CN112865180A - Flexible grid-connected auxiliary device for power transformer - Google Patents
Flexible grid-connected auxiliary device for power transformer Download PDFInfo
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- CN112865180A CN112865180A CN202110200868.6A CN202110200868A CN112865180A CN 112865180 A CN112865180 A CN 112865180A CN 202110200868 A CN202110200868 A CN 202110200868A CN 112865180 A CN112865180 A CN 112865180A
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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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
- H02H7/04—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 transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/001—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection limiting speed of change of electric quantities, e.g. soft switching on or off
- H02H9/002—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection limiting speed of change of electric quantities, e.g. soft switching on or off limiting inrush current on switching on of inductive loads subjected to remanence, e.g. transformers
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Abstract
The invention discloses a flexible grid-connected auxiliary device of a power transformer, which comprises a first switch, a second switch, a third switch, a three-phase short-circuit bar and an auxiliary transformer, wherein when the power transformer is switched on and connected to the grid in a no-load mode, the auxiliary device is used for pre-magnetizing the power transformer, and the short-circuit impedance provides a current-limiting impedance for the power transformer due to the fact that the auxiliary transformer has a certain short-circuit impedance, so that the excitation surge current of the power transformer during the no-load switching on and connection to the grid is inhibited, and the flexible switching on and connection to the grid. In addition, when the power transformer is in no-load open-circuit and open-circuit, the short-circuit impedance of the auxiliary device becomes a small load of the power transformer, and provides an excitation current follow current loop for the power transformer, so that the operation overvoltage caused by the no-load open-circuit of the power transformer is reduced, and the flexible open-circuit and open-circuit are realized. The device ensures the stability of a local power grid and the insulation safety of equipment.
Description
Technical Field
The invention relates to the technical field of power systems, in particular to a flexible grid-connected auxiliary device of a power transformer.
Background
The transformer is a device for changing alternating voltage to transmit electric energy by using the principle of electromagnetic induction. In an electric power system, a transformer and a switch cabinet are connected with power grids with different voltage levels from power generation and transmission to a distribution network. Among them, a high-voltage large-capacity power transformer and a vacuum circuit breaker are widely used. Due to the influence of the residual magnetism of the iron core of the power transformer, when no switching-on measures are taken, the no-load excitation inrush current level of the power transformer reaches 6-10 times of the rated current, so that the voltage of a local power grid is reduced, other electric equipment is influenced, and even the switching-on is difficult to smoothly take place.
The vacuum circuit breaker has the advantages of small maintenance workload, large current breaking capacity, suitability for frequent operation and the like, and is widely applied to grid connection and protection of a large-capacity power transformer. The vacuum circuit breaker may generate an operation overvoltage both when closing and breaking a circuit. When the contact of the vacuum circuit breaker is closed, the phenomenon of bouncing in different degrees generally exists, so that overvoltage occurs. However, when the circuit is switched on, the distance between the contacts is increased along with the time and disappears quickly, and the transition voltage appearing between the contacts is in a descending trend, so that the peak value of overvoltage is generally low, and the overvoltage has no great threat to equipment. The main factors influencing the safe operation of the equipment are the overvoltage generated when the circuit is cut off, namely the interception overvoltage, the arc reignition overvoltage and the three-phase interception overvoltage. Accidents such as insulation breakdown of high-voltage equipment, circuit equipment failure, even damage of a circuit breaker and the like, burning of a high-voltage switch cabinet and the like can be caused.
The invention provides a chance for the problems of reducing the excitation inrush current level of the no-load closing of the power transformer, inhibiting the overvoltage level of the opening and closing operation, and the like.
Disclosure of Invention
The invention aims to provide a flexible grid-connected auxiliary device for a power transformer, which can realize flexible switch-on and grid-connection and flexible switch-off and grid-disconnection of the power transformer and solve the technical problems that the stability of a local power grid is reduced due to overlarge no-load switch-on excitation surge current and high operation overvoltage level of the power transformer and the insulation of a line or equipment is damaged due to the overlarge operation overvoltage level when the power transformer is switched off in a no-load mode.
