A new differential protection method for variable frequency motors

Abstract

The present invention relates to a novel variable frequency motor differential protection method, comprising the following steps: Step 1: Collect analog quantity: set multiple sampling points between the high voltage switch port and the neutral point of the motor, respectively collect the three-phase power frequency voltage, three-phase power frequency current, three-phase variable frequency voltage and three-phase variable frequency current of the motor; Step 2: use a signal processing algorithm for the analog quantity in step 1, respectively perform orthogonal transformation on the three-phase power frequency voltage and the three-phase power frequency current to obtain the instantaneous active power on the power frequency side; perform orthogonal transformation on the three-phase variable frequency voltage and the three-phase variable frequency current to obtain the instantaneous active power on the frequency conversion side; Step 3: determine whether the motor has differential protection under variable frequency working and power frequency working conditions. The protection range of the variable frequency motor of the above technical solution is from the outgoing circuit breaker to the neutral point of the motor, and according to the auxiliary node of the bypass switch, the working state of the motor can be automatically determined, and the protection can be automatically switched, thereby improving the reliability and safety of the inverter electric relay protection.

Description

Technical Field

[0001] The invention relates to the field of relay protection of electric power systems, and in particular to a novel variable frequency motor differential protection method. Background Art

[0002] For power plants, the power consumption rate is a key indicator for assessing their operating level. The power consumption rate of general thermal power plants is about 5%. Under the premise of ensuring the safe and reliable operation of the units, reducing the power consumption rate and thus reducing the power generation cost and improving the price competitiveness when connected to the grid has become one of the goals pursued by various power generation companies. Frequency conversion of motors is a very effective means to reduce the power consumption rate. Based on the advantages of frequency conversion speed regulation, more and more power plants are beginning to transform high-voltage motors with frequency conversion speed regulation technology. Newly built units of 300MW and above in China all use high-voltage frequency conversion devices (such as condensate pumps, three major fans, feed water pumps, etc.) to varying degrees, which is universal. The relay protection of the high-voltage frequency conversion speed regulation system is the cornerstone to ensure the safe and reliable operation of its equipment. So far, this protection function has been relatively weak, and it is mainly aimed at the protection of power electronic devices. For high-voltage motors, this protection is difficult to meet the actual operation requirements.

[0003] At present, variable frequency driven high-voltage motors all use current quick-break protection as the main protection for the motor, which is not sensitive enough for high-voltage motor protection and does not comply with the provisions of the national relay protection regulations. Therefore, after using the frequency converter to drive the high-voltage motor, the research and application of differential protection for high-voltage motors has important practical significance and broad application prospects.

[0004] The Technical Regulations for Power Design of Thermal Power Plants stipulate that motors of 2MW and above should be equipped with motor differential protection. For motors with split-phase leads at the neutral point below 2MW, motor differential protection should also be installed when the current quick-break protection is not sensitive enough.

[0005] The problems of the current traditional differential protection solution for variable frequency motors are as follows: (1) The protection device used on site collects the current of the switch cabinet CT and the CT at the neutral point of the motor. When the motor is running at the industrial frequency, the differential protection of the protection device works normally. When the motor is switched to the variable frequency operation mode, the CT on the circuit breaker side is the 50HZ current, while the CT on the motor side is the current after the frequency conversion. The frequencies are different. At this time, if the original differential protection is activated, its protection range will not only be reduced to the AC part before the frequency converter, but also the differential principle of the original differential protection device is only based on the AC current with a frequency of 50HZ. Since the CT on the motor side is not a constant 50HZ current, it will cause the differential protection to malfunction and trip the power circuit breaker. Therefore, the method adopted by the power plant at this time is to withdraw the differential protection pressure plate and activate the quick-break protection function of the original protection device, using the quick-break protection as the main protection for the motor during variable frequency operation. This does not comply with the provisions of the technical regulations for relay protection and safety automatic devices. (2) High-voltage motors have no reliable backup protection under variable frequency conditions. The equivalent impedance of the high-voltage variable frequency speed regulation system changes greatly. Due to the DC isolation effect in the AC-DC link of the inverter, the scope of comprehensive protection is narrowed to the AC part before the inverter. The inverter output cable and motor are basically in an unprotected state and can only rely on the protection inside the inverter. The protection configured for high-voltage inverters is for the inverter power components, not the motor, and cannot guarantee that it can respond to various motor faults under any circumstances. Moreover, its set value and characteristics cannot be verified on site. (3) Conventionally configured TA has low-frequency saturation phenomenon. The traditional electromagnetic TA uses the principle of electromagnetic induction to achieve primary and secondary current conversion through iron core coupling. Because the iron core has magnetic saturation characteristics, it is not suitable for operation under wide-frequency conditions, especially under low-frequency conditions. It is very easy to saturate, thus affecting the correctness of the protection device.

