CN111799757A - Longitudinal zero-sequence voltage turn-to-turn protection method for generator - Google Patents
Longitudinal zero-sequence voltage turn-to-turn protection method for generator Download PDFInfo
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- 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
- H02H7/06—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 for dynamo-electric generators; for synchronous capacitors
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Abstract
The invention provides a longitudinal zero-sequence voltage turn-to-turn protection method for a generator, which comprises the following steps: firstly, redesigning a longitudinal zero-sequence voltage turn-to-turn protection circuit structure, wherein the generator is provided with two sets of common terminal TVs, namely TV1 and TV2, wherein a neutral point of the TV1 is grounded, a neutral point of the TV2 is grounded, and grounding circuits of the TV1 and the TV2 form terminal zero-sequence voltage; the neutral point of the generator is grounded, a grounding transformer is arranged on the connecting line, and the partial grounding circuit forms zero sequence voltage of the neutral point of the generator; the self-generating longitudinal zero-sequence voltage U is formed by the zero-sequence voltage at the generator end and the neutral point zero-sequence voltage of the generatorzo(ii) a Calculating short-circuit resistance and short-circuit reactance of grounding transformer at neutral point of generator, and correcting by impedance of grounding transformer
And
Description
Technical Field
The invention belongs to the technical field of turn-to-turn protection of generators in thermal power plants, and particularly relates to a method for realizing turn-to-turn protection of a generator without a special TV at a generator end.
Background
Stator winding of modern large-scale generator can not avoid stator bar with same phase and different turns on upper and lower layer bar in same slot of stator, which inevitably leads to turn-to-turn short circuit fault of generator stator winding, and turn-to-turn protection is installed for the large-scale generator.
For turn-to-turn protection of a stator of a thermal power generating unit, longitudinal zero-sequence voltage turn-to-turn protection and negative-sequence power direction turn-to-turn protection are generally adopted in China. The negative sequence power direction inter-turn protection has the defects of low sensitivity, no action before grid connection of a machine set, susceptibility to influence of frequency change and the like, so the negative sequence power direction inter-turn protection is generally rarely used independently and is more used as an auxiliary criterion of longitudinal zero sequence voltage protection for preventing inter-turn protection misoperation caused by one-time line break of a special TV. The longitudinal zero sequence voltage turn-to-turn protection is generally adopted abroad. Therefore, the current stator turn-to-turn protection mode which is most widely applied is the longitudinal zero-sequence voltage turn-to-turn protection mode. Special TV opening triangle for longitudinal zero-sequence voltage turn-to-turn protection machine end
The electrical capacity protection of the generator of the large-scale unit is based on a dual configuration principle, and the conventional inter-turn protection system only has one group of special TVs at the generator end, and the two protection systems are all taken from the opening triangles of the special TVs. The circuit structure of turn-to-turn protection in the prior art is as follows: as shown in fig. 1, the generator neutral point is directly connected to the generator end dedicated TV neutral point, and the generator end TV primary side is grounded; the secondary side of a generator terminal TV is connected in an open mode as follows: x → b, y → c, then a and z are led out to form an open triangle, or a grounding transformer is arranged on the primary side for measuring the zero sequence voltage on the primary side of the generator TV
Further obtaining a zero sequence voltage sampling value 3U of the position0. Zero sequence voltage measured by the secondary side opening triangle of the circuit structure is related to the turns of the neutral point of the TV terminal and the generator. Once the TV special for the terminal is abnormal, neither set of turn-to-turn protection can be correctly judgedIf the turn-to-turn short circuit occurs, the collected data cannot be correctly reflected, and the erroneous judgment of the protection device may be caused.
For example, when a dedicated TV is disconnected once, an open triangle of a terminal TV generates unbalanced voltage, and inter-turn protection acts to trip a main switch, but at the moment, the generator does not actually generate inter-turn short circuit, so that the generator stops, and the inter-turn protection malfunction causes unnecessary influence. For example, the generator itself generates turn-to-turn short circuit, but the secondary side opening triangle of the generator terminal TV generates disconnection, which causes the 3U actually collected by the protection device0And the value is 0, so that the protection device cannot correctly judge the protection fault, and the protection device refuses to operate. The protection device refuses to move, can cause damage to the generator body, enlarges the accident range and possibly influences the stability of the power plant.
