Annual maintenance mode of converter station and preventive test method of DC equipment

In the operation of the power system, the converter station plays a vital role, it is like a large power converter, the high-voltage power into low voltage power suitable for users [1–2]. Whether it is the city’s skyscrapers, or the fields in the countryside, can not be separated from the silent contribution of the converter station. Therefore, the regular maintenance of the converter station has become an important task for power workers. This work is not only related to the normal operation of power equipment, but also related to the stable operation of the entire power system, the operation status directly affects the stability and reliability of power supply [3–5].

Annual maintenance, i.e., inspection and repair, is the process of comprehensive inspection, maintenance and safeguard of the equipment and system of the converter station, which is of great significance to ensure the normal operation of the converter station and prolong the service life of the equipment, and through the test, the potential problems can be found and repaired in a timely manner, so as to prevent the occurrence of accidents [6–8]. Due to the differences between the main equipment and conventional AC equipment in the principle structure, operation mode, pre-testing and inspection projects, cycles are also different, how to scientifically and reasonably arrange and organize the relevant work matters, is the key guarantee for the orderly and efficient completion of the work task. This is a complex and detailed work, which requires professional engineers to operate and debug in the field. The annual maintenance work of the converter station has a short outage time, including the maintenance of primary equipment, secondary equipment, valve hall equipment, DC equipment, and also includes the primary equipment test, secondary circuit, protection calibration, and automation, communication system debugging, and various types of analysis, detection and evaluation of faults to provide safe and reliable operation [9–12]. Among them, the preventive test is in order to find the hidden danger of the equipment in operation, to prevent accidents or equipment damage, the inspection, testing or monitoring of equipment, also includes taking oil samples or gas samples for the test; it is an important link in the operation and maintenance of electric power equipment, and it is one of the effective means to ensure the safe operation of electric power system.

In this paper, the test workload of converter station equipment is divided into one part per year, and the annual maintenance work of converter station is completed in a cycle of three years. Preventive tests are carried out on DC equipment such as converter transformer, in which converter transformer adopts different wiring methods such as HD and LY according to different voltage levels, and sets up two types of voltage addition methods such as unilateral voltage addition and bilateral voltage addition. At the same time, for the problem of port voltage inconsistency caused by the unbalance of bilateral pressurization wukong, this paper proposes to compensate reactance, adjust the intermediate transformer input and other tests to solve it. The double-arm bridge method is selected to measure the DC resistance of the DC equipment winding. Considering that the raw signal noise of partial discharge detection is more, this paper uses the normalization processing method to preprocess the raw detection data. The partial discharge of DC equipment is detected according to the relationship between the partial discharge actually voltage, extinction voltage and operating voltage. The effectiveness of the DC equipment preventive test method proposed in this paper is verified by constructing a simulation model or field testing.

2

Annual maintenance of the converter station

2.1

Power outages

In accordance with the maintenance test procedures converter, DC winding, AC and DC filters and other equipment B1 repair cycle for 1 year, the test cycle for 3 years, converter valves, valves, cold equipment cycle for 1 year, 2 years, 4 years, etc., DC must be out of power each year to carry out the maintenance work of the project for a period of 1 year, and due to the concentration of power outages, short time, equipment, operation of a wide range of control and control of the difficulty of the work, it is desirable to test the workload is divided into a part of each year, cycle three years to complete all of the way to prioritize the arrangement of test time for each interval, and test arrangements as the skeleton of reasonable arrangements for other B repair, elimination work. Therefore, it is appropriate to divide the test workload into one part per year, and complete all the work in a cycle of three years, prioritize the test time for each interval, and reasonably arrange other B repair and defect elimination work with the test arrangement as the skeleton.