[0006] There are great limitations in the practical application of magnetic balance differential protection: (1) Due to the characteristics of the installation of magnetic balance current transformers, magnetic balance differential protection is only aimed at internal faults of the motor. The power supply line of the motor is not within the scope of magnetic balance differential protection and needs to be equipped with separate line protection. (2) Since the magnetic balance current transformer is installed on the motor body, if the protection device is far away from the motor, the secondary cable will be relatively long. In this case, if the capacity of the magnetic balance current transformer is selected to be relatively small, the secondary load of the current transformer may not match, and the protection will fail to operate. (3) Due to the structural characteristics of the magnetic balance current transformer (core-through structure), when it is installed on the motor body, a main junction box must be used. Due to the limitation of electrical clearance, the main junction box cannot be made too small. Therefore, it is impossible to install a magnetic balance transformer for motors with a low center height, and thus it is impossible to adopt magnetic balance differential protection. Summary of the invention

[0007] The purpose of the present invention is to provide a novel variable frequency motor differential protection method to solve the problem that the existing variable frequency motor has a small protection range and cannot automatically judge the working state of the motor.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A novel variable frequency motor differential protection method comprises the following steps:

[0010] Step 1: Collecting analog quantities: setting multiple sampling points between the high-voltage switch port and the neutral point of the motor to respectively collect the three-phase power frequency voltage, three-phase power frequency current, three-phase variable frequency voltage and three-phase variable frequency current of the motor;

[0011] Step 2: using a signal processing algorithm for the analog quantity in step 1, respectively performing orthogonal transformation on the three-phase power frequency voltage and the three-phase power frequency current to obtain instantaneous active power on the power frequency side; performing orthogonal transformation on the three-phase variable frequency voltage and the three-phase variable frequency current to obtain instantaneous active power on the variable frequency side;

[0012] Step 3: Determine whether the motor has differential protection under variable frequency working conditions and under power frequency working conditions:

[0013] When the bypass switch is open, the motor protection is in the instantaneous active power differential protection mode, and the instantaneous active power on the power frequency side and the instantaneous active power on the variable frequency side are subjected to the instantaneous active power differential element. When the condition is met, the output is true. When the condition is true and the bypass switch opening condition is met, the protection outlet trips and outputs the variable frequency differential action signal.

[0014] When the bypass switch is closed, the three-phase power frequency current and the three-phase variable frequency current are respectively calculated for phase current differential element protection. When at least one differential element is actuated, the condition is true. When the condition is true, the bypass switch closing condition, and the CT disconnection condition are met, the protection outlet trips and outputs the power frequency differential action signal.

[0015] Preferably, the three-phase power frequency voltage is collected by a voltage transformer arranged on the bus, which is U a工频 , U b工频 and U c工频 The three-phase power frequency current is collected by a current sensor provided at the high voltage outlet of the motor, which are I a工频 ,I b工频 and I c工频 The three-phase variable frequency voltage is collected by the frequency converter with a wide-band voltage sensor, which are U a变频 , U b变频 and U c变频The three-phase variable frequency current is collected by a broadband current sensor arranged at the neutral point of the motor, and is respectively I a变频 ,I b变频 and I c变频 .

[0016] Preferably, the three-phase power frequency voltage U a工频 , U b工频 and U c工频 Perform an orthogonal transformation: Get the instantaneous voltage U of phase α and phase β α工频 , U β工频 ;

[0017] The three-phase power frequency current I a工频 ,I b工频 and I c工频 Perform an orthogonal transformation: Get the instantaneous current I of phases α and β α工频 ,I β工频 .

[0018] Preferably, the instantaneous currents of the α and β phases I α工频 ,I β工频 And the instantaneous voltage U of the α and β phases α工频 , U β工频 Orthogonal transformation: Get the power frequency instantaneous active power p 工频 , Instantaneous reactive power q 工频 .

[0019] Preferably, the three-phase variable frequency voltage U a变频 , U b变频 and U c变频 Perform an orthogonal transformation: Get the instantaneous voltage of phases α and β on the inverter side

[0020] The three-phase variable frequency current I a变频 ,I b变频 and I c变频 Perform an orthogonal transformation: The instantaneous current I of the α and β phases on the inverter side is obtained α变频 ,I β变频 .

