Parameter checking method for neutral point grounding system of generator with extended unit connection

Abstract

The present application discloses a method for checking the parameters of a generator neutral grounding system with an expanded unit connection, including checking the parameters of a two-generator neutral grounding system using the test parameters of a single-machine grounding system, which comprises the following steps: Step 1: Establish a mathematical model of the generator neutral grounding system with an expanded unit connection; Step 2: Determine the method for checking the parameters of the generator neutral grounding system with an expanded unit connection; wherein, the parameters of the generator neutral grounding system with an expanded unit connection in Step 2 include the generator neutral unbalanced voltage, neutral point displacement voltage, single-phase grounding capacitive current, and single-phase grounding maximum fault current; The present application can effectively verify the performance of the generator neutral grounding system with an expanded unit connection and determine whether the generator neutral grounding method meets the technical requirements.

Description

Technical Field

[0001] This application belongs to the technical field of power systems, and particularly relates to a method for checking the parameters of a generator neutral grounding system with an extended unit connection. Background Art

[0002] The extended unit connection is a unit composed of two generators connected to one transformer. When a single-phase grounding occurs at the generator outlet, the single-phase grounding fault current is the vector sum of the single-phase grounding capacitive current and the neutral point currents of the two generators. The single-phase grounding capacitive current is the sum of the capacitive currents to the ground of all equipment from the outlets of the two generators to the low-voltage side of the transformer, including the stator windings of the generators, the low-voltage windings of the main transformer, and the excitation transformer, auxiliary transformer, cables, busbars, etc. The grounding device parameters of the extended unit connection are different from those of the unit connection, and it is impossible to conduct tests using the parallel operation of two generators in an isolated network.

[0003] Therefore, it is necessary to develop a method for checking the parameters of a generator neutral grounding system with an extended unit connection. Summary of the Invention

[0004] In view of the above problems, the embodiments of this application provide a method for checking the parameters of a generator neutral grounding system with an extended unit connection, which can effectively verify the performance of the generator neutral grounding system with an extended unit connection and determine whether the generator neutral grounding method meets the technical requirements. The technical solution is as follows:

[0005] This application provides a method for checking the parameters of a generator neutral grounding system with an extended unit connection, including using the test parameters of a single-machine grounding system to check the parameters of a two-machine neutral grounding system, including the following steps: Step 1: Establish a mathematical model of the generator neutral grounding system with an extended unit connection; Step 2: Determine the method for checking the parameters of the generator neutral grounding system with an extended unit connection; where the parameters of the generator neutral grounding system with an extended unit connection in Step 2 include the asymmetric voltage of the generator neutral point, the neutral point displacement voltage, the single-phase grounding capacitive current, and the maximum single-phase grounding fault current.

[0006] For example, in the method for checking the parameters of the generator neutral grounding system with an extended unit connection provided in an embodiment, in Step 2, the calculation formula for the single-phase capacitance C to the ground of two machines is:

[0007] C = C 1 + C 2

[0008] where C 1 and C 2 are the total single-phase capacitances to the ground of the stator windings of the two generators, in F.

[0009] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the total inductance L of the two generators is:

[0010]

[0011] where L 1 and L 2 are the inductances of the two generators respectively.

[0012] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the total single-phase capacitance current I c to the ground of the two generators is:

[0013]

[0014] where U n is the rated line voltage of the generator stator winding, in V; ω is the angular velocity.

[0015] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the total resistive current I R at the neutral point of the two generators is:

[0016]

[0017] where R 1 and R 2 are the neutral grounding resistances of the two generators respectively.

[0018] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the total inductive current I L at the neutral point of the two generators is:

[0019]

[0020] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the single-phase grounding fault current I d of the two generators is:

[0021]

[0022] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the degree of detuning v is:

[0023]

[0024] For example, in the method for checking the parameters of the generator neutral point grounding system with extended unit connection provided in an embodiment, in step 2, the calculation formula for the damping ratio d is:

[0025]

[0026] For example, in the method for checking the parameters of the generator neutral point grounding system with extended unit connection provided in an embodiment, in step 2, the calculation formula for the displacement voltage coefficient m is:

[0027]