At present, it mainly adopts the double-pole stopping method to complete the overhaul and pre-testing work of all DC equipment in a cycle of three years. Each year, one pole of equipment pre-testing and inspection is completed, and the other pole with the principle of overhaul and maintenance is completed in three years on a rolling basis: i.e., in the first year, the pre-testing of pole 1 equipment is the main focus, and pole 2 is in line with B1 repair; in the second year, pre-testing is carried out mainly in pole 3, and pole 1 is in line with B1 repair; in the third year, the pre-testing is carried out mainly in metal return line and grounding pole, and positive and negative double-pole equipment is in line with the principle of B1 repair, and the other three major groups of typical configuration of AC filters ( DT11/24, TT3/13/36, SC) each year to carry out a group of three-year pre-test, the other two groups with the B1 repair and circuit breaker 1-year DC resistance test work.

2.2

Content and organization of maintenance pre-testing

DC annual inspection needs to coordinate the work of various operating units in conflict and cross, verify the site safety measures and precautions, in order to ensure that the annual maintenance project is complete and safety measures, should be a reasonable arrangement of the operation plan, refine the work of each unit content and responsibilities.

The annual power outage operation tasks are broken down to the specific work that needs to be carried out every day during the construction period, so that it is easy to track the progress and find problems on a daily basis, and coordinate the corresponding work in advance to ensure that the work is completed on schedule. Because overhaul and high-voltage test must be handled separately for the work ticket, the DC area can be divided into pole 1, pole 2, neutral area, grounding pole area, and is divided into overhaul, pre-test, secondary, special rectification and defects in several categories, the number of work tickets for the annual inspection to optimize the reduction of the number of blackout operations, reduce the arrangement of safety measures and the work ticket for the time to greatly save time for the operation, to overhaul, the second, the test ticket as a The order of starting work is formulated in the order of overhaul, secondary and test tickets, and the limited resources are integrated to improve operational efficiency.

3

Preventive test methods for DC equipment

3.1

Converter transformer test

A converter transformer is the main equipment of a DC transmission system, which serves to send the electric power from the AC power system at the transmission end to a rectifier or to receive power from an inverter to the AC power system at the receiving end [13]. It uses the magnetic coupling of the windings on both sides to transmit the power, and at the same time realizes the electrical insulation and isolation of the AC system and the DC part to avoid the short-circuiting of some components caused by the neutral grounding of the AC power system and the grounding of the DC part. On the other hand, it realizes the voltage conversion, so that the AC bus voltage on the net side of the converter transformer and the DC voltage on the DC side of the converter valve can conform to the rated voltage and the permissible voltage deviation on both sides respectively. The maintenance and pre-testing time of the converter transformer determines the duration of the station-wide outage and maintenance, and it is crucial to complete the maintenance and pre-testing of the converter transformer efficiently.

3.1.1

Test wiring scheme and power frequency

1)

Wiring scheme

Converter ACLD test principle can be from the network side of the pressurization, can also be from the valve side of the pressurization. Due to the field handover test net side pressurization requires a larger voltage level of the test transformer, test operability is not large. Therefore, the field test generally adopts the valve-side pressurization method. Considering the existence of Y-type and D-type two wiring methods on the valve side of the converter, resulting in differences in the ratio of the converter, and at the same time in a different position of the converter, the valve side voltage withstand value also has differences, ± 800kV DC power transmission with the converter wiring as an example, the converter exists 800kV (HY), 600kV (HD), 400kV (LY) and 200kV (LD) four different wiring type and voltage level. There are four different wiring types and voltage levels. Therefore, the converter needs to use two different pressurization methods, i.e., unilateral pressurization and bilateral pressurization, and the pressurization methods of converters of different voltage levels are shown in Table 1.

2)

Selection of test power supply frequency

In the converter ACLD test, the applied voltage should be higher than the operating voltage, if the frequency of the test power supply using industrial frequency, it is very easy to cause the converter core magnetic saturation phenomenon, resulting in excessive excitation current.

In order to effectively avoid the occurrence of converter core saturation phenomenon in the test process, the frequency of the test power supply needs to be appropriately greater than the rated operating frequency of the converter; if the frequency of the test power supply is too large, it is also easy to cause the core of the residual magnetism of the iron is too large; the comprehensive consideration of the two aspects of the field test of the power supply frequency is generally in the range of 120Hz ~ 280Hz.

