The 110kV Dalong substation in Dongguan has three main transformers and four busbars. At this stage, there are 16 10kV outlets (about 100km long), all of which are composed of overhead lines and cables. The station uses three KD-XH01-315/10.5 type arc extinguishing systems (including two DDS-type distribution network ground fault intelligent detection device), 10kV distribution network to achieve a ground fault integration solution.
From October 2002 to early 2006, it has been running for more than three years. In the first year, as planned, only the selected line was not automatically tripped. The automatic trip function was started on January 7, 2004, and the trip time was tentatively set to 10 seconds.
Comprehensive three-year operation, it has been proved that under the harsh conditions of small capacitance current, high resistance grounding, and unstable voltage and current in the grounding process, rapid arc extinguishing and rapid correctness are achieved when the capacitance current changes over a wide range. Line selection, quick tripping effect.
2 Operational analysis
2.1 Ground fault conditions and data analysis
As of January 20, 2006, there were 238 grounding records in the running arc suppression system. Record data analysis is as follows:
1 The majority of transient ground faults (202 trips, accounting for 84.87%) are eliminated automatically after arc suppression coil compensation. Non-instantaneous ground faults requiring tripping last more than 10 seconds account for only a few (36, 15.13%). ).
2 The grounding with very short intervals accounts for a considerable majority of the total, and the interval between successive groundings is recorded within 112 seconds in 5 seconds, accounting for 47.06%.
3 Ground faults are concentrated in severe weather conditions such as thunderstorm days and typhoon strikes. At this time, ground faults not only occur frequently but also have short intervals and most of the duration is short. Most of them can be eliminated due to the compensation of the arc suppression coil. Table 1 lists the ground faults that occurred during thunderstorm days on June 13, 2003, and during the typhoon attacks from September 2 to September 3, 2003.
Table 1 Data Analysis of Ground Fault Concentration
4 When the grounding occurs, the dispersion of the capacitor current amplitude of the distribution network is large, and the capacitor current is very small. Statistics are shown in Table 2. (Note: The maximum capacitive current of this station is about 35A)
Table 2. Statistics of Capacitance Current Amplitude and Number of Times of Grounding
5 When the grounding voltage distribution network neutral voltage amplitude dispersion, including the lower amplitude of the majority, which is mostly high-resistance grounding. For example, a total of 142 times the voltage of the neutral point of the distribution network below 4 kV is used for grounding, accounting for 60%. Data statistics are shown in Table 3
Table 3. Statistics of amplitude and frequency of neutral voltage Un at grounding
2.2 Arc suppression coil operation
The arc-suppression coil in operation has never experienced any abnormal behavior and has never caused any adverse effects such as system resonance. As can be seen from the large number of fault recordings, the KD-XH arc suppression system fully demonstrates the fast response characteristics. When the interval time is very short and continuous frequent grounding, the KD-XH arc-suppression system can operate quickly and continuously, continuously and quickly withdraw from the compensation. One of the typical oscillograms is shown in Fig. 1. Six times of grounding occurs within 6 seconds. The arc suppression coils are compensated in time and exit in time.
Fig. 1 Recorded SH858 at 10:02:11 on July 24, 2003 (F2 is ground line)
Another typical example is that in the evening of September 2, 2003, five times of instantaneous grounding of different lines on the mother I and II mothers occurred within 15 seconds, and two KD-XH arc suppression coils respectively arranged on two bus bars were Can be compensated quickly and exit quickly accordingly.
In addition, the records of the KD-XH arc-extinguishing system show that all the compensation currents are very close to the capacitance current, indicating that the arc suppression system is accurate, the residual current is small, and the arc suppression effect is good.
It is the good characteristics of the KD-XH arc suppression system that guarantees the quick elimination of transient ground faults and enables all transient ground faults to automatically disappear without tripping.
2.3 analysis of the operation of the line selection device
The line selection device is set to: line selection is not performed for ground faults that disappear when the duration is less than 1 second; line selection is performed when the ground fault duration is longer than 1 second; and tripping is performed if the ground fault persists for 10 seconds.
Of the 238 groundings recorded over the past three years, most of the durations were less than 1 second without the need for line selection. Only 91 times were greater than 1 second. The line selection device was selected. There is a large dispersion of the capacitor current amplitude during the grounding line selection. There are quite a few line selections under the condition that the capacitor current is very small. The statistical data is shown in Table 4. There is also a large dispersion of the neutral-point voltage amplitude when grounding is selected. There are quite a few times that the line selection is performed under high-resistance grounding conditions. Statistics are shown in Table 5.
