CN110601140A - Differential protection device and algorithm of variable frequency motor - Google Patents
Differential protection device and algorithm of variable frequency motor 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
- H02H7/08—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 motors
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Abstract
The invention relates to a differential protection device and an algorithm of a variable frequency motor, which comprise a frequency converter, a motor, a current transformer CT1, a CT2 and a CT3, wherein the CT1, the frequency converter, the CT2, the motor and the CT3 are sequentially connected in series, the frequency converter is provided with a bypass, the bypass is provided with a bypass breaker, a closing signal contact of the bypass breaker is connected into the differential protection device, the CT2 and the CT3 form differential protection when the bypass breaker is disconnected, and the CT1 and the CT3 form differential protection when the bypass breaker is closed. The invention has the beneficial effects that: differential protection is realized under two modes of power frequency operation and variable frequency operation.
Description
Technical Field
The present invention relates to differential protection of a variable frequency motor, and more particularly, to a differential protection apparatus and an algorithm for a variable frequency motor.
Background
The main equipment of the large-scale thermal power plant comprises a boiler, a steam turbine and a generator, wherein the boiler is provided with a primary fan, a secondary fan and an induced draft fan, the steam turbine is provided with a water feeding pump, and the fan and the water pump are driven by a large-scale motor. The power plant also consumes a large amount of power to maintain the operation of the equipment while generating power, wherein the power consumption of the large motor accounts for about 85% of the total amount. Under the large background of energy-saving development, the frequency conversion transformation of a large motor in a large-scale thermal power plant is very common, and the purpose is to reduce the service power ratio, reduce the power generation cost and further improve the competitiveness. At present, the power of a motor is larger than 2000kW, after a frequency converter is added to improve and finish the motor, the motor protection of the original configuration is not suitable for a frequency converter dragging motor system, wherein the differential protection cannot be normally input under the non-power frequency condition, and only a high-voltage frequency converter is utilized to protect the motor, so that huge potential safety hazards exist, and whether the sensitivity of the frequency converter to the motor protection meets the requirements or not can not be determined.
After the motor is transformed by frequency conversion, the protection device configured originally is continuously used, as shown in fig. 1, a group of current transformers are installed at the motor end of the motor, differential protection is formed by the motor end and a neutral point, and the backup protection device still uses the protection function of the original motor. The primary side currents of the two groups of current transformers are the same, although the conditions for forming differential protection are met, the working voltage frequency is not the power frequency, the current transformers are designed to be used in a power frequency system, the protection device is the same as the current transformers, the current sampling loop is designed to be used under the power frequency, the current sampling loop is not applicable under the non-power frequency working condition, and the reliability of the protection function is uncertain. In part of fields, differential protection is formed in such a way, and misoperation frequently occurs in operation.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the defects in the prior art and provide a differential protection device and an algorithm of a variable frequency motor, so that the differential protection is suitable for the motor after variable frequency transformation.
The invention is realized by the following technical scheme:
the utility model provides a differential protection device of inverter motor, includes converter, motor, current transformer CT1, CT2 and CT3, CT1, converter, CT2, motor and CT3 are established ties in proper order, the converter is equipped with the bypass, the bypass is equipped with the bypass circuit breaker, the closing signal contact access differential protection device of bypass circuit breaker, CT2 and CT3 constitute differential protection when the bypass circuit breaker breaks off, and CT1 and CT3 constitute differential protection when the bypass circuit breaker is closed.
According to the technical scheme, preferably, the current transformers CT2 and CT3 are both broadband current transformers.
The invention has the beneficial effects that: differential protection is realized under two modes of power frequency operation and variable frequency operation.