Table 1.

The size of equalizing ring of different voltage grades

Voltage rating(kV)	Double ring type(mm)			Hemispherical form(mm)
Voltage rating(kV)	d	H	D	Hemispherical form(mm)
110	150	680	550	480
220	200	890	720	710
500	250	1130	1480	1420
800	300	1560	1690	1630

In addition, according to the high-voltage test procedures: when the test power supply frequency is equal to or less than 2 times the rated frequency, its full voltage test frequency duration should be 120s, when the test frequency is greater than 2 times the rated frequency, the test voltage duration: 1 $t = 240 \frac{f_{r a t e d}}{f_{t e s t}} (s)$

However, it should not be less than 30s.

3.1.2

Bilateral Pressurization Imbalance and Compensation

In the process of ACLD test in the field of converter, it is often faced with the phenomenon of inconsistency in the output side port voltage of both excitation transformers. There are two main reasons for this phenomenon through preliminary analysis. 1)

The inlet equivalent capacitance of the two casings on the valve side of the converter is inconsistent, i.e., the reactive power is unbalanced.

2)

The loss generated at both ends of the converter is inconsistent, mainly related to the manufacturing process and winding structure of the converter, i.e. active unbalance.

In this paper, based on a DC project converter bilateral pressurization scheme and parameters, the establishment of the corresponding Matlab simulation model, the test power supply frequency 320Hz, the upper end of the entrance capacitance of 25nF, the lower end of the entrance capacitance of 35 nF, in the middle of the two capacitors grounded to simulate the terminal entrance capacitance, the two sides of the compensating reactance of 11.64H, respectively, port capacitance affects the voltage simulation model shown in Figure 1. When the two ends of the converter port capacitance is not the same, one end of the excitation variable due to the capacitance rise effect voltage rises to 162 kV, the other end of the port voltage is only 128 kV.

To address the above bilateral pressurized reactive power imbalance causing port voltage inconsistency, two solution measures are proposed: 1)

For the problem of unequal inlet capacitance, compensating reactance is used to make the reactive current input to the intermediate variable zero.

2)

For the voltage imbalance problem, according to the field test experience, the output voltage can be artificially raised by adjusting the different gears of the input and output terminals of the intermediate transformer on both sides, so as to make the voltage of both terminals remain symmetrical. The general principle is to increase the ratio of the intermediate variable at the lower voltage end or decrease the ratio of the intermediate variable at the higher voltage end.

3.2

DC resistance test

As the stator winding of DC equipment has inductance, its winding can be regarded as an equivalent circuit of inductance L and resistance R in series. DC equipment winding DC resistance measurement principle shown in Figure 2.

When switch S is closed and DC power supply U_s is turned on instantly, the current in inductor L cannot be changed abruptly, and the currents in resistor R and inductor L are both zero, there is a transition process in the DC resistance measurement circuit of DC equipment winding, and the relationship of the circuit currents in the transition process is as follows: 2 $U_{s} = i_{L} R + L \frac{d i}{d t}$ 3 $i_{L} = \frac{U_{S}}{R} (1 - e^{- 7 / τ})$

Where: U_s is the externally applied DC voltage (V). R is the DC resistance of the winding (Ω). L is the inductance of the winding (H). i_L is the DC current through the winding (A).

Where $τ = \frac{L}{R}$ is called the circuit time constant, the length of time it takes for the charging current i_L to reach the steady state value $\frac{U_{S}}{R}$ depends on the τ value. The larger the τ value, the longer it takes i_L to reach the steady state value. The smaller the value of τ, the shorter the time for i_L to reach a steady state value. Therefore, when measuring the DC resistance of DC equipment windings, the DC resistance will gradually become smaller and smaller and finally stabilize at a value.