[0021] Preferably, the instantaneous currents of the α and β phases I α变频 ,I β变频 And the instantaneous voltage of α and β phases Orthogonal transformation: Get the instantaneous active power p on the frequency conversion side 变频 , Instantaneous reactive power q 变频 .

[0022] Preferably, the instantaneous active differential protection action equation of the motor under variable frequency working conditions is: Among them, P 启动值 The minimum instantaneous active action value set by the user, p 差值 is the instantaneous active power difference between the power frequency side and the variable frequency side, p 差值 =|p 工频侧 -p 变频侧 |;p 工频侧 is the instantaneous active power at the switch outlet, p 变频侧 is the instantaneous active power of the motor neutral point, p 制动 is the instantaneous active braking amount, p 制动 =|P 工频侧 +P 变频侧 | / 2;p 拐点 is the inflection point value; K 斜率 is the slope of the action curve.

[0023] Preferably, the action equation of the current differential protection element of the motor under the power frequency working condition is: Among them, I 启动值 The minimum current action value set by the user, I 差值 is the current difference between the power frequency side and the variable frequency side, is the switch output current value, The effective value of the current measured at the neutral point of the motor; I 制动 is the current braking amount, I 拐点 is the inflection point value; K 斜率 is the slope of the action curve.

[0024] Preferably, the p 启动值 It is 0.1 times of the rated instantaneous power of the motor, K 斜率 is 0.5, p 拐点 It is 0.7 to 1 times the rated instantaneous power of the motor.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The novel variable frequency motor differential protection method provided in the above technical solution extends the protection range of the variable frequency motor from the outgoing circuit breaker to the neutral point of the motor, and can automatically judge the working state of the motor and automatically switch the protection according to the auxiliary node of the bypass switch, thereby improving the reliability and safety of the inverter electric relay protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 A novel variable frequency motor differential protection action logic block diagram provided in an embodiment of the present invention.

[0029] Figure 2 The instantaneous active differential protection action curve under the motor variable frequency working condition provided by the embodiment of the present invention.

[0030] Figure 3 The present invention provides a current differential protection action curve for a motor under power frequency working conditions.

[0031] Figure 4 A schematic diagram of an analog quantity acquisition method provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0034] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0035] The embodiment of the present invention provides a novel variable frequency motor differential protection method, as shown in the attached Figure 1 As shown in the figure, when the bypass switch is turned on, the motor protection is in the instantaneous active power differential protection mode. The new variable frequency motor differential protection method includes the following steps:

[0036] Step 1: Collect analog quantities, including:

[0037] Three-phase power frequency voltage U a工频 , U b工频 and U c工频 ;

[0038] Three-phase power frequency current I a工频 ,I b工频 and I c工频 ;

[0039] Three-phase variable frequency voltage U a变频 , U b变频 and U c变频 ;

[0040] Three-phase variable frequency current I a变频 ,I b变频 and I c变频 ;

[0041] Among them, the three-phase industrial frequency voltage is collected through the voltage transformer installed on the bus, the three-phase industrial frequency current is collected through the current sensor installed on the high-voltage outlet of the motor, the three-phase variable frequency voltage is collected through the wide-band voltage sensor installed on the inverter, and the three-phase variable frequency current is collected through the wide-band current sensor installed at the neutral point of the motor.

[0042] Step 2: Three-phase power frequency voltage U a工频 , U b工频 and U c工频 Perform an orthogonal transformation: Get the instantaneous voltage U of phase α and phase β α工频 , U β工频 ;

[0043] For three-phase power frequency current I a工频 ,I b工频 and I c工频 Perform an orthogonal transformation: Get the instantaneous current I of phases α and β α工频 ,I β工频 ;

[0044] The α and β phase instantaneous current I α工频 ,I β工频 And the instantaneous voltage U of the α and β phases α工频 , U β工频 Orthogonal transformation: Get the power frequency instantaneous active power p工频 , Instantaneous reactive power q 工频 .