In order to solve the technical problem, the invention provides a flexible grid-connected auxiliary device of a power transformer, which is applied to a main loop of a power grid, wherein the main loop comprises a vacuum circuit breaker, the power transformer and a circuit breaker which are sequentially connected from a power grid end to an electrical equipment end, and the power transformer is used for connecting the electrical equipment to the power grid when the vacuum circuit breaker and the circuit breaker are closed; the auxiliary device comprises a first switch, a second switch, a third switch, a three-phase short-circuit bar and an auxiliary transformer;
the primary terminal of the auxiliary transformer is connected with a power grid through the first switch and is connected with the three-phase short-circuit bar through the second switch, and the secondary terminal of the auxiliary transformer is connected with the secondary terminal of the power transformer through the third switch;
further comprising:
the control module is used for controlling the first switch and the third switch to be closed when the power transformer is switched on and connected to the grid in a no-load mode, providing current-limiting impedance for the power transformer, enabling a power grid to pre-magnetize the power transformer through the auxiliary transformer, controlling the vacuum circuit breaker to be switched on, and controlling the first switch and the third switch to be switched off after the power transformer is connected to the grid; and when the power transformer is in no-load brake-off and network-off, controlling the second switch and the third switch to be closed so as to provide short-circuit impedance for the power transformer and serve as a load of the power transformer, providing a follow current loop for exciting current of the power transformer, controlling the vacuum circuit breaker to be disconnected, and controlling the second switch and the third switch to be disconnected after the power transformer completes brake-off and network-off.
Preferably, the control module is specifically configured to, when the power transformer is switched on and connected to the grid, sequentially control the first switch to be closed and the third switch to be closed according to a time sequence, so as to provide a current-limiting impedance for the power transformer, pre-magnetize the power transformer through the auxiliary transformer by a power grid, and control the vacuum circuit breaker to be closed, the first switch to be opened, and the third switch to be opened according to the time sequence after the third switch is closed for a first preset time; and when the power transformer is in no-load opening and network breaking, the second switch, the third switch and the vacuum circuit breaker are sequentially controlled to be closed, closed and opened according to a time sequence so as to provide short-circuit impedance for the power transformer, provide a follow current loop for a load and exciting current of the power transformer, and control the second switch and the third switch to be opened after the vacuum circuit breaker is opened for a second preset time.
Preferably, the method further comprises the following steps:
a fourth switch disposed between a secondary tap terminal of the auxiliary transformer and a low voltage device;
the control module is further used for controlling the first switch and the fourth switch to be closed after the power transformer completes grid connection;
the auxiliary transformer is used for converting the power grid voltage to supply power for the low-voltage equipment.
Preferably, when the power transformer is in no-load open-circuit and open-circuit, controlling the second switch and the third switch to be closed to provide short-circuit impedance for the power transformer and provide a freewheeling loop for a load and for an excitation current of the power transformer, and after the power transformer completes the open-circuit and open-circuit, controlling the second switch and the third switch to be open-circuit includes:
and when the power transformer is in no-load opening and network breaking, the breaker, the fourth switch, the first switch, the second switch, the third switch and the vacuum breaker are sequentially controlled to be opened according to a time sequence, so that short-circuit impedance is provided for the power transformer and is used as a load, a follow current loop is provided for exciting current of the power transformer, and the second switch and the third switch are controlled to be opened after the vacuum breaker is opened for a second preset time.
Preferably, the first switch, the second switch, the third switch and the fourth switch are load switches or circuit breakers.
Preferably, the short-circuit impedance is N% of the rated impedance of the auxiliary transformer, and N is 5-20.
Preferably, the capacity of the auxiliary transformer is M% of the capacity of the power transformer, and M is not less than 0.5.
