CN108134398B - Method for inhibiting subsynchronous oscillation of thermal power generating unit based on current signal feedback - Google Patents
Method for inhibiting subsynchronous oscillation of thermal power generating unit based on current signal feedback Download PDFInfo
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Abstract
The invention belongs to the technical field of subsynchronous oscillation analysis and suppression of power systems, and particularly relates to a method for suppressing subsynchronous oscillation of a thermal power generating unit based on current signal feedback. The method comprises the following steps: performing phase locking on a current signal of an alternating current circuit based on a phase-locked loop to obtain a current phase angle, and performing abc-dq conversion by taking the current phase angle as a conversion angle to obtain a subsynchronous frequency current deviation signal; designing a static synchronous series compensator SSSC controller, and enabling the static synchronous series compensator SSSC to output additional subsynchronous frequency voltage and generate a positive damping torque in the electromagnetic torque of a generator set through additionally generating a damping control loop of each subsynchronous frequency component in a q-axis control loop of the controller based on a subsynchronous frequency current deviation signal. The invention selects the frequency deviation signal as the extraction signal, can correctly reflect the state of the system, enables the static synchronous series compensator SSSC to suppress according to the severity of the subsynchronous oscillation, and achieves the purpose of suppressing the subsynchronous oscillation.
Description
Technical Field
The invention belongs to the technical field of subsynchronous oscillation analysis and suppression of power systems, and particularly relates to a method for suppressing subsynchronous oscillation of a thermal power generating unit based on current signal feedback.
Background
Subsynchronous Oscillation (SSO) of a steam turbine generator unit is an important factor influencing safe and stable operation of the unit and a power grid, and is an electromechanical coupling Oscillation phenomenon existing between a power network and a generator unit. When the SSO is researched, the large shaft of the generator set is equivalent to a plurality of elastically connected concentrated mass blocks. When the mass blocks of the generator set shafting rotate synchronously, if disturbance exists, relative torsional vibration may occur. If the damping of the torsional vibration is negative, the shaft system of the generator set will continuously twist, and the torsional vibration will be amplified gradually, which finally results in the fatigue damage and even breakage of the shaft system.
The interaction between the steam turbine generator unit and the traditional high-voltage direct-current transmission system (LCC-HVDC) may cause subsynchronous torsional vibration of the unit shafting and lose the mechanical life of the unit shafting. Sub-synchronous oscillation of the thermal power generating unit may also be caused by large-capacity power electronic devices near the thermal power generating unit, such as a Static Var Compensator (SVC), a static var synchronous generator (SVG) and the like.
Disclosure of Invention
Aiming at the problems, the invention provides a method for inhibiting sub-synchronous oscillation of a thermal power generating unit based on current signal feedback, which comprises the following steps:
step 1: performing phase locking on an alternating current line current signal based on a phase-locked loop principle to obtain a line current phase angle, performing abc-dq conversion on three-phase alternating current line current by taking the current phase angle as a conversion angle to obtain a subsynchronous frequency current signal, and realizing extraction of the subsynchronous frequency current signal in an alternating current line;
step 2: designing a static synchronous series compensator SSSC controller, taking the subsynchronous frequency current signal obtained in the step 1 as an input signal, adding a damping control loop for generating each subsynchronous frequency component on a reference current value in a reactive power control loop of the static synchronous series compensator SSSC controller, and enabling the static synchronous series compensator SSSC to output an additional subsynchronous frequency voltage through the additional damping control loop aiming at each natural torsional vibration subsynchronous frequency component;
and step 3: according to a time domain simulation test, determining parameters of a damping control loop of each subsynchronous frequency component in a static synchronous series compensator SSSC controller, wherein the parameters of the loop comprise an optimal compensation phase angle, an optimal phase shift link time constant and an optimal gain constant of each natural torsional vibration mode subsynchronous frequency; the damping control loop of each subsynchronous frequency component enables the static synchronous series compensator SSSC to output additional subsynchronous frequency voltage, and a positive damping torque is generated in the electromagnetic torque of the generator set, so that the purpose of inhibiting subsynchronous oscillation is achieved.