Disclosure of Invention
The invention aims to solve the technical problems that the generator electrical quantity protection TV in the prior art cannot correctly judge whether turn-to-turn short circuit occurs or not, cannot correctly reflect the acquired data, and has the condition of turn-to-turn protection misoperation.
In order to solve the technical problem and provide a reliable voltage inter-turn protection measure, the invention provides the following scheme:
a longitudinal zero-sequence voltage inter-turn protection method for a generator comprises the following steps:
firstly, the longitudinal zero-sequence voltage turn-to-turn protection circuit structure is set as follows:
the generator is provided with two sets of ordinary terminal TVs, namely TV1 and TV2, wherein a neutral point of the TV1 is grounded, a neutral point of the TV2 is grounded, and grounding circuits of the TV1 and the TV2 form terminal zero-sequence voltage; the neutral point of the generator is grounded, a grounding transformer is arranged on the connecting line, and the partial grounding circuit forms zero sequence voltage of the neutral point of the generator;
the self-generating longitudinal zero-sequence voltage U is formed by the zero-sequence voltage at the generator end and the neutral point zero-sequence voltage of the generatorzo：
Order: zero sequence voltage at the machine end is
The value is taken from a secondary side opening triangle of a terminal TV;
neutral point zero sequence voltage of generator is
The value is extracted from the grounding transformer for the second time;
the self-generating longitudinal zero-sequence voltage inter-turn protection action equation is as follows:
in the formula:
second, the impedance of the grounding transformer is calculated and corrected by the short-circuit resistance and the short-circuit reactance of the grounding transformer
And
a value of (d); the calculation process of the short-circuit resistance and the short-circuit reactance is as follows:
the zero sequence voltage transformation ratio at the generator end is generally
Wherein
Rated phase voltage for the generator;
because the neutral point zero sequence voltage transformation ratio is related to the voltage transformation ratio and the tap position of the grounding transformer, the neutral point zero sequence voltage transformation ratio of the generator is set as
Wherein U isn0.NA neutral zero-sequence voltage secondary rated value;
zero sequence voltage at end of definer
And neutral zero sequence voltage
Correlation coefficient k of (a):
wherein, Un0.NA neutral zero-sequence voltage secondary rated value;
in the equivalent circuit of the neutral grounding transformer after the short-circuit impedancenIs the secondary load resistance value of the grounding transformer, rkAnd xkSecondary values, U, of short-circuit resistance and short-circuit reactance, respectively, of the grounding transformern0Is neutral zero-sequence voltage, wherein the neutral zero-sequence voltage is measured
Then, the actual secondary voltage of the grounding transformer should be:
zero sequence voltage at generator terminal when generator stator single-phase earth fault occurs
And neutral zero sequence voltage
The power frequency components are approximately equal to each other to obtain
Namely, it is
Wherein k is a zero-sequence voltage correlation coefficient of a machine end and a neutral point;
in formula (3)
And
expressed by its sine and cosine components, respectively, then
Wherein, Uf0-cAnd Uf0-sRespectively the magnitude of cosine and sine components of the zero sequence voltage at the machine end; u shapen0-cAnd Un0-sThe sizes of cosine and sine components of the zero sequence voltage of the neutral point are respectively, and the following full-period Fourier algorithm based on a cosine model is adopted for calculating the sine and cosine components:
in the above formula: a (k) and b (k) are respectively cosine component and sine component, N is the number of sampling points of power frequency weekly wave, and x (k) is discrete sampling value;
bringing formula (4) and formula (5) into formula (3) to obtain
Order to
Can obtain the product
The following equations can be obtained according to the above equation system:
thereby obtaining the short-circuit resistance rkAnd short-circuit reactance xkCalculating formula:
rk＝(k1·k-1)·rn(11)
xk＝k2·k·rn(12)
thirdly, the self-generated longitudinal zero-sequence voltage inter-turn protection rule is as follows:
TV one-time wire-break locking
When the TV is normally operated and the TV is disconnected for one time, the opening angle can generate zero sequence voltage due to the unbalance of three-phase voltage of the TV for one time, if the opening angle reaches the protection fixed value, turn-to-turn protection misoperation can be caused,