The application provides a flexible grid-connected auxiliary device of a power transformer, which comprises a first switch, a second switch, a third switch, a three-phase short-circuit bar and an auxiliary transformer, wherein when the power transformer is in no-load switch-on and grid-connected, the auxiliary device pre-magnetizes the power transformer, and the auxiliary transformer has certain short-circuit impedance which provides a current-limiting impedance for the power transformer, so that the excitation surge current of the power transformer during no-load switch-on and grid-connection is inhibited, and the flexible switch-on and grid-connection is realized. In addition, when the power transformer is in no-load open-circuit and open-circuit, the short-circuit impedance of the auxiliary device becomes a small load of the power transformer, and provides an excitation current follow current loop for the power transformer, so that the operation overvoltage caused by the no-load open-circuit of the power transformer is reduced, and the flexible open-circuit and open-circuit are realized. The device ensures the stability of a local power grid and the insulation safety of equipment.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed in the prior art and the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.
Fig. 1 is a schematic diagram of a power transformer flexible grid-connected auxiliary device provided by the invention;
fig. 2 is a schematic diagram of another power transformer flexible grid-connected auxiliary device provided by the invention.
Detailed Description
The core of the invention is to provide a flexible grid-connected auxiliary device which can realize flexible switch-on and grid-connection and flexible switch-off and grid-disconnection of a power transformer, thereby ensuring the stability of a local power grid and the insulation safety of equipment.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, fig. 1 is a schematic diagram of a power transformer flexible grid-connected auxiliary device provided by the present invention, the device is applied to a main circuit of a power grid, wherein the main circuit includes a vacuum circuit breaker QF1, a power transformer T1 and a circuit breaker QF2, which are sequentially connected from a power grid end to an electrical equipment end, and the power transformer T1 is used for connecting the electrical equipment to the power grid when the vacuum circuit breaker QF1 and the circuit breaker QF2 are closed; the auxiliary device comprises a first switch QL1, a second switch QL2, a third switch QL3, a three-phase short-circuit bar and an auxiliary transformer T2;
a primary terminal of the auxiliary transformer T2 is connected to the grid through a first switch QL1, and is connected to the three-phase short-circuit bar through a second switch QL2, and a secondary terminal of the auxiliary transformer T2 is connected to a secondary terminal of the power transformer T1 through a third switch QL 3;
further comprising:
the control module is used for controlling the first switch QL1 and the third switch QL3 to be closed when the power transformer T1 is switched on and connected to the grid in an idle load mode, so that current-limiting impedance is provided for the power transformer T1, the power grid is enabled to pre-charge the power transformer T1 through the auxiliary transformer T2, the vacuum circuit breaker QF1 is controlled to be switched on, and the first switch QL1 and the third switch QL3 are controlled to be switched off after the power transformer T1 is connected to the grid; when the power transformer T1 is unloaded, the brake is opened and the network is disconnected, the second switch QL2 and the third switch QL3 are controlled to be closed to provide short-circuit impedance for the power transformer T1 and serve as a load of the power transformer T1, a follow current loop is provided for the exciting current of the power transformer T1, the vacuum circuit breaker QF1 is controlled to be disconnected, and after the power transformer T1 is disconnected, the second switch QL2 and the third switch QL3 are controlled to be disconnected.
Considering that when the power transformer T1 is in no-load grid connection, the no-load magnetizing inrush current level of the power transformer T1 will reach 6-10 times of the rated current, which causes a local power grid voltage drop, affects other electric devices, and even is difficult to smoothly switch on. And when no-load network breaking, operation overvoltage can be generated, and accidents such as insulation breakdown of high-voltage equipment, circuit equipment failure, even damage of a circuit breaker and the like, burning of a high-voltage switch cabinet and the like can be caused.