The step 1 specifically comprises:
step 1.1: phase-locked feedback principle based on phase-locked loop is used for phase-locking alternating current line current signals at the installation position of SSSC (static synchronous series compensator) to obtain line current phase angle theta0，
The ac line current signal is:
wherein, ImIs the amplitude of the current signal of the AC line; theta1In order to be the actual phase position,
t is time, ω1In order to be the actual frequency of the frequency,
is the initial phase angle of phase a of the AC line current signal,
phase-locking the AC line current by using the phase-locking feedback principle to output the current phase angle
ω0To correspond to a synchronous angular velocity of 50Hz at power frequency,
for phase-locked feedback output phase, the measured phase and the actual phase satisfy the relation theta during normal steady state operation0＝θ1；
Step 1.2: line current phase angle theta calculated in step 1.10Performing abc-dq conversion on the three-phase AC line current for the conversion angle to obtain iqAs an output signal, the extraction and conversion of the extracted subsynchronous frequency current signal are completed,
current signal of three-phase AC line in theta0Performing an abc-dq transformation on the transformation angle to obtain:
the resulting q-axis component is:
will iq(t) subtracting the amplitude to obtain an output signal:
the step 2 specifically comprises: subsynchronous frequency current output variable i obtained from step 1q(t) inputting a static synchronous series compensator SSSC, adding damping control loops for generating each subsynchronous frequency component on a reference current value in a reactive control loop in the static synchronous series compensator SSSC controller, determining the number of the damping control loops according to the number of the natural torsional vibration subsynchronous frequencies of the shafting of the thermal power generating unit, wherein each damping control loop corresponds to one natural torsional vibration subsynchronous frequency, each damping control loop comprises a filtering link, a stopping link, a gain link and a phase compensation link, and performing natural torsional vibration subsynchronous frequency signal of the corresponding shaftingThe feedback control is carried out by controlling the feedback,
each damping control loop is aimed at each natural torsional vibration subsynchronous frequency component fiGenerating additional electromagnetic torque DeltaT 'by feedback control'eSo that it is related to Δ TeOf (2) a resultant vector Δ TeThe phase angle difference between the rotor shaft system of the generator set and the rotation difference delta omega of the shaft system of the generator set is within 90 degrees, so that a positive damping torque can be generated on the rotor shaft system of the generator set, and the purpose of inhibiting subsynchronous oscillation is achieved.
The step 3 specifically includes:
step 3.1: determining the optimal compensation phase angle of the natural torsional vibration sub-synchronous frequency of each generator set shafting
Establishing an electromagnetic transient simulation model of a power system with generator sets sent out through a static synchronous series compensator SSSC, adopting a multi-quality block model for each generator set at a sending end, and obtaining an optimal compensation phase angle in the static synchronous series compensator SSSC suppression subsynchronous oscillation controller through testing
fiThe thermal power generating unit has n natural torsional vibration subsynchronous frequencies in total.
The specific method comprises the following steps:
respectively at 10 deg. intervals, and setting 36 angles theta in the range of 0-360 degjAnd j is 1,2,3, 7, 36, substituting the static synchronous series compensator SSSC obtained in the step 2 for restraining a phase shifting link in a subsynchronous oscillation controller, and determining the optimal compensation phase angle theta by comparing the restraining effects of the 36 anglesbest.1；
At thetabest.1The optimal angle is measured again at intervals of 1 degree within the range of 5 degrees of positive and negative angles;
determining an optimal compensated phase angle θbest.final，θbest.finalNamely the natural torsional vibration frequency f of the generator set1Corresponding optimum compensation angleDegree;
sequentially testing to obtain the optimal compensation phase angle of the subsynchronous frequencies of other respective natural torsional vibration modes
Step 3.2: determining optimal phase shift element time constant of damping control loop of subsynchronous frequency component in static synchronous series compensator SSSC
The structure of the SSSC phase compensation of the static synchronous series compensator is as follows:
for the subsynchronous frequency fiIn the phase addition of alphaiAnd TiRespectively as follows:
wherein, ω isi＝2π×fi，fiIs the ith natural torsional vibration subsynchronous frequency; alpha is alphaiAnd TiCalculated according to the formulas (6) and (7);
step 3.3: determining optimal gain factor for damping control loop of subsynchronous frequency components in SSSC
For the subsynchronous frequency fiSuppression of gain factor K in subsynchronous oscillation controller by SSSC (static synchronous series compensator)wiComprises the following steps:
wherein, Uq.maxFor q-axis transmissionThe maximum value of the reference voltage is obtained; i.e. iq.maxFor the subsynchronous frequency current output variable i obtained in step 1q(t) a maximum current value; 1,2, n, wherein the thermal power generating unit has n natural torsional vibration subsynchronous frequencies; krel.iFor a reliability factor, Krel.iThe value is 0.8 or 0.2.