action criterion is as follows: TV2 negative sequence voltage of 3U'2＜3V
Negative sequence voltage 3U of TV12＞8V
TV1 self-generating zero-sequence voltage 3U0＞8V
TV1 opening angle zero sequence voltage 3U0＞8V；
TV secondary wire break block
When a fault occurs, zero sequence voltage of the protection device is collected to be zero when the TV is subjected to secondary line break, so that the protection cannot operate correctly, the accident is enlarged, further secondary line break locking of the TV is increased,
the TV anomaly discrimination criterion is as follows:
the positive sequence voltage is less than 18V, and any phase current is more than 0.04 In;
the negative sequence voltage 3U2 is greater than 8V;
the generator end TV and the main transformer high-voltage side TV meet any one of the conditions, the corresponding TV disconnection alarm signal disappears abnormally after the time delay of 10s, and the signal returns automatically after the time delay of 10 s;
c. main transformer high zero sequence voltage
Prevent the inter-turn protection from being operated by mistake when in an external fault, set the zero sequence voltage locking fixed value at the high-voltage side of the main transformer,
action criterion is as follows: u shapet0＜Ut0zodWherein U ist0For main change of opening angle zero sequence voltage, Ut0zodMeasuring a locking zero-sequence voltage setting value for the high voltage of the main transformer;
TV slow-melt latch
If the fuse is blown under normal working current, the blowing time of the fuse is longer, and the amplitude of the phase voltage is reduced in the process that the resistance value of the fuse is gradually increased, so that the related protection malfunction is caused, the high-voltage side fuse of the TV is not completely blown within the specified time, so that the resistance of a fuse tube is continuously increased, and the condition that the secondary output voltage of the TV is continuously reduced occurs,
the distinguishing method comprises the following steps: when the zero sequence voltage judgment method is used for monitoring PT secondary side opening triangular voltage, comparing a generator outlet zero sequence voltage value with a neutral point zero sequence voltage value, when the difference between the absolute value of the generator outlet zero sequence voltage and the absolute value of the generator neutral point zero sequence voltage value monitored by the secondary side of a voltage transformer is larger than a parameter set value, the PT slow melting is considered to occur, and meanwhile, a stator negative sequence current criterion is locked.
The invention has the beneficial effects that: the invention does not depend on the practical turn-to-turn protection of the stator of the special TV, and improves the line break criterion of the special TV by comparing the self-produced longitudinal zero-sequence voltage with the special TV open triangular voltage. And comparing the self-generated longitudinal zero sequence voltage with the special TV open triangular voltage to perfect the special TV disconnection criterion.
Drawings
FIG. 1 is an original longitudinal zero-sequence voltage inter-turn protection circuit;
FIG. 2 is a longitudinal zero sequence voltage turn-to-turn protection circuit of the present invention;
FIG. 3 is an equivalent circuit of a grounding transformer;
fig. 4 is a logic diagram of a zero-sequence voltage determination method.
Detailed Description
In the prior art, the longitudinal zero-sequence voltage turn-to-turn protection of the generator is generally formed by using a special TV. It is to take: a. and (b) utilizing the open delta zero voltage and the neutral point zero sequence voltage to form longitudinal zero sequence voltage, thereby realizing the turn-to-turn protection of the generator stator.
The turn-to-turn short circuit protection system utilizes zero-sequence voltage action generated when the turn-to-turn short circuit or branch short circuit is generated in the generator to cut off short circuit faults. In order to prevent malfunction of the protection device caused by zero sequence voltage during ground fault, the invention redesigns the structure of the protection circuit. The design concept of the invention is as follows: the neutral point on the high-voltage side of the voltage transformer is not grounded but is directly connected with the neutral point of the generator, and the neutral point of the generator is grounded through an arc suppression coil or a lightning arrester; two groups of conventional (common) TVs, TV1 and TV2 are adopted to replace the original special TVs, and neutral points of the TVs 1 and 2 are directly grounded.
