A method for detecting partial discharge of a cable transformer group

Abstract

The invention provides a cable transformer group partial discharge detection method, belonging to the technical field of partial discharge detection. The invention utilizes traditional cable oscillation wave partial discharge test equipment and technology, obtains the short-circuit inductance of the three-phase transformer according to the basic parameters of the three-phase transformer, connects the three-phase transformer in series into the circuit, forms an LC oscillation circuit with the distributed capacitance of the cable line, the additional resonant inductance and the inductance of the three-phase transformer, and simultaneously detects the partial discharge of the cable line and the three-phase transformer in the form of oscillation wave partial discharge detection. There is no need to isolate the three-phase transformer and the cable separately for separate detection, thus greatly reducing the detection time. The invention is mainly applied to on-site detection of new energy power generation stations and large industrial and mining enterprises.

Description

Technical Field

[0001] The invention relates to the technical field of partial discharge detection, in particular to a method for detecting partial discharge of a cable transformer group. Background Art

[0002] Partial discharge is widely used in insulation testing of power equipment, especially cables and transformers. Cable partial discharge detection usually uses cable oscillation wave partial discharge detection technology, which has gradually become the mainstream technology since the 1990s and has developed into DC and AC excitation types. It uses resonant inductors and cables to form an LC oscillation circuit, and then uses partial discharge sensors and partial discharge data acquisition and analysis units to collect and analyze the signals of the oscillation circuit. The traveling wave method is used to locate the local signal, and the location of the partial discharge can be determined.

[0003] Transformer partial discharge detection is generally used for testing high-voltage oil-fired transformers and dry-type transformers before leaving the factory. Non-electrical methods such as ultrasound and chromatographic analysis are often used for on-site detection.

[0004] At present, with the rapid development of new energy technologies such as wind power and photovoltaics and large-scale working conditions enterprises, the wiring method of medium and high voltage 35~66kV (cable) line transformer group has become a major wiring method. Conventional preventive tests require the cables and transformers to be separated for separate testing of the cables and transformers, which requires a lot of electrical isolation and safety measures. In addition, since partial discharge detection is generally not used in the pre-test of 35~66kV and below transformers, and the power grid generally adopts a neutral point non-effective grounding system, the cable transformer group with a high overvoltage level needs to be disassembled for testing, the joints are disassembled, and the transformer windings are tested for DC withstand voltage and dielectric loss according to the national standards. The cables are tested for AC withstand voltage and partial discharge according to the national standards. The testing process is cumbersome and increases the testing time. Therefore, a new detection method is needed that can directly perform partial discharge detection on the cable transformer group without disassembling the joints of the cable and transformer. Summary of the invention

[0005] According to the above technical problems, a cable transformer group partial discharge detection method is provided. The present invention utilizes traditional cable oscillation wave partial discharge test equipment and technology, obtains the short-circuit inductance of the transformer according to the basic parameters of the transformer, connects the transformer in series into the circuit, and forms an LC oscillation circuit with the distributed capacitance of the cable line, the additional resonant inductance, and the transformer inductance, and adopts the oscillation wave partial discharge detection form to simultaneously detect the partial discharge of the cable line and the transformer, without isolating the transformer and the cable separately for separate detection, which greatly reduces the detection time, and does not use multiple devices at the same time, and is mainly used for on-site detection of new energy power generation stations and large industrial and mining enterprises.

[0006] The technical means adopted by the present invention are as follows:

[0007] A method for detecting partial discharge of a cable transformer group, wherein the device used in the method includes a power supply, a switch, an additional resonant inductor, a partial discharge sensor and a partial discharge data acquisition and analysis unit, and the method includes the following steps:

[0008] S1: A three-phase transformer is Y-connected to one end of a three-phase cable, the other end of the three-phase cable is open and not grounded, and the single-phase cable to be tested in the three-phase cable is connected to the three-phase transformer, and the remaining two-phase cables are disconnected from the three-phase transformer; the power supply is connected in series with the neutral point of the Y-connected winding of the three-phase transformer through the switch and the additional resonant inductor to charge the Y-connected winding and the single-phase cable to be tested, and the switch is connected to the ground. When the switch is closed, one end of the additional resonant inductor applying voltage and the ground are short-circuited; the partial discharge sensor is connected to the single-phase cable to be tested, and is close to the connection between the single-phase cable to be tested and the three-phase transformer; the partial discharge sensor is electrically connected to the partial discharge data acquisition and analysis unit;

[0009] S2: Calculate the adjustment value required for the additional resonant inductor so that the distributed capacitance of the single-phase cable line to be tested, the additional resonant inductor, and the inductance of the Y-connected winding of the three-phase transformer form an LC oscillation circuit; the oscillation frequency of the LC oscillation circuit f G 20~300Hz; further lower, f G Preferably, it is an industrial frequency, such as 50 Hz or 60 Hz;

