EP2807664B1

EP2807664B1 - Combined transformer for power system

Info

Publication number: EP2807664B1
Application number: EP12813352.7A
Authority: EP
Inventors: Fang Min LIN
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG; Siemens Corp
Priority date: 2011-12-31
Filing date: 2012-12-21
Publication date: 2016-05-11
Anticipated expiration: 2032-12-21
Also published as: HRP20160996T1; WO2013098226A1; EP2807664A1; ES2586827T3

Description

The present invention relates to a combined transformer for a power system, and in particular to an independent electronic combined transformer.
It is necessary to use transformers in a power system to measure high voltage large current and high voltage, in order to achieve protection, control and electric energy measurement of the power system. During this process, it is also necessary to carry out electric isolation on the primary high voltage and the secondary low voltage. The combined transformer is an apparatus that has a current and voltage measurement function while being capable of carrying out electric isolation on the primary high voltage and the secondary low voltage.
In conventional combined transformers, the electromagnetic transformer technology with an iron core based on the Faraday electromagnetic induction principle is usually adopted for the measurement of both current and voltage. At present, such a combined transformer has been widely applied in power systems all over the world. As the voltage transformer therein is electromagnetic, the inductive element formed by the iron core and the primary winding may cause ferromagnetic resonance with capacitance elements (e.g., breakers , capacitors, etc.) on the power grid under certain operating conditions, thus affecting the stable operation of the power grid. Secondly, on the secondary side, no short-circuit is allowed for the voltage transformer while no open-circuit is allowed for the current transformer, otherwise the large current formed due to short-circuit or the over-voltage formed due to open-circuit will cause serious damage to the transformer. In addition, the output interface of the conventional transformer is suitable for electromechanical relays, and the output power of a single coil is relatively large sometimes, so that the cross-sectional area of the magnetic core used in the transformers is increased, the loss is large, and the size of the transformer is increased.
For a high voltage side active electronic combined transformer, the current measurement part is located on the high voltage side at the upper part of the transformer, the primary current passes through the center of the current transformer via a primary conducting rod, wherein a hollow coil is generally used as the transformer for protection level, and a low power coil is used as the transformer for measurement level, while the secondary output from the current transformer is converted to a digital optical signal to be sent to the low voltage side via an optical fiber. The voltage measurement part is generally located in an insulator, and an electrode-type capacitive divider is usually adopted. Such a combined transformer has poor stability, the current measurement error is influenced directly by the power voltage on the high voltage side and the working state of each electronic device, and, as the voltage measurement is based on the principle of the capacitive divider and the output error needs to be calibrated after on-site installation, it is unable to achieve plug and play. The designed service life of the transformer in the power system is generally required to reach up to 30 years, while the electronic device in the high voltage side active electronic combined transformer generally has a service life of just 4 to 5 years and can hardly be maintained normally but can be replaced or repaired only after being powered off; however, abnormal power-off will have a major impact on the stable operation of the power grid. Secondly, the high voltage side active electronic combined transformer has poor resistance to electromagnetic interference, the over-voltage formed due to the operation of the isolating switch or the like on the power grid may make the high voltage side power supply or electronic modules crash or directly break down. Secondly, a hollow coil is adopted for the current protection function of the current transformer, and it is necessary to utilize quadratic integration to achieve the linear relation between the secondary output voltage and the primary current, but regardless of the type of the integrator, the time constant of the integrator will cause a certain amount of distortion to the output wave-form; in particular, in the case of the time constant of the integrator not being large enough, when the short-circuit current passes through the Rogowski coil primarily, the trailing of the output of the Rogowski coil is very serious, and will even cause abnormal protection actions, affecting the safe operation of the power grid.
For a high voltage side passive pure photoelectric combined transformer, the current measurement part is located on the high voltage side at the upper part of the transformer, and full optical fiber type or magneto-optical glass type sensitive rings based on Faraday magneto-optical effects are adopted. Such a combined transformer has the disadvantage that the transformer has relatively high costs, and quite high requirements on materials (particularly on polarization-maintaining optical fiber of the current transformer and the piezoelectric crystal of the voltage transformer), several times, even more than 10 times, higher than that of the conventional transformer. The optical fiber sensitive ring in the current measurement part and the crystal in the voltage measurement part are sensitive to both the magnetic field formed by the primary current and the electric field formed by the primary voltage, but also are sensing elements with respect to voltage, temperature or the like, and vibration, temperature change or the like during the operation of the transformer may have a direct impact on the error. In addition, the processing loop of the secondary signal is complex, there is white noise in the output signal, and the combined transformer will also output a secondary signal due to the interference of the white noise when there is no primary current, therefore the requirements of billing cannot be met. If a technology combining the capacitive divider with the Pockels effects is used for voltage measurement, the measurement error of the voltage part also has to be calibrated on site, and it is unable to achieve plug and play.