A differential protection algorithm of a variable frequency motor comprises the following steps:
step 000: the automatic switching device comprises a frequency converter, a motor, a current transformer CT1, a CT2 and a CT3, wherein the CT1, the frequency converter, the CT2, the motor and the CT3 are sequentially connected in series, the frequency converter is provided with a bypass, the bypass is provided with a bypass breaker, a closing signal contact of the bypass breaker is connected into a differential protection device, the CT2 and the CT3 form differential protection when the bypass breaker is disconnected, and the CT1 and the CT3 form differential protection when the bypass breaker is closed;
step 010: sampling value differential is adopted, and sampling points of all channels are instantaneous values of current at the same moment;
step 020: the vector sum of the currents measured by the current transformers forming the differential protection is differential current, the braking current is calculated according to half of the vector difference of the currents on two sides, the differential quick-break and the ratio differential adopt an abrupt change starting element and an overcurrent starting element, when the differential current is abruptly changed or the maximum value of the differential current is larger than a corresponding overcurrent fixed value, the starting element acts and keeps 10s, and a starting relay is opened.
According to the above technical solution, preferably, when the bypass breaker is opened, the balancing coefficient K is passedph2And the problem of inconsistent transformation ratios of the current transformers on the two sides of the motor is solved.
According to the above technical solution, preferably, when the bypass breaker is closed, the balancing coefficient K is passedph1And the problem of inconsistent transformation ratios of the current transformers on the two sides of the motor is solved.
According to the technical scheme, preferably, the differential quick-break protection action criterion is that the maximum value in the three-phase differential current is greater than the differential quick-break protection fixed value.
According to the above technical solution, preferably, the ratio differentiation uses a three-fold line ratio differentiation principle.
According to the technical scheme, the method preferably further comprises an algorithm for monitoring the disconnection of the current transformer, wherein the judgment logic is that one phase or two phases of current on any side of the three-phase current of the system is less than 0.125 times of rated current, and other two phases or one phase of current are all greater than 0.2 times of rated current.
According to the above technical solution, preferably, the algorithm for monitoring disconnection of the current transformer is started after the rate differential element is started.
According to the above technical solution, preferably, the algorithm for monitoring disconnection of the current transformer satisfies any one of the following three conditions, and does not perform discrimination:
(1) before starting, if the maximum phase current of a certain side is less than 0.2Ie, the disconnection judgment of the current transformer of the side is not carried out;
(2) the maximum phase current after starting is more than 1.2 Ie;
(3) the current on either side increases after start-up compared to before start-up.
The invention has the beneficial effects that: differential protection is carried out in two modes of power frequency operation and variable frequency operation, and a sampling value differential calculation method is applied, so that the method is suitable for a variable frequency dragging motor system.
Drawings
Fig. 1 shows a schematic diagram of a prior art connection scheme for motor differential protection.
Fig. 2 shows a schematic diagram of an electric system of a variable frequency motor according to an embodiment of the differential protection device of the variable frequency motor of the present invention.
Fig. 3 shows a schematic diagram of a variable frequency differential protection current loop of an embodiment of the differential protection apparatus of the variable frequency motor of the present invention.
Fig. 4 shows a schematic diagram of a differential protection system of a variable frequency motor according to an embodiment of the differential protection device of the variable frequency motor.
Fig. 5 shows a differential quick-break logic diagram of an embodiment of the differential protection algorithm of the variable frequency motor of the present invention.
Fig. 6 shows a differential protection operation characteristic diagram of an embodiment of the differential protection algorithm of the variable frequency motor according to the present invention.
Fig. 7 shows a ratio differential logic diagram of an embodiment of the differential protection algorithm of a variable frequency motor of the present invention.
Fig. 8 shows a schematic diagram of a through-current test of an embodiment of the differential protection device and algorithm of the variable frequency motor of the present invention.
Fig. 9 shows a through-current test current phasor diagram of an embodiment of the differential protection device and algorithm of the variable frequency motor of the present invention.
Detailed Description
In order to make the technical solutions of the present invention better understood by those skilled in the art, the present invention will be further described in detail with reference to the accompanying drawings and preferred embodiments.
A differential protection device of a variable frequency motor comprises a frequency converter, a motor, a current transformer CT1, a CT2 and a CT3, wherein the CT1, the frequency converter, the CT2, the motor and the CT3 are sequentially connected in series, the frequency converter is provided with a bypass, the bypass is provided with a bypass breaker, a closing signal contact of the bypass breaker is connected into the differential protection device, the CT2 and the CT3 form differential protection when the bypass breaker is disconnected, and the CT1 and the CT3 form differential protection when the bypass breaker is closed.