In order to solve the technical problem, the application provides an auxiliary device, when a power transformer T1 is switched on and connected to a grid, the auxiliary device pre-magnetizes a power transformer T1, that is, a power grid pre-magnetizes a power transformer T1 through an auxiliary transformer T2, and since a certain short-circuit impedance exists between a primary terminal and a secondary terminal of the auxiliary transformer T2 in the auxiliary device, the short-circuit impedance provides a current-limiting impedance for the power transformer T1, so that an excitation inrush current when the power transformer T1 is switched on in an idle load state is suppressed, and flexible switching-on and grid connection are realized.
When the power transformer T1 is disconnected, that is, before the vacuum short-circuit device QF1 is disconnected, the short-circuit impedance of the auxiliary device becomes a small load of the power transformer T1, and provides an excitation current freewheeling circuit for the power transformer T1, and since the circuit formed by the power transformer T1 and the auxiliary transformer T2 has a capacitance to ground, the magnetic energy stored in the T1 will oscillate and decay in the LCR circuit for a period of time and then be released. Therefore, the device can ensure the stability of a local power grid and the insulation safety of equipment, further reduce the operation overvoltage caused by no-load network breaking and realize flexible brake-separating and network breaking.
It should be noted that the specific implementation of the auxiliary device in the present application is not limited to include the first switch QL1, the second switch QL2, the third switch QL3, the three-phase short-circuit bar, the auxiliary transformer T2 and the control module, and other implementations are also possible, and the present application is not limited thereto. The auxiliary transformer T2 is also connected to the power transformer T1 in the same group.
In conclusion, the flexible grid-connected auxiliary device for the power transformer T1 provided by the application can realize flexible switch-on and grid-connection and flexible switch-off and grid-disconnection of the power transformer T1, avoid the situation of overlarge excitation current or overvoltage, and further ensure the stability of a local power grid and the insulation safety of equipment.
On the basis of the above-described embodiment:
as a preferred embodiment, the control module is specifically configured to, when the power transformer T1 is switched on and connected to the grid, sequentially control the first switch QL1 to be closed and the third switch QL3 to be closed according to a time sequence, so as to provide a current-limiting impedance for the power transformer T1, pre-magnetize the power transformer T1 for the grid through the auxiliary transformer T2, and, after the third switch QL3 is closed for a first preset time, sequentially control the vacuum circuit breaker QF1 to be closed, the first switch QL1 to be opened, and the third switch QL3 to be opened according to a time sequence; when the power transformer T1 is unloaded, the brake is opened and the network is disconnected, the second switch QL2 is controlled to be closed, the third switch QL3 is controlled to be closed and the vacuum circuit breaker QF1 is controlled to be opened according to the time sequence, short-circuit impedance is provided for the power transformer T1 and is used as a load, a follow current loop is provided for the exciting current of the power transformer T1, and the second switch QL2 and the third switch QL3 are controlled to be opened after the vacuum circuit breaker QF1 is opened for a second preset time.
Specifically, the first switch QL1 is closed first to connect the primary terminal of the auxiliary transformer T2 to the grid, then the third switch QL3 is closed, and the secondary terminal of the auxiliary transformer T2 is connected to the secondary terminal of the power transformer T1, so that the grid pre-charges the power transformer T1 through the auxiliary transformer T2, and a flexible closing grid connection is realized by a magnetizing inrush current when the power transformer T1 is closed in an idle load state.
Considering that in practical applications, the second switch QL2 may be a load switch, and the third switch QL3 may be a circuit breaker, and at this time, the capacity of the second switch QL2 for withstanding inrush current is much smaller than that of the third switch QL3, therefore, when the power transformer T1 is unloaded to open the grid, the second switch QL2 is controlled to be closed, and then the third switch QL3 is controlled to be closed, so as to implement flexible grid opening and grid breaking, thereby avoiding generating an operation overvoltage.
In summary, the reliability of the power transformer T1 in the grid connection process is further improved by the sequence of controlling the switches in the embodiment.