[0010] Calculating the adjustment value required for the additional resonant inductor includes:

[0011] S21: Calculate the short-circuit impedance of the Y-connected winding of the three-phase transformer X S ,

[0012]

[0013] Where: S N is the capacity of the three-phase transformer, U N is the rated voltage of the three-phase transformer, ∆U k is the impedance voltage of the three-phase transformer;

[0014] S22: According to the short-circuit impedance of the Y-connected winding of the three-phase transformer Xs Get the inductance value of the Y-connected winding of the three-phase transformer LS ,

[0015]

[0016] S23: Calculate the overall inductance of the LC oscillation circuit Lp ;

[0017]

[0018] Where: Cc is the distributed capacitance value of the single-phase cable to be tested in the three-phase cable;

[0019] S24: Calculate the additional resonant inductance value, which is Lp-Ls ;

[0020] S3: adjusting the additional resonant inductance value according to the calculation result of step S2, then turning on the power supply and disconnecting the switch, so that the power supply charges the three-phase transformer and the single-phase cable to be tested, and then closing the switch to short-circuit one end of the additional resonant inductance voltage application and the ground, so that the distributed capacitance of the single-phase cable line to be tested, the additional resonant inductance, and the inductance of the Y-connected winding of the three-phase transformer form an LC oscillation circuit;

[0021] S4: The partial discharge sensor and the partial discharge data acquisition and analysis unit acquire and analyze the high-frequency signal formed by the LC oscillation circuit to achieve simultaneous detection of partial discharge of the three-phase transformer and the three-phase cable.

[0022] Preferably, at least one winding of the three-phase transformer is ∆-connected or short-circuited to ensure a zero-sequence short circuit.

[0023] Preferably, a protection resistor is connected in series between the power supply and the additional resonant inductor.

[0024] Preferably, the partial discharge sensor is a current type sensor or a voltage type sensor.

[0025] Preferably, the current type sensor is an HFCT high frequency current sensor, and the voltage type sensor is a coupled capacitance sensor.

[0026] Preferably, the HFCT high-frequency current sensor is an open-type HFCT high-frequency current sensor.

[0027] Preferably, the power supply is a high voltage power supply, and the voltage and current generated by the power supply satisfy the requirement that the additional resonant inductor, three-phase transformer and three-phase cable can generate LC oscillation. The power supply is an AC high voltage generator, and an ultra-low frequency, variable frequency series harmonic, industrial frequency series harmonic or other types of high voltage generators can be selected.

[0028] Preferably, the switch is a high-voltage switch, which may be a mechanical switch or a power electronic switch, and is capable of withstanding the maximum test voltage and maximum impact current.

[0029] Preferably, the detection frequency range of the partial discharge sensor and the partial discharge data acquisition and analysis unit is preferably 30 kHz to 3 MHz.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] 1. The present invention can simultaneously perform partial discharge detection on three-phase transformers and cable lines.

[0032] 2. Reduce the inspection time and cost of line transformer group equipment.

[0033] 3. The existing oscillating wave pressurization equipment (power supply, additional resonant inductor, partial discharge sensor and partial discharge data acquisition and analysis unit) can be used to realize simultaneous detection of three-phase transformers and cables.

[0034] Based on the above reasons, the present invention can be widely promoted in the field of partial discharge detection and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0036] Figure 1 This is a wiring diagram of a cable transformer group partial discharge detection method device (using a current sensor) according to the present invention.

[0037] Figure 2 This is a wiring diagram of a cable transformer group partial discharge detection method device (using a voltage sensor) according to the present invention. DETAILED DESCRIPTION

[0038] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail with reference to the accompanying drawings and in combination with the embodiments. In order to make the purpose, 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 in combination with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0039] like Figure 1~2As shown, the present invention provides a method for detecting partial discharge of a cable transformer group, and the equipment used includes a power supply 1, a switch 2, an additional resonant inductor 3, a partial discharge sensor 4 and a partial discharge data acquisition and analysis unit 5 (a partial discharge detector can be used, which is not described in detail for the prior art); the power supply 1 is a high-voltage power supply, and the power supply 1 is an AC high-voltage generator, and an ultra-low frequency, variable frequency series harmonic, industrial frequency series harmonic or other types of high-voltage generators can be selected, and the voltage and current generated by the generator meet the requirements that the additional resonant inductor 3, the three-phase transformer 6 and the three-phase cable 7 can generate LC oscillation. The partial discharge sensor 4 uses a current type sensor (such as Figure 1 ) or voltage type sensors (such as Figure 2 ). The current type sensor is a HFCT high frequency current sensor. In order to reduce wiring, it is preferably an open type HFCT high frequency current sensor. The voltage type sensor is a coupling capacitor sensor. The switch 2 is a high voltage switch. A mechanical switch or a power electronic switch can be selected, and it can withstand the maximum test voltage and maximum impact current. The detection frequency range of the partial discharge sensor 4 and the partial discharge data acquisition and analysis unit 5 is preferably 30kHz~3MHz.