Prior Art is disclosed in DE 198 32 707 A1 , EP 1 624 312 A1 , DE 195 08 582 A1 , EP 2 136 216 A1 , WO 99/15906 A1 and DE 198 41 164 A1 .
Therefore, there is a need in the art for a combined transformer for a power system, with reasonable costs, high reliability and convenient installation.
The object of the present invention is to provide a combined transformer, by which the error calibration after installation of the existing combined transformer is avoided, with reasonable costs, high reliability, long service life and plug and play capability.
This object is achieved by a combined transformer as claimed in claim 1.
Such a combined transformer for a power system provided in the present invention comprises: a shell, a base and an insulator connecting the shell and the base. A current transformer is arranged in the shell, wherein the current transformer can detect the current in a primary conductor in the power system, and the current transformer sends a first signal reflecting the value of the current in the primary conductor to the base. A voltage transformer is arranged in the insulator, wherein the voltage transformer comprises: an output branch circuit comprising a first input end and a second input end, wherein the output branch circuit sends a second signal reflecting the value of the voltage in the primary conductor to the base; a resistive division circuit, one end of which is electrically connected to the primary conductor, the other end thereof being electrically connected to the first input end; a capacitive division circuit formed by a plurality of layers of capacitive screens serially connected, one end of which is electrically connected to the primary conductor, the other end thereof being electrically connected to the second input end, and the first input end being electrically connected to a secondary division potential point of the capacitive division circuit; and a secondary division resistor, one end of which is electrically connected to the first input end, the other end thereof being electrically connected to the second input end. As a voltage division method achieved by connecting series resistors and series capacitors in parallel and a plurality of layers of capacitive screens are used in the present invention, the frequency domain range in which the combined transformer can measure the value of the voltage in the primary conductor accurately is broadened, and the error of the voltage transformer need not be calibrated after on-site installation, i.e. the combined transformer can plug and play. In addition, the combined transformer can be used for measurement of not only AC voltage but also DC voltage.
In a schematic implementation of the combined transformer in the present invention, the extension direction of the electrodes of each layer of capacitive screen in the plurality of layers of capacitive screens is the same as the extension direction of the insulator, so that capacitive screens with a large enough electrode area may be set, and the processing and assembling of the plurality of layers of capacitive screens can be simplified, according to actual demands.
In another schematic implementation of the combined transformer in the present invention, the combined transformer comprises a high voltage flange and a grounding flange, wherein the insulator is connected to the shell through the high voltage flange, and the insulator is connected to the base through the grounding flange.
In still another schematic implementation of the combined transformer in the present invention, the combined transformer comprises a fairlead, a current double shielded twisted-pair cable transmitting the first signal to the base and a voltage double shielded twisted-pair cable transmitting the second signal to the base, wherein the current double shielded twisted-pair cable and the voltage double shielded twisted-pair cable are passed through the fairlead. The fairlead can protect the double shielded twisted-pair cables passed therethrough.
In yet another schematic implementation of the combined transformer in the present invention, the capacitive screens are wound onto the fairlead or assembled onto the fairlead after being pre-molded, the electrodes of the capacitive screens are one of aluminum foil, copper foil, thin-film semiconductor or paper semiconductor, while the insulating layer between the electrodes is one of sulfur hexafluoride gas composite thin-film, insulation oil composite cable paper, epoxy resin composite crepe paper or silicone oil composite polytetrafluoroethylene tape.
In yet another schematic implementation of the combined transformer in the present invention, the second input end of the output branch circuit is electrically connected to ground potential.
In yet another schematic implementation of the combined transformer in the present invention, the resistive division circuit comprises a plurality of thick-film resistors, wherein the thick-film resistors are connected in parallel to both ends of the capacitive screen after being connected to each other in series, one end of the thick-film resistors connected in series is electrically connected to the primary conductor, and the other end thereof is electrically connected to the first input end of the output branch circuit.
