CN110112743B - Isolation type variable frequency transformer and starting control and capacity expansion method thereof - Google Patents
Isolation type variable frequency transformer and starting control and capacity expansion method thereof Download PDFInfo
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- CN110112743B CN110112743B CN201910270897.2A CN201910270897A CN110112743B CN 110112743 B CN110112743 B CN 110112743B CN 201910270897 A CN201910270897 A CN 201910270897A CN 110112743 B CN110112743 B CN 110112743B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
- H02J3/08—Synchronising of networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P21/00—Arrangements or methods for the control of electric machines by vector control, e.g. by control of field orientation
- H02P21/22—Current control, e.g. using a current control loop
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P21/00—Arrangements or methods for the control of electric machines by vector control, e.g. by control of field orientation
- H02P21/34—Arrangements for starting
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P6/00—Arrangements for controlling synchronous motors or other dynamo-electric motors using electronic commutation dependent on the rotor position; Electronic commutators therefor
- H02P6/20—Arrangements for starting
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Abstract
The invention relates to an isolation type variable frequency transformer and a starting control and capacity expansion method thereof, wherein the transformer comprises a synchronous motor and a wound rotor asynchronous motor, the synchronous motor and the wound rotor asynchronous motor are connected through a coupling unit, the secondary side of the transformer is directly led out from a stator winding of the synchronous motor, the primary side of the transformer is directly led out from a stator winding of the wound rotor asynchronous motor, a back-to-back converter is further connected between the stator winding of the wound rotor asynchronous motor and a rotor winding thereof, the rotor winding of the wound rotor asynchronous motor is further connected with a starting unit, and the starting unit and the back-to-back converter are both connected with a detection and control unit. Compared with the prior art, the invention has the advantages of capability of realizing secondary side variable voltage frequency conversion, flexible configuration of required frequency reference value according to application occasions, wider secondary side frequency conversion range, electromagnetic isolation of primary side and secondary side, no pollution of power electronic harmonic wave on the secondary side, stronger overload capability, capability of realizing on-load starting of the synchronous motor, and the like.
Description
Technical Field
The invention relates to the technical field of transformers, in particular to an isolation type variable frequency transformer and a starting control and capacity expansion method thereof.
Background
The prior frequency conversion technology between alternating current electric energy mainly has two schemes, namely, the first type of frequency conversion device is composed of power electronic devices, and the electric energy which is required to be converted generally needs to pass through the frequency conversion device, namely, the power of the frequency conversion device is equal to the power of the converted electric energy; the second scheme is a variable frequency transformer, wherein a three-phase rotor winding and a three-phase stator winding of a wound-rotor type asynchronous motor are adopted to respectively form a primary side and a secondary side of the variable frequency transformer, and a variable speed driving device is adopted to drive a rotor of the wound-rotor type asynchronous motor. Compared with the two schemes, the full-power frequency conversion scheme formed by the power electronic devices has the highest control degree of freedom, and can realize a wide frequency conversion range; the existing frequency conversion transformer scheme is mainly used for connecting two asynchronous alternating current power supplies with close frequency, and flow control between the two asynchronous alternating current power supplies is achieved by controlling the rotating speed through a variable speed driving device. The invention and the variable frequency transformer are basically the same as rotary transformers, so the variable frequency transformer is the implementation scheme which is the most similar to the invention.
The main defects of the existing variable frequency transformer are as follows: 1. the frequency conversion reference value is fixed. When the frequency on one side is substantially determined, the frequency on the other side can only be varied on the basis of the frequency. Assuming that the primary frequency is f, the secondary frequency can only float up and down at f; 2. the frequency conversion range is narrower. The suitable rotating speed working range is near 0 speed (slip ratio is close to 1), and the device is not suitable for the situation that the primary side and the secondary side have very different frequencies. This is because when the primary and secondary frequencies differ greatly, the slip ratio of the wound-rotor asynchronous motor is small in absolute value, slip power flowing through the rotor winding is small, and most of the power needs to be provided by mechanical power of a variable-speed driving device (a frequency conversion device usually formed by power electronic devices), so that the capacity requirement of the variable-speed driving device is increased; 3. electromagnetic coupling exists between the primary side and the secondary side of the existing variable frequency transformer, and the disturbance of any side can directly affect the other side.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide an isolation type variable frequency transformer and a starting control and capacity expansion method thereof. Functionally, the main application scenario of the existing variable frequency transformer is to connect two asynchronous alternating current power grids with similar frequencies, and the main application scenario of the variable frequency transformer is variable frequency speed regulation, namely one end is a power grid, and the other end is an alternating current motor.