[0028] The beneficial effects brought by the method for checking the parameters of the generator neutral point grounding system with extended unit connection of the present application are as follows: By establishing a mathematical model of the generator neutral point grounding system with extended unit connection; determining the method for checking the parameters of the generator neutral point grounding system with extended unit connection, for exploring the laws of the generator neutral point grounding method with extended unit connection and determining the parameters and performance of the generator neutral point grounding device, conducting parameter tests on the generator neutral point grounding system with extended unit connection, including tests on the asymmetric voltage of the generator neutral point, the displacement voltage of the neutral point, the single-phase grounding capacitive current, and the maximum single-phase grounding fault current, a method for checking the parameters of the double-machine neutral point grounding system by using the test parameters of the single-machine grounding system is proposed. The present application can effectively evaluate the parameter performance of the generator neutral point grounding device with extended unit connection and verify whether the generator neutral point grounding method with extended unit connection and the grounding device parameters meet the technical requirements. Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0030] Figure 1 is the schematic diagram of the generator neutral point grounding system;

[0031] Figure 2 is the schematic wiring diagram of the generator neutral point grounding system parameter test. Detailed Embodiments

[0032] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0033] Unless otherwise defined, the technical terms or scientific terms used in this disclosure shall have the ordinary meaning as understood by those of ordinary skill in the art to which this disclosure pertains. The terms "first", "second" and similar words used in this disclosure do not denote any order, quantity or importance, but are only used to distinguish different components. Words such as "including" or "comprising" mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects. Words such as "connected" or "coupled" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Upper", "lower", "left", "right", etc. are only used to represent relative position relationships, and when the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0034] The present application provides a method for checking the parameters of a generator neutral point grounding system with an extended unit wiring, including checking the parameters of a double-machine neutral point grounding system by using the test parameters of a single-machine grounding system, and comprising the following steps: Step 1: Establish a mathematical model of the generator neutral point grounding system with an extended unit wiring; Step 2: Determine the method for checking the parameters of the generator neutral point grounding system with an extended unit wiring; wherein, the parameters of the generator neutral point grounding system with an extended unit wiring in Step 2 include the generator neutral point asymmetric voltage, neutral point displacement voltage, single-phase grounding capacitive current, and single-phase grounding maximum fault current.

[0035] By establishing a mathematical model of the generator neutral point grounding system with an extended unit wiring in the present application; determining the method for checking the parameters of the generator neutral point grounding system with an extended unit wiring, in order to explore the laws of the generator neutral point grounding method with an extended unit wiring and determine the parameters and performance of the generator neutral point grounding device, carrying out the parameters test of the generator neutral point grounding system with an extended unit wiring, including the tests of the generator neutral point asymmetric voltage, neutral point displacement voltage, single-phase grounding capacitive current, and single-phase grounding maximum fault current, a method for checking the parameters of a double-machine neutral point grounding system by using the test parameters of a single-machine grounding system is proposed. The present application can effectively evaluate the parameter performance of the generator neutral point grounding device with an extended unit wiring, and verify whether the generator neutral point grounding method and the grounding device parameters with an extended unit wiring meet the technical requirements.

[0036] The parameter tests of the generator neutral grounding system include tests on the generator neutral unbalanced voltage, neutral displacement voltage, maximum single-phase grounding capacitive current, and maximum single-phase grounding fault current. The neutral unbalanced voltage refers to the voltage of the neutral point to the ground when the generator operates no-load at the rated voltage with the neutral point ungrounded; the neutral displacement voltage refers to the voltage of the neutral point to the ground when the generator neutral point is connected to the grounding device at the rated voltage; the maximum single-phase grounding capacitive current refers to the current flowing through the grounding point when a single-phase metallic grounding occurs at the rated voltage with the generator neutral point ungrounded; the maximum single-phase grounding fault current refers to the current flowing through the grounding point when a single-phase grounding occurs at the generator terminal at the rated voltage after the generator neutral point is connected to the grounding device; the displacement voltage coefficient refers to the ratio of the displacement voltage to the neutral unbalanced voltage of the generator at the rated voltage.

[0037] For convenient analysis, the distributed capacitance between the stator winding and the ground is regarded as a concentrated capacitance and is connected in parallel between the outgoing line end of the generator stator winding and the ground. The generator neutral grounding device can be equivalent to a parallel equivalent circuit of a resistor and an inductor. The general schematic diagram of the generator neutral grounding system is as shown in Figure 1 and the wiring schematic diagram of the parameter test of the generator neutral grounding system is as shown in Figure 2 .