3.2.1

Test methods

Bridge method refers to a method of measuring DC resistance with a DC bridge [14], which has high sensitivity and accuracy, and the commonly used DC bridges are single-arm and double-arm bridges. 1)

Single-arm bridge

The principle wiring of the single-arm bridge is shown in Fig. 3(a). When the voltage drop on the measured resistor R_X is equal to the voltage drop on R₃, there is no potential difference between the two points A and B, i.e., there is no current in the current detector G, and the bridge is balanced. When the bridge is balanced, U_CA = U_CB, and $U_{C A} = \frac{R_{X} U_{C D}}{R_{X} + R_{2}}$ , $U_{C B} = \frac{R_{3} U_{C D}}{R_{3} + R_{4}}$ , so $\frac{R_{X}}{R_{X} + R_{2}} = \frac{R_{3}}{R_{3} + R_{4}}$ , so $R_{X} = \frac{R_{2}}{R_{4}} R_{3}$ . R₂ and R₄ for a certain proportion of adjustable resistance, R₃ is a smooth adjustable resistance, adjust R₃ can make the bridge to balance. R_X including the influence resistance, so the actual measured DC equipment winding DC resistance should be equal to R_X minus the lead resistance. When the measured resistance is smaller, the measurement error caused by the lead resistance is larger. Therefore, the measurement should minimize the impact of lead resistance.

2)

Double-arm bridge

The principle wiring of a two-arm bridge is shown in Fig. 3(b). When the bridge is balanced, no current passes through the current detector G and the potentials at points C and D are equal. Due to $R_{3} = {R^{'}}_{3}$ , $R_{4} = {R^{'}}_{4}$ , so $R_{X} = \frac{R_{3}}{R_{4}} R_{N}$ . Double arm bridge will be measured resistance of the lead and the standard resistor lead resistance value made equal, at the same time to ensure that the measured resistance voltage lead contact is good, so as to eliminate the voltage lead resistance and contact resistance brought about by the error.

3.2.2

Bridge selection

Generally, when the measured resistance is above 1Ω, a single-arm bridge is selected, and when it is below 1Ω, a double-arm bridge is selected. As the DC equipment winding DC resistance of the converter station is very small, in order to avoid the error caused by the lead resistance and contact resistance, the double-arm bridge is chosen to measure its DC resistance.

3.2.3

Cautions

There are many factors affecting the DC resistance test results, such as lead tightness, instrument accuracy, temperature level, contact condition and stabilization time, etc. Therefore, the following matters should be noted in the test: 1)

The accuracy level of digital DC bridge is not less than 0.03 grade.

2)

DC equipment winding leads should have sufficient cross-section and good contact.

3)

Accurately measure the temperature of the winding and convert the measured DC equipment DC resistance value to the value at the same temperature for comparison, the conversion formula is as follows: 4 $R_{2} = R_{1} \frac{T_{k} + t_{2}}{T_{k} + t_{1}}$

Where: R₁, R₂ are the resistance values at temperatures of t₁ and t₂, respectively. T_k is a constant, copper winding T_k = 237, aluminum winding T_k = 218.

3.3

Local discharge test

3.3.1

Measured signal pre-processing

Considering more noise in the original signal, the partial discharge characteristics are not obvious enough, and the original signal type is more, there is a magnitude between the data, so it is first normalized to eliminate the magnitude between the measurement data, which is expressed by the formula: 5 $x = \frac{x_{0} - \min x_{0}}{\max x_{0} - \min x_{0}}$

Where: x is the normalized partial discharge test measurement signal. x₀ is the original signal. min x₀ and max x₀ are the minimum and maximum values of the original signal, respectively. By normalizing the original measurement signal, all the measurement signal values are normalized to 0~1, eliminating the data scale. Subsequently, the mean value filtering method is used to filter the partial discharge signal, assuming that there is a 3mm×3mm filter (kernel), for a given measurement signal, the value of the partial discharge test measurement signal of electrical equipment after mean value filtering is expressed as: 6 $y = \frac{1}{M} \sum_{i \in S}^{S} x u_{i}$

Where: y is the filtered partial discharge test measurement signal value of the electrical equipment. M is the sum of neighboring signal values centered on the measurement signal. S is the neighborhood set. i is the number of signals in the neighborhood centered on the measurement signal. u_i is the i th signal value in the neighborhood. Filtering of all measurement signals according to the above lays the foundation for subsequent partial discharge detection.