[0045] Step 3: The three-phase variable frequency voltage U a变频 , U b变频 and U c变频 Perform an orthogonal transformation: Get the instantaneous voltage of phases α and β on the inverter side

[0046] The three-phase variable frequency current I a变频 ,I b变频 and I c变频 Perform an orthogonal transformation: The instantaneous current I of the α and β phases on the inverter side is obtained α变频 ,I β变频

[0047] The α and β phase instantaneous current I α变频 ,I β变频 And the instantaneous voltage of α and β phases Orthogonal transformation: Get the instantaneous active power p on the frequency conversion side 变频 , Instantaneous reactive power q 变频 .

[0048] Step 3: Calculate the instantaneous active power p on the power frequency side 工频 , Instantaneous active power on the frequency conversion side p 变频 The instantaneous active power differential element is calculated, and the instantaneous active power differential protection action equation under variable frequency working conditions is: Among them, P 启动值 The minimum instantaneous active action value set by the user, p 差值 is the instantaneous active power difference between the power frequency side and the variable frequency side, p 工频侧 is the instantaneous active power at the switch outlet, p 变频侧 is the instantaneous active power of the motor neutral point, p 差值 =|p 工频侧 -p 变频侧 |;p 制动 is the instantaneous active braking amount, p 制动 =|P 工频侧 +P 变频侧 | / 2;p 拐点 is the inflection point value; K 斜率 is the slope of the action curve.

[0049] Among them, p 启动值 It is 0.1 times of the rated instantaneous power of the motor, K 斜率 is 0.5, p 拐点 It is 0.7 to 1 times the rated instantaneous power of the motor.

[0050] The above conditions are met and the output is true.

[0051] When the output is true and the bypass switch opening condition is met, the protection output trips and outputs the variable frequency differential action signal.

[0052] As attached Figure 1 As shown, when the bypass switch is closed, the motor protection is in the current differential protection mode. The new variable frequency motor differential protection method includes the following steps:

[0053] Step 1: Collect analog quantities, including:

[0054] Three-phase power frequency voltage U a工频 , U b工频 and U c工频 ;

[0055] Three-phase power frequency current I a工频 ,I b工频 and I c工频 ;

[0056] Three-phase variable frequency voltage U a变频 , U b变频 and U c变频 ;

[0057] Three-phase variable frequency current I a变频 ,I b变频 and I c变频 ;

[0058] Among them, the three-phase industrial frequency voltage is collected through the voltage transformer installed on the bus, the three-phase industrial frequency current is collected through the current sensor installed on the high-voltage outlet of the motor, the three-phase variable frequency voltage is collected through the wide-band voltage sensor installed on the inverter, and the three-phase variable frequency current is collected through the wide-band current sensor installed at the neutral point of the motor.

[0059] Step 2: Three-phase power frequency current I a工频 ,I b工频 and I c工频 Perform an orthogonal transformation: Get the instantaneous current I of phases α and β α工频 ,I β工频 ;

[0060] The three-phase variable frequency current I a变频 ,I b变频 and I c变频 Perform an orthogonal transformation: The instantaneous current I of the α and β phases on the inverter side is obtained α变频 ,I β变频 ;

[0061] The action equation of the current differential protection element of the motor under the power frequency working condition is: Among them, I 启动值 The minimum current action value set by the user, I 差值 is the current difference between the power frequency side and the variable frequency side, is the switch output current value, The effective value of the current measured at the neutral point of the motor; I 制动 is the current braking amount, I 拐点 is the inflection point value; K 斜率 is the slope of the action curve.

[0062] Among them, I 拐点 K is the inflection point value, and the user can set this parameter by himself; 斜率 The slope of the action curve. Users can set this parameter by themselves. 斜率 Usually 0.5.

[0063] When at least one differential element is activated, the output is true.

[0064] When the output is true, when the bypass switch is closed and the CT is not disconnected, the protection output trips and outputs the power frequency differential action signal.

[0065] The protection range of the variable frequency motor in the above technical solution is from the outgoing circuit breaker to the neutral point of the motor, and according to the auxiliary node of the bypass switch, the working state of the motor can be automatically judged and the protection can be automatically switched, thereby improving the reliability and safety of the inverter electric relay protection.

[0066] The above-mentioned embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by technicians in this field on the basis of the present invention shall fall within the scope of protection required by the present invention.