The protection device also needs to consider the operation mode of the frequency converter dragging motor system. The motor normally operates in a frequency conversion mode, but the problem of tripping of a motor circuit caused by the fault of the frequency converter can occur, but in order to maintain the motor to continuously operate, the electric system is designed with a bypass switching function, the fan can be continuously maintained to operate through a power frequency bypass after the frequency converter exits, and meanwhile, conditions are created for technicians to process the fault. As shown in fig. 2, when the frequency converter normally operates, QF1, KM1, QS1, QS2 and KM2 are switched on, and QF2 is switched off; when the frequency converter has a fault, KM1, QS1, QS2 and KM2 are switched off, and QF1 and QF2 are switched on.
When the frequency conversion operation and the bypass operation are carried out, an electric primary system changes, and the differential protection device needs to normally work under two working conditions. In a frequency conversion operation mode, the CT1 operates at power frequency, and primary side currents of the CT2 and the CT3 are non-power frequency and are the same current; when the bypass circuit operates, the primary side currents of the CT1, the CT2 and the CT3 are all power frequency and are the same current. By analyzing the current flowing in and out of the motor stator winding, we can determine the mode of differential protection configuration according to the principle of differential protection. The motor operates in a variable frequency mode, differential protection is formed by CT2 and CT3, and the protection range is from the output side of the frequency converter to the neutral point of the motor; the motor operates at power frequency, differential protection is formed by the CT1 and the CT3, the protection range is from the lower port of the high-voltage switch to the neutral point of the motor, and the principle of a protection current loop is shown in figure 3.
In consideration of the problem of switching the working modes of the device, as shown in fig. 4, the protection device can be switched in on site by using a switching-on signal contact of the QF2 circuit breaker, and when the contact is closed after the QF2 is switched on, an opening signal is provided for the device, so that automatic switching between a power frequency mode and a frequency conversion mode can be realized.
According to the technical scheme, preferably, the current transformers CT2 and CT3 are both broadband current transformers.
The power frequency current transformer can be saturated gradually along with the reduction of current frequency, and the saturation degree is constantly deepened and is caused the continuous increase of secondary current error, and the output current wave form is seriously out of shape. For a large motor driven by a frequency converter, differential protection formed by a power frequency current transformer cannot work normally. To address this issue, we chose to use a wide-band current transformer that can accommodate variable frequency operating conditions, such as the model lzbj 9-10E2 wide-band current transformer manufactured by jiangsu jing transformer corporation.
The broadband current transformer well solves the problem of measuring the running current of the motor under the working condition of the frequency converter, can be arranged on the output side of the frequency converter and the neutral point of the motor, obtains the maximum protection range and provides differential protection for a high-voltage cable and a motor winding. When the field is modified, the primary current range of the current transformer is selected according to the rated current of the motor, and generally 1.5 times of the rated current of the motor is selected. For site operation, a group of broadband current transformers need to be installed in a frequency converter bypass cabinet, a control cable from a frequency converter to a high-voltage switch is laid for uploading secondary current for protection, primary wiring of the current transformers at the motor end can be unchanged, secondary side wiring cables are dismantled and short-circuit output terminals at the secondary side of the transformers, the current transformers at the neutral point side are replaced by the broadband current transformers, and the original control cable is utilized.
A differential protection algorithm of a variable frequency motor comprises the following steps:
step 000: the automatic switching device comprises a frequency converter, a motor, a current transformer CT1, a CT2 and a CT3, wherein the CT1, the frequency converter, the CT2, the motor and the CT3 are sequentially connected in series, the frequency converter is provided with a bypass, the bypass is provided with a bypass breaker, a closing signal contact of the bypass breaker is connected into a differential protection device, the CT2 and the CT3 form differential protection when the bypass breaker is disconnected, and the CT1 and the CT3 form differential protection when the bypass breaker is closed;
step 010: sampling value differential is adopted, and sampling points of all channels are instantaneous values of current at the same moment;
step 020: the vector sum of the currents measured by the current transformers forming the differential protection is differential current, the braking current is calculated according to half of the vector difference of the currents on two sides, the differential quick-break and the ratio differential adopt an abrupt change starting element and an overcurrent starting element, when the differential current is abruptly changed or the maximum value of the differential current is larger than a corresponding overcurrent fixed value, the starting element acts and keeps 10s, and a starting relay is opened.
