CN110661238B - Multi-terminal flexible direct-current power distribution network protection method based on current-limiting inductive voltage - Google Patents
Multi-terminal flexible direct-current power distribution network protection method based on current-limiting inductive voltage Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/28—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for meshed systems
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Abstract
A multi-terminal flexible direct-current power distribution network protection method based on current-limiting inductance voltage comprises fault starting, fault detection, fault identification and fault pole selection; the fault starting is judged by du/dt and di/dt; the fault detection is to further detect whether the line has faults by adopting low-voltage and overcurrent protection; the fault identification is to reliably identify the internal and external faults of the area by using the current-limiting inductive voltage at two ends of a direct-current line; the fault pole selection is to determine whether the system has positive pole, negative pole or bipolar fault by using the current-limiting inductance voltage at two ends of a direct current line; and when a corresponding fault is detected, a tripping signal is sent out, and the corresponding direct current breaker acts to remove the fault. The invention solves the problem of low protection speed of the traditional direct-current power distribution network. The invention identifies the fault through the voltage at the two ends of the current-limiting inductor.
Description
Technical Field
The invention relates to a direct current protection method, in particular to a multi-terminal flexible direct current distribution network protection method based on a Voltage Source Converter (VSC).
Background
In recent years, with the development of power electronic technology and direct current energy storage devices, power supplies and loads in power systems have changed greatly, the proportion of distributed power supplies in the power supplies is higher and higher, and various direct current loads are connected with a power grid through a converter. When the traditional alternating current power distribution network is used for realizing the consumption of the distributed power supply and the direct current load, the distributed power supply and the direct current load need to be connected with the alternating current power distribution network through the multistage converter station, and the investment cost is increased due to the investment of a large number of power electronic equipment. Compared with an alternating-current power distribution system, the direct-current power distribution network has the advantages of large electric energy transmission capacity, convenience for distributed energy access, low line loss, high reliability, high power supply quality and the like.
The multi-end flexible direct-current power distribution network can be in butt joint with an alternating-current power distribution network through a direct-current power distribution line, and the method is an important development direction of a direct-current power distribution system. At present, relay protection research on a multi-terminal flexible direct-current power distribution network at home and abroad is still in a theoretical stage, and compared with an alternating-current power distribution network, a perfect protection method is lacked, so that protection and fault identification of the multi-terminal flexible direct-current power distribution network are one of core technologies to be solved urgently. Because the system damping is small and natural zero crossing points do not exist in the direct-current power distribution network, once a direct-current side fails, the fault current peak value is high, the rising speed is high, and the fault current can quickly reach the whole power grid. For a multi-terminal flexible direct-current power distribution network, after a fault occurs, the fault current superposition of a plurality of converter stations brings further damage to the system, so that higher requirements are put forward on the quick action of a protection method, and under a normal condition, the fault needs to be identified within 3 ms. ABB and SIMENS company propose to regard as the main protection with travelling wave protection and differential undervoltage protection as, differential protection as the protection method of backup protection to high-voltage direct-current transmission system, but direct-current distribution network because the circuit is shorter, the travelling wave method is difficult to catch the ripples head, has great error, therefore, direct-current distribution network protection adopts the travelling wave method seldom. The holding method can be used for cutting off the direct current side fault by using the alternating current side circuit breaker under the condition that the direct current circuit breaker is not developed, and has important reference value for the early direct current protection research, but the protection method needs to cut off all the alternating current circuit breakers, causes short-time power failure in a non-fault area, reduces the power supply reliability, has low fault cutting speed, and cannot meet the requirements of direct current protection on selectivity and rapidity. The direct-current fault protection method based on the direct-current breaker and the converter self-clearing can meet the requirement of direct-current protection on quick action, but the core idea of the method is still a 'holding method', so that the requirement of protection on selectivity cannot be met. Aiming at the defect of selectivity of a direct current power grid, the direct current side fault can be identified and the severity of the fault can be judged by using an inverse time-limited current variance protection method, but the method is only suitable for a double-end direct current power distribution network and cannot identify the faults inside and outside a multi-end flexible direct current power distribution system area. For a multi-terminal direct-current power grid, a fault can be identified by using the change rate of fault current at a direct-current side, but the feed current of an adjacent power grid in the multi-terminal flexible direct-current power grid can have certain influence on a protection method.
To sum up, the main difficult problem of research of the multi-end flexible direct current distribution network is: the direct-current power distribution network has small system damping and no natural zero crossing point, once a direct-current side fails, the fault current peak value is high, the rising speed is high, and the fault current peak value can quickly reach the whole power grid.