As a preferred embodiment, the method further comprises the following steps:
a fourth switch QL4 provided between the secondary tap terminal of the auxiliary transformer T2 and the low-voltage device;
the control module is also used for controlling the first switch QL1 and the fourth switch QL4 to be closed after the power transformer T1 is connected to the grid;
the auxiliary transformer T2 is used to convert the grid voltage to power the low voltage devices.
Referring to fig. 2, fig. 2 is a schematic diagram of another power transformer flexible grid connection auxiliary device provided by the present invention.
In consideration of the fact that the power transformer T1 converts the grid voltage to supply power to the electrical devices when the vacuum circuit breaker QF1 and the circuit breaker QF2 are closed, the auxiliary transformer T2 may also serve as an auxiliary power source to supply power to low-voltage devices, such as lighting devices in a wind power generation system.
Based on this, according to the present application, a fourth switch QL4 is further disposed between the secondary tap terminal of the auxiliary transformer T2 and the low-voltage device, and at this time, when the auxiliary transformer T2 is in the power distribution condition, the fourth switch QL4 is in the closed state, and the auxiliary transformer T2 converts the output voltage of the power grid to supply power to the low-voltage device. The auxiliary transformer T2 is a double-winding transformer.
In summary, in the manner of this embodiment, the auxiliary transformer T2 can be directly used to supply power to the low-voltage device, and no additional power module needs to be configured for the low-voltage device.
As a preferred embodiment, when the power transformer T1 is unloaded to open the gate and disconnect the network, the method of controlling the second switch QL2 and the third switch QL3 to close to provide short-circuit impedance for the power transformer T1 and to provide a freewheeling circuit for the load and for the excitation current of the power transformer T1, and after the power transformer T1 completes the open-gate and disconnect the network, the method of controlling the second switch QL2 and the third switch QL3 to open includes:
when the power transformer T1 is unloaded, the brake is opened and the network is disconnected, the breaker QF2, the fourth switch QL4, the first switch QL1, the second switch QL2, the third switch QL3 and the vacuum breaker QF1 are sequentially controlled to be opened according to the time sequence, short-circuit impedance is provided for the power transformer T1 and serves as a load, a follow current loop is provided for the exciting current of the power transformer T1, and the second switch QL2 and the third switch QL3 are controlled to be opened after the vacuum breaker QF1 is opened for a second preset time.
Specifically, when the auxiliary device in the present application includes the fourth switch QL4, the fourth switch QL4 should be controlled to be opened first to make the auxiliary transformer T2 be in an unloaded state, and then the steps of controlling the first switch QL1 to be opened, the second switch QL2 to be closed, and the third switch QL3 to be closed are performed, so that the reliability and safety of the auxiliary device during operation are ensured.
As a preferred embodiment, the first switch QL1, the second switch QL2, the third switch QL3 and the fourth switch QL4 are load switches or circuit breakers.
It should be noted that, the sequence of controlling the on and off of each switch in the present application may be implemented by using a time relay, and in addition, a software program control mode may also be adopted, so that the switch state of the switch is changed according to a certain time sequence.
In a preferred embodiment, the short-circuit impedance is N% of the rated impedance of the auxiliary transformer T2, and N is 5-20.
The embodiment aims to limit the short-circuit impedance of the auxiliary transformer T2, and if the short-circuit impedance is too large, the voltage regulation rate is large and the output voltage is low when the power distribution working condition is met, that is, the first switch QL1 and the fourth switch QL4 are both closed, so that the operation of the electrical equipment is affected; if the short-circuit impedance is too small, the pre-magnetizing current is too large under the auxiliary switching-on working condition, namely before the vacuum circuit breaker QF1 is switched on, and the T2 short-circuit current is large under the auxiliary switching-off working condition, namely before the vacuum circuit breaker QF1 is switched off, so that the dynamic stability and the thermal stability of the T2 are reduced.