Advantageous effects
According to the static synchronous series compensator, the SSSC is selected to serve as an input variable, the subsynchronous oscillation state in a power system can be correctly reflected, the more serious the subsynchronous oscillation problem is, the larger the input variable signal is, and therefore the static synchronous series compensator SSSC can be restrained according to the severity of subsynchronous oscillation; a damping control loop in the SSSC suppression subsynchronous oscillation controller is designed, each subsynchronous modal frequency signal is subjected to a filtering link, a blocking link, a gain link and a phase compensation link to obtain a control signal, and the control signal controls a reference current value through the SSSC reactive control loop, so that a positive electric damping torque is provided for a system, the development of subsynchronous oscillation is suppressed, and the purpose of suppressing subsynchronous oscillation is achieved.
Drawings
FIG. 1 is a diagram of a system for delivering a large steam turbine generator unit through a static synchronous series compensator SSSC;
FIG. 2 is a schematic diagram of an elastic multi-mass block model of a steam turbine generator shafting;
FIG. 3 is a schematic view of a torsional relationship of a single mass;
FIG. 4 is a schematic diagram of phase-locking an AC line current signal and extracting a sub-synchronous frequency current signal based on the phase-locked loop principle;
FIG. 5 is a schematic diagram of a static synchronous series compensator SSSC damping subsynchronous oscillation controller;
fig. 6 is a schematic diagram of a damping control loop in which a static synchronous series compensator SSSC additionally generates subsynchronous frequency components at a reference current value in a reactive control loop;
FIG. 7 is a phase-frequency characteristic diagram of a static synchronous series compensator SSSC control loop;
FIG. 8 is a modal diagram of the unit when the SSSC is not put into the damping control loop;
fig. 9 is a unit mode diagram after the static synchronous series compensator SSSC is put into the damping control loop.
Detailed Description
The embodiments are described in detail below with reference to the accompanying drawings.
Example 1
As shown in fig. 1, for a typical large steam turbine generator unit, the steam turbine generator unit is analyzed by a static synchronous series compensator SSSC sending system, 2400MW full occurs to four 600MW steam turbine generator units, and the power plant generator unit is affected by nearby high voltage direct current HVDC, alternating current series compensation and other factors, so that sub-synchronous oscillation SSO of a shaft system may occur, and the safety of the shaft system of the unit is threatened. A static synchronous series compensator SSSC is installed on a power plant generator set output line, and the static synchronous series compensator SSSC on the output line is applied to restrain the subsynchronous oscillation of the generator set.
When the SSO is researched, the large shaft of the generator set is equivalent to a plurality of elastically connected concentrated mass blocks. When the mass blocks of the generator set shafting rotate synchronously, if disturbance exists, relative torsional vibration may occur. If the damping of the torsional vibration is negative, the shaft system of the generator set will continuously twist, and the torsional vibration will be amplified gradually, which finally results in the fatigue damage and even breakage of the shaft system.
The elastic multi-mass-block model of the steam turbine generator shafting is shown in fig. 2, the number of concentrated mass blocks is n, the rotational inertia of each mass block is Mi (i is 1,2, …, n), the rigidity of each shaft section is ki (i is 1,2, …, n-1), the input mechanical torque of each mass block of the steam turbine is Ti (i is 1,2, …, n), the output electromagnetic torque of the generator is Te, and the mechanical damping of each mass block is Di,j(i-1, 2, …, n-1; j-1, 2, …, n-1; (i ≦ j)), the mechanical damping includes self-damping of the masses and mutual damping between adjacent masses.
The torsional relationship of any one of the masses is shown in FIG. 3, the left-hand shaft torque K in FIG. 3i-1,i(θi-1-θi) In the same direction as the input amount Ti of the external torque, and accelerating torque
Damping torque D on the massiωiAnd right axial moment Ki,i+1(θi-θi+1) In the opposite direction.