Disclosure of Invention
The invention provides a multi-terminal flexible direct-current power distribution network protection method based on current-limiting inductive voltage, aiming at solving the defect that the prior art can not meet the requirements of a multi-terminal flexible direct-current power distribution network on protection speed and selectivity.
The invention is realized by adopting the following technical scheme: a multi-terminal flexible direct-current power distribution network protection method based on the voltage of a current-limiting inductor is characterized in that the voltage and the current of the current-limiting inductor of a direct-current line are used for judging fault starting, fault detection, fault identification and fault pole selection.
S1: analysis of fault characteristics
(1) Bipolar short circuit fault
When the system has bipolar short-circuit fault or unipolar ground fault, the short-circuit current fed in from AC side is only the follow current of inductor, the fault current is mainly the discharge current of capacitor, and the frequency domain i of the fault current of DC linel(s) the expression is:
in the formula: u. ofc(0)、iL(0) Indicating the moment of occurrence of a faultThe capacitor voltage and the inductor current are R, L, C, which are equivalent resistance, inductance and capacitance of the direct-current side fault loop; s is the laplace operator.
Under normal conditions, the transition resistance of the bipolar short-circuit fault is 0, the resistance of the direct current line is also very small, and the fault loop is in an underdamped state, namely
Time domain il of fault current(t) the expression is:
in the formula: req、Leq、CeqRespectively an equivalent resistor, an inductor and a capacitor of the direct current line; for the convenience of expression of the formula, intermediate variables alpha, beta and omega without practical meaning are introduced for simplifying the formula in the text0、ωd(ii) a e is the base of the exponential function In and t represents time.
The differential equation for the fault current is:
if the initial fault time is t equal to 0+, the differential value of the fault current at this time is:
for a direct-current power distribution network, a difference between a direct current and a voltage at an initial fault moment is more than two orders of magnitude, so that a differential value of the current at the moment when t is 0+ is as follows:
as can be seen from equation (5), the current differential value at the initial failure time is greater than 0.
(2) Single pole ground fault
When a single-pole ground fault occurs in a direct-current power distribution network, the transient process can be divided into 2 stages, and when the transition resistance is small, the circuit is in an underdamping state, namely
The discharging loop is an RLC oscillating circuit with the amplitude gradually reduced, and the expression of the fault current at the direct current side is similar to that of the double-pole short-circuit fault; when the transition resistance is large, the circuit is in an over-damped discharge state, i.e.
The time domain expression of the fault current is as follows:
in the formula: p1、P2Is a defined intermediate variable; rfRepresents the transition resistance; req、Leq、 CeqAs defined for formula (2); e is the base of the exponential function In and t represents time.
The differential equation for the fault current is:
assuming that the initial fault time is t equal to 0+, the differential value of the fault current at the time is:
for a direct-current power distribution network, a direct current and a voltage at the initial time of a fault are different by more than two orders of magnitude, so that the differential value of the current at the time when t is 0 is:
as can be seen from equations (5) and (9), the current differential expressions of the unipolar ground fault and the bipolar ground fault are the same regardless of whether the circuit is in the underdamped state or the overdamped state at the fault initial time, and the current differential value is greater than 0 at the fault initial time.
S2: protection criterion
In a direct current system, a direct current circuit comprises a positive circuit and a negative circuit, a protection device and a current-limiting inductor are arranged at the outlet of each circuit, the positive direction of the positive circuit is defined as that a bus points to the circuit, and the positive direction of the negative circuit is defined as that the circuit points to the bus.
When the positive earth fault occurs on the direct current side, the current increases rapidly, so that for the current-limiting inductor, when the positive direction fault occurs, the fault has d at the momenti/dt>0, the voltage at the two ends of the positive line inductor is also greater than 0, and similarly, when the negative pole has a single-pole ground fault, the voltage at the two ends of the negative line inductor is also greater than 0 in a specified positive direction; on the contrary, when the protection device has a fault in the reverse direction, the fault current increases in the reverse direction, and the voltage of the current-limiting inductor at the protection device is less than 0. Thus, pilot protection is constructed from the current-limiting inductor voltage across the line. The specific process is as follows:
(1) and (3) starting a fault: collecting the voltage and current of a Line1-Line4, and adopting du/ dt、di/dtAs a fault start criterion when du/dt、di/dtWhen the threshold is exceeded, protection is enabled.
(2) And (3) fault detection: the fault detection is realized by using low-voltage and overcurrent protection, and the fault detection has the function of further detecting whether a line has a fault or not on the basis of fault starting criteria.