The aim of the invention can be achieved by the following technical scheme:
the utility model provides an isolation type variable frequency transformer, this transformer includes synchronous motor and wound rotor asynchronous motor, synchronous motor with be connected through the shaft coupling unit between the wound rotor asynchronous motor, the secondary of this transformer is by synchronous motor's stator winding is direct draws forth, and its primary is by wound rotor asynchronous motor's stator winding is direct draws forth, wound rotor asynchronous motor's stator winding still is connected between its rotor winding and is provided with back-to-back converter, wound rotor asynchronous motor's rotor winding still is connected and is provided with the start-up unit, the start-up unit with back-to-back converter all be connected with detection and control unit, can produce voltage and frequency continuously adjustable and have constant voltage frequency ratio characteristic's alternating voltage through control at the secondary, back-to-back converter includes machine side converter and net side converter, back-to-back converter passes through machine side converter with wound rotor asynchronous motor's rotor winding is connected, it is through the net side converter is connected with rotor asynchronous motor's stator winding through the transformer of optional configuration.
Further, the starting unit comprises a circuit breaker, a three-phase rectifier bridge, a fixed resistor and a variable resistor, wherein the fixed resistor and the variable resistor are connected in parallel to the direct current end of the three-phase rectifier bridge, the alternating current end of the three-phase rectifier bridge is connected to the rotor winding of the wound rotor asynchronous motor through the circuit breaker, the variable resistor comprises a full-control switch and a fixed resistor, the fixed resistor and the full-control switch are connected in series, and the three-phase rectifier bridge is composed of a plurality of diodes.
Further, the synchronous motor adopts a permanent magnet synchronous motor or an electric excitation synchronous motor, and when the synchronous motor is the electric excitation synchronous motor, a direct current bus of the back-to-back converter is connected with a synchronous motor excitation unit in a bridging way.
Further, the ac frequency reference value of the secondary side of the transformer has a calculation formula as follows:
f 0 ＝p SG f 1 /(p WRIM n gb )
wherein f 0 For the ac frequency reference value, f 1 For primary AC frequency, p SG For the pole pair number, n, of synchronous machines gb For the gear ratio of the coupling unit, p WRIM Is the pole pair number of the wound rotor asynchronous motor.
Further, the rotating speed of the wound rotor asynchronous motor is controlled by the back-to-back converter to change in the normal working rotating speed range, and the alternating voltage with continuously adjustable voltage and frequency and constant voltage frequency ratio characteristics can be obtained at the secondary side of the isolation type variable frequency transformer, wherein the calculation formula of the secondary side frequency of the transformer is as follows:
wherein f is the secondary frequency, ω WRIM The rotating speed of the wound rotor asynchronous motor is obtained.
Further, the adjustable range of the secondary frequency of the transformer is as follows:
(1-s max )f 0 ＜f＜(1+s max )f 0
s max ＝U VSC /(k r/s U s )
in U VSC Effective value k of maximum alternating voltage provided by alternating current ports of back-to-back converters r/s U is the ratio of the number of turns of a rotor winding to the number of turns of a stator winding of the wound rotor asynchronous motor s The voltage of an alternating current power supply connected with a stator winding of the wound rotor asynchronous motor.