The oscillation frequency of the shafting is a natural vibration mode of the shafting when the external torque is zero. Neglecting mechanical damping, the torsional equation of motion of each mass is:
at the same time, the user can select the desired position,
for a shafting containing n mass blocks, combining equations (1) and (2), the torsional motion equation is 2 n:
writing equation (3) in matrix form
And let X be [ ω 1 … ω n, θ 1 … θ n]，
The matrix form of equation (3) is then:
for standard equation of state of the type (4)
Its general solution is
Wherein p isi＝αi±jωiAlpha is damping performance, omega is oscillation frequency, uiIs piThe corresponding right feature vector. For the shafting with n mass blocks, n-1 non-zero characteristic roots are provided, corresponding to n-1 shafting torsional vibration modes, and 1 characteristic root with a value of 0 is provided, corresponding to a shafting rigid body mode, so that the inherent oscillation frequency of the shafting is obtained by the formula (4). Step 1: extracting signals for suppressing subsynchronous oscillation of generator set
The input signal of the static synchronous series compensator SSSC restraining the subsynchronous oscillation controller should contain subsynchronous oscillation characteristic information, and the signals which can be taken generally comprise: 1) the shafting slip delta omega of the generator set, the a-phase bus voltage, the line current, the power and the like at the installation position of the static synchronous series compensator SSSC, and meanwhile, as the static synchronous series compensator SSSC for restraining the SSO problem of the subsynchronous oscillation of the generator set, the input signal of the static synchronous series compensator SSSC has the characteristics of realizability and effectiveness.
According to two indexes of realizability and effectiveness, meanwhile, as a power plant is usually far away from a static synchronous series compensator SSSC installation station, shafting slip delta omega can cause interference due to long-distance transmission, and when a plurality of power plants exist near the sending end side of the static synchronous series compensator SSSC, slip signals can have certain interference; the voltage and the power of a bus at the installation position of the SSSC of the static synchronous series compensator have larger fluctuation; meanwhile, the SSSC is not suitable for being used as an input signal because partial subsynchronous information can be lost due to the action of the SSSC.
Through the analysis, the alternating current line current signal at the SSSC mounting position of the static synchronous series compensator is easy to collect, has obvious signal characteristics, can reflect a plurality of subsynchronous oscillation mode components of a plurality of power plant units at a sending end, accords with two indexes of realizability and effectiveness, and is an optimal input signal, so the alternating current line current signal at the SSSC mounting position of the static synchronous series compensator is taken as the input signal.
The method for locally controlling the static synchronous series compensator SSSC adopts an alternating current line current signal at an installation place as an input signal. In engineering practice, even when relatively serious SSO occurs, subsynchronous components in line current and bus voltage are still far smaller than power frequency components, which brings great influence on the detection capability of a subsynchronous signal by a static synchronous series compensator SSSC, especially when a small disturbance fault occurs, the subsynchronous signal is easily submerged in noise and is difficult to extract, so a signal extraction link is a key and difficult point in the design of the static synchronous series compensator SSSC. Meanwhile, when the shafting of the generating set generates subsynchronous oscillation, subsynchronous/supersynchronous component current and voltage can be generated at the same time, and the subsynchronous frequency component is mainly detected because the supersynchronous analysis frequency is higher, the impedance is higher, and the supersynchronous component is smaller than the subsynchronous component.
The purpose of extracting the SSSC signal of the static synchronous series compensator is to extract a required subsynchronous signal from alternating current line current, convert the subsynchronous signal into a signal reflecting the system state and easy to implement and control, and transmit the signal to the next link, and simultaneously reduce the adverse effect on the signal in the extraction process.
The subsynchronous oscillation problem is more serious, the frequency deviation is larger, and therefore the static synchronous series compensator SSSC can restrain according to the severity of the subsynchronous oscillation. The power frequency content in the line current is far larger than the subsynchronous component, and if the frequency is adopted, the subsynchronous component information can be covered. According to the above conditions, the signal extraction unit is considered to be optimal for constructing the frequency deviation, so that the suppression effect can be better achieved.
From the above analysis, the present invention selects and adopts the line current signal as the input signal of the extraction link, and the current frequency deviation as the output, so the measurement of the frequency deviation becomes the key of the link.
Fig. 4 is a schematic diagram illustrating phase-locking and extracting a sub-synchronous frequency current signal for an ac line current signal based on the phase-locked loop principle, where fig. 4 is mainly composed of two parts, the first part is to phase-lock an ac line current at a static synchronous series compensator SSSC installation position by phase-locked feedback to obtain a current phase angle θ0The part mainly extracts the subsynchronous frequency signal in the line current signal; second part at theta0Performing abc-dq transformation on dq transformation angle, and finally taking iqThis section mainly performs extraction and conversion of the subsynchronous frequency current signal as an output signal, as follows.
