Alternating-current ultralow-frequency oscillatory wave partial discharge detector for cable transformer bank

Abstract

The utility model relates to the electric power field, and discloses a cable transformer group AC ultralow frequency oscillation wave partial discharge detector comprising an ultralow frequency AC high voltage generator, a high voltage switch device, an additional resonant inductor, a three-phase cable line transformer group, a partial discharge sensor and a partial discharge data acquisition and analysis unit; the three-phase cable line transformer bank is formed by connecting a three-phase transformer Y-connection winding and a three-phase cable line, at least one winding of the three-phase transformer is connected or short-circuited, the other end of the cable line is open-circuited and is not grounded, and the ultralow-frequency alternating-current high-voltage generator is connected with the additional resonant inductor and a neutral point of the three-phase transformer Y-connection winding in series. And when only a single-phase cable is connected, the other two phases need to be disconnected from the transformer winding. According to the utility model, transformer and cable line partial discharge detection can be carried out simultaneously, and line transformer group equipment detection time and cost are reduced.

Description

Technical Field

[0001] The present utility model relates to the field of electric power, and specifically to an AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group. Background Art

[0002] With the rapid development of new energy technologies such as wind power and photovoltaic power, and large-scale industrial enterprises, the wiring mode of the 35 - 66 kV (cable) line transformer group in medium and high voltage has become a main wiring mode. For conventional preventive tests, the cable and the transformer need to be separated, and a large number of electrical isolations and safety measures are required.

[0003] In addition, since partial discharge detection is generally not used for the pre-test of transformers below 35 - 66 kV, and the power grid generally adopts a non-effectively grounded neutral system with a relatively high overvoltage level, the current test methods have problems such as poor effectiveness and complex operation. Content of the Utility Model

[0004] In order to overcome the above deficiencies, the present utility model provides an AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group.

[0005] The technical solution adopted by the present utility model is as follows:

[0006] An AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group includes an ultra-low frequency AC high voltage generator, a high voltage switch device, an additional resonant inductor, a three-phase cable line transformer group, a partial discharge sensor, and a partial discharge data acquisition and analysis unit.

[0007] The three-phase cable line transformer group is composed of a Y-connected winding of a three-phase transformer and three-phase cable lines. At least one winding of the three-phase transformer is connected in Δ or short-circuited, and the other end of the cable line is open and not grounded.

[0008] The ultra-low frequency AC high voltage generator is connected in series with the additional resonant inductor and the neutral point of the Y-connected winding of the three-phase transformer. When only one-phase cable is connected, the other two phases need to be disconnected from the transformer winding.

[0009] The high voltage switch device is connected in series between the connection point of the ultra-low frequency AC high voltage generator and the additional resonant inductor and the ground.

[0010] The partial discharge sensor is connected in series between the partial discharge data acquisition and analysis unit and the Y-connected winding of the three-phase transformer, and the current sensor and the voltage sensor are directly connected to the connection point and the ground.

[0011] The maximum output voltage of the ultra-low frequency AC high voltage generator is higher than the test voltage. The test voltage is based on the AC withstand voltage values of the transformer winding and the cable that are not higher than the preventive test standard, and not lower than the operating voltage and partial discharge test voltage of the transformer winding and the cable.

[0012] The maximum output current of the ultra-low frequency AC high-voltage generator is greater than the test current. The test current refers to the test voltage and the distributed capacitance of the power cable, and a certain margin is reserved.

[0013] The high-voltage switch device selects a mechanical switch or a power electronic switch, and can withstand the highest test voltage and the maximum impulse current, and closes and grounds at the ultra-low frequency AC peak point.

[0014] The partial discharge sensor is three-phase or single-phase. It can adopt a current type, such as an HFCT high-frequency current sensor, or a voltage type, such as a coupling capacitor sensor. When using an HFCT high-frequency current sensor, an open type is optimally used.

[0015] The beneficial effects of the present utility model:

[0016] The present utility model can simultaneously detect the partial discharge of the transformer and the cable line, reducing the detection time and cost of equipment such as line transformer groups. Description of the Drawings

[0017] Figure 1 is the schematic diagram of the present utility model;

[0018] Figure 2 is the wiring diagram of the present utility model. Detailed Implementation Manner

[0019] As Figure 1-2 shown: An AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group includes an ultra-low frequency AC high-voltage generator, a high-voltage switch device, an additional resonance inductor, a three-phase cable line transformer group, a partial discharge sensor, and a partial discharge data acquisition and analysis unit.