Table 4. Statistics of Capacitance Current Amplitude and Number of Times when Grounding is Wired
Table 5. Statistics of amplitude and frequency of neutral voltage Un at line grounding
The correctness of the majority of the line selection results (74 times) can be reliably judged by the data of the oscilloscope; the remaining two times have been proved by the actual situation; the other 15 times refer to the experience of the correctness of line selection by the line selection record data. The comparison can be inferred for correctness. In all the line selection results, there was no case of mis-selection, and only two times failed to select the grounding line, and the others were all correctly selected. See Table 6 for statistics on correctness of line selection.
Table 6 Number of correctness of line selection
The above data shows that after more than three years of operation, the correct line rate of the device has reached over 97%; the correct rate of line selection in the past two years after targeted improvement has exceeded 98%; further improvement is expected to make it It is closer to 100%. It is worth noting that the error rate of this device is zero, that is, the device has never been misplaced in all cases where the line should be selected, and even if it cannot be selected due to a disturbance of the voltage waveform in some cases, it will not be misplaced. Therefore, it will not have any negative impact on the operation of the system. At the same time, it is worth noting that the statistical results of the above-mentioned line correctness rates are obtained from the line selection grounding conditions reflected in Table 4 and Table 5. It can be seen that the line selection device has a large range of capacitive current changes in the grid, small capacitive current, and high Resistance to ground and other harsh occasions still have a high rate of accuracy. There is no specific definition of "high impedance" with respect to the value of the ground impedance, because the same ground impedance will cause different neutral voltages in systems with different parameters. A more significant requirement is to ensure accurate line selection at the lowest neutral point voltage (usually 25% of the system operating voltage) at which the arc suppression coil operates. The device meets this requirement.
For the only two cases in which failed lines could not be selected, improvements have been made in a targeted manner and no similar phenomenon has occurred since.
2.4 Operation after Automatic Trip Function
Since January 7, 2004, after the automatic tripping function of the line selection was put into operation, as of January 20, 2006, there were 28 trips requiring ground trips (grounding duration greater than 10 seconds). Twenty-seven of them were correctly and promptly tripped by the DDS line selection device. Only one time when the voltage waveform of the neutral point was grounded, the ground line could not be selected because it was very turbulent. It was manually opened. As mentioned above, similar problems did not arise again after the improvement.
3 Operational Practice Effect Analysis
The practice of the grounding operation mode for three years has fully proved its feasibility, enabling the superiority of the model analyzed in the previous article to be concretely realized. Among them, the following points have more profound experience.
(1) greatly reduce the tripping rate and improve the reliability of power supply
Dalong Station avoids a large number of trips due to the use of fast arc suppression coil grounding. More than three years of practice have shown that 202 transient ground faults recorded by the KD-XH (about 85% of the total grounding record) are automatically eliminated after compensation. If a small resistance is used for grounding, it is obvious that these faults will cause tripping, which will increase the tripping rate.
Among them, the quick response performance of the arc extinguishing coil is an essential condition for eliminating frequent continuous earth faults that occupy a considerable majority of the total number of groundings and are short in duration and short in duration. The fast arc-extinguishing coil can properly handle such failures, making it quickly disappear without tripping and without causing resonance, which can greatly reduce the tripping rate and improve the reliability of power supply. Practice has proved that in the southern regions where thunderstorms and typhoons are common, the need for such rapid response is particularly prominent and benefits are particularly evident. For example, the 11 consecutive ground faults on thunderstorm days on June 13, 2003 were all automatically eliminated due to rapid compensation and did not cause tripping; 73 consecutive ground faults during typhoon blowouts from the evening of September 2 to September 3 were observed. 71 times were automatically eliminated and only 2 ground trips due to wire landing.
(2) Reduce the time and labor for finding ground fault lines
In the traditional arc suppression coil grounding method, the elimination of ground faults mainly depends on the “test pull†method. It usually takes at least several minutes to find the faulty line. In the process, it not only affects the normal power transmission of other lines, but also The staff on duty is in a state of high tension, and it can not help but sometimes make mistakes on the fly and even cause an accident to increase, causing the operating personnel to have a headache. After the line selection trip mode is adopted, the attendant only needs to perform the tripping operation on the selected line (only 7 times when the line is not automatically tripped) or it can be automatically tripped without operation (27 times for line selection and automatic tripping). ), quickly isolated the fault line, so that the watchman from the busy work out of the work, the main focus will be used to assist in the search for faults, as soon as possible to restore fault line transmission. For those groundings (55 times) that do not need to trip due to a short duration but have been routed, the attendant can accurately grasp which line has a short-term failure and is conducive to operation management. Therefore, the ground fault integration solution is very popular with field workers.
After the successful implementation of Dongguan Dalong Substation, the Dongguan Power Supply Bureau and the Huizhou Power Supply Bureau began to promote the use of the single-phase ground fault integration solution. As of December 2007, Dongguan Power Supply Bureau had 61 substations, and the Huizhou Power Supply Bureau had 17 substations that were put into the line selection tripping function.