The invention also provides a starting control method adopting the isolation type variable frequency transformer, which realizes the starting of preset acceleration by means of the starting unit, and switches the isolation type variable frequency transformer from a static state to a normal working rotating speed and then controls the isolation type variable frequency transformer by the back-to-back converter, and comprises the following steps:
step 1: pre-charging a direct current bus capacitor through power grid vector control of a grid-side converter in the back-to-back converter and stabilizing the direct current bus capacitor at a rated value of the direct current bus capacitor, so that a machine-side converter in the back-to-back converter is in a locking state;
step 2: the detection and control unit detects the rotating speed of the wound rotor asynchronous motor in real time and judges whether the rotating speed is smaller than the normal working rotating speed, if so, a breaker in the starting unit is kept closed, and a locking state of a machine side converter in the back-to-back converter is maintained;
step 3: if not, the full-control switch in the variable resistor in the starting unit is in an off state, the locking state of the machine side converter in the back-to-back converter is relieved, the vector control of the wound rotor asynchronous motor is adopted to enable the motor to normally work for a period of time, and then the circuit breaker in the starting unit is disconnected, so that the starting of the isolation type variable-frequency transformer is completed.
Preferably, the step 2 specifically includes: and the detected rotating speed of the wound rotor asynchronous motor is subjected to low-pass filtering to remove noise, then differential is obtained, and the error of the differentiated rotating speed and the acceleration reference value of the wound rotor asynchronous motor is subjected to PI regulator and then is compared with a triangular carrier wave to obtain a pulse width modulation signal of a full-control switch in a variable resistor in the starting unit.
The invention also provides a capacity expansion method adopting the isolation type variable frequency transformer, which comprises the steps of respectively connecting the primary side and the secondary side of a plurality of sets of isolation type variable frequency transformers in parallel or utilizing a power distribution device to collect power on the mechanical rotating shafts of a plurality of wound rotor asynchronous motors so as to drag one synchronous motor.
The coordination control method corresponding to the capacity expansion method comprises the following steps: selecting one set of isolation type variable frequency transformers as a main variable frequency transformer, and using the rest sets as auxiliary variable frequency transformers, wherein the d-axis of vector control of a machine side converter of the main variable frequency transformer adopts three-loop control of rotating speed loop, torque loop and current loop, and the reference value of the rotating speed loop
According to the secondary side alternating current frequency reference value f * Setting, namely: />
The d-axis of the vector control of the side converter of the variable frequency transformer adopts torque closed loop-current closed loop double-loop control, and the torque command is distributed according to a set principle, namely: the total power P shared by multiple sets of slave variable frequency transformers should satisfy P e∑ *(m-1)/m≤P<P e∑ Wherein m is the total number of sleeves of the isolation type variable frequency transformer, and P e∑ The total power transmitted by a plurality of sets of isolation type variable frequency transformers is provided.
Compared with the prior art, the invention has the following advantages:
(1) The main application of the isolation type variable frequency transformer is to drag one or more AC motors by the secondary side, realize the open-loop constant voltage frequency ratio control of the one or more AC motors, and be applicable to multi-motor variable frequency speed regulation occasions with low speed regulation performance requirements of ventilators, chemical spinning and the like.
(2) Can realize a wider frequency conversion range, and the required back-to-back converter capacity is only S max *P n Wherein P is n For isolating the rated capacity of the transformer (S max Is typically 0.3 to 0.5)
(3) The disclosed starting unit can realize the starting of preset acceleration, not only limits starting torque, starting power and starting current, but also can establish the alternating voltage with slowly rising frequency at the secondary side of the isolation type variable frequency transformer, so that the on-load starting of the synchronous motor can be realized at the secondary side.
(4) The primary side and the secondary side of the transformer are electromagnetically isolated, so that the transformer has strong overload capacity, and the secondary side is not polluted by power electronic harmonic waves.