(3) Fault identification: the voltage value of the current-limiting inductance at the two ends of each line is measured in the specified positive direction, the voltage value of the current-limiting inductance at the two ends of each line is calculated, the voltage of the current-limiting inductance at the two ends of the direct current line is utilized to form a fault identification and fault pole selection criterion, and the fault identification criterion is used for reliably identifying faults inside and outside the area.
(4) And (3) fault pole selection: the function of the fault pole selection criterion is to determine whether the system has positive pole, negative pole or bipolar fault. And when a corresponding fault is detected, a tripping signal is sent out, and the corresponding direct current breaker acts to remove the fault.
The invention solves the problem of low protection speed of the traditional direct-current power distribution network. A boundary element is constructed by installing current-limiting reactors at two ends of a direct current line, and fault identification is carried out through voltages at two ends of a current-limiting inductor. The invention is not influenced by transition resistance, fault distance and inverter power reversal, can accurately identify the internal and external faults and select the fault pole after the direct current side fault, and can meet the requirements of direct current protection on selectivity, quick action, sensitivity and reliability.
Drawings
(1) Fig. 1 is a schematic diagram of a multi-terminal flexible direct current distribution network.
(2) Fig. 2 is a diagram for calibrating the directions of the positive and negative electrode lines.
(3) Fig. 3 is a flow chart of a protection method.
Detailed Description
The invention comprises a multi-end annular direct current distribution system, wherein the multi-end annular direct current distribution system comprises a converter, a direct current breaker and a direct current circuit, and the converter adopts a two-level Voltage Source Converter (VSC) as shown in figure 1.
In fig. 1: S1-S4 represent an AC system; VSC 1-VSC 4 are converters; bus 1-Bus 4 are direct current buses; 12. 14, 21, 23, 32, 34, 41, 43 denote dc breaker numbers, respectively; line1-Line4 are direct current transmission lines; F1-F4 are fault points, wherein: f1 is on Line1, F2 is on Line2, F3 is on Line4, and F4 is on Bus2 DC Bus. The direct current circuit breaker is installed in the direct current bus export, and the current-limiting inductance is installed between direct current circuit breaker and direct current transmission line, and every transmission line includes two circuits of positive negative pole.
For the four-terminal flexible direct-current power distribution system shown in fig. 1, a master-slave control mode is adopted, and a converter VSC1 adopts constant direct-current voltage control to support the voltage of the whole direct-current system as a balance node of the system; and the converters VSC 2-VSC 4 adopt constant power control as power nodes of the system.
A multi-terminal flexible direct-current power distribution network protection method based on current-limiting inductive voltage comprises the following steps:
s1: fail to start
After the direct current side has a fault, the voltage of the direct current bus is reduced and the current is increased at the moment of the fault. Therefore, a fault starting criterion is formed according to the variable quantities of the direct-current voltage and the direct-current after the fault, as shown in the formula (1), and when the conditions of the formula (1) are simultaneously met, the protection scheme is started.
In the formula: u, i represent positive or negative DC line voltage, current, Un、InRated values representing direct voltage and current; t represents time.
S2: fault detection
When the system is disturbed or interfered, malfunction of a fault starting element can be caused, and therefore, a fault detection criterion needs to be constructed to identify disturbance and fault. Compared with a fault transient process, the system disturbance has long time, and cannot cause serious system under-voltage and over-current, so that a fault detection element is formed according to low-voltage and over-current protection, when each direct-current line continuously has 3 sampling points meeting low-voltage and over-current conditions, a fault can be determined, a fault detection criterion is shown as a formula (2), and when the conditions of the formula (2) are met simultaneously, the fault is detected.
In the formula: i represents the positive or negative DC line current, UnRepresents a direct voltage, InIndicating rated value of current, UdcIndicating the dc side inter-pole voltage.
S3: fault detection
(1) Positive and negative line direction calibration
As shown in fig. 2, taking the converter station 1 as an example, each line outlet is provided with a protection device and a current-limiting inductor, 12P and 12N are respectively a positive and negative protection device, Lr is a current-limiting inductor, and the positive direction of the positive line is defined as the line pointing to the bus, and the positive direction of the negative line is defined as the line pointing to the bus.
When a positive ground fault occurs on the dc side F1, the current increases rapidly, so for the current-limiting inductor, when a positive fault occurs, there is a fault instant di/dt>0, so:
in the formula: u shapeL12PThe voltage across the current-limiting inductor is the positive line.
Similarly, when the negative pole has a single-pole ground fault, the voltage across the negative line inductor is also greater than 0 in a specified positive direction.