3.3.2

Local Discharge Detection

Based on the preprocessed partial discharge test measurement signals, the presence of partial discharge phenomenon in the DC equipment is determined [15]. In order to define the validity boundary of the test, a limit value of the partial discharge charge is set at a_max. This setting implies that, if the observed partial discharge charge is lower than a_max during the test, the DC equipment is considered to have no significant partial discharge phenomenon under the test conditions, and the performance is considered to be up to standard in this aspect. Even if the above conditions are met, it can not be completely sure that the DC equipment does not have partial discharge, so the next need to analyze the two cases of partial discharge detection judgment logic. 1)

The starting voltage and extinction voltage are greater than the working voltage of the DC equipment. Normally, in order to ensure the safe operation of DC equipment under the operating voltage, the onset voltage and extinction voltage of partial discharge of DC equipment must be at least 15% higher than the maximum operating voltage. If this condition is met, it means that the insulation of the DC device is strong enough to withstand the potential risk of partial discharge, so it can be determined that the device passes the test.

2)

If the onset and disappearance voltages do not reach the high thresholds mentioned above, i.e. they are below or close to the maximum operating voltage, the test turns to the immediate dissipation of the partial discharge charge. In this case, even if the voltage conditions are relatively unfavorable, as long as the measured partial discharge charge can quickly and completely fall below the value of a_max, the DC equipment can still be considered to have the ability to manage the risk of partial discharge, and thus be judged to have passed the test.

Through the above analysis, under the condition that the charge value is less than the limit value in the DC equipment test, the partial discharge of DC equipment is detected according to the relationship between the partial discharge starting voltage, extinction voltage and operating voltage of DC equipment, which is expressed by the formula: 7 $E = {\begin{array}{l} 1, V_{0} + V_{*} \geq V \\ 0, V_{0} + V_{*} \leq V \end{array}$

Where: E is the partial discharge detection result of DC equipment. 1 is passed and 0 is not passed. V₀ is the partial discharge starting voltage of DC equipment. V_* is the extinction voltage. V is the working voltage. The formula for the partial discharge starting voltage of DC equipment is: 8 $V_{0} = V_{w e} \cdot \frac{h}{e_{1}} [\frac{h}{e_{1}} + \frac{L}{e_{2}}]$

Where: V_we is the voltage applied for partial discharge test of DC equipment. h is the spacing. L is the thickness of DC equipment. e₁ is the relative capacitance of air. e₂ is the relative capacitance rate of DC equipment. According to the above logic to detect DC equipment partial discharge, so as to realize DC equipment partial discharge detection test.

4

Experimental analysis

In order to verify the effectiveness of the DC equipment preventive test method proposed in this paper, the method of this paper is applied to a converter station, respectively, the converter transformer, converter transformer windings, and converter transformer partial discharge test analysis of the converter station.

4.1

Converter transformer test analysis

4.1.1

Simulation results

In the MATLAB/SIMULINK environment to establish a converter transformer model consisting of six centralized capacitors, centralized capacitors using resistance-capacitance parallel module, the use of the model for the positive wiring case of the converter transformer capacitance and dielectric loss tan δ test simulation simulation simulation, simulation and calculation results are shown in Table 2, the table HV, GD, GND, MU, LM, VM, CC, CL, TK represent pressurized terminals, shielded terminals, grounding terminals, measurement terminals, net-side windings, valve-side windings, clamps, cores and tanks, respectively, and the following expressions are the same. According to the simulation results, the maximum value of the error between the calculation results of the electric capacity and the theoretical value is 0.180%, and the calculated value is extremely close to the theoretical value. tan δ The maximum value of the relative deviation between the calculation results and the theoretical value is 1.198%, and the calculated value is extremely close to the theoretical value. The simulation results show that the wiring method and the compensation method of bilateral voltage unbalance in this paper can effectively realize the measurement of capacitance and dielectric loss of the converter transformer winding.