Drawings
FIG. 1 is a schematic diagram of an isolated variable frequency transformer employing a permanent magnet synchronous motor;
FIG. 2 is a schematic diagram of an isolated side frequency transformer employing an electro-magnetic synchronous motor;
FIG. 3 is a schematic diagram of a process for startup control of an isolated variable frequency transformer;
FIG. 4 is a schematic diagram of a parallel capacity expansion connection of multiple sets of isolated variable frequency transformers;
FIG. 5 is a schematic illustration of a capacity expansion connection using a power distribution device;
FIG. 6 is a schematic diagram of the variation of the related parameters of the isolated variable frequency transformer when two load synchronous motors are idle in the simulation of the present invention, wherein FIG. 6-a is a schematic diagram of the variation of the rotation speed, FIG. 6-b is a schematic diagram of the variation of the secondary frequency and the voltage, FIG. 6-c is a schematic diagram of the variation of the rotation speed of the secondary load motor, and FIG. 6-d is a schematic diagram of the variation of the secondary three-phase voltage;
fig. 7 is a schematic diagram of the change of related parameters of the isolated variable frequency transformer when two load synchronous motors have load torque in the invention, wherein fig. 7-a is a schematic diagram of the change of the rotation speed, fig. 7-b is a schematic diagram of the change of the secondary frequency and the voltage, fig. 7-c is a schematic diagram of the change of the rotation speed of the secondary load motor, and fig. 7-d is a schematic diagram of the change of the secondary three-phase voltage.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are some, but not all embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.
Examples
The embodiment of the invention comprises a constitution, a starting method and a capacity expansion method of an isolated variable frequency transformer.
A first part: isolation type variable frequency transformer
The isolation type variable frequency transformer is composed of the following components: (1) a wound rotor asynchronous motor; (2) the synchronous motor can be a permanent magnet synchronous motor or an electric excitation synchronous motor, and when the electric excitation synchronous motor is adopted, the synchronous motor also comprises a synchronous motor excitation unit; (3) a coupling unit comprising a coupling and an optional gearbox; (4) a back-to-back converter comprising a grid-side converter and a machine-side converter; (5) a detection and control unit; (6) the transformer is used for reducing the voltage level of the back-to-back voltage source type converter and can be used for selecting whether the transformer is equipped or not according to the needs; (7) starting unit comprising a circuit breaker T 1 The three-phase uncontrolled rectifier bridge is composed of diodes, a fixed resistor and a controllable resistor. Wherein the resistor R is fixed 1 The resistor with larger resistance is adopted, so that a rotor winding of the wound rotor asynchronous motor keeps a passage in the starting process; controllable resistance is controlled by full control switch T 2 And a resistor R with smaller resistance 2 Series composition, by controlling T 2 The starting torque is adjusted.
The isolated variable frequency transformer using the permanent magnet synchronous motor is shown in fig. 1, and the isolated variable frequency transformer using the electric excitation synchronous motor is shown in fig. 2. The synchronous machine excitation unit shown in fig. 2 is connected across the dc bus of the back-to-back converter in a preferred topology.
The primary side of the isolation type variable frequency transformer is connected with a stator winding of a wound rotor asynchronous motor, a rotor winding of the wound rotor asynchronous motor is connected with the primary side in a bridging way through a back-to-back converter and an optional transformer, and the primary side is usually connected with a power grid; the secondary side of the isolation type variable frequency transformer is led out from the stator winding of the synchronous motor. The starting unit is connected with a rotor winding of the wound rotor asynchronous motor and is used for starting the variable frequency transformer. The coupling unit is used for connecting the rotating shafts of the synchronous motor and the wound rotor asynchronous motor, the speed change gear box is an optional component of the coupling unit, and the frequency reference value of the secondary alternating current can be designed more flexibly by adopting the speed change gear box. The detection and control unit applies control for the starting unit and the back-to-back converter.
Assume that the pole pair number of a wound rotor asynchronous motor in an isolated variable frequency transformer is p WRIM The pole pair number of the synchronous motor is p SG Speed change ratio 1 of the speed change gearbox: n is n gb (in the case of not using a speed change gear box, n gb =1). Under the control of the back-to-back converter, the rotating speed of the wound rotor asynchronous motor can be within a slip range of |s|<S max Internal continuous variation, where s max ＝U VSC /(k r/s U s )，U VSC Effective value k of maximum alternating voltage provided by alternating current ports of back-to-back converters r/s U is the ratio of the number of turns of a rotor winding to the number of turns of a stator winding of the wound rotor asynchronous motor s The voltage of an alternating current power supply connected with a stator winding of the wound rotor asynchronous motor.