The invention mainly carries out application research on the self-generated longitudinal zero-sequence voltage inter-turn protection of the generator, and specifically comprises the following steps: 1) carrying out theoretical research on a self-generated longitudinal zero-sequence voltage turn-to-turn protection circuit of the generator; 2) measuring and calculating research on short-circuit resistance and short-circuit reactance of a generator neutral point grounding transformer; 3) and formulating a self-generating longitudinal zero-sequence voltage inter-turn protection method of the generator.
The following detailed description of the technical protocol of this study:
1) the invention provides a self-generating longitudinal zero-sequence voltage inter-turn protection circuit which is designed as follows:
the wiring of the improved turn-to-turn protection circuit is shown in fig. 2, and the generator is provided with two sets of TV terminals, namely TV1 and TV 2; neutral points of the TV1 and the TV2 are grounded, and grounding circuits of the TV1 and the TV2 form terminal zero sequence voltage; the neutral point of the generator is grounded, a grounding transformer is arranged on the wiring, and the partial grounding circuit forms zero sequence voltage of the neutral point of the generator.
The TV1 and the TV2 respectively form two sets of completely independent inter-turn protection of the generator, and the configuration requirement of doubling protection is met.
Order: zero sequence voltage at the machine end is
The value is taken from a secondary side opening triangle of a terminal TV;
neutral point zero sequence voltage of generator is
The value is extracted from the grounding transformer for the second time;
the self-generating longitudinal zero-sequence voltage inter-turn protection action equation is as follows:
in the formula:
The invention adopts the UzoAs the judgment basis of turn-to-turn protection.
2) In actual operation, the grounding resistance of the system is considered, reactance errors possibly caused by factors such as an operating environment and the like are caused, and incorrect protection actions caused by operating environment changes are effectively compensated through the algorithm.
When the generator stator single-phase earth fault occurs, the primary values of the generator end and the neutral point power frequency zero sequence voltage are approximately equal, therefore, the generator stator single-phase earth fault is simulated, the waveform data of the generator end zero sequence voltage and the neutral point zero sequence voltage when the fault occurs are recorded through the generator protection device, and the short-circuit resistance and the short-circuit reactance value of the generator neutral point earth transformer can be obtained by combining the transformation ratio of the generator end zero sequence voltage TV and the neutral point zero sequence voltage TV and the secondary load resistance value of the earth transformer.
The experimental method is that a single-phase grounding point is arranged at a TV position at the generator end under the shutdown state, a separate excitation power supply is switched on, the excitation current is slowly increased until the zero-sequence voltage of the neutral point of the generator exceeds the grounding protection action fixed value of a fundamental wave zero-sequence voltage stator of the generator, the generator protection device records the waveform data of the zero-sequence voltage at the generator end and the zero-sequence voltage of the neutral point when the fault occurs, the proportional relation between the zero-sequence voltage at the generator end and the fundamental wave amplitude of the neutral point zero-sequence voltage can be calculated according to the waveform data, a basis is provided for setting calculation of the grounding protection of the fundamental wave zero-sequence voltage stator, and meanwhile, preparation is.
Self-generated longitudinal zero sequence voltage U obtained through sampling in TV1 and TV2zoBecause the neutral point of the generator has the grounding transformer, the actual situation of the longitudinal zero-sequence voltage cannot be accurately reflected, and the accurate longitudinal zero-sequence voltage needs to be obtained through algorithm compensation calculation so as to judge whether the turn-to-turn short circuit fault of the generator exists.
The calculation process of the short-circuit resistance and the short-circuit reactance of the generator neutral point grounding transformer is as follows:
the zero sequence voltage transformation ratio at the generator end is generally
(
Voltage rating for the generator);
because the neutral point zero sequence voltage transformation ratio is related to the voltage transformation ratio and the tap position of the grounding transformer, the neutral point zero sequence voltage transformation ratio of the generator is set as
(Un0.NNeutral zero sequence voltage quadratic rating);
zero sequence voltage at end of definer
And neutral zero sequence voltage
In (2) correlation ofCoefficient k:
wherein, Un0.NThe zero sequence voltage secondary rated value of the neutral point.