In yet another schematic implementation of the combined transformer in the present invention, the resistive division circuit comprises a resistance band, wherein the resistance band is formed by continuously attaching resistive slurry to the interior surface of the insulator by laser printing or spray coating, one end of the resistance band is electrically connected to the high voltage flange, and the other end thereof is electrically connected to the first input end of the output branch circuit. Using the insulator as a part of the voltage transformer not only ensures long service life, reliability and stability of the insulating system and insulating materials of the combined transformer, but also saves materials of the voltage transformer, so that the design costs are reduced.
In yet another schematic implementation of the combined transformer in the present invention, the current transformer is a low power electronic current transformer comprising a magnetic core, a secondary winding wound onto the magnetic core and a shunt resistor connected to the tail end of the secondary winding.
In yet another schematic implementation of the combined transformer in the present invention, there is an insulating medium filling the space between the current transformer and the shell, and the insulating medium is one of sulfur hexafluoride gas, insulation oil composite cable paper, epoxy resin composite crepe paper or silicone oil composite polytetrafluoroethylene tape.
In yet another schematic implementation of the combined transformer in the present invention, the base is provided therein with an outlet box, wherein the outlet box comprises: an input module, a secondary first signal being obtained after the first signal is processed by the input module in terms of voltage division, and a secondary second signal being obtained after the second signal is processed by the input module in terms of voltage division; a sampling module electrically isolated from the input module, the sampling module sampling the secondary first signal and the secondary second signal and converting these into a first digital signal reflecting the first signal and a second digital signal reflecting the second signal respectively; a conversion module, the conversion module receiving the first digital signal and the second digital signal outputted by the sampling module, and integrating the two digital quantities into a message based on communication agreements or communication protocols; an output module, the output module receiving the message outputted by the conversion module and outputting the message to the outside via a network interface of an optical fiber or a cable; a power supply module, the power supply module providing electric energy required for working to the input module, the sampling module, the conversion module and the output module. In the combined transformer, the input module, the sampling module and the conversion module are all located within the base on the low voltage side, which helps to improve the resistance to electromagnetic interference and the reliability of the combined transformer; furthermore, the maintenance is convenient, and modules can be maintained or replaced without power being shut off once.
In yet another schematic implementation of the combined transformer in the present invention, the base is provided therein with an outlet box, wherein the outlet box comprises: an input module, a secondary first signal being obtained after the first signal is processed by the input module in terms of voltage division, and a secondary second signal being obtained after the second signal is processed by the input module in terms of voltage division; a sampling module electrically isolated from the input module, the sampling module sampling the secondary first signal and the secondary second signal and converting these into a first digital signal reflecting the first signal and a second digital signal reflecting the second signal respectively; an output module, the output module receiving the first digital signal and the second digital signal outputted by the sampling module, and outputting the first digital signal and the second digital signal to the outside via a network interface of an optical fiber or a cable; a power supply module, the power supply module providing electric energy required for working to the input module, the sampling module and the output module. In the combined transformer, both the input module and the sampling module are located within the base on the low voltage side, which helps to improve the resistance to electromagnetic interference and the reliability of the combined transformer; furthermore, the maintenance is convenient, and modules can be maintained or replaced without power being shut off once.
In yet another schematic implementation of the combined transformer in the present invention, the outlet box also comprises: a synchronization module, the synchronization module receiving a synchronization signal from outside of the combined transformer and controlling the sampling module to synchronize according to the synchronization signal.
The preferred embodiments will be described below with reference to the accompanying drawings in a clear and understandable way, and the features, technical characteristics, advantages and implementations of the combined transformer in the present invention will be further described.
The following figures are only for schematic description and explanation of the present invention and are not to limit the scope of the present invention.