The main technical parameters of the secondary side of the isolation type variable frequency transformer are as follows:
the normal working rotating speed range of the wound rotor asynchronous motor in the isolation type variable frequency transformer is [ omega ] 1 ，ω 2 ]Wherein omega 1 ＝(1-s max )2πf 1 /p WRIM ，ω 2 ＝(1+s max )2πf 1 /p WRIM Units of rad/s.
Ac frequency reference value f of secondary side of isolation type frequency conversion transformer 0 (in Hz),
f 0 ＝p SG f 1 /(p WRIM n gb )
the alternating current frequency of the secondary side of the isolation type variable frequency transformer can be set at a frequency reference value f 0 And (5) adjusting up and down.
The adjustable range of the secondary frequency of the isolation type variable frequency transformer is as follows:
(1-s max )f 0 ＜f＜(1+s max )f 0
secondary frequency f of isolation type variable frequency transformer and rotation speed omega of asynchronous motor with wound rotor WRIM Relationship between:
in the rotating speed range omega of the wound rotor asynchronous motor 1 To omega 2 Any rotational speed omega between WRIM The secondary side frequency of the isolation type variable frequency transformer is
Open circuit phase voltage omega WRIM ψ/n gb For a permanent magnet synchronous motor, psi is the flux linkage coefficient of a permanent magnet, and the unit is V s/rad; for an electrically excited synchronous machine, ψ is the excitation current i f Acting on the flux linkage coefficient generated by the rotor windings. Therefore, the isolation type variable frequency transformer can realize the variable frequency and the frequency conversion of the secondary side alternating current, and the frequency and the voltage have constant ratio, namely the characteristic of constant voltage frequency ratio.
A second part: control method of isolation type variable frequency transformer
The function of the detection and control unit is to control the isolated variable frequency transformer. The detection quantity comprises, but is not limited to, the rotating speed of the wound rotor asynchronous motor, the three-phase voltage of the stator of the wound rotor asynchronous motor, the three-phase current of the rotor of the wound rotor asynchronous motor and the three-phase current of the grid-side converter. The control of the isolation type variable frequency transformer comprises starting control and normal operation control.
(1) Start control
Before entering into a normal working state, the isolation type variable frequency transformer needs to be accelerated from a static state to the rotating speed omega for normal working set ，ω set The range of the values is as follows: (1-s) max )2πf 1 /p WRIM ＜ω set ＜2πf 1 /p WRIM . A starting method comprises the following steps:
the first step: the grid-side converter in the back-to-back voltage source converter firstly completes the pre-charging of the direct current bus capacitor under the vector control of the grid voltage orientation and controls the voltage of the direct current bus capacitor to be stabilized at a rated value, and the machine-side converter is in a locking state;
and a second step of: the detecting and controlling unit detects the rotating speed omega of the wound rotor asynchronous motor in real time WRIM If the rotation speed is lower than omega set Then keep T 1 Closing and maintaining the locked state of the machine side converter. Setting acceleration reference value of wound rotor asynchronous motor
Full control switch T in controllable resistor 2 Controlled according to fig. 3. In particular, when omega WRIM Less than omega set At the same time, the detected rotational speed omega WRIM Filtering noise by low-pass filtering, taking differential, and +.>
The error of the PI regulator is compared with the triangular carrier wave to obtain the full-control switch T 2 Pulse Width Modulation (PWM) signals of (a);
and a third step of: when omega WRIM Greater than omega set At the time, the full control switch T is switched by the mode 2 In an off state; meanwhile, the machine side converter is unlocked and adopts vector control of stator voltage orientation of a wound rotor asynchronous motor, a d axis of the vector control comprises three-loop control of a rotating speed loop, a torque loop and a current loop, wherein a reference value of the rotating speed loop is omega set The method comprises the steps of carrying out a first treatment on the surface of the The q-axis of the vector control contains a reactive loop and a current loop, and the reactive reference value is taken to be 0. Delay one after the machine side converter is put into normal working stateOff T for a period of time (e.g. 50 ms) 1 The isolation type variable frequency transformer is started up.
It should be noted that the core idea of the starting process is to use a controllable resistor to realize starting with preset acceleration (which may be variable acceleration), and switch the isolated variable frequency transformer to be controlled by the back-to-back converter after accelerating to its normal rotation speed range, and the general improvement based on the idea still falls into the scope of the present invention.