The equivalent circuit of the neutral grounding transformer after considering the short-circuit impedance is shown in FIG. 3, where rnIs the secondary load resistance value of the grounding transformer, rkAnd xkSecondary values, U, of short-circuit resistance and short-circuit reactance, respectively, of the grounding transformern0Is neutral zero sequence voltage. As can be seen from FIG. 3, the zero sequence voltage measured value at the neutral point is
Then, the actual secondary voltage of the grounding transformer should be:
zero sequence voltage at generator terminal when generator stator single-phase earth fault occurs
And neutral zero sequence voltage
The power frequency components are approximately equal to obtain
Namely, it is
And k is a zero-sequence voltage correlation coefficient of the machine end and the neutral point.
In formula (3)
And
represented by its sine and cosine components, respectively.
Wherein, Uf0-cAnd Uf0-sRespectively the magnitude of cosine and sine components of the zero sequence voltage at the machine end; u shapen0-cAnd Un0-sThe sizes of cosine and sine components of the zero sequence voltage of the neutral point are respectively, and the following full-period Fourier algorithm based on a cosine model is adopted for calculating the sine and cosine components:
in the above formula: a (k) and b (k) are respectively cosine component and sine component, N is the number of sampling points of power frequency per cycle, and x (k) is discrete sampling value.
Bringing formula (4) and formula (5) into formula (3) to obtain
Order to
And
can obtain the product
From the above system of equations
Thereby obtaining the short-circuit resistance rkAnd short-circuit reactance xkCalculating formula:
rk＝(k1·k-1)·rn(11)
xk＝k2·k·rn(12)
the invention adopts a digital algorithm, calculates the short-circuit impedance of the neutral point grounding transformer of the generator in real time through sampling and analyzing the grounding transformer, and is used for calculating the self-generated longitudinal zero-sequence voltage of the generator without using an external device to measure the impedance of the grounding transformer in the traditional measurement mode.
3) The invention adopts TV disconnection logic, and locks inter-turn protection when the TV disconnection is judged:
the turn-to-turn protection measure is to judge that when the TV1 and the TV2 have faults, the sampled longitudinal zero-sequence voltage is abnormal, so that the compensated longitudinal zero-sequence voltage is abnormal, and the false operation of turn-to-turn protection can be caused. In order to avoid the above situation, the invention improves the reliability of the turn-to-turn protection of the self-generated longitudinal zero-sequence voltage of the generator, and the specific measures are to adopt the following judgment rules to improve the protection sensitivity and reduce the risk of protection misoperation, and the specific measures comprise:
one-time broken wire locking of e.TV
When the TV is normally operated and the TV is disconnected for one time, the opening angle can generate zero sequence voltage due to the unbalance of three-phase voltage of the TV for one time, if the opening angle reaches the protection fixed value, turn-to-turn protection misoperation can be caused,
action criterion is as follows: TV2 negative sequence voltage of 3U'2＜3V
Negative sequence voltage 3U of TV12＞8V
TV1 self-generating zero-sequence voltage 3U0＞8V
TV1 opening angle zero sequence voltage 3U0＞8V。
f.TV secondary wire break lock
When a fault occurs, zero sequence voltage acquisition of the protection device can be zero when the TV is subjected to secondary line break, so that the protection cannot operate correctly, the accident is enlarged, further secondary line break locking of the TV is increased,
the TV anomaly discrimination criterion is as follows:
the positive sequence voltage is less than 18V, and any phase current is more than 0.04 In;
the negative sequence voltage 3U2 is greater than 8V;
the generator end TV and the main transformer high-voltage side TV meet any one of the conditions, the corresponding TV disconnection alarm signal disappears abnormally after the time delay of 10s, and the signal returns automatically after the time delay of 10 s.
g. Main transformer high zero sequence voltage
Prevent the inter-turn protection from being operated by mistake when in an external fault, the zero sequence voltage locking fixed value at the high-voltage side of the main transformer can be set,
action criterion is as follows: u shapet0＜Ut0zodWherein U ist0For main change of opening angle zero sequence voltage, Ut0zodAnd measuring a locking zero-sequence voltage setting value for the high voltage of the main transformer.
h.TV slow-melt latch
The fuse wire is aged under the action of gravity and heat accumulation due to severe operating environment on site, so that the fuse wire is likely to be broken under normal working current, the fuse wire is fused under the normal working current, the fusing time of the fuse wire is long, the amplitude of the phase voltage is reduced in the process that the resistance value of the fuse wire is gradually increased, the related protection is caused to be mistaken, the fuse wire on the high-voltage side of the TV is not completely fused within the specified time, the resistance of a fuse tube is continuously increased, and the secondary output voltage of the TV is continuously reduced.