Fig. 1 is a structure diagram of a schematic implementation of a combined transformer for a power system.
Fig. 2 is a circuit structure diagram illustrating a schematic implementation of a voltage transformer of the combined transformer as shown in Fig. 1.
Fig. 3 is a partial structure diagram showing a schematic implementation of a capacitive screen in the combined transformer for a power system.
Fig. 4 is a structure diagram illustrating a schematic implementation of a base of the combined transformer for a power system.

In order to understand the technical features, objects and effects of the present invention more clearly, particular embodiments of the present invention are described here with reference to the accompanying drawings, in which like numerals in the figures represent the same parts or parts with a similar structure but the same function.
To make the figures look concise, only parts related to the present invention are schematically shown in each of the figures, and they do not represent the actual structure of the product. In addition, to make the figures look concise and easy to understand, in some figures, only one of components with the same structure or function is schematically drawn or marked.
In this context, "a" or "an" represents not only "only one" but also "more than one". In this context, "parallel" between two objects does not mean absolutely parallel in a geometric sense; instead, it may comprise deviation allowable during assembling and processing.
In this context, the ground potential, the base 20, the grounding flange 33 and the second input end 384 may be electrically connected to the ground, and they have the same potential. The shell 10, the primary conductor 40 and the high voltage flange 31 have the same potential.
Fig. 1 is a structure diagram showing a schematic implementation of a combined transformer for a power system. As shown in Fig. 1, the combined transformer in the present invention comprises a shell (10), a base (20) and an insulator (30). The shell 10 and the base 20 are connected via the insulator 30. In an implementation of the combined transformer in the present invention, the insulator 30 comprises a ceramic bush. The end of the insulator 30 connected to the shell 10 is provided with a high voltage flange 31, while the end thereof connected to the base 20 is provided with a grounding flange 33, wherein connecting bolts (not shown) connect the insulator 30 to the shell 10 and the base 20 via the high voltage flange 31 and the grounding flange 33, respectively. The insulator 30 may also be connected to the shell 10 and the base 20 by means of pouring or bonding.
A current transformer 12 is arranged in the shell 10. The current transformer 12 can detect the current in a primary conductor 40 in the power system. The primary conductor 40 is connected to the power supply line in the power system, and the potential of the primary conductor and the current passing therethrough are the same as the power supply line. The current transformer 12 sends a first signal reflecting the value of the current in the primary conductor 40 to the base 20. In a schematic implementation of the combined transformer in the present invention, the current transformer is a low power electronic current transformer disclosed in Chinese Patent ZL 200510024292.3 , comprising a magnetic core, a secondary winding and a shunt resistor. The leads of the secondary winding are evenly wound onto the magnetic core, the tail end of the secondary winding is connected to the shunt resistor, and the first signal induced in the secondary winding and reflecting the value of the current in the primary conductor is sent to the base, transmitted by the double shielded twisted-pair cable. The specific structure of the current transformer may be found in the specification of that invention patent and will not be described again here. Other types of current transformers may also be used, for example, current transformers based on magneto-optical effects.
In addition, the current transformer may have the following structures, with corresponding ways for measurement: i) it consists of two independent coils, each of the coils has at least one independent output, one of the coils is used for measuring the value of the current in the power system, and the other coil is used for triggering a protection action when there is overload current in the power system; ii) only one output is designed in the same low power coil, and this output value can meet the requirements of measurement and also precision requirements of protection for different currents in the primary conductor, for example, when the current in the primary conductor is less than or equal to 200% of the rated primary conductor current, the coil output meets the error requirements of the measurement level, while when the primary current is between 200% of the rated primary conductor current and the system short-circuit current, the secondary output meets the error requirements of the protection level; and iii) two paths of outputs are designed in the same low power coil, wherein one path meets the error requirements of the measurement level, and the other path meets the error requirements of the protection level.
There is an insulating medium 11 filling the space between the current transformer 12 and the shell 10, and the insulating medium 11 may be one of sulfur hexafluoride gas, insulation oil composite cable paper, epoxy resin composite crepe paper or silicone oil composite polytetrafluoroethylene tape.