(2) Normal operation control
In the normal operation control, T 1 The off state is maintained. The machine side converter can adopt vector control of stator voltage orientation of the wound rotor asynchronous motor, and d-axis control of the machine side converter comprises a rotating speed ring, a torque ring and a current ring, wherein the reference value of the rotating speed ring of the wound rotor asynchronous motor is omega 1 To omega 2 The secondary side alternating current frequency and the secondary side alternating current voltage can be continuously adjusted through the arrangement; the q-axis control includes a reactive loop and a current loop, and the reactive reference value can be set according to the power factor. The grid-side converter can adopt vector control of grid voltage orientation, d-axis control of the grid-side converter comprises direct current bus voltage ring and current ring control, q-axis control of the grid-side converter comprises reactive power ring and current ring, and reactive power reference values can be set according to power factors. For an isolation type variable frequency transformer adopting an electrically excited synchronous motor, a synchronous motor excitation unit can adopt secondary side voltage closed-loop control. The vector control of the machine side converter, the network side converter and the control of the synchronous machine excitation unit can be used for referencing the prior art.
Third section: parallel capacity expansion method for isolation type variable frequency transformer
If the capacity of a single set of isolation type variable frequency transformers is insufficient to meet application requirements due to the design and manufacturing level of the motor, the single set of isolation type variable frequency transformers need to be expanded, and coordination control is needed among multiple sets of isolation type variable frequency transformers. There are two expansion methods: the first method adopts a plurality of complete isolated variable frequency transformers, and the primary side and the secondary side of the complete isolated variable frequency transformers are respectively connected in parallel, as shown in fig. 4; the second method is to collect the power on the mechanical rotating shafts of the plurality of wound rotor asynchronous motors by means of a power distribution device and drag a large synchronous machine, as shown in fig. 5. The two parallel capacity expansion methods are consistent in starting control and coordination control in normal operation, and specifically comprise the following steps:
assuming that a total of m sets of isolation variable frequency transformers are connected in parallel, one set of isolation variable frequency transformers is selected as a master variable frequency transformer, and the other sets of isolation variable frequency transformers are selected as slave variable frequency transformers. In the starting control, the starting can be completed by only a certain set of starting units of the isolation type variable frequency transformer, and the starting can also be completed by a plurality of sets of starting units of the isolation type variable frequency transformer together (the carrying capacity in the starting process can be enhanced). In normal operation, multiple sets of isolation type variable frequency transformers adopt master-slave coordination control, wherein the d-axis of vector control of the side converter of the main variable frequency transformer adopts three-loop control of rotating speed loop-torque loop-current loop, and the reference value of the rotating speed loop
Setting +.>
The d-axis of the vector control of the converter from the side of the variable frequency transformer adopts 'torque (power) closed loop-current closed loop' double-loop control, and the torque (power) command is distributed according to a certain algorithm. The general principle of distribution is to detect the total power P transmitted by a plurality of sets of isolation type variable frequency transformers e∑ The total power P shared by the variable frequency transformers should satisfy P e∑ *(m-1)/m≤P<P e∑ Suggested as p=p e∑ * (m-1)/m. The power reference values among the frequency conversion transformers of each set can be equally divided, or the residual power can be distributed to the next set after a specified set reaches rated power, and so on. The q-axis control of the machine side converter vector control and the network side converter vector control are the same as those of the single isolated variable frequency transformer.
The specific simulation verification of the embodiment of the invention is as follows:
electromagnetic transient simulation verification is carried out on PSCAD software, the capacity of a wound rotor asynchronous motor in an isolated variable-frequency transformer is 20MW, the capacity of a permanent magnet synchronous motor is 20MW, the rated frequency is 25Hz, the rated voltage is 10kV, and R in a starting unit 1 3000 ohm, R 2 For 30 ohm, set ω set The start acceleration was set to 0.05pu/s at 0.9 pu. In the simulation, the primary side of the isolation type variable frequency transformer is connected with a power frequency alternating current power grid, and the secondary side drags two load synchronous motors, wherein the rated capacity of the load synchronous motors is 2MW, and the rated voltage is 10kV.