The phase-locked loop PLL input is a three-phase alternating current line current signal, and the line current signal is made to be:
wherein, ImIs the amplitude of the current signal of the AC line; theta1In order to be the actual phase position,
ω1in order to be the actual frequency of the frequency,
is the initial phase angle of phase A of the current signal of the AC line, and t is time;
phase-locked and output circuit current based on phase-locked feedback principle
ω0The power frequency is 50Hz,
the phase is fed back for phase locking. As known from the phase-locked feedback working principle of the phase-locked loop PLL, the relation theta is satisfied between the measured phase and the actual phase in a steady state0＝θ1(ii) a In the event of a fault, θ0≠θ1But since the subsynchronous component is typically small, θ can be approximated as0≈θ1。
Phase-locked feedback output phase theta0And outputting the three-phase alternating current line current signals to the next link, and converting the three-phase alternating current line current signals by taking the output phase as an abc-dq conversion angle, wherein the abc-dq conversion is as follows:
the q-axis signal is calculated as:
the amplitude of the q-axis signal is truncated, and the final output signal is:
as can be seen from the expression, first, the formula includes (ω)0-ω1) The component is the frequency deviation of the required structure, and the deviation contains subsynchronous information; secondly, this process also completes the frequency conversion of alternating current AC-direct current DC. Step 2: design static synchronous series compensator SSSC inhibits subsynchronous oscillation controller
The static synchronous series compensator SSSC suppresses the subsynchronous oscillation controller, as shown in fig. 5, and can achieve the purpose of suppressing the subsynchronous oscillation by modulating a current reference value and providing positive electrical damping in a subsynchronous frequency range by modulating a control angle, and the static synchronous series compensator SSSC has a reasonable design and can play a good role in suppressing.
(1) Basic loop of static synchronous series compensator SSSC for restraining subsynchronous oscillation
The physical structure of the static synchronous series compensator SSSC suppression subsynchronous oscillation controller is as shown in fig. 6, based on the subsynchronous frequency current signal obtained in step 1, a damping control loop generating each subsynchronous frequency component is added to a reference current value in a reactive control loop of the static synchronous series compensator SSSC controller, and for each natural torsional oscillation subsynchronous frequency component, the static synchronous series compensator SSSC outputs an additional subsynchronous frequency voltage through the additional damping control loop, so as to generate a positive damping torque in the electromagnetic torque of the generator set, thereby achieving the purpose of suppressing the subsynchronous oscillation.
The static synchronous series compensator SSSC is applied to a dq shaft for control, and subsynchronous oscillation suppression is performed on a plurality of generator sets at a sending end, so that a plurality of modal frequency parallel structures are selected. From step 1To output variable iq(T) inputting a reactive control loop in the SSSC control strategy, generating damping to restrain subsynchronous oscillation by controlling reactive power, and generating additional electromagnetic torque delta T 'by an additional loop'eSo that it is related to Δ TeOf (2) a resultant vector Δ TeThe phase angle difference between the rotor difference delta omega and the shafting of the generator set is in the range of 90 degrees, so that the system can generate a positive damping torque, and the purpose of inhibiting subsynchronous oscillation is achieved.
(2) Setting of basic loop parameters of static synchronous series compensator SSSC controller
When a static synchronous series compensator SSSC restrains a subsynchronous oscillation controller, the phase difference between an SSSC input link and electromagnetic torque at a frequency to be compensated needs to be calculated, and then the phase is compensated through a static synchronous series compensator SSSC phase correction link; after determining the structure of the static synchronous series compensator SSSC and the phase lag required to be compensated, the basic loop parameters of the static synchronous series compensator SSSC controller can be obtained.
1) Determining the optimal compensation phase angle of the natural torsional vibration sub-synchronous frequency of each generator set shafting
A detailed electromagnetic transient simulation model of the generator set is established on the basis of a PSCAD/EMTDC software platform and is sent to a power system through a SSSC (static synchronous series compensator), and each generator set at a sending end adopts a multi-quality block model. Test results show that the optimal compensation phase angle in the SSSC suppression subsynchronous oscillation controller of the static synchronous series compensator
The thermal power generating unit has n natural torsional vibration subsynchronous frequencies.