[0020] The three-phase cable line transformer group is composed of a three-phase transformer Y-connected winding and a three-phase cable line connected. At least one winding of the three-phase transformer is in △ connection or short-circuited, and the other end of the cable line is open and not grounded;

[0021] The ultra-low frequency AC high-voltage generator is connected in series with the neutral point of the additional resonance inductor and the three-phase transformer Y-connected winding. When only one-phase cable is connected, the other two phases need to be disconnected from the transformer winding;

[0022] The high-voltage switch device is connected in series between the connection point of the ultra-low frequency AC high-voltage generator and the additional resonance inductor and the ground;

[0023] The partial discharge sensor is connected in series between the partial discharge data acquisition and analysis unit and the three-phase transformer Y-connected winding. The current sensor and the voltage sensor are directly connected to the connection point and the ground.

[0024] The highest output voltage of the ultra-low frequency AC high-voltage generator is higher than the test voltage. The test voltage is based on the AC withstand voltage values of the transformer windings and cables that are not higher than the preventive test standards, and is not lower than the operating voltage and partial discharge test voltage of the transformer windings and cables.

[0025] The maximum output current of the ultra-low frequency AC high-voltage generator is greater than the test current. The test current refers to the test voltage and the distributed capacitance of the power cable, and a certain margin is reserved.

[0026] The high-voltage switch device is selected as a mechanical switch or a power electronic switch, and can withstand the highest test voltage and the maximum impact current, and is switched to ground at the peak point of the ultra-low frequency AC.

[0027] The partial discharge sensor can be three-phase or single-phase. It can adopt a current type, such as an HFCT high-frequency current sensor, or a voltage type, such as a coupling capacitor sensor. When using an HFCT high-frequency current sensor, an open type is preferably used.

[0028] After the Y-connected windings of the three-phase transformer under test and the three-phase cable line are charged to a certain voltage value, the high-voltage switch device is switched on, short-circuiting the connection point between the ultra-low frequency AC high-voltage generator and the resonant inductor and the ground. The distributed capacitance of the three-phase cable line, the additional resonant inductor, and the Y-connected windings of the three-phase transformer under test form an LC oscillation, exciting partial discharges at the insulation defect sites; the partial discharge sensor and the partial discharge data acquisition and analysis unit collect the high-frequency signals in the loop for analysis, and at the same time detect the partial discharges in the transformer and the cable line.

[0029] For the additional resonant inductor, calculate the value according to the (cable) line parameters and the target resonant frequency, such as 20 - 300 Hz, determine the insulation strength according to the maximum output voltage, and determine the wire cross-section according to the maximum output current and the working time.

[0030] The detection frequency range of the partial discharge sensor and the partial discharge data acquisition and analysis unit should be 30 kHz to 3 MHz; when used for cable partial discharge detection, it is preferably 30 to 500 kHz; the partial discharge detection frequency of the transformer winding can be appropriately increased, or two types of partial discharge sensors can be used to detect the transformer winding and the power cable respectively.

[0031] In an embodiment, AC withstand voltage or partial discharge tests are carried out on the step-up transformer of a 4 MW / 35 kV wind turbine and the transformer bank of the collector cable line.

[0032] The capacity of the step-up dry-type transformer is 5 MVA, the voltage is 37.5 / 3.3 kV, the connection group is Y△-11, and the short-circuit impedance is 7%;

[0033] The power cable is cross-linked polyethylene insulated aluminum core, model YJLV 22 -26 / 25-3*70mm 2, The length is 1000m.

[0034] YJLV 22 -26 / 25 - 3*70mm 2 The distributed capacitance of the power cable is about 125nF (single core).