Table 2.

Simulation results

Measuring object	C_x1	C_x2	C_x3	C_x4
HV	LW	LW	LW	LW
GD	VM	CC+CL	LW	CC+CL
GND	TK	TK	TK	TK
MU	CC+CL	VM	CC+CL	LW
C_x/nF	22.17	5.47	2.46	5.53
tan δ / %	0.198	0.147	0.169	0.146
C₀ / nF (Theoretical value)	22.21	5.49	2.48	5.54
tan δ / % (Theoretical value)	0.197	0.146	0.167	0.145

4.1.2

Field test results

In the converter station using the wiring method of this paper on a random group of converter transformer capacity and tanδ field test, the test results are shown in Table 3. In the field test results, the maximum relative deviation between the measured and theoretical values of the electric capacity is 0.158%, which is basically the same as the theoretical value, and the maximum relative deviation between the measured and theoretical values of tanδ is 0.738%, which is very close to the theoretical value. The measured results further show that the wiring method in this paper can effectively realize the measurement of converter transformer winding capacitance and tanδ firstly.

Table 3.

Actual measurement results

Measuring object	C_x1	C_x2	C_x3	C_x4
HV	LW	LW	LW	LW
GD	VM	CC+CL	LW	CC+CL
GND	TK	TK	TK	TK
MU	CC+CL	VM	CC+CL	LW
C_x/nF	25.36	3.74	6.43	3.71
tan δ / %	0.273	0.139	0.243	0.138
C / nF (Theoretical value)	25.32	3.75	6.44	3.72
tan₀ δ / % (Theoretical value)	0.271	0.140	0.244	0.139

4.2

DC resistance test analysis

A 550kV on-load voltage-regulated converter transformer B-phase medium-voltage winding test process as an example for DC resistance test analysis. The transformer is a three-phase common structure, rated capacity of 720/720/220MVA, voltage combination of 550/220 ± 8 × 1.29%/35kV, a total of 21 blocks. The test instrument is HCR 3140X DC resistance demagnetization tester, which has the functions of testing DC resistance and demagnetization at the same time. In order to ensure the accuracy of the test, demagnetization after the completion of the test using conventional methods, and then re-test with the test methods in this paper for comparison. The test results of the conventional test method and the improved test method are shown in Table 4. From the table, it can be seen that the converter transformer winding 21 stops DC resistance of the factory average value of 85.52mΩ, using conventional methods to measure the average value of DC resistance of 68.63mΩ, while the use of this paper’s method of testing the average value of DC resistance of 83.68mΩ. The two methods and the average value of DC resistance measurement of the average value of the factory average value of the error of 19.75% and 1.20%, respectively. It can be clearly seen that using the method of this paper to test the DC resistance of the converter transformer can get a smaller measurement error.

Table 4.

Comparison of test results

Stay position	Factory value(mΩ)	Conventional method(mΩ)	Ours(mΩ)
1	83.69	68.94	84.11
2	87.2	72.28	81.41
3	81.69	60.07	81.4
4	80.03	68.48	81.29
5	84.65	65.73	85.35
6	84.61	65.07	83.39
7	81.59	61.49	88.3
8	84.29	68.5	81.88
9	89.27	70.43	82.36
10	81.95	68.61	80.8
11	85.14	61.17	84.74
12	86.38	74.33	80.48
13	81.06	61.56	89.67
14	88.5	73.77	85.23
15	88.48	74.69	85.55
16	86.3	69.87	81.27
17	82.41	68.43	81.43
18	83.47	72.84	80.75
19	81.61	71.56	89.81
20	89.94	72.67	86.61
21	82.6	70.68	81.37

Table 5 shows the statistics of the time consumed by the two test and measurement methods in the DC resistance test. The data in the table show that it takes about 27min to test the single-phase DC resistance of the MV winding in the conventional method, and 12min to test the DC resistance of the MV winding in the method of this paper, which is about 4/9 of the time required in the conventional test method, and the DC resistance testing time is shortened partly due to the reasonable selection of the bridge. The above results show that the DC resistance testing method in this paper is scientific and effective, and can successfully complete the preventive test of DC equipment of converter.