The specific method comprises the following steps:
to determine
For example, 36 angles θ are set in the range of 0-360 ° at intervals of 10 ° respectivelyjAnd j is 1,2,3, 36, substituted into a phase shifting link in a SSSC suppression subsynchronous oscillation controller in a PSCAD/EMTDC model, and the suppression effects of the 36 angles are comparedDetermining an optimal compensated phase angle θbest.1。
Is in thetabest.1The angle is within the range of plus or minus 5 degrees, and the optimal angle is measured again by taking 1 degree as an interval, namely:
determining an optimal compensated phase angle θbest.finalI.e. the natural torsional vibration subsynchronous frequency f of the generator set1The corresponding optimal compensation angle.
Thirdly, the optimal compensation phase angle of other respective natural torsional vibration sub-synchronous frequencies is obtained through sequential testing
2) Determining the optimal phase shift element time constant in a static synchronous series compensator SSSC restraining subsynchronous oscillation controller
The structure adopted by the first-order lead and lag links of the static synchronous series compensator SSSC phase compensation is as follows:
for natural torsional vibration subsynchronous frequency fiIn the phase addition of alphaiAnd TnRespectively as follows:
wherein, ω isi＝2π×fi，fiIs the ith natural torsional vibration subsynchronous frequency; alpha is alphaiAnd TiCalculated according to the formulas (10) and (11);
3) determining an optimal gain constant in a static synchronous series compensator SSSC-suppressed subsynchronous oscillation controller
For natural torsional vibration subsynchronous frequency fiSuppression of gain factor K in subsynchronous oscillation controller by SSSC (static synchronous series compensator)w.iComprises the following steps:
wherein, Uq.maxOutputting the maximum value of the reference voltage for the q axis; i.e. iq.maxFor the subsynchronous frequency current output variable i obtained in step 1q(t) a maximum current value; 1,2,3, n, wherein the thermal power generating unit has n natural torsional vibration subsynchronous frequencies; krel.iIs a reliability factor.
Krel.iFor a reliable coefficient, 0.8 is taken for a torsional vibration modal frequency component with a large risk; for the less risky torsional mode frequency component, 0.2 is taken.
Example 2
In this embodiment, the first and second power plants adopt a typical generator set shown in fig. 1 and send out a system through the static synchronous series compensator SSSC, the total capacity of the first power plant is 1200MW (2 × 600MW), the total capacity of the second power plant is 1200MW (2 × 600MW), the system capacity of the sending out line on which the static synchronous series compensator SSSC is installed is set to be 300MW through 500kV line outgoing power, and the voltage class is 500 kV. In the system, a first plant unit and a second plant unit have subsynchronous risks, the first plant unit comprises 2 torsional vibration modes, and the natural torsional vibration frequencies of the first plant unit and the second plant unit are 19.07Hz (mode 1) and 23.65Hz (mode 2) in sequence; the axis of the plant-B unit comprises 2 torsional vibration modes, and the natural torsional vibration frequencies are 12.66Hz (mode 1) and 21.28Hz (mode 2) in sequence.
According to the design method of the static synchronous series compensator SSSC restraining subsynchronous oscillation controller, the transfer function of a control link is obtained,
transfer function HdampThe phase-frequency characteristics of(s) are shown in fig. 7, and the compensation angles required for the respective modes are shown in table 1:
TABLE 1 STATIC SYNCHRONOUS SERIES COMPENSATOR SSSC controller PHASE COMPENSATION LIST
| Modal frequency (Hz) | 12.66 | 19.07 | 21.28 | 23.65 |
| Angle to be compensated | -66.1 | -82.38 | -97.3 | -121.75 |
| Compensating phase angle | -62.1 | -83.1 | -94.5 | -124 |
| Gain of | 8.5 | 8.5 | 8.5 | 34.0 |
As can be seen from fig. 7 and table 1, the phase-frequency characteristic approaches the optimal phase shift target at the sub-synchronous modal frequency in the phase, and meets the design requirement of the static synchronous series compensator SSSC.
The SSSC suppression effect of the static synchronous series compensator is simulated by using a PSCAD/EMTDC tool, and simulation analysis is performed on different working conditions of large disturbance and small disturbance of a system in various operation modes and different gain conditions of the static synchronous series compensator SSSC.