As a preferred embodiment, the short-circuit impedance is N% of the rated impedance of the auxiliary transformer, and N is 5-20%. In the embodiment of the present application, N is preferably about 10, and other values may also be adopted, which is determined according to the actual situation, and the present application is not limited herein.
As a preferred embodiment, the capacity of the auxiliary transformer T2 is M% of the capacity of the power transformer T1, M being not less than 0.5.
The application provides a specific embodiment to specifically introduce the flexible grid-connected auxiliary device provided by the application, and the embodiment is as follows:
the system is applied to the field of high-power wind power generation, and a certain 7MW wind generating set is provided with a power transformer T1 and an auxiliary transformer T2:
the primary terminal of the power transformer T1 is connected to the collector line via a vacuum breaker QF1, and the secondary terminal is connected to the wind power converter via a breaker QF 2. Wherein, the rated capacity of the power transformer T1 is 8000kVA, the primary rated voltage is 35kV, the secondary rated voltage is 0.69kV, the short-circuit impedance is 8%, the no-load loss is 8kW, the no-load current is 0.3%, and the no-load excitation capacity is 24kVA, so that the secondary excitation impedance can be deduced to be about 20 omega.
The rated capacity of the auxiliary transformer T2 is 200kVA, the primary rated voltage is 35kV, the secondary rated voltage is 0.69kV, the secondary tap voltage is 0.4kV, the primary rated current is 3.3A, the secondary rated current is 167A, the secondary tap rated current is 289A, the short-circuit impedance is 10%, the reactance component is 9.9%, and the resistance component is 1.4%, so that the secondary short-circuit impedance is deduced to be about 0.24 omega.
The working condition I is as follows: during power distribution, when the first switch QL1 and the fourth switch QL4 are both closed;
at this time, the second switch QL2 and the third switch QL3 must be in an open state, the first switch QL1 and the fourth switch QL4 must be in a closed state, and the auxiliary transformer T2 supplies power to other auxiliary electrical equipment of the wind turbine generator, such as heating equipment and other various auxiliary electrical equipment of the wind turbine generator, through a power distribution loop; the power transformer T1 is connected with the power grid and the wind power generation converter and used for transmitting electric energy to the power grid.
Working conditions are as follows: auxiliary closing, i.e. before the vacuum breaker QF1 closes;
before the power transformer T1 is incorporated into the grid, all switches (QF1, QF2, QL1, QL2, QL3 and QL4) are in an open state.
The secondary terminal of the power transformer T1 adopts a pre-magnetizing mode to assist closing and grid connection, a time relay or a microcomputer protection program is used for controlling the switching sequence, and the sequence of the switching actions is as follows:
1. closing the QL1 to bring the primary terminal of the auxiliary transformer T2 into the grid;
2. QL3 is closed to connect and conduct the secondary terminal of auxiliary transformer T2 to the secondary terminal of power transformer T1, and the grid pre-charges power transformer T1 via auxiliary transformer T2. After pre-magnetizing is finished, the fracture voltage of the vacuum circuit breaker QF1 is greatly reduced, and the specific calculation process is as follows:
the power factor of the power transformer T1 is 8kW/(8000kVA 0.3%) -0.333;
the voltage regulation factor Δ U of the auxiliary transformer T2 is (1.4% × 0.333+ 9.9% × (1-0.3332)0.5) × (24/200) 1.176%;
the primary voltage of T1 is 35 × by (1-1.176%) ═ 34.384kV, i.e. the port voltage of the vacuum interrupter QF1 in the main circuit is only 0.412kV (═ 35kV × 1.176%), and without the auxiliary device in this application, the port voltage of QF1 is the grid voltage.
3. After a plurality of seconds, the QF1 is closed, and a primary terminal of the power transformer T1 is connected and conducted with a power grid, so that flexible switching-on and grid connection of the power transformer are realized;
4. immediately breaking the QL 1;
5. finally, QL3 is broken.