In order to solve the problem of data calculation of sampling points which are constantly changed, an algorithm which takes a plurality of sampling data at the same time as an action criterion is provided, and then a sampling value differential is provided. Sampling points of all channels of the protection device are instantaneous values of current at the same moment, and when an interphase short circuit fault occurs outside protected equipment, the sum of the values of all sampled currents is zero; when the inter-phase short circuit fault occurs in the equipment, the sum of the sampling current values is not zero. The sampling value differential protection is formed by using the sum of sampling value currents according to a certain action criterion, and compared with the conventional phasor differential protection, the sampling value differential protection has the advantages that the differential calculation amount is reduced, the protection action time can be shortened, and the damage degree of fault equipment is reduced. The sampling value differential principle does not carry out Fourier calculation, and the problem that harmonic waves generated by a frequency converter circuit influence the calculation precision can be well solved. Therefore, for motor systems operating at varying frequencies, differential protection can be achieved using this algorithm.
According to the above technical solution, preferably, when the bypass breaker is opened, the balancing coefficient K is passedph2And the problem of inconsistent transformation ratios of the current transformers on the two sides of the motor is solved.
The situation that the transformation ratios of CT on two sides of the motor are inconsistent can exist, and the balance coefficient K converted to the neutral side of the motor is utilizedph2To correct for this effect.
Frequency conversion method
Differential current calculation formula
Icd＝|Ict3+Kph2Ict2|
Calculation formula of brake current
Izd＝|Ict3-Kph2Ict2|/2
Formula for calculating balance coefficient
In the formula Ict2Is the secondary current of the current transformer CT2, and Ict3 is the secondary current of the current transformer CT 3.
According to the above technical solution, preferably, when the bypass breaker is closed, the balancing coefficient K is passedph1And the problem of inconsistent transformation ratios of the current transformers on the two sides of the motor is solved.
The situation that the transformation ratios of CT on two sides of the motor are inconsistent can exist, and the balance coefficient K converted to the neutral side of the motor is utilizedph1To correct for this effect.
Power frequency mode
Differential current calculation formula
Icd＝|Ict3+Kph1Ict1|
Rule current calculation formula
Izd＝|Ict3-Kph1Ict1|/2
Formula for calculating balance coefficient
In the formula, Ict1 is the secondary current of a current transformer CT 1.
According to the technical scheme, preferably, the differential quick-break protection action criterion is that the maximum value in the three-phase differential current is greater than the differential quick-break protection fixed value. The protection logic is shown in fig. 5.
According to the above technical solution, preferably, the ratio differentiation uses a three-fold line ratio differentiation principle. The action curve is shown in fig. 6, the slope of the third fold line is fixed to 1, and the protection logic is shown in fig. 7.
The criteria for the protection action are as follows:
DI>Icdqd HI≤0.5Ie
DI-Icdqd>K(HI-05Ie) 0.5Ie<HI≤3Ie
DI-Icdqd-2.5KIe>HI-3Ie HI>3Ie
wherein DI is a differential current calculated value, IcdqdFor differential current start-up, K is slope, HI is brake current, IeIs the rated current secondary value of the motor.
According to the technical scheme, the method preferably further comprises an algorithm for monitoring the disconnection of the current transformer, wherein the judgment logic is that one phase or two phases of current on any side of the three-phase current of the system is less than 0.125 times of rated current, and other two phases or one phase of current are all greater than 0.2 times of rated current. The lockout function is based on two assumptions: firstly, the CT disconnection does not occur when all loops are the same, and secondly, the CT disconnection and the fault do not occur simultaneously.
According to the above technical solution, preferably, the algorithm for monitoring disconnection of the current transformer is started after the rate differential element is started. This also prevents false locking of instantaneous CT disconnect. When only one phase or two phases of current of a certain group of current transformers of the system are monitored to be zero and the other two phases or one phase of current of the certain group of current transformers are equal to the current before starting, the current is considered to be broken, and the instantaneous CT broken wire locking rate differential function can be selected to be put into or taken out through setting of control words.