The simulation time is set to be 15s, the unit of the first plant and the unit of the second plant are full, the single-phase earth fault occurs on the alternating current bus on the rectifying side at the moment of 4s, and the fault duration time is 0.08 s. Fig. 8a and b are respectively a model attenuation diagram of a unit of a first plant and a unit of a second plant under the condition that the static synchronous series compensator SSSC is not put into a damping control loop under the fault; fig. 9a and b are respectively model attenuation diagrams of a unit of a first plant and a unit of a second plant when the static synchronous series compensator SSSC is put into a damping control loop under the fault.
As can be seen from a comparison of fig. 8 and 9, when the static synchronous series compensator SSSC is not used, the attenuation of each mode is slow; after the static synchronous series compensator SSSC is put into, each modal frequency component is rapidly converged, and the static synchronous series compensator SSSC has a good inhibition effect on subsynchronous oscillation.
In the invention, a subsynchronous oscillation damping feedback loop is added in a control strategy of a static synchronous series compensator SSSC, and subsynchronous torsional oscillation signals of a thermal power unit shafting are acquired to carry out feedback control to inhibit subsynchronous oscillation. A line current subsynchronous frequency signal is extracted and converted based on a phase-locked feedback principle, a static synchronous series compensator SSSC is designed to inhibit a subsynchronous oscillation controller, the subsynchronous oscillation signal is subjected to a filtering link, a blocking link, a gain link and a phase compensation link to obtain a control signal, the control signal is controlled through a reactive control loop, and finally a positive electrical damping torque is provided for a generator set rotor, the development of subsynchronous oscillation is inhibited, and the purpose of inhibiting subsynchronous oscillation is achieved.
The present invention is not limited to the above embodiments, and any changes or substitutions that can be easily made by those skilled in the art within the technical scope of the present invention are also within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (5)
1. A method for suppressing subsynchronous oscillation of a thermal power generating unit based on current signal feedback is characterized by comprising the following steps of:
step 1: performing phase locking on an alternating current line current signal based on a phase-locked loop principle to obtain a line current phase angle, performing abc-dq conversion on three-phase alternating current line current by taking the current phase angle as a conversion angle to obtain a subsynchronous frequency current signal, and realizing extraction of the subsynchronous frequency current signal in an alternating current line;
step 2: designing a static synchronous series compensator SSSC controller, taking the subsynchronous frequency current signal obtained in the step 1 as an input signal, adding a damping control loop for generating each subsynchronous frequency component on a reference current value in a reactive power control loop of the static synchronous series compensator SSSC controller, and enabling the static synchronous series compensator SSSC to output an additional subsynchronous frequency voltage through the additional damping control loop aiming at each natural torsional vibration subsynchronous frequency component;
and step 3: according to a time domain simulation test, determining parameters of a damping control loop of each subsynchronous frequency component in a static synchronous series compensator SSSC controller, wherein the parameters of the loop comprise an optimal compensation phase angle, an optimal phase shift link time constant and an optimal gain constant of each natural torsional vibration mode subsynchronous frequency; the damping control loop of each subsynchronous frequency component enables the static synchronous series compensator SSSC to output additional subsynchronous frequency voltage, and a positive damping torque is generated in the electromagnetic torque of the generator set, so that the purpose of inhibiting subsynchronous oscillation is achieved.