In the first group of simulation, two load synchronous motors are idle, and the starting process is shown in fig. 6-a, 6-b, 6-c and 6-d;
a second set of simulations, the first load synchronous motor load torque 0.2pu, the second synchronous motor load torque 0.3pu, the start-up process shown in FIGS. 7-a, 7-b, 7-c and 7-d;
two groups of simulation results show that the designed isolation type variable frequency transformer has correct functions, the secondary side has constant voltage frequency ratio characteristics, the starting of preset acceleration can be realized, and the secondary side can drag the synchronous motor with load to start.
The invention has been considered preferable from the viewpoint of saving the capacity of the converter, and a wound rotor asynchronous motor and partial power converter control are adopted in the isolated variable frequency transformer. If the wound rotor asynchronous motor in the isolation type variable frequency transformer is replaced by other types of motors and is matched with the corresponding converter for control, the invention still belongs to the scope of the patent.
The isolated variable frequency transformer can realize energy bidirectional flow, and besides the simulation case that the alternating current motor is dragged at the secondary side, the alternating current generator is also connected to the secondary side, so that the isolated variable frequency transformer is also a possible application of the invention.
While the invention has been described with reference to certain preferred embodiments, it will be understood by those skilled in the art that various changes and substitutions of equivalents may be made and equivalents will be apparent to those skilled in the art without departing from the scope of the invention. Therefore, the protection scope of the invention is subject to the protection scope of the claims.
Claims (9)
1. The isolation type variable frequency transformer is characterized by comprising a synchronous motor and a wound rotor asynchronous motor, wherein the synchronous motor and the wound rotor asynchronous motor are connected through a coupling unit, the secondary side of the transformer is directly led out from a stator winding of the synchronous motor, the primary side of the transformer is directly led out from the stator winding of the wound rotor asynchronous motor, a back-to-back converter is further connected between the stator winding of the wound rotor asynchronous motor and a rotor winding of the wound rotor asynchronous motor, the rotor winding of the wound rotor asynchronous motor is further connected with a starting unit, the starting unit and the back-to-back converter are both connected with a detection and control unit, the back-to-back converter comprises a machine side converter and a network side converter, and the back-to-back converter is connected with the rotor winding of the wound rotor asynchronous motor through the machine side converter and is connected with the stator winding of the wound rotor asynchronous motor through a selectively configurable transformer;
the starting unit comprises a circuit breaker, a three-phase rectifier bridge, a fixed resistor and a variable resistor, wherein the fixed resistor and the variable resistor are connected in parallel to the direct current end of the three-phase rectifier bridge, the alternating current end of the three-phase rectifier bridge is connected with a rotor winding of the wound rotor asynchronous motor through the circuit breaker, the variable resistor comprises a full-control switch and a fixed resistor, the fixed resistor and the full-control switch are connected in series, and the three-phase rectifier bridge is composed of a plurality of diodes.
2. An isolated variable frequency transformer according to claim 1, wherein the synchronous motor is a permanent magnet synchronous motor or an electrically excited synchronous motor, and when the synchronous motor is an electrically excited synchronous motor, the dc bus of the back-to-back converter is connected across to a synchronous motor excitation unit.
3. The isolated variable frequency transformer of claim 1, wherein the ac frequency reference value of the secondary side of the transformer has a calculation formula:
f 0 ＝p SG f 1 /(p WRIM n gb )
wherein f 0 For the ac frequency reference value, f 1 For primary AC frequency, p SG For the pole pair number, n, of synchronous machines gb For the gear ratio of the coupling unit, p WRIM Is the pole pair number of the wound rotor asynchronous motor.
4. An isolated variable frequency transformer according to claim 1, wherein the secondary frequency of the transformer is adjustable over a range of frequencies:
(1-s max )f 0 ＜f＜(1+s max )f 0
s max ＝U VSC /(k r/s U s )
wherein f 0 Is the reference value of alternating current frequency, f is the secondary frequency, U VSC Effective value k of maximum alternating voltage provided by alternating current ports of back-to-back converters r/s U is the ratio of the number of turns of a rotor winding to the number of turns of a stator winding of the wound rotor asynchronous motor s The voltage of an alternating current power supply connected with a stator winding of the wound rotor asynchronous motor.