Table 5.

Time compared

Stay position	Conventional method(s)	Ours(s)
1	74.16	39.88
2	83.32	24.30
3	78.65	34.08
4	82.99	34.48
5	81.97	35.26
6	75.21	25.20
7	71.89	36.09
8	77.80	34.65
9	76.79	29.25
10	82.78	37.22
11	80.09	39.92
12	79.28	25.91
13	68.79	32.95
14	78.46	37.00
15	80.06	21.51
16	75.93	35.46
17	81.16	34.96
18	81.10	35.99
19	80.19	37.18
20	80.86	31.93
21	68.54	31.11

4.3

Local discharge test analysis

4.3.1

Measurement signal processing

A converter transformer is randomly selected for testing in the converter station, model: S13-M-100/10-NX3, main parameters: rated capacity 120kVA, rated voltage 15000/500V, rated frequency 100Hz, number of phases 3, coupling group labeling Dyn11, no-load loss 142W, load loss 1507W, short circuit impedance 4.09%, insulation level LI75AC/35AC/5kV.

Extract the converter transformer ultrasonic detection of the signal, due to which there is no partial discharge, so the ultrasonic signal obtained is entirely by the converter transformer vibration caused by noise and other interference signals, selected one of the way the signal is converted into the time domain and frequency domain waveforms as shown in Figure 4. Figure 4(a) can be seen from the converter vibration noise and other interference signals sound level amplitude is very large, Figure 4(b) can be seen in the converter noise and other interference signals such as converter noise frequency range is wide, mainly concentrated in the 50kHz or so, which overlaps with the frequency range of the signal of the local discharge. Therefore, the converter due to vibration and other local discharge detection and localization of interference in the field measurement and subsequent signal processing can not be ignored, but also to local discharge detection and localization of great interference.

As the selected converter station converter transformer in this paper are all indoors, the vibration of the converter is larger than that of the AC transformer, and these vibration noises due to reflection, superposition caused by the interference signal is larger than that of the outdoor, and these noises will have a great impact on the partial discharge detection. The partial discharge signals collected by the on-site charged detection are extracted from the field measured waveforms of two channels as shown in Fig. 5, and (a) and (b) represent the signal waveforms of channel 5 and channel 6, respectively. From the figure, it can be found that the ultrasonic signals collected by these two channels have obvious local discharge signals, due to the vibration noise of the converter and other interference and mixed in the local discharge signal, which brings great interference to the judgment of the local discharge signal between the various channels of the time-delay relationship between the local discharge signals, which in turn affects the precise location of the local discharge point, resulting in a large localization error can not be accurately locate the position of the discharge point of the local discharge. Therefore, there is an urgent need to analyze and process the mixed signal of the partial discharge in the converter to filter out the interference signal and effectively restore the partial discharge signal.

In order to facilitate the analysis and processing of the signal, the extraction of which all the way to the waveform signal for the time and frequency domain analysis, the time domain waveform shown in Figure 6(a), can be seen in the emergence of the local discharge signal will also be accompanied by a lot of other interfering signals mixed with them, from the amplitude of the interfering signal amplitude is larger. Figure 6(b) is converted to frequency domain waveforms, it can be seen that the interference signal band range is wider, but also in the local discharge signal band range. From Figure 6, it can be seen that the interference signal is mainly concentrated between 50kHz to 100kHz, while the local discharge signal is in the range of tens to 370kHz, which shows that the interference signal caused by vibration both in terms of the size of the sound level and the frequency of the local discharge signal will produce a lot of mixed interference, which will cause a lot of trouble in the judgment of the local discharge signal time delay, so it is necessary to optimize the processing of the signal Therefore, it is necessary to optimize the signal processing and restore the original LEC signal.