Claims

1. A novel variable frequency motor differential protection method, characterized in that: The following steps are involved: Step 1: Collecting analog quantities: setting multiple sampling points between the high-voltage switch port and the neutral point of the motor to respectively collect the three-phase power frequency voltage, three-phase power frequency current, three-phase variable frequency voltage and three-phase variable frequency current of the motor; Step 2: using a signal processing algorithm for the analog quantity in step 1, respectively performing orthogonal transformation on the three-phase power frequency voltage and the three-phase power frequency current to obtain instantaneous active power on the power frequency side; performing orthogonal transformation on the three-phase variable frequency voltage and the three-phase variable frequency current to obtain instantaneous active power on the variable frequency side; Step 3: Determine whether the motor has differential protection under variable frequency working conditions and under power frequency working conditions: When the bypass switch is turned on, the motor protection is in the instantaneous active power differential protection mode, and the instantaneous active power on the power frequency side and the instantaneous active power on the variable frequency side are calculated by the instantaneous active power differential element protection. When the condition is met, the output is true. When the condition is true and the bypass switch opening condition is met, the protection outlet trips and outputs the variable frequency differential action signal. When the bypass switch is closed, the three-phase power frequency current and the three-phase variable frequency current are respectively calculated for phase current differential element protection. When at least one differential element is actuated, the condition is true. When the condition is true, the bypass switch closing condition and the CT disconnection condition are met, the protection outlet trips and outputs a power frequency differential action signal. The instantaneous active differential protection action equation of the motor under variable frequency working conditions is: Among them, P 启动值 The minimum instantaneous active action value set by the user, p 差值 is the instantaneous active power difference between the power frequency side and the variable frequency side, p 差值 =|p 工频侧 -p 变频侧 |;p 工频侧 is the instantaneous active power at the switch outlet, ρ 变频侧 is the instantaneous active power of the motor neutral point, p 制动 is the instantaneous active braking amount, p 制动 =|P 工频侧 +P 变频侧 | / 2;p 拐点 is the inflection point value; K 斜率 is the slope of the action curve; The action equation of the current differential protection element of the motor under the power frequency working condition is: Among them, I 启动值 The minimum current action value set by the user, I 差值 is the current difference between the power frequency side and the variable frequency side, is the switch output current value, The effective value of the current measured at the neutral point of the motor; I 制动 is the current braking amount, I 拐点 is the inflection point value; K 斜率 is the slope of the action curve.

2. The novel variable frequency motor differential protection method according to claim 1 is characterized in that: The three-phase power frequency voltage is collected by a voltage transformer arranged on the bus, which is U a工频 , U b工频 and U c工频 The three-phase power frequency current is collected by a current sensor provided at the high voltage outlet of the motor, which are I a工频 ,I b工频 and I c工频 The three-phase variable frequency voltage is collected by the frequency converter with a wide-band voltage sensor, which are U a变频 , U b变频 and U c变频 The three-phase variable frequency current is collected by a broadband current sensor arranged at the neutral point of the motor, and is respectively I a变频 ,I b变频 and I c变频 .

3. The novel variable frequency motor differential protection method according to claim 1 is characterized in that: The three-phase power frequency voltage U a工频 , U b工频 and U c工频 Perform an orthogonal transformation: Get the instantaneous voltage U of phase α and phase β α工频 , U β工频 ; The three-phase power frequency current I a工频 ,I b工频 and I c工频 Perform an orthogonal transformation: Get the instantaneous current I of phases α and β α工频 ,I β工频 .

4. The novel variable frequency motor differential protection method according to claim 3 is characterized in that: The α and β phase instantaneous current I α工频 ,I β工频 And the instantaneous voltage U of the α and β phases α工频 , U β工频 Orthogonal transformation: Get the power frequency instantaneous active power p 工频 , Instantaneous reactive power q 工频 .

5. The novel variable frequency motor differential protection method according to claim 1 is characterized in that: The three-phase variable frequency voltage U a变频 , U b变频 and U c变频 Perform an orthogonal transformation: Get the instantaneous voltage of phases α and β on the inverter side The three-phase variable frequency current I a变频 ,I b变频 and I c变频 Perform an orthogonal transformation: The instantaneous current I of the α and β phases on the inverter side is obtained α变频 ,I β变频 .

6. The novel variable frequency motor differential protection method according to claim 5 is characterized in that: The α and β phase instantaneous current I α变频 ,I β变频 And the instantaneous voltage of α and β phases Orthogonal transformation: Get the instantaneous active power p on the frequency conversion side 变频 , Instantaneous reactive power q 变频 .

7. The novel variable frequency motor differential protection method according to claim 1 is characterized in that: The p 启动值 It is 0.1 times of the rated instantaneous power of the motor, K 斜率 is 0.5, p 拐点 It is 0.7 to 1 times the rated instantaneous power of the motor.

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