Working conditions are as follows: auxiliary opening, namely when the vacuum circuit breaker is opened;
the power transformer flexible grid-connected auxiliary device adopts a short circuit T2 mode when assisting in the opening working condition.
Specifically, the short-circuit impedance of the auxiliary transformer T2/the excitation impedance of the power transformer T1 is 0.24/20 is 1.2%;
because the proportion of the short-circuit impedance of the T2 in the freewheel circuit of the T1 is very small and can be ignored, the freewheel circuit of the T1 can be ensured to be smooth, and the operation overvoltage of the primary terminal of the T1 can be avoided.
When T2 is short-circuited, the short-circuit current is about 10 times (1/10%) of its rated current, and then T1 has a load of 25% (200 kVA 10 times/8000 kVA) before the vacuum circuit breaker QF1 is opened, thus avoiding the empty opening of T1. The third switch QL3 selects the nominal value according to 2000A and the second switch QL2 selects the nominal value according to 3150A.
After the T1 is disconnected, the magnetic energy stored in the core will oscillate in the free-wheeling circuit formed by the secondary winding of T1 and the secondary winding of T2, and after a while, will decay and be depleted.
The short circuit duration of the auxiliary transformer T2 is much shorter than the short circuit duration (2 seconds) specified in the power transformer standard, and there should be sufficient margin to satisfy the dynamic and thermal stabilities caused by 10 times of the short circuit current.
The power transformer T1 is disconnected in accordance with a strict operation sequence of the above-mentioned switches, wherein the switch sequence may be controlled by a time relay or a microcomputer protection program. The operation sequence of each switch operation is as follows:
1. breaking QF2 to ensure T1 to be in a no-load state;
2. breaking the QL 1;
3. breaking the QL 4;
4. closing QL2, connecting the secondary terminal of the auxiliary transformer T2 with the three-phase short-circuit bar, and short-circuiting the secondary terminal of T2;
5. closing the QL3, connecting and conducting the primary terminal of the auxiliary transformer T2 and the secondary terminal of the power transformer T1, and enabling the T2 to be in a short-circuit operation state;
6. immediately breaking a vacuum circuit breaker QF1 to disconnect a primary terminal of a power transformer T1 from a power grid;
7. breaking QL2 after five minutes;
8. finally, QL3 is broken.
According to the embodiment, the flexible grid-connected auxiliary device provided by the application has three working conditions of flexible switch-on grid connection, flexible switch-off grid disconnection and power distribution. When the power transformer T1 is switched on in a no-load mode, the auxiliary transformer T2 serves as current-limiting impedance to suppress excitation inrush current and operation overvoltage generated by the no-load switching on of the power transformer T1, and flexible switching-on grid connection is achieved. When the power transformer T1 is opened in no-load, the short-circuited auxiliary transformer T2 becomes a small load of the power transformer T1 and provides a follow current loop for the exciting current of the power transformer, so that the overvoltage of opening operation is suppressed, and flexible opening and network breaking are realized. In addition, due to the existence of the capacitance to ground in the T1 and T2 loops, the magnetic energy stored in the iron core of the power transformer T1 is released after oscillation in the LCR loop decays for a period of time, and therefore the stability of a local power grid and the insulation safety of equipment can be ensured.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. The device disclosed by the embodiment corresponds to the method disclosed by the embodiment, so that the description is simple, and the relevant points can be referred to the method part for description.