Application of the signal processing technology proposed in this paper on the field measured local discharge signal processing time, frequency diagram shown in Figure 7, and Figure 6 mixed local discharge signal comparison can be found using this processing technology by the converter vibration noise and other interference brought about by the optimization of the processing has achieved significant results for which the suppression of useless signals is obvious. In this paper, the key to the signal processing technology is the selection of the threshold value and value, the selection of the threshold value is analyzed through a number of tests, many times to select the appropriate threshold for comparison and analysis, and then get the more ideal local discharge waveform, and the original local discharge signal is better restored and retained intact.

4.3.2

Local Discharge Detection

Using the scheme proposed in this paper on the converter station converter transformer scaling model for partial discharge detection, ultrasonic sensors are uniformly arranged in the power transformer tank side wall, height from the ground 150mm, spacing 50mm, a total of 8, was “L” type arrangement. In the winding Z direction to set the power transformer local discharge signal.

Figure 8 shows the partial discharge waveform of high-frequency current of the converter transformer, and Table 6 shows the partial discharge detection results of ultrasonic method based on high-frequency current cycle compensation. From the table, it can be seen that the converter transformer ultrasonic-high-frequency current detection and localization of partial discharge signal error within 0.5 °, to meet the detection requirements. From the figure, it can be seen that the processed partial discharge signal retains the real high-frequency discharge signal inside the converter transformer, which improves the accuracy of partial discharge detection in the field converter transformer.

Table 6.

Local discharge detection results

Group number	Local discharge azimuth/°	Measured result/°	Error/°
1	(88.2,29.3)	(87.9,29.1)	(0.3.0.2)
2		(88.1,29.1)	(0.1,0.2)
3		(87.8,28.9)	(0.4,0.4)
4		(87.9,28.8)	(0.3,0.5)
5		(88.0,29.2)	(0.2,0.1)
6		(87.7,29.2)	(0.5,0.1)
7		(88.1,29.0)	(0.1,0.3)
8		(88.0,29.1)	(0.2,0.2)

5

Conclusion

This paper focuses on the preventive test methods of DC equipment. In the test of converter transformer, the maximum relative deviation of the capacitance calculation result of the simulation model from the theoretical value is 0.180%, and the maximum relative deviation of the dielectric loss calculation result from the theoretical value is 1.198%. The maximum relative deviation of the capacitance calculation result from the theoretical value during the field test is 0.158%, and the maximum relative deviation of the measured dielectric loss from the theoretical value is 0.738%. It shows that the preventive test method of converter transformer proposed in this paper is reasonable and effective. The average value of DC resistance measured by the method of this paper is 83.68mΩ, and the error of the average value of DC resistance from the factory is 1.20%, which is 18.55% lower than that of the conventional method. This shows the superiority of the DC equipment DC resistance preventive test method in this paper. In the partial discharge detection, the pre-processed data better restore and retain the original partial discharge signal, detection and localization of partial discharge signal and the real signal position error within 0.5°. It shows that the method of this paper can effectively test the preventive test of partial discharge of DC equipment.

Language:: English

Publication timeframe:: 1 times per year
Journal Subjects:: Life Sciences, Life Sciences, other, Mathematics, Applied Mathematics, General Mathematics, Physics, Physics, other

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Annual maintenance mode of converter station and preventive test method of DC equipment

Chao Kang

Hao Huo

Ningrui Li

Bingbing Liu

Fuqiang Guo

Published Online: Mar 19, 2025

Received: Oct 23, 2024

Accepted: Feb 13, 2025

DOI: https://doi.org/10.2478/amns-2025-0512

KeywordsAnnual maintenance, DC equipment, Partial discharge, Preventive test, Converter station

© 2025 Chao Kang et al., published by Sciendo

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Keywords
Annual maintenance, DC equipment, Partial discharge, Preventive test, Converter station