[0040] The steps adopted by the method are as follows:

[0041] S1: The three-phase transformer 6Y is connected to one end of the three-phase cable 7, the other end of the three-phase cable 7 is open and not grounded, and the single-phase cable to be tested in the three-phase cable 7 is connected to the three-phase transformer 6, and the remaining two-phase cables are disconnected from the three-phase transformer 6; at least one winding of the three-phase transformer 6 is ∆-connected or short-circuited to ensure zero-sequence short circuit. The power supply 1 is connected in series with the neutral point of the Y-connected winding of the three-phase transformer 6 through the switch 2 and the additional resonant inductor 3, the switch 2 is connected to the ground, and when the switch 2 is closed, one end of the additional resonant inductor applying voltage and the ground are short-circuited; a protective resistor 8 is connected in series between the power supply 1 and the additional resonant inductor 3. The partial discharge sensor 4 is connected to the single-phase cable to be tested, and is close to the connection between the single-phase cable to be tested and the three-phase transformer 6; the partial discharge sensor 4 is electrically connected to the partial discharge data acquisition and analysis unit 5;

[0042] S2: Calculate the adjustment value required for the additional resonant inductor 3 so that the distributed capacitance of the single-phase cable line to be tested, the additional resonant inductor 3, and the inductance of the Y-connected winding of the three-phase transformer 6 form an LC oscillation circuit, and the oscillation frequency of the LC oscillation circuit is f G 20~300Hz, preferably 50Hz or 60Hz;

[0043] Calculating the adjustment value required for the additional resonant inductor 3 includes:

[0044] S21: Calculate the short-circuit impedance of the Y-connected winding of the three-phase transformer 6 X S ,

[0045]

[0046] Where: S N is the capacity of the three-phase transformer 6, U N is the rated voltage of the three-phase transformer 6, ∆U k is the impedance voltage of the three-phase transformer 6;

[0047] S22: According to the short-circuit impedance of the Y-connected winding of the three-phase transformer 6 Xs Get the inductance value of the Y-connected winding of the three-phase transformer 6 LS ,

[0048]

[0049] S23: Calculate the overall inductance of the LC oscillation circuit Lp ;

[0050]

[0051] Wherein: Cc is the distributed capacitance value of the single-phase cable to be tested in the three-phase cable 7;

[0052] S24: Calculate the additional resonant inductance value, which is Lp-Ls ;

[0053] S3: adjusting the additional resonant inductance value according to the calculation result of step S2, then turning on the power supply 1 and disconnecting the switch 2, so that the power supply 1 charges the three-phase transformer 6 and the single-phase cable to be tested, and then closing the switch to short-circuit one end of the additional resonant inductance 3 applying voltage and the ground, so that the distributed capacitance of the single-phase cable line to be tested, the additional resonant inductance 3, and the inductance of the Y-connected winding of the three-phase transformer 6 form an LC oscillation circuit;

[0054] S4: The partial discharge sensor 4 and the partial discharge data acquisition and analysis unit 5 collect and analyze the high-frequency signal formed by the LC oscillation circuit to achieve simultaneous detection of partial discharges of the three-phase transformer 6 and the three-phase cable 7. Example

[0055] Partial discharge detection for 4MW / 35kV wind turbine step-up transformer and collector cable line group:

[0056] The step-up transformer has a capacity of 5MVA, a voltage of 3.3 / 37.5kV, a wiring group of △Y-1, and a short-circuit impedance (also called impedance voltage, expressed as a percentage of the rated voltage) of 7%; that is S N =5MVA, U N =37.5KV, ∆U k =7%.

[0057] Cable model, YJLV22-26 / 25-3×70mm 2 , length 1000m, check the cable manufacturer's manual, YJLV22-26 / 25-3×70mm 2 , the distributed capacitance of 1000m power cable is about 125 nF (single core / single phase), that is, Cc=125×10 -9 F .

[0058] f G Take the power frequency as 50Hz;

[0059] By calculation, we can know the short-circuit impedance of the Y-connected winding of the transformer

[0060]

[0061] Ignore the resistance in the short-circuit impedance and the inductance value of the short-circuit impedance

[0062]

[0063] If no additional resonant inductance is added, the oscillation frequency of the single-phase cable to be tested is

[0064]

[0065] This far exceeds the range requirement of 20-300 Hz for cable series harmonic or oscillation wave detection frequency, so the present invention adds an additional resonant inductor to make the oscillation frequency of the single-phase cable to be tested meet the specification requirements.

