD obtained through detailed simulation calculations. The total harmonic distortion rate THD obtained through the rapid evaluation calculation of the present invention is not much different from the total harmonic distortion rate THD obtained through simulation calculations, indicating that the rapid evaluation method for the harmonics of the static frequency converter under the floating power station black start method proposed by the present invention is reasonable and reliable.

[0042] Table 1

[0043]

[0044] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A rapid evaluation method for the harmonics of a static frequency converter under the black start mode of a floating power station, characterized in that, The specific steps include: 1) For the floating power station black start system, the active power output of the static inverter required during the start-up process is calculated based on the rotor inertia of the generator set and the pre-designed rotor speed-up curve; 2) Based on the short-circuit capacity of the black start system power supply and the maximum active power output of the static inverter, the harmonic action coefficient is calculated to characterize the impact of the short-circuit capacity of the black start system power supply and the SFC output power on the harmonic level; 3) According to the fitting formula between the total harmonic distortion rate at the common connection point PCC and the harmonic action coefficient, the total harmonic distortion rate corresponding to the harmonic action coefficient is interpolated and calculated; 4) Evaluate whether the total harmonic distortion rate meets the requirements: If the total harmonic distortion rate does not exceed the threshold, it indicates that the harmonic level of the floating power station in the black start mode meets the requirements and the speed-up time is within the allowable range, and the evaluation ends. If the total harmonic distortion rate exceeds the threshold, it indicates that the harmonic level does not meet the requirements, and go to step 5) to optimize and adjust the speed-up curve; 5) Optimize and adjust the speed-up curve, repeating steps 1) to 4). If the total harmonic distortion rate does not exceed the threshold, it indicates that the harmonic level of the floating power station in the black start mode meets the requirements. If the speed-up time is within the allowable range, the evaluation ends. If the speed-up time is close to the upper limit of the allowable value and still cannot meet the harmonic requirements, it indicates that the optimized speed-up curve cannot completely solve the harmonic problem. Harmonic suppression measures need to be added, and then the evaluation of steps 1) to 5) should be repeated. The calculation formula for the harmonic action coefficient is as follows: where the maximum value P of the active power output by the static frequency converter max = max(P SFC (kT)), P SFC (kT) is the active power output by the static frequency converter, k = 1, 2, 3,..., represents the sampling point, and T is the sampling period; S sc is the short-circuit capacity of the black start system power supply; The fitting formula between the total harmonic distortion THD and the harmonic action coefficient HIC is: THD=a·HIC+b, wherein the coefficients a and b are obtained by fitting the results of a limited number of numerical simulation calculations.

2. The rapid evaluation method for the harmonics of the static frequency converter under the black start mode of the floating power station according to claim 1, wherein, The method for optimizing and adjusting the speed-up curve is: reducing the angular acceleration so that the maximum value of the active power output by the static inverter is reduced, and the total harmonic distortion rate is reduced accordingly.

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