According to the above technical solution, preferably, the algorithm for monitoring disconnection of the current transformer satisfies any one of the following three conditions, and does not perform discrimination:
(1) before starting, if the maximum phase current of a certain side is less than 0.2Ie, the disconnection judgment of the current transformer of the side is not carried out;
(2) the maximum phase current after starting is more than 1.2 Ie;
(3) the current on either side increases after start-up compared to before start-up.
For backup protection of a variable frequency motor system, a protection current sampling object is the running current of a lower port of a high-voltage switch, and the working frequency is always 50Hz when the motor runs at power frequency or variable frequency. Although the protection working condition is not changed, the protection objects corresponding to the two modes are changed, and the variable-frequency running motor is a phase-shifting transformer, and the power-frequency running motor is a motor. Among the backup protections, the protection subject is mainly overcurrent protection and motor-specific protection, such as start protection. The problem can be solved by using two protection logic protection devices of a built-in transformer and a built-in motor, the same method as frequency conversion differential motion can be adopted, a closing signal contact of a bypass breaker QF2 is used as a signal, different protection types are started under different working conditions, and the purpose that a protection object corresponds to backup protection is achieved.
After the debugging work of the single device is finished, the wiring correctness of the current loops participating in the differential protection needs to be confirmed, the number of the transformers is increased from two groups to three groups, and the difficulty is brought to the polarity checking work. After the primary electric system and the protection system are connected, in order to complete the current loop connection checking work before the whole system is put into operation for the first time, the correctness of the connection can be determined by using a through-current test.
Before a test, a high-voltage switch QF1 is sent to an overhaul position to ensure personnel safety, an isolating switch QS1 and a QS2 are separated, a frequency converter bypass circuit breaker QF2 is switched on, a switching-on signal of the QF2 is disconnected to a connection wire of a protection device, a high-voltage cable on the neutral point side of the motor is disconnected, three cables 2 with the cross sections not smaller than 10mm are prepared, a resistance gear of a multimeter is used for checking and marking three-phase stator windings of the motor, the connection wire on the neutral point side is recovered, each phase of stator winding is short-circuited by the cables, and the test principle is shown in. Two test wires of the current generator are respectively connected with an A phase and a B phase above a primary wiring terminal of a lower port CT1 of a high-voltage switch QF1, a 50Hz current with the amplitude of 50A is introduced, sampling and differential current values of the protection device are checked, the current phase relation is shown in figure 9, I in figure 9 is that a primary side of the current transformer is introduced with current, and I isA CT2IA CT3 IB CT2 IC CT3Sampling a deviceThe current, the secondary side current amplitude of CT2 and CT3 is the same and the phase difference is 180 degrees, the differential current A phase and B phase are both zero to prove that the polarity of CT2 and CT3 is correct, the short circuit QF2 closing signal is connected to the wiring of the device, the device is switched to a power frequency mode, the sampling value and the differential current value of CT1 and CT3 are checked, and the polarity of CT1 and CT3 is determined. And when the differential current is not zero, stopping the through-flow work immediately, determining the mutual inductor with the wrong polarity according to the data of the frequency conversion and power frequency modes, changing the wiring, and checking the test again until the current is correct. When checking the phase C, the current is passed through the primary sides of the phase A and the phase C by the same method.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (10)
1. A differential protection device of a variable frequency motor is characterized in that: including converter, motor, current transformer CT1, CT2 and CT3, CT1, converter, CT2, motor and CT3 establish ties in proper order, the converter is equipped with the bypass, the bypass is equipped with the bypass circuit breaker, the closing signal contact access differential protection device of bypass circuit breaker, CT2 and CT3 constitute differential protection when the bypass circuit breaker breaks off, and CT1 and CT3 constitute differential protection when the bypass circuit breaker is closed.
2. According to a differential protection device of a variable frequency motor, characterized in that: the current transformers CT2 and CT3 are both broadband current transformers.