The distinguishing method comprises the following steps: and the zero sequence voltage judgment method is to compare the zero sequence voltage value of the outlet of the generator with the zero sequence voltage value of the neutral point when monitoring the PT secondary side opening triangular voltage. When the difference between the absolute value of the zero sequence voltage at the outlet of the generator and the absolute value of the zero sequence voltage at the neutral point of the generator is monitored by the secondary side of the voltage transformer, the PT slow melting is considered to occur. The stator negative sequence current criterion needs to be locked at the moment.
The logic of the zero sequence voltage determination method is shown in FIG. 4, in which U0Zero sequence voltage at the generator end; u shapeLIs composed ofA neutral point voltage; u shape0setSetting a threshold value; i is0Zero sequence current at the generator end; u shape0setSetting a threshold value for the zero sequence current at the machine end; and t is the zero sequence voltage judgment action time.
The invention has the beneficial effects that: in the prior art, primary side neutral points of TV1 and TV2 are directly connected with the ground, and originally, the sampling method can only be used in logic operations of overvoltage, reverse power, loss of field, loss of step and other protections of a generator in actual production, and the sampling method has no compensation algorithm, so that the sampling method of TV1 and TV2 can not be used in calculation of turn-to-turn protection of the generator. By the scheme provided by the invention, on the basis of the existing TV, the requirement of turn-to-turn protection is met, the dual standard is met, and the cost is greatly reduced.
If the self-generated longitudinal zero-sequence voltage inter-turn protection of the generator is practically applied, the problem of sharing of a long-term inter-turn protection dualized configuration channel on a thermal power generating unit can be solved, the TV disconnection criterion can be perfected, the overall performance of a protection scheme is obviously improved, the development of the industry is deduced, the application prospect is wide, and the economic and social benefits are very obvious.
Claims (1)
1. A longitudinal zero-sequence voltage inter-turn protection method for a generator comprises the following steps:
firstly, the longitudinal zero-sequence voltage turn-to-turn protection circuit structure is set as follows:
the generator is provided with two sets of ordinary terminal TVs, namely TV1 and TV2, wherein a neutral point of the TV1 is grounded, a neutral point of the TV2 is grounded, and grounding circuits of the TV1 and the TV2 form terminal zero-sequence voltage; the neutral point of the generator is grounded, a grounding transformer is arranged on the connecting line, and the partial grounding circuit forms zero sequence voltage of the neutral point of the generator;
the self-generating longitudinal zero-sequence voltage U is formed by the zero-sequence voltage at the generator end and the neutral point zero-sequence voltage of the generatorzo：
Order: zero sequence voltage at the machine end is
The value is taken from a secondary side opening triangle of a terminal TV;
neutral point zero sequence voltage of generator is
The value is extracted from the grounding transformer for the second time;
the self-generating longitudinal zero-sequence voltage inter-turn protection action equation is as follows:
in the formula:
second, the impedance of the grounding transformer is calculated and corrected by the short-circuit resistance and the short-circuit reactance of the grounding transformer
And
a value of (d); the calculation process of the short-circuit resistance and the short-circuit reactance is as follows:
the zero sequence voltage transformation ratio at the generator end is generally
Wherein
Rated phase voltage for the generator;
because the neutral point zero sequence voltage transformation ratio is related to the voltage transformation ratio and the tap position of the grounding transformer, the neutral point zero sequence voltage transformation ratio of the generator is set as
Wherein U isn0.NA neutral zero-sequence voltage secondary rated value;
zero sequence voltage at end of definer
And neutral zero sequence voltage
Correlation coefficient k of (a):
wherein, Un0.NA neutral zero-sequence voltage secondary rated value;
in the equivalent circuit of the neutral grounding transformer after the short-circuit impedancenIs the secondary load resistance value of the grounding transformer, rkAnd xkSecondary values, U, of short-circuit resistance and short-circuit reactance, respectively, of the grounding transformern0Is neutral zero-sequence voltage, wherein the neutral zero-sequence voltage is measured