In contrast, when a positive ground fault occurs at the back side F4 of the protection device 12P in fig. 1, the fault current increases in the reverse direction, so that there are:
therefore, the protection criterion for the positive direction fault of the protection device m is as follows:
in the formula: u shapeLmtRepresenting the current-limiting inductor voltage; m is the number of the direct current breaker, m is 12,21,14, 23, 41, 32, 43, 34; t represents the positive and negative poles of the direct current breaker, t is P, N, P represents the positive pole, and N represents the negative pole; rmtFor positive direction fault identification signals, R mP1 indicates positive direction failure of the protection device, R mN1 indicates that the positive direction of the negative electrode of the protection device fails, and Rmt0 indicates that a reverse direction fault has occurred; u shapesetIs a protection criterion threshold value.
(2) Threshold setting
UsetFor protecting the threshold value of the criterion, according to theoretical analysis, the voltage at two ends of the current limiting inductor is 0 under normal conditions, UsetIt may be set to 0 but the threshold should be greater than 0 in view of the unbalanced voltage of the voltage transformer. According to the national power grid company enterprise standard (Q/GDW10531-2016), the amplitude error of the DC electronic voltage transformer is +/-0.2%, the error of one transformer is-0.2%, the error of the other transformer is + 0.2%, the unbalanced voltage under a load state is not more than 0.4%, the reliability coefficient is considered to be 2, and the protection criterion threshold value U issetThe following can be taken:
Uset＝2×0.4％×Un (6)
(3) fault detection
The faults inside and outside the area can be judged according to the protection devices at the two ends of the direct current line, and the specific criteria are as follows:
in the formula: n represents a direct current breaker of the opposite end of the line where m is located; rmtFor m positive direction fault recognition signals of protection devices, RntIdentifying a signal for a fault in the positive direction of the protection device n; stIdentifying signals for faults inside or outside the zone, StAn intra-zone fault is indicated by 1, otherwise, an out-of-zone fault.
S4: fault pole selection
The purpose of setting a fault pole selection criterion is to further identify whether a fault occurs in the positive pole or the negative pole, and if a positive pole ground fault occurs, the direct current circuit breakers at two ends of the positive pole circuit act; if the negative earth fault occurs, the direct current circuit breakers at two ends of the negative line act; if a bipolar short circuit fault occurs, the direct current circuit breakers at the two ends of the positive and negative electrode lines act. The fault pole selection criterion is as follows:
in the formula: sPIdentifying the signal for positive pole faults, SNIdentifying a signal for a negative fault; sPWhen the value is 1, the positive pole fault is judged, and SNWhen the current is equal to 1, the negative pole is judged to be failed, and SP*SNWhen the value is 1, a bipolar fault is determined.
The flow of the protection method of the multi-end flexible direct-current distribution network is shown in fig. 3.
And (3) starting a fault:
when the system runs, the system starts to enter a protection link, and a direct current breaker is used for reading a voltage value and a current value; and the voltage and current values are differentiated by time t respectivelyu/dt、 di/dtCalculating, starting protection when the conditions meeting the formula (1) are all met, and carrying out fault detection in the next step;
in the formula: u and i respectively represent the voltage and current of the positive or negative DC line, Un、 InRated values of direct-current voltage and current are respectively represented; t represents time.
And (3) fault detection:
the fault detection criterion is as follows:
in the formula: i represents the positive or negative DC line current, UnAnd In represent rated values of the dc voltage and the current, respectively.
When the conditions are met, the current converter is locked, and next fault identification is carried out; otherwise, reading the voltage value and the current value circularly;
fault identification:
the voltage value of the current-limiting inductance at the two ends of each line is measured in the specified positive direction, the voltage value of the current-limiting inductance at the two ends of the line is calculated, and the faults inside and outside the area can be judged according to the protection devices at the two ends of the direct current line, wherein the specific criterion is as follows:
in the formula: n represents a direct current breaker of the opposite end of the line where m is located; rmtFor m positive direction fault recognition signals of protection devices, RnIdentifying a signal for a fault in the positive direction of the protection device n; stIdentifying signals for faults inside or outside the zone, StAn intra-zone fault is indicated by 1, otherwise, an out-of-zone fault.
And (3) fault pole selection:
the purpose of setting a fault pole selection criterion is to further identify whether a fault occurs in the positive pole or the negative pole, and if a positive pole ground fault occurs, the direct current circuit breakers at two ends of the positive pole circuit act; if the negative earth fault occurs, the direct current circuit breakers at two ends of the negative line act; if a bipolar short circuit fault occurs, the direct current circuit breakers at the two ends of the positive and negative electrode lines act. The fault pole selection criterion is as follows:
in the formula: s. thePIdentifying the signal for positive pole faults, SNIdentifying a signal for a negative fault; sPWhen the value is equal to 1, the positive pole is judged to be failed, SNWhen the current is equal to 1, the negative pole is judged to be failed, and SP*SNWhen the value is 1, a bipolar fault is determined.