It is further noted that, in the present specification, relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (7)
1. The flexible grid-connected auxiliary device of the power transformer is characterized by being applied to a main loop of a power grid, wherein the main loop comprises a vacuum circuit breaker, a power transformer and a circuit breaker which are sequentially connected from a power grid end to an electrical equipment end, and the power transformer is used for connecting the electrical equipment to the power grid when the vacuum circuit breaker and the circuit breaker are closed; the auxiliary device comprises a first switch, a second switch, a third switch, a three-phase short-circuit bar and an auxiliary transformer;
the primary terminal of the auxiliary transformer is connected with a power grid through the first switch and is connected with the three-phase short-circuit bar through the second switch, and the secondary terminal of the auxiliary transformer is connected with the secondary terminal of the power transformer through the third switch;
further comprising:
the control module is used for controlling the first switch and the third switch to be closed when the power transformer is switched on and connected to the grid in a no-load mode, providing current-limiting impedance for the power transformer, enabling a power grid to pre-magnetize the power transformer through the auxiliary transformer, controlling the vacuum circuit breaker to be switched on, and controlling the first switch and the third switch to be switched off after the power transformer is connected to the grid; and when the power transformer is in no-load brake-off and network-off, controlling the second switch and the third switch to be closed so as to provide short-circuit impedance for the power transformer and serve as a load of the power transformer, providing a follow current loop for exciting current of the power transformer, controlling the vacuum circuit breaker to be disconnected, and controlling the second switch and the third switch to be disconnected after the power transformer completes brake-off and network-off.
2. The power transformer flexible grid-connection auxiliary device according to claim 1, wherein the control module is specifically configured to, when the power transformer is switched on and connected to the grid, sequentially control the first switch to be closed and the third switch to be closed according to a time sequence, so as to provide a current-limiting impedance for the power transformer, pre-magnetize the power transformer through the auxiliary transformer by a power grid, and after the third switch is closed for a first preset time, sequentially control the vacuum circuit breaker to be closed, the first switch to be opened, and the third switch to be opened according to the time sequence; and when the power transformer is in no-load opening and network breaking, the second switch, the third switch and the vacuum circuit breaker are sequentially controlled to be closed, closed and opened according to a time sequence so as to provide short-circuit impedance for the power transformer, provide a follow current loop for a load and exciting current of the power transformer, and control the second switch and the third switch to be opened after the vacuum circuit breaker is opened for a second preset time.
3. The flexible grid-tie assist device of claim 2, further comprising:
a fourth switch disposed between a secondary tap terminal of the auxiliary transformer and a low voltage device;
the control module is further used for controlling the first switch and the fourth switch to be closed after the power transformer completes grid connection;
the auxiliary transformer is used for converting the power grid voltage to supply power for the low-voltage equipment.
4. The flexible grid-connected auxiliary device according to claim 3, wherein when the power transformer is unloaded, disconnected and disconnected, the second switch and the third switch are controlled to be closed to provide short-circuit impedance for the power transformer and to serve as a load and to provide a freewheeling loop for an excitation current of the power transformer, and after the power transformer is disconnected and disconnected, the second switch and the third switch are controlled to be opened, including:
and when the power transformer is in no-load opening and network breaking, the breaker, the fourth switch, the first switch, the second switch, the third switch and the vacuum breaker are sequentially controlled to be opened according to a time sequence, so that short-circuit impedance is provided for the power transformer and is used as a load, a follow current loop is provided for exciting current of the power transformer, and the second switch and the third switch are controlled to be opened after the vacuum breaker is opened for a second preset time.
5. The flexible grid connection auxiliary device according to claim 4, wherein the first switch, the second switch, the third switch and the fourth switch are load switches or circuit breakers.
6. The flexible grid-connected auxiliary device according to any one of claims 2-5, wherein the short-circuit impedance is N% of the rated impedance of the auxiliary transformer, and N is 5-20.
7. The flexible grid-connection auxiliary device according to any one of claims 2-5, wherein the capacity of the auxiliary transformer is M% of the capacity of the power transformer, M being not less than 0.5.
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US11955782B1 (en) | 2022-11-01 | 2024-04-09 | Typhon Technology Solutions (U.S.), Llc | System and method for fracturing of underground formations using electric grid power |
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| CN112865180B (en) | 2023-06-20 |
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