The insulator 30 is provided therein with a voltage transformer 32. Fig. 2 is a circuit structure diagram illustrating a schematic implementation of a voltage transformer 32 of the combined transformer as shown in Fig. 1. As shown, the voltage transformer 32 comprises a resistive division circuit 34, a capacitive division circuit 36, an output branch circuit 38 and a secondary division resistor 37, wherein the capacitive division circuit 36 is realized by a plurality of layers of capacitive screens connected in series. The output branch circuit 38 comprises a first input end 382 and a second input end 384. The output branch circuit 38 can send a second signal reflecting the value of the voltage in the primary conductor 40 to the base 20. The resistive division circuit 34 is connected in parallel to the capacitive division circuit 36, i.e. one end of the resistive division circuit 34 is electrically connected to the primary conductor 40 to have the same potential as the primary conductor 40, and the other end of the resistive division circuit 34 is electrically connected to the first input end 382. One end of the capacitive division circuit 36 is electrically connected to the primary conductor 40 and the other end thereof is electrically connected to the second input end 384, and the first input end 382 is electrically connected to the secondary division potential point 361 of the capacitive division circuit 36. One end of the secondary division resistor 37 is electrically connected to the first input end 382 and the other end thereof is electrically connected to the second input end 384. In the schematic implementation as shown in the figure, the second input end 384 is grounded.
As a voltage division method achieved by connecting series resistors and series capacitors in parallel is used, the frequency domain range in which the combined transformer can measure the value of the voltage in the primary conductor accurately is broadened, and in particular the loss of the high frequency voltage signals is avoided. At the same time, compared with conventional electrode type capacitive dividers, the electrode area of each layer of capacitor in the plurality of layers of capacitive screens is increased greatly, and the thickness of the insulating layer between electrodes may be very small, so that the capacitance of the capacitive screens is greatly increased. In addition, the capacitive screens have low susceptibility to interference by external stray capacitance due to small parasitic capacitance, and therefore it is unnecessary to calibrate the error of the voltage transformer after on-site installation, i.e. the combined transformer can plug and play. At the same time, the combined transformer can be used for measurement of not only AC voltage but also DC voltage.
Fig. 3 is a partial structure diagram showing a schematic implementation of a capacitive screen in the combined transformer for a power system. As shown, the extension direction of the electrodes 362 of each layer of capacitive screen in the plurality of layers of capacitive screens is the same as the extension direction of the insulator 30. With such an arrangement method, capacitive screens with a large enough electrode area may be set, thereby avoiding too small a thickness of the insulating layer between electrodes. In addition, the extension direction of electrodes 362 may also form a certain included angle with the extension direction of the insulator 30.
With reference to Fig. 1 and Fig. 3, the insulator 30 is also provided with a fairlead 35, a current double shielded twisted-pair cable 352 transmitting the first signal to the base 20 and a voltage double shielded twisted-pair cable 354 transmitting the second signal to the base 20 are passed through the fairlead 35; the use of double shielded twisted-pair cables can reduce interference to the transmitted signals from external electromagnetic fields, but other transmission methods with a shielding function may also be used, and the fairlead 35 can protect the double shielded twisted-pair cables passed therethrough.
With reference to Fig. 2 and Fig. 3, the capacitive screens are wound onto the fairlead 35 or assembled onto the fairlead after being pre-molded, and the common electrodes between each layer of capacitors are connected in series. The electrodes of the capacitive screens may be flexible thin film with conductivity, such as aluminum foil, copper foil, thin-film semiconductor or paper semiconductor. The insulating layer between the electrodes may be flexible insulating thin film, such as sulfur hexafluoride composite thin-film, insulation oil composite cable paper, epoxy resin composite crepe paper or silicone oil composite polytetrafluoroethylene tape. The outermost layer of electrodes of the capacitive screens is electrically connected to the primary conductor 40, and the last layer of electrodes of the capacitive screens is electrically connected to the ground potential. The second input end 384 of the output branch circuit 38 is electrically connected to the last layer of electrodes of the capacitive screens, the first input end 382 thereof is electrically connected to the tap of the electrode in the capacitive division circuit 36 corresponding to the secondary division potential point 361, i.e. the potential difference between the secondary division potential point 361 and the ground potential is sampled as the voltage output signal so as to output the second signal reflecting the value of the voltage in the primary conductor, wherein the secondary division potential point is a potential point, the potential of which corresponds to the division ratio specified by the primary conductor potential according to standards. It will be understood by those skilled in the art that the resistance value of the output branch circuit 38 connected to the resistive division circuit 34 may be adjusted according to actual demands, to obtain a suitable potential.
With reference to Fig. 2 and Fig. 3, the resistive division circuit 34 comprises a plurality of thick-film resistors, and after these thick-film resistors are connected in series, one end is connected to the outermost layer of electrodes of the capacitive screens and the other end is connected to the secondary division potential point 361 of the capacitive screens, so that the resistive division circuit 34 is connected to the capacitive division circuit 36 in parallel, wherein one end of the thick-film resistors is electrically connected to the primary conductor 40 and the other end thereof is electrically connected to the first input end 382 of the output branch circuit 38. A curved resistance band can be formed by continuously attaching resistive slurry to the interior surface of the insulator by laser printing or spray coating. One end of the resistance band is electrically connected to the high voltage flange, and the other end thereof is electrically connected to the first input end 382 of the output branch circuit 38 and insulated from the grounding flange 33. Using the insulator 30 as a part of the voltage transformer not only ensures long service life, reliability and stability of the insulating system and insulating materials of the combined transformer, but also saves materials of the voltage transformer, so that the design costs are reduced.