3. A differential protection algorithm of a variable frequency motor is characterized by comprising the following steps:
step 000: the automatic switching device comprises a frequency converter, a motor, a current transformer CT1, a CT2 and a CT3, wherein the CT1, the frequency converter, the CT2, the motor and the CT3 are sequentially connected in series, the frequency converter is provided with a bypass, the bypass is provided with a bypass breaker, a closing signal contact of the bypass breaker is connected into a differential protection device, the CT2 and the CT3 form differential protection when the bypass breaker is disconnected, and the CT1 and the CT3 form differential protection when the bypass breaker is closed;
step 010: sampling value differential is adopted, and sampling points of all channels are instantaneous values of current at the same moment;
step 020: the vector sum of the currents measured by the current transformers forming the differential protection is differential current, the braking current is calculated according to half of the vector difference of the currents on two sides, the differential quick-break and the ratio differential adopt an abrupt change starting element and an overcurrent starting element, when the differential current is abruptly changed or the maximum value of the differential current is larger than a corresponding overcurrent fixed value, the starting element acts and keeps 10s, and a starting relay is opened.
4. The differential protection algorithm of the variable frequency motor according to claim 3, characterized in that: when the bypass breaker is opened, the balance coefficient K is passedph2And the problem of inconsistent transformation ratios of the current transformers on the two sides of the motor is solved.
5. The differential protection algorithm of the variable frequency motor according to claim 3, characterized in that: when the bypass breaker is closed, pass the equilibrium coefficient Kph1And the problem of inconsistent transformation ratios of the current transformers on the two sides of the motor is solved.
6. The differential protection algorithm of the variable frequency motor according to claim 3, characterized in that: the criterion of the differential quick-break protection action is that the maximum value in the three-phase differential current is greater than the differential quick-break protection fixed value.
7. The differential protection algorithm of the variable frequency motor according to claim 3, characterized in that: the ratio differential adopts a three-fold line ratio differential principle.
8. The differential protection algorithm of the variable frequency motor according to claim 3, characterized in that: the system also comprises an algorithm for monitoring the disconnection of the current transformer, and the judgment logic of the algorithm is that one-phase or two-phase current of the three-phase current on any side of the system is less than 0.125 times of rated current, and the other two-phase or one-phase current is more than 0.2 times of rated current.
9. The differential protection algorithm for the variable frequency motor according to claim 8, wherein: the algorithm for monitoring the disconnection of the current transformer is started after the ratio differential element is started.
10. The differential protection algorithm for the variable frequency motor according to claim 8, wherein: the algorithm for monitoring the disconnection of the current transformer meets any one of the following three conditions without discrimination:
(1) before starting, if the maximum phase current of a certain side is less than 0.2Ie, the disconnection judgment of the current transformer of the side is not carried out;
(2) the maximum phase current after starting is more than 1.2 Ie;
(3) the current on either side increases after start-up compared to before start-up.
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| CN111208418A (en) * | 2020-01-10 | 2020-05-29 | 中国南方电网有限责任公司超高压输电公司广州局 | Phase selection switching-on and switching-off state monitoring system and method for converter station alternating current filter |
| CN111208418B (en) * | 2020-01-10 | 2022-05-13 | 中国南方电网有限责任公司超高压输电公司广州局 | Phase selection switching-on and switching-off state monitoring system and method for converter station alternating current filter |
| CN111999583A (en) * | 2020-08-24 | 2020-11-27 | 南京工程学院 | Fault trip judging method of safety and stability control device suitable for alternating current power grid |
| CN111999583B (en) * | 2020-08-24 | 2023-03-28 | 南京工程学院 | Fault trip judging method of safety and stability control device suitable for alternating current power grid |
| CN112653108A (en) * | 2020-12-23 | 2021-04-13 | 国网江苏省电力有限公司镇江供电分公司 | Operation protection method for replacing transformer circuit breaker with bypass circuit breaker without shutdown protection |
| CN112653108B (en) * | 2020-12-23 | 2022-06-10 | 国网江苏省电力有限公司镇江供电分公司 | Operation protection method for replacing transformer circuit breaker with bypass circuit breaker without shutdown protection |
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Application publication date: 20191220 |