The protection scheme comprises fault starting, fault detection, fault identification and fault pole selection criteria. Wherein, adopt du/dt、di/dtAs a failed startCriterion is carried out; the low-voltage and overcurrent protection is used for realizing fault detection, and the function of the fault detection is to further detect whether a line has a fault or not on the basis of fault starting criteria; the method comprises the following steps that current-limiting inductive voltages at two ends of a direct-current line are utilized to form fault identification and fault pole selection criteria, and the fault identification criteria are used for reliably identifying faults inside and outside a region; the function of the fault pole selection criterion is to determine whether the system has positive pole, negative pole or bipolar fault. And when a corresponding fault is detected, a tripping signal is sent out, and the corresponding direct current breaker acts to remove the fault.
Claims (1)
1. A multi-end flexible direct-current power distribution network protection method based on a current-limiting inductor comprises fault starting, fault detection, fault identification and fault pole selection; the fault starting is judged by adopting du/dT and di/dT; the fault detection is to further detect whether the line has faults by adopting low-voltage and overcurrent protection; the fault identification is to reliably identify the internal and external faults of the area by using the current-limiting inductive voltage at two ends of a direct-current line; the fault pole selection is to determine whether the system has positive pole, negative pole or bipolar fault by using the current-limiting inductance voltage at two ends of a direct current line; when a corresponding fault is detected, a tripping signal is sent out, and the corresponding direct current breaker acts to remove the fault; the method is characterized by comprising the following steps:
(1) and (3) starting a fault: when the system runs, the system starts to enter a protection link, and a direct current breaker is used for reading a voltage value and a current value; respectively carrying out differential du/dT and di/dT calculation on the voltage and the current values for the time T, and carrying out the next fault detection when the protection starting is carried out when the conditions of the formula (1) are met;
in the formula: u represents a positive or negative DC line voltage, i represents a positive or negative DC line current, UnRated value, I, representing DC voltagenA rated value representing a direct current; t represents time;
(2) and (3) fault detection:
the fault detection criterion is as follows:
in the formula: u shapedcRepresents the DC side inter-pole voltage;
when the conditions are met, the current converter is locked, and next fault identification is carried out; otherwise, reading the voltage value and the current value circularly;
(3) fault identification:
the positive direction of the positive electrode circuit is defined as a bus pointing circuit, and the positive direction of the negative electrode circuit is defined as a circuit pointing bus;
the protection criterion for the positive direction fault of the protection device m is as follows:
in the formula: u shapeLmtRepresenting the current-limiting inductor voltage; m is the number of the direct current breaker, and m is 12,21, 14, 23, 41, 32, 43 and 34; t represents the positive and negative poles of the direct current breaker, t is P, N, P represents the positive pole, and N represents the negative pole; rmtFor positive direction fault identification signals, Rmp1 indicates positive direction failure of the protection device, RmN1 indicates that the positive direction of the negative electrode of the protection device fails, and Rmt0 indicates that a reverse direction fault has occurred; u shapesetA protection criterion threshold;
criterion threshold value UsetThe following can be taken:
Uset＝2×0.4％×Un (4)
the voltage value of the current-limiting inductance at the two ends of each line is measured in the specified positive direction, the voltage value of the current-limiting inductance at the two ends of the line is calculated, and the faults inside and outside the area can be judged according to the protection devices at the two ends of the direct current line, wherein the specific criterion is as follows:
in the formula: n represents a direct current breaker of the opposite end of the line where m is located; rmtFor m positive direction fault recognition signals of protection devices, RntIdentifying a signal for a fault in the positive direction of the protection device n; stIdentifying signals for faults inside or outside the zone, St1 indicates that an intra-area fault occurs, otherwise, the intra-area fault is an out-area fault;
(4) and (3) fault pole selection: the purpose of setting a fault pole selection criterion is to further identify whether a fault occurs at the positive pole or the negative pole, and if a positive pole ground fault occurs, the direct current circuit breakers at two ends of a positive pole line act; if the negative earth fault occurs, the direct current circuit breakers at two ends of the negative line act; if a bipolar short circuit fault occurs, the direct current circuit breakers at two ends of the positive and negative circuits act; the fault pole selection criterion is as follows:
in the formula: s. thepIdentifying the signal for positive pole faults, SNIdentifying a signal for a negative fault; spWhen the value is 1, the positive pole fault is judged, and SNWhen the current value is 1, it is determined that the negative electrode is defective.
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