Then, the actual secondary voltage of the grounding transformer should be:
zero sequence voltage at generator terminal when generator stator single-phase earth fault occurs
And neutral zero sequence voltage
The power frequency components are approximately equal to each other to obtain
Namely, it is
Wherein k is a zero-sequence voltage correlation coefficient of a machine end and a neutral point;
in formula (3)
And
expressed by its sine and cosine components, respectively, then
Wherein, Uf0-cAnd Uf0-sRespectively the magnitude of cosine and sine components of the zero sequence voltage at the machine end; u shapen0-cAnd Un0-sThe sizes of cosine and sine components of the zero sequence voltage of the neutral point are respectively, and the following full-period Fourier algorithm based on a cosine model is adopted for calculating the sine and cosine components:
in the above formula: a (k) and b (k) are respectively cosine component and sine component, N is the number of sampling points of power frequency weekly wave, and x (k) is discrete sampling value;
bringing formula (4) and formula (5) into formula (3) to obtain
Order to
Can obtain the product
The following equations can be obtained according to the above equation system:
thereby obtaining the short-circuit resistance rkAnd short-circuit reactance xkCalculating formula:
rk＝(k1·k-1)·rn(11)
xk＝k2·k·rn(12)
thirdly, the self-generated longitudinal zero-sequence voltage inter-turn protection rule is as follows:
TV one-time wire-break locking
When the TV is normally operated and the TV is disconnected for one time, the opening angle can generate zero sequence voltage due to the unbalance of three-phase voltage of the TV for one time, if the opening angle reaches the protection fixed value, turn-to-turn protection misoperation can be caused,
action criterion is as follows: TV2 negative sequence voltage of 3U'2<3V
Negative sequence voltage 3U of TV12>8V
TV1 self-generating zero-sequence voltage 3U0>8V
TV1 opening angle zero sequence voltage 3U0>8V；
TV secondary wire break block
When a fault occurs, zero sequence voltage of the protection device is collected to be zero when the TV is subjected to secondary line break, so that the protection cannot operate correctly, the accident is enlarged, further secondary line break locking of the TV is increased,
the TV anomaly discrimination criterion is as follows:
the positive sequence voltage is less than 18V, and any phase current is more than 0.04 In;
the negative sequence voltage 3U2 is greater than 8V;
the generator end TV and the main transformer high-voltage side TV meet any one of the conditions, the corresponding TV disconnection alarm signal disappears abnormally after the time delay of 10s, and the signal returns automatically after the time delay of 10 s;
c. main transformer high zero sequence voltage
Prevent the inter-turn protection from being operated by mistake when in an external fault, set the zero sequence voltage locking fixed value at the high-voltage side of the main transformer,
action criterion is as follows: u shapet0<Ut0zodWherein U ist0For main change of opening angle zero sequence voltage, Ut0zodMeasuring a locking zero-sequence voltage setting value for the high voltage of the main transformer;
TV slow-melt latch
If the fuse is blown under normal working current, the blowing time of the fuse is longer, and the amplitude of the phase voltage is reduced in the process that the resistance value of the fuse is gradually increased, so that the related protection malfunction is caused, the high-voltage side fuse of the TV is not completely blown within the specified time, so that the resistance of a fuse tube is continuously increased, and the condition that the secondary output voltage of the TV is continuously reduced occurs,
the distinguishing method comprises the following steps: when the zero sequence voltage judgment method is used for monitoring PT secondary side opening triangular voltage, comparing a generator outlet zero sequence voltage value with a neutral point zero sequence voltage value, when the difference between the absolute value of the generator outlet zero sequence voltage and the absolute value of the generator neutral point zero sequence voltage value monitored by the secondary side of a voltage transformer is larger than a parameter set value, the PT slow melting is considered to occur, and meanwhile, a stator negative sequence current criterion is locked.
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| CN114200376A (en) * | 2021-11-13 | 2022-03-18 | 平顶山姚孟发电有限责任公司 | Fault pre-judging system for generator terminal voltage transformer and control method thereof |
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