[0038] For example, in the method for checking the parameters of the generator neutral grounding system with the wiring of the expansion unit provided in an embodiment, in step 2, the calculation formula for the double-machine single-phase capacitance to ground C is:

[0039] C = C 1 + C 2

[0040] where C 1 and C 2 are the total single-phase capacitances to ground of the stator windings of the two generators, in F.

[0041] where the calculation formula for the single-machine single-phase capacitance to ground C 1 is:

[0042]

[0043] where C 1 —— the total single-phase capacitance to ground of the generator stator winding, in F;

[0044] I C1 —— the total single-phase capacitive current of the generator stator winding, in A;

[0045] U n —— the rated line voltage of the generator stator winding, in V.

[0046] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit connection provided in an embodiment, in step 2, the calculation formula for the total inductance L of the two generators is:

[0047]

[0048] where L 1 and L 2 are the inductances of the two generators respectively.

[0049] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit connection provided in an embodiment, in step 2, the calculation formula for the total single-phase capacitance current I c to the ground of the two generators is:

[0050]

[0051] where U n is the rated line voltage of the generator, in V; ω is the angular velocity.

[0052] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit connection provided in an embodiment, in step 2, the calculation formula for the total resistive current I R at the neutral point of the two generators is:

[0053]

[0054] where R 1 and R 2 are the neutral grounding resistances of the two generators respectively.

[0055] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit connection provided in an embodiment, in step 2, the calculation formula for the total inductive current I L at the neutral point of the two generators is:

[0056]

[0057] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit connection provided in an embodiment, in step 2, the calculation formula for the single-phase ground fault current I d of the two generators is:

[0058]

[0059] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit connection provided in an embodiment, in step 2, the calculation formula for the degree of detuning v is:

[0060]

[0061] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the damping ratio d is:

[0062]

[0063] For example, in the method for checking the parameters of the generator neutral grounding system with the extended unit wiring provided in an embodiment, in step 2, the calculation formula for the displacement voltage coefficient m is:

[0064]

[0065] It can be seen from the above mathematical model that the double - machine single - phase grounding fault current is the sum of the grounding fault currents of the two generators, and the single - machine single - phase grounding fault current cannot be used to design the parameters of the grounding device.

[0066] Example of checking the parameters of the extended unit wiring grounding system

[0067] 1) Parameters of the generator neutral grounding system

[0068] For a certain hydropower station, the rated capacities of generators #5 and #6 are 110 MW, the rated voltage is 15.75 kV, the single - phase capacitance to ground of the stator winding is 0.946 μF, the single - phase capacitance to ground of the capacitor on the generator side is 0.132 μF, the single - phase capacitance to ground of the capacitor on the low - voltage side of the main transformer is 0.260 μF. The generator neutral uses a high - impedance grounding device, with the grounding device model being FDLG - 50 kVA / 15.75 kV / 0.3 kV, the rated capacity is 50 kVA, the rated voltage is 15.75 kV / 0.4 kV, the turns ratio is 52.5, the resistance value of the resistor on the secondary side of the grounding transformer is 0.66 Ω, and the inductance value of the reactor is 1.52 mH.

[0069] 2) Test results of the generator single - machine

[0070] A. Measurement of the neutral point unbalanced voltage

[0071] At 100% rated stator voltage, the neutral point unbalanced voltage of the generator is 238.73 V, which is relatively small.

[0072] B. Measurement of the neutral point displacement voltage

[0073] At 100% rated stator voltage, the neutral point displacement voltage of the generator is 280.71 V, which does not exceed the 10% rated phase voltage value of 796.70 V specified by the standard. The displacement voltage coefficient is 1.176, which does not exceed the standard - specified value of 1.5.

[0074] C. Measurement of the maximum single - phase grounding capacitance current of the generator

[0075] Under 100% rated stator voltage, the maximum single-phase grounding capacitive current of the generator operating alone is 12.69 A.