Fig. 4 is a structure diagram illustrating a schematic implementation of a base 20 of the combined transformer for a power system. As shown, the base 20 (not shown) is provided therein with an outlet box 22 comprising: an input module 24, a sampling module 26, a conversion module 25, an output module 28, a synchronization module 27 and a power supply module 29.
The input module 24 can receive a first signal reflecting the value of the current in the primary conductor 40 and a second signal reflecting the value of the voltage in the primary conductor, which are inputted by the voltage double shielded twisted-pair cable 354 and the current double shielded twisted-pair cable 352 of the output branch circuit 38, respectively, and process the first signal and the second signal in terms of voltage division to further reduce their potentials, with a secondary first signal and a secondary second signal respectively obtained after voltage division.
The sampling module 26 samples the secondary first signal and the secondary second signal by means of isolating and coupling, for example, inputs the secondary first signal and the secondary second signal by means of magnetoelectric coupling or photoelectric coupling. The electric isolation between the sampling module 26 and the input module 24 is used for ensuring the maintenance safety of the outlet box 22 and preventing high voltage from influencing the devices within the outlet box 22. At the same time, the sampling module 26 also converts the secondary first signal and the secondary second signal in the analog form into a first digital signal and a second digital signal in a digital form.
The conversion module 25 receives the first digital signal and the second digital signal, carries out standardized processing in the form of re-sampling and format conversion on the first digital signal and the second digital signal in accordance with an industrial standard of power systems, for example, IEC61850-9-1/2 or IEC60044-1(FT3) to obtain formatted signals meeting communication agreements/protocols, and integrates two digital quantities (the first digital signal and the second digital signal) into a message based on communication agreements or communication protocols. The conversion module 25 may or may not be used according to the specific condition of a power system.
The output module 28 can receive formatted signals or directly receive digital signals; furthermore, the output module 28 converts formatted signals or digital signals into signals suitable for Ethernet or point-to-point transmission and outputs same to the outside via a network interface of an optical fiber or a cable.
As shown in Fig. 4, the outlet box 22 also comprises a synchronization module 27. It can receive a synchronization signal from outside of the combined transformer according to IEC61588 and control the sampling module 26 to synchronize according to the synchronization signal, to coordinate the measurement actions of each of the combined transformers and test terminals in the power system. After losing the external synchronization signal, the outlet box 22 switches automatically, so that the input module 24, the sampling module 26, the conversion module 25 and the output module 28 within the outlet box 22 continue working, to ensure the sampling is continuous and uninterrupted.
The power supply module 29 can provide electric energy required for working to the input module 24, the sampling module 26, the conversion module 25, the output module 28 and the synchronization module 27. The power supply module 29 may convert externally-provided DC ± 110V or AC 220V into low voltage DC that may be used by the sampling module 26, the input module 24 and the conversion module 25 to work. They may also be powered by batteries.
In the combined transformer for a power system, the shell 10 on the high voltage side is of a passive structure, no energy-obtaining, sampling conversion module is arranged within the shell 10, the input module 24, the sampling module 26 and the conversion module 25 are all located within the base 20 on the low voltage side, which helps to improve the resistance to electromagnetic interference and the reliability of the combined transformer; furthermore, the maintenance is convenient, and modules can be maintained or replaced without shutting power off once.
In this context, "schematic" indicates "serving as an example, instance or description", and no illustration or implementation described as "schematic" in this context should be interpreted as a more preferred or more advantageous technical solution.
It should be understood that, although the specification is given according to each of the embodiments, it is by no means the case that each embodiment only comprises one independent technical solution; this narration manner of the specification is only for clarity, and for those skilled in the art, the specification should be regarded as a whole, and the technical solutions in each of the embodiments may also be suitably combined to form other implementations that may be understood by those skilled in the art.
The series of detailed descriptions set forth above are only specific descriptions of feasible embodiments of the present invention, and are not intended to limit the protective scope of the present invention; and all the equivalent embodiments or modifications made without departing from the protective scope of the present invention as claimed.

Claims

A combined transformer for a power system, comprising a shell (10), a base (20) and an insulator (30) connecting the shell (10) and the base (20), wherein
a current transformer (12) is arranged in the shell (10), which current transformer (12) can detect the current in a primary conductor (40) in the power system, and which current transformer (12) sends a first signal reflecting the value of the current in the primary conductor (40) to the base (20);
a voltage transformer (32) is arranged in the insulator (30), wherein the voltage transformer (32) comprises:
an output branch circuit (38), comprising a first input end (382) and a second input end (384), wherein the output branch circuit (38) sends a second signal reflecting the value of the voltage in the primary conductor (40) to the base (20);

a resistive division circuit (34), one end of which is electrically connected to the primary conductor (40), the other end thereof being electrically connected to the first input end (382);

a capacitive division circuit (36) formed by a plurality of layers of capacitive screens serially connected, one end of which is electrically connected to the primary conductor (40), the other end thereof being electrically connected to the second input end (384), and the first input end (382) being electrically connected to a secondary division potential point (361) of the capacitive division circuit (36); and

a secondary division resistor (37), one end of which is electrically connected to the first input end (382), the other end thereof being electrically connected to the second input end (384)
characterized in that
the capacitive division circuit (36) is formed by a plurality of electrode layers of capacitive screens serially connected, wherein the outermost electrode layer is electrically connected to the primary conductor (40), wherein the second input end (384) of the output branch circuit (38) is electrically connected to the last electrode layer, wherein the first input end (382) of the output branch circuit (38) is electrically connected to a tap of an electrode layer corresponding to the secondary division potential point (361).
The combined transformer as claimed in claim 1, wherein the extension direction of electrode layers (362) of capacitive screens is the same as the extension direction of the insulator (30).
The combined transformer as claimed in claim 1 or 2, wherein the combined transformer comprises a high voltage flange (31) and a grounding flange (33), wherein the insulator (30) is connected to the shell (10) through the high voltage flange (31), and the insulator (30) is connected to the base (20) through the grounding flange (33).
The combined transformer as claimed in any of claims 1 to 3, wherein the combined transformer comprises a fairlead (35), a current double shielded twisted-pair cable (352) transmitting the first signal to the base (20) and a voltage double shielded twisted-pair cable (354) transmitting the second signal to the base (20), wherein the current double shielded twisted-pair cable (352) and the voltage double shielded twisted-pair cable (354) are passed through the fairlead (35).
The combined transformer as claimed in claim 4, wherein the capacitive screens are wound onto the fairlead (35) or assembled onto the fairlead (35) after being pre-molded, the electrode layers (362) of capacitive screens are one of aluminum foil, copper foil, thin-film semiconductor or paper semiconductor, while the insulating layer between the electrode layers (362) is one of sulfur hexafluoride gas composite thin-film, insulation oil composite cable paper, epoxy resin composite crepe paper or silicone oil composite polytetrafluoroethylene tape.
The combined transformer as claimed in any of claims 1 to 5, wherein the second input end (384) of the output branch circuit (38) is electrically connected to ground potential.
The combined transformer as claimed in any of claims 1 to 6, wherein the resistive division circuit (34) comprises a plurality of thick-film resistors, wherein the thick-film resistors, after being connected to each other in series, are connected in parallel to both ends of the capacitive division circuit (36), one end of the thick-film resistors connected in series is electrically connected to the primary conductor (40), and the other end thereof is electrically connected to the first input end (382) of the output branch circuit (38).
The combined transformer as claimed in any of claims 1 to 7, wherein the resistive division circuit (34) comprises a resistance band, wherein the resistance band is formed by continuously attaching resistive slurry to the interior surface of the insulator (30) by laser printing or spray coating, one end of the resistance band is electrically connected to the high voltage flange (31), and the other end thereof is electrically connected to the first input end (382) of the output branch circuit (38).
The combined transformer as claimed in any of claims 1 to 8, wherein the current transformer (12) is a low power electronic current transformer comprising a magnetic core, a secondary winding wound onto the magnetic core and a shunt resistor connected to the tail end of the secondary winding.
The combined transformer as claimed in any of claims 1 to 9, wherein there is an insulating medium (11) filling the space between the current transformer (12) and the shell (10), wherein the insulating medium (11) is one of sulfur hexafluoride gas, insulation oil composite cable paper, epoxy resin composite crepe paper or silicone oil composite polytetrafluoroethylene tape.
The combined transformer as claimed in any one of claims 1 to 10, wherein the base (20) is provided therein with an outlet box (22), wherein the outlet box (22) comprises:
an input module (24), a secondary first signal being obtained after the first signal is processed by the input module (24) in terms of voltage division, and a secondary second signal being obtained after the second signal is processed by the input module (24) in terms of voltage division;

a sampling module (26) electrically isolated from the input module (24), the sampling module (26) being able to sample the secondary first signal and the secondary second signal and convert these into a first digital signal reflecting the first signal and a second digital signal reflecting the second signal respectively;

a conversion module (25), the conversion module (25) receiving the first digital signal and the second digital signal outputted by the sampling module (26) and integrating the two digital quantities into a message based on communication agreements or communication protocols;

an output module (28), the output module (28) receiving the message outputted by the conversion module (25) and outputting the message to the outside via a network interface of an optical fiber or a cable; and

a power supply module (29), the power supply module (29) providing electric energy required for working to the input module (24), the sampling module (26), the conversion module (25) and the output module (28).
The combined transformer as claimed in any one of claims 1 to 10, wherein the base (20) is provided therein with an outlet box (22), wherein the outlet box (22) comprises:
an input module (24), a secondary first signal being obtained after the first signal is processed by the input module (24) in terms of voltage division, and a secondary second signal being obtained after the second signal is processed by the input module (24) in terms of voltage division;

a sampling module (26) electrically isolated from the input module (24), the sampling module (26) being able to sample the secondary first signal and the secondary second signal and convert these into a first digital signal reflecting the first signal and a second digital signal reflecting the second signal respectively;

an output module (28), the output module (28) receiving the first digital signal and the second digital signal outputted by the sampling module (26), and outputting the first digital signal and the second digital signal to the outside via a network interface of an optical fiber or a cable; and

a power supply module (29), the power supply module (29) providing electric energy required for working to the input module (24), the sampling module (26) and the output module (28).
The combined transformer as claimed in claim 11 or 12, wherein the outlet box (22) also comprises:
a synchronization module (27), the synchronization module (27) receiving a synchronization signal from outside of the combined transformer and controlling the sampling module (26) to synchronize according to the synchronization signal.