In the operation of the oilfield power grid, the 35kV system and the 10kV system are grounded systems or non-grounded systems through arc suppression coils at neutral points, and the system parameters may be in the resonance zone during the development period of the power grid, and the electromagnetic voltage transformer is prone to ferromagnetic resonance, resulting in overvoltage or overcurrent, and the voltage transformer will be burned in serious cases. In response to this problem, Wei Bin and Pu Jun of Sinopec Northwest Oilfield Branch wrote an article in the second issue of "Electrical Technology" in 2024 to study the three voltage transformer burnout faults in the 35kV substation of the second substation of Shunbei Oilfield, and concluded that the power system is in the resonance region, and the ferromagnetic resonance of the voltage transformer is the cause of the voltage transformer burnout. By taking measures such as the introduction of arc suppression coils, the application of voltage transformers with good saturation characteristics and the transformation of 4PT, the ferromagnetic resonance is effectively controlled.
The Shunbei Oilfield is located in the northern part of the Taklamakan Desert, and the underground oil and gas distribution is striped. In the early stage of the development of the four belts, due to the formation pressure is sufficient, the oil wells are mostly self-blowing wells, and the 110kV substation in Shunbei District 2 was built earlier than the oil and gas processing station, and its 35kV distribution network is in a light-load state, which is prone to the ferromagnetic resonance of the voltage transformer (PT), and in serious cases, the 35kV voltage transformer burns out the fault three times.
1 Introduction to Shunbei Oilfield Power Grid
The power supply of the Shunbei Oilfield Power Grid is drawn from the 220kV Hailou Substation in Aksu Shaya County, Xinjiang. At present, the power grid of Shunbei Oilfield consists of three 110kV substations, 110kV transmission network, 35kV distribution network and 10kV distribution network. The 110kV transmission network is connected by three stages of two 110kV lines, with a total length of 223km. Each 110kV substation is divided into 35kV line and 10kV line distribution. The total capacity of the power grid is 110MV∙A, and the average annual load is 26.8MV∙A.
1.1 The power grid structure of the substation in Shunbei District 2
The substation in Shunbei District 2 is a 110kV prefabricated cabin substation, with two main transformers with a capacity of 20MV∙A, both of which are three-turn transformers, and the main transformer connection group is marked with YNyn0D11.
The 110kV system of the substation is a high-current grounding system, the 35kV system is a grounding system through an arc suppression coil, and the 10kV system is an ungrounded system.
The 35kV system of the substation in Shunbei District 2 is composed of a transformer 35kV three-phase winding, busbar, circuit breaker, electromagnetic voltage transformer, power line, cable and arc suppression coil. The simplified main wiring of the substation in Shunbei District 2 is shown in Figure 1.
Fig.1 Simplified main wiring of the substation in Shunbei District 2
1.2 35kV distribution network system parameter model
Through the integration of the parameters of the main system components of the substation, the 35kV system parameter model is established, as shown in Figure 2.
Fig.2 Parameter model of the 35kV system
If the arc suppression coil is not invested, the voltage transformer winding and the system ground capacitor form a series circuit. If the arc suppression coil is put in, the voltage transformer winding and the system capacitor to the ground are connected in parallel, and then the 35kV winding of the transformer and the arc suppression coil winding of the neutral point of the main transformer form a series circuit.
See parameter (1) for the parameter values of each component:
Parameters(1)
2 Failure process
2.1 How the system operated before the failure
During the resonance, the operation mode of the substation in Shunbei District 2 is as follows: the two main transformers run in parallel, the 110kV bus joint position, the 35kV bus joint position, and the 35kV arc suppression coil at the neutral point of the main transformer exits. The two 35kV lines have a total load of 85kW.
2.2 Failure History
1) On July 11, 2022, the backstage of the oilfield ESC reported that the 35kV section I and section II 3U0 of the substation in Shunbei District 2 had exceeded the limit, and notified the power personnel on duty to inspect the site. The power duty personnel inspected the 35kV voltage transformer body, protection device, fault recording and monitoring background, and found that the secondary voltage of the 35kV II section A phase had dropped to about 40V, and at the same time smelled the burning smell of the 35kV section II section PT cabinet, and the power outage inspection found that the 35kV section II phase A phase PT was burned out.
2) On September 16, 2022, the background of the oilfield ESC reported that the voltage of the 35kV II section of the substation in Shunbei District 2 was abnormal and the voltage of phase A was reduced. The on-site inspection of the electric power duty personnel found that the fuse of the 35kVI section PT phase A phase fuse was blown out, the phase A PT burned out, and the high-voltage neutral point detuning resistor burned out.
3) On September 27, 2022, the background of the oilfield ESC reported that the 35kV C phase voltage of the substation in Shunbei District 2 was reduced. The on-site inspection of the power duty personnel found that the 35kV I. section C phase PT inter-turn short circuit burned out, and the high-voltage neutral point detuning resistor burned out.
3. Find the cause of the failure
After the fault occurs, the power maintenance personnel inspect the voltage transformer body and secondary circuit, and at the same time review the arc suppression coil device and the fault recorder records, as follows.
3.1 Inspection of voltage secondary circuit
Carry out the insulation test and short circuit inspection of the secondary circuit cable, pass the test, and eliminate the fault of the secondary circuit.
3.2 35kV电压互感器本体试验
The test was carried out on the unburned voltage transformer, and the test data are as follows.
1) Insulation and withstand voltage test data
The insulation test data of the voltage transformer are shown in Table 1, and the withstand voltage test data are shown in Table 2. As can be seen from Table 1 and Table 2, the insulation test data meets the requirements of the State Grid DL/T 596-2021 "Preventive Test Regulations for Power Equipment" The insulation resistance of the primary winding to the secondary winding and the ground is ≥ 2500MW, and the insulation resistance between the secondary windings and to the ground is ≥ 1000MW; The withstand voltage test data meets 80% of the factory test value.
Table 1 Insulation test data of voltage transformer