[0076] d Maximum single-phase grounding fault current test

[0077] The maximum single-phase grounding fault current under the rated stator voltage of the generator is 8.44 A, which is less than the short-time allowable value of the maximum single-phase grounding fault current of 15 A. The maximum single-phase grounding fault current of two generators may exceed the short-time allowable value of the maximum single-phase grounding fault current of 15 A. Therefore, it is necessary to set a high-impedance grounding device at the generator neutral point to offset part of the capacitive current and limit the maximum single-phase grounding fault current within 15 A.

[0078] 3) Two-generator check

[0079] A Single-phase capacitance to ground of a single generator

[0080] Under 100% rated stator voltage of the generator, the measured value of the maximum single-phase grounding capacitive current of the generator operating alone is 12.69 A. The single-phase capacitance to ground of a single generator (including the capacitance of one group of capacitors on the generator side and the capacitance to ground of two groups of capacitors on the transformer side) is:

[0081]

[0082] B. Total single-phase capacitance to ground C of two generators

[0083] C = 2×1.481 - 2×0.260 = 2.442 μF

[0084] C. Total single-phase capacitive current I of two generators c

[0085]

[0086] D. Total resistive current I at the neutral point of two generators R

[0087]

[0088] E. Total inductive current I at the neutral point of two generators L

[0089]

[0090] F. Maximum single-phase grounding fault current I of two generators d

[0091]

[0092] G. Degree of detuning v

[0093]

[0094] H. Damping ratio d

[0095]

[0096] I. Displacement voltage coefficient

[0097]

[0098] Check result: The maximum fault current of single-phase grounding for two generators is 12.26 A, which meets the allowable value of 15 A for the maximum fault current of single-phase grounding. The displacement voltage coefficient is 1.706, and the displacement voltage coefficient is on the high side (reference value 1.5). The main reason for the large displacement voltage coefficient is caused by the internal resistance of the grounding transformer. However, the measured value of the displacement voltage of a single generator is 280.71 V, and the displacement voltage during the operation of two generators is 1.706×280.71 = 478.89 V, which does not exceed 10% of the rated phase voltage. The displacement voltage coefficient only reflects the performance of the grounding device. As long as the displacement voltage does not exceed the allowable value, a slightly larger displacement voltage coefficient is allowed.

[0099] Although the embodiments of the present application have been disclosed for illustrative purposes, those skilled in the art will recognize that various modifications, additions, and substitutions can be made without departing from the scope and spirit of the invention as disclosed in the appended claims.

Claims

1. A method for checking the parameters of a generator neutral grounding system with an extended unit connection, characterized in that, using the test parameters of a single-machine grounding system to check the parameters of a double-machine neutral grounding system, including the following steps: Step 1: Establish a mathematical model of the generator neutral grounding system with an extended unit connection; Step 2: Determine the method for checking the parameters of the generator neutral grounding system with an extended unit connection; wherein, the parameters of the generator neutral grounding system with an extended unit connection in Step 2 include the generator neutral unbalanced voltage, neutral point displacement voltage, single-phase grounding capacitive current, and single-phase grounding maximum fault current. In Step 2, the calculation formula for the double-machine single-phase-to-ground capacitance C is: C=C 1 +C 2 Among them, C 1 and C 2 are respectively the total single-phase-to-ground capacitances of the stator windings of the two generators, in F. In the second step, the calculation formula for the total inductance L of the two machines is: Among them, L 1 and L 2 are the inductances of the two generators respectively. In the second step, the calculation formula for the total single-phase-to-ground capacitive current I c is as follows: Among them, U n is the rated line voltage of the generator stator winding, V; ω is the angular velocity. In the second step, the total resistive current I of the neutral points of the two machines R is calculated by the formula: Among them, R 1 and R 2 are the neutral point grounding resistances of the two generators respectively. In the second step, the calculation formula for the total inductive current I L of the neutral points of the two generators is: In the second step, the formula for the double machine single-phase grounding fault current I d is as follows:

2. The method for checking the parameters of the generator neutral grounding system with an extended unit connection according to claim 1, characterized in that, in Step 2, the calculation formula for the degree of detuning v is:

3. The method for checking the parameters of the generator neutral grounding system with an extended unit connection according to claim 2, characterized in that, in Step 2, the calculation formula for the damping ratio d is:

4. The method for checking the parameters of the generator neutral grounding system with an extended unit connection according to claim 3, characterized in that, in Step 2, the calculation formula for the displacement voltage coefficient m is:

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