2. The method for suppressing the subsynchronous oscillation of the thermal power generating unit based on the current signal feedback as claimed in claim 1, wherein the step 1 specifically comprises:
step 1.1: based on phase-locked loopsThe phase-locking feedback principle is used for carrying out phase locking on an alternating current line current signal at the mounting position of the SSSC of the static synchronous series compensator to obtain a line current phase angle theta0，
The ac line current signal is:
wherein, ImIs the amplitude of the current signal of the AC line; theta1In order to be the actual phase position,
t is time, ω1In order to be the actual frequency of the frequency,
is the initial phase angle of phase a of the AC line current signal,
phase-locking the AC line current by using the phase-locking feedback principle to output the current phase angle
ω0To correspond to a synchronous angular velocity of 50Hz at power frequency,
for phase-locked feedback output phase, the measured phase and the actual phase satisfy the relation theta during normal steady state operation0＝θ1；
Step 1.2: line current phase angle theta calculated in step 1.10Performing abc-dq conversion on the three-phase AC line current for the conversion angle to obtain iqAs an output signal, the extraction and conversion of the extracted subsynchronous frequency current signal are completed,
current signal of three-phase AC line in theta0Performing an abc-dq transformation on the transformation angle to obtain:
the resulting q-axis component is:
will iq(t) subtracting the amplitude to obtain an output signal:
3. the method for suppressing the subsynchronous oscillation of the thermal power generating unit based on the current signal feedback as claimed in claim 1, wherein the step 2 specifically comprises: subsynchronous frequency current output variable i obtained from step 1q(t) inputting a static synchronous series compensator SSSC, adding damping control loops for generating each subsynchronous frequency component on a reference current value in a reactive control loop in the static synchronous series compensator SSSC controller, determining the number of the damping control loops according to the number of the natural torsional vibration subsynchronous frequencies of the shafting of the thermal power generating unit, wherein each damping control loop corresponds to one natural torsional vibration subsynchronous frequency, each damping control loop comprises a filtering link, a stopping link, a gain link and a phase compensation link, and performs feedback control on corresponding natural torsional vibration subsynchronous frequency signals of the shafting,
each damping control loop is aimed at each natural torsional vibration subsynchronous frequency component fiGenerating additional electromagnetic torque DeltaT 'by feedback control'eSo that it is related to Δ TeOf (2) a resultant vector Δ TeThe phase angle difference between the rotor shaft system of the generator set and the rotation difference delta omega of the shafting of the generator set is in a range of 90 degrees, so that a positive damping torque can be generated on the rotor shaft system of the generator set, and the purpose of inhibiting subsynchronous oscillation is achieved, wherein delta TeThe electromagnetic torque of the generator is not applied when the method of suppressing subsynchronous oscillation is applied.
4. The method for suppressing the subsynchronous oscillation of the thermal power generating unit based on the current signal feedback as claimed in claim 1, wherein the step 3 specifically comprises:
step 3.1: determining the optimal compensation phase angle of the natural torsional vibration sub-synchronous frequency of each generator set shafting
Establishing an electromagnetic transient simulation model of a power system with generator sets sent out through a static synchronous series compensator SSSC, adopting a multi-quality block model for each generator set at a sending end, and obtaining an optimal compensation phase angle in the static synchronous series compensator SSSC suppression subsynchronous oscillation controller through testing
fiThe thermal power generating unit is the ith natural torsional vibration subsynchronous frequency of a thermal power generating unit shafting, and the thermal power generating unit has n natural torsional vibration subsynchronous frequencies;
the specific method comprises the following steps:
respectively at 10 deg. intervals, and setting 36 angles theta in the range of 0-360 degjAnd j is 1,2,3, 7, 36, substituting the static synchronous series compensator SSSC obtained in the step 2 for restraining a phase shifting link in a subsynchronous oscillation controller, and determining the optimal compensation phase angle theta by comparing the restraining effects of the 36 anglesbest.1；
At thetabest.1The optimal angle is measured again at intervals of 1 degree within the range of 5 degrees of positive and negative angles;
determining an optimal compensated phase angle θbest.final，θbest.finalNamely the natural torsional vibration frequency f of the generator set1The corresponding optimal compensation angle;
sequentially testing to obtain the optimal compensation phase angle of the subsynchronous frequencies of other respective natural torsional vibration modes
i＝1,2,...,n；
Step 3.2: determining optimal phase shift element time constant of damping control loop of subsynchronous frequency component in static synchronous series compensator SSSC
The structure of the SSSC phase compensation of the static synchronous series compensator is as follows:
for the subsynchronous frequency fiIn the phase addition of alphaiAnd TiRespectively as follows:
wherein, ω isi＝2π×fi，fiIs the ith natural torsional vibration subsynchronous frequency; alpha is alphaiAnd TiCalculated according to the formulas (6) and (7);
step 3.3: determining optimal gain factor for damping control loop of subsynchronous frequency components in SSSC
For the subsynchronous frequency fiSuppression of gain factor K in subsynchronous oscillation controller by SSSC (static synchronous series compensator)w.iComprises the following steps:
wherein, Uq.maxOutputting the maximum value of the reference voltage for the q axis; i.e. iq.maxFor the subsynchronous frequency current output variable i obtained in step 1q(t) a maximum current value; 1,2, n, wherein the thermal power generating unit has n natural torsional vibration subsynchronous frequencies; krel.iIs a reliability factor.
5. The method for suppressing the subsynchronous oscillation of the thermal power generating unit based on the current signal feedback as claimed in claim 4, wherein K isrel.iThe value is 0.8 or 0.2.
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