5. The isolated variable frequency transformer according to claim 1, wherein the rotation speed of the wound rotor asynchronous motor is controlled by the back-to-back converter to change in a normal working rotation speed range, and an alternating voltage with continuously adjustable voltage and frequency and constant voltage frequency ratio characteristic can be obtained at the secondary side of the isolated variable frequency transformer, wherein the secondary side frequency of the transformer has a calculation formula:
wherein f is the secondary frequency, ω WRIM For the rotating speed of the wound rotor asynchronous motor, p SG For the pole pair number, n, of synchronous machines gb The speed ratio of the coupling unit.
6. A start control method using the isolated variable frequency transformer according to any one of claims 1 to 5, wherein the start control method realizes a preset acceleration start by means of the start unit, and switches the isolated variable frequency transformer from a stationary state to a normal operating rotation speed and then to be controlled by the back-to-back converter, comprising the steps of:
step 1: pre-charging a direct current bus capacitor through power grid vector control of a grid-side converter in the back-to-back converter and stabilizing the direct current bus capacitor at a rated value of the direct current bus capacitor, so that a machine-side converter in the back-to-back converter is in a locking state;
step 2: the detection and control unit detects the rotating speed of the wound rotor asynchronous motor in real time and judges whether the rotating speed is smaller than the normal working rotating speed, if so, a breaker in the starting unit is kept closed, and a locking state of a machine side converter in the back-to-back converter is maintained;
step 3: if not, the full-control switch in the variable resistor in the starting unit is in an off state, the locking state of the machine side converter in the back-to-back converter is relieved, the vector control of the wound rotor asynchronous motor is adopted to enable the motor to normally work for a period of time, and then the circuit breaker in the starting unit is disconnected, so that the starting of the isolation type variable-frequency transformer is completed.
7. The method for controlling the start-up of an isolated variable frequency transformer according to claim 6, wherein the step 2 specifically comprises: and the detected rotating speed of the wound rotor asynchronous motor is subjected to low-pass filtering to remove noise and then is differentiated, and the error between the differentiated rotating speed and the acceleration reference value of the wound rotor asynchronous motor is compared with a triangular carrier wave after passing through a PI regulator to obtain a pulse width modulation signal of a full-control switch in a variable resistor in the starting unit.
8. A capacity expansion method adopting the isolated variable frequency transformer as claimed in any one of claims 1 to 5, wherein the capacity expansion method comprises the steps of respectively connecting primary sides and secondary sides of a plurality of sets of isolated variable frequency transformers in parallel or utilizing a power distribution device to collect power on mechanical rotating shafts of a plurality of wound rotor asynchronous motors so as to drag one synchronous motor.
9. The capacity expansion method of an isolated variable frequency transformer according to claim 8, wherein the coordination control method corresponding to the capacity expansion method is as follows: selecting one set of isolation type variable frequency transformers as a main variable frequency transformer, and using the rest sets as auxiliary variable frequency transformers, wherein the d-axis of vector control of a machine side converter of the main variable frequency transformer adopts three-loop control of rotating speed loop, torque loop and current loop, and the reference value of the rotating speed loop
According to the secondary side alternating current frequency reference value f * Setting, namely:
wherein p is SG For the pole pair number, n, of synchronous machines gb The speed ratio of the coupling unit is;
the d-axis of the vector control of the side converter of the variable frequency transformer adopts torque closed loop-current closed loop double-loop control, and the torque command is distributed according to a set principle, namely: the total power P shared by multiple sets of slave variable frequency transformers should satisfy P e∑ *(m-1)/m≤P<P e∑ Wherein m is the total number of sleeves of the isolation type variable frequency transformer, and P e∑ The total power transmitted by a plurality of sets of isolation type variable frequency transformers is provided.
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| CN110718896B (en) * | 2019-08-16 | 2022-03-29 | 南京南瑞继保工程技术有限公司 | Short-circuit fault protection method and device in variable-frequency starting process of phase modulator |
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