[0035] Calculate the short - circuit impedance of the transformer (single - phase (UN√3, SN÷3))

[0036]

[0037] Ignoring the resistance in the short - circuit impedance, the inductance value of the short - circuit impedance is about

[0038]

[0039] According to the frequency close to the power frequency 50 / 60Hz, the additional resonant inductance value

[0040]

[0041] Since the additional resonant inductance value is much larger than the short - circuit impedance of the transformer, the oscillating wave voltage applied on the transformer winding is approximately equal to the voltage on the cable, which can effectively excite the partial discharge at the insulation defect.

[0042] Check the withstand voltage value of the 35kV transformer during factory test. For dry - type transformers, the withstand voltage is 60kV, and 80% of the voltage is 48.0kV respectively;

[0043] The voltage applied for the partial discharge of the 35kV cable can be taken as 1.7U0. U0 is taken as 26kV between the cable and the ground, that is, the AC effective value is 44.2kV, which is about 2.08 times of the operating phase voltage (1.05U N / √3), which can excite the partial discharge of the insulation defect and does not exceed the withstand voltage values of the cable and the dry - type transformer.

[0044] Taking into account the hand - over withstand voltage test, the maximum output voltage U of the ultra - low - frequency AC high - voltage generator Max is taken as 52kV.

[0045] The maximum output current of the ultra - low - frequency AC high - voltage generator, referring to the capacitance current between the 0.1Hz cable and the ground

[0046] I C =3×2πf L C×U Max =3×2×3.14×0.1×125×10 -9 ×52×10 3 =0.012

[0047] A

[0048] The value of the additional resonant inductance current is referred to the capacitance current between the cable and the ground at 50Hz of the oscillating wave

[0049] I C = 3 × 2πfC × U Max = 3 × 314 × 125 × 10 -9 × 52 × 10 3 = 6.13 A

[0050] To reduce the wiring workload, it is recommended to use an open-type HFCT for the partial discharge sensor, but the frequency range should preferably maintain a high sensitivity between 30 kHz and 3 MHz. When using a capacitive voltage sensor for coupling, the withstand voltage of the capacitor should meet the requirements.

[0051] It is possible to measure the three phases of the transformer and the three cores of the power cable simultaneously. If only one (two) phase (core) needs to be measured, the number of sensors can be reduced and the other cable cores can be disconnected, which will not be elaborated further.

[0052] The above embodiments are only used to illustrate the technical solutions of this patent, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group, characterized in that, It includes an ultra-low frequency AC high-voltage generator, a high-voltage switching device, an additional resonance inductor, a three-phase cable line transformer bank, a partial discharge sensor, and a partial discharge data acquisition and analysis unit. The three-phase cable line transformer bank is composed of a three-phase transformer Y-connected winding connected to a three-phase cable line. At least one winding of the three-phase transformer is in Δ connection or short-circuited, and the other end of the cable line is open and not grounded. The ultra-low frequency AC high-voltage generator is connected in series with the additional resonance inductor and the neutral point of the three-phase transformer Y-connected winding. When only one-phase cable is connected, the other two phases need to be disconnected from the transformer winding. The high-voltage switching device is connected in series between the connection point of the ultra-low frequency AC high-voltage generator and the additional resonance inductor and the ground. The partial discharge sensor is connected in series between the partial discharge data acquisition and analysis unit and the three-phase transformer Y-connected winding. The current sensor and voltage sensor are directly connected to the connection point and the ground.

2. The AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group according to claim 1, wherein The maximum output voltage of the ultra-low frequency AC high-voltage generator is higher than the test voltage. The test voltage is based on the AC withstand voltage values of the transformer winding and the cable that are not higher than the preventive test standard, and not lower than the operating voltage and partial discharge test voltage of the transformer winding and the cable.

3. The AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group according to claim 1, characterized in that, The maximum output current of the ultra-low frequency AC high-voltage generator is greater than the test current. The test current refers to the test voltage and the distributed capacitance of the power cable, and a certain margin is reserved.

4. An AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group according to claim 1, characterized in that, The high-voltage switching device is selected as a mechanical switch or a power electronic switch, and can withstand the highest test voltage and the maximum impact current, and is switched on and grounded at the peak point of the ultra-low frequency AC.

5. An AC ultra-low frequency oscillating wave partial discharge detector for a cable transformer group according to claim 1, characterized in that, The partial discharge sensor is three-phase or single-phase.

Images