[0066] The overall inductance value Lp of the LC oscillation circuit

[0067]

[0068] The inductance value of the additional resonant inductor is adjusted to Lp-Ls= 80.94 H .

[0069] Since the impedance of the additional resonant inductor is much larger than the short-circuit impedance of the transformer, the voltage applied to the transformer is approximately equal to the voltage on the cable, which can effectively stimulate local discharge at the insulation defects.

[0070] The voltage of the power supply can stimulate partial discharge at the insulation defect and must not exceed the withstand voltage of the cable and transformer. For example, if you check the factory test withstand voltage of a 35kV transformer, the dry-type transformer is 60kV, the oil-immersed transformer is 68kV, and the 80% voltage is 48.0 and 54.4kV respectively; the partial discharge applied voltage of a 35kV cable can be 1.7U 0 , U 0 Take the cable phase voltage as 26kV, that is, the AC effective value is 44.2kV, which is approximately the operating phase voltage It is 2.08 times of that of the insulation defect, which can stimulate local discharge of insulation defects and does not exceed the withstand voltage of cables and transformers, meeting the requirements of "DL / T596-2021 Preventive Test Procedure for Power Equipment".

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting partial discharge of a cable transformer group, wherein the device used in the method comprises a power supply, a switch, an additional resonant inductor, a partial discharge sensor and a partial discharge data acquisition and analysis unit, wherein: The method comprises the following steps: S1: A three-phase transformer is Y-connected to one end of a three-phase cable, the other end of the three-phase cable is open and not grounded, and the single-phase cable to be tested in the three-phase cable is connected to the three-phase transformer, and the remaining two-phase cables are disconnected from the three-phase transformer; the power supply is connected in series with the neutral point of the Y-connected winding of the three-phase transformer through the switch and the additional resonant inductor to charge the Y-connected winding and the single-phase cable to be tested, and the switch is connected to the ground, and when the switch is closed, one end of the additional resonant inductor applying voltage and the ground are short-circuited; the partial discharge sensor is connected to the single-phase cable to be tested, and is close to the connection between the single-phase cable to be tested and the three-phase transformer; the partial discharge sensor is electrically connected to the partial discharge data acquisition and analysis unit; at least one winding of the three-phase transformer Connect or short-circuit to ensure zero-sequence short circuit; S2: Calculate the adjustment value required for the additional resonant inductor so that the distributed capacitance of the single-phase cable line to be tested, the additional resonant inductor, and the inductance of the Y-connected winding of the three-phase transformer form an LC oscillation circuit, and the oscillation frequency of the LC oscillation circuit is f G 20~300Hz; Calculating the adjustment value required for the additional resonant inductor includes: S21: Calculate the short-circuit impedance of the Y-connected winding of the three-phase transformer X S , ， Where: S N is the capacity of the three-phase transformer, U N is the rated voltage of the three-phase transformer, ∆U k is the impedance voltage of the three-phase transformer; S22: According to the short-circuit impedance of the Y-connected winding of the three-phase transformer Xs Get the inductance value of the Y-connected winding of the three-phase transformer LS , ， S23: Calculate the overall inductance of the LC oscillation circuit Lp , ， Where: Cc is the distributed capacitance value of the single-phase cable to be tested in the three-phase cable; S24: Calculate the additional resonant inductance value, which is Lp-Ls ; S3: adjusting the additional resonant inductance value according to the calculation result of step S2, then turning on the power supply and disconnecting the switch, so that the power supply charges the three-phase transformer and the single-phase cable to be tested, and then closing the switch to short-circuit one end of the additional resonant inductance voltage application and the ground, so that the distributed capacitance of the single-phase cable line to be tested, the additional resonant inductance, and the inductance of the Y-connected winding of the three-phase transformer form an LC oscillation circuit; S4: The partial discharge sensor and the partial discharge data acquisition and analysis unit acquire and analyze the high-frequency signal formed by the LC oscillation circuit to achieve simultaneous detection of partial discharge of the three-phase transformer and the three-phase cable.

2. A cable transformer group partial discharge detection method according to claim 1, characterized in that: A protection resistor is connected in series between the power supply and the additional resonant inductor.

3. A cable transformer group partial discharge detection method according to claim 1, characterized in that: The partial discharge sensor adopts a current type sensor or a voltage type sensor.

4. A cable transformer group partial discharge detection method according to claim 3, characterized in that: The current type sensor is an HFCT high frequency current sensor, and the voltage type sensor is a coupling capacitance sensor.

5. A cable transformer group partial discharge detection method according to claim 4, characterized in that: The HFCT high-frequency current sensor is an open-type HFCT high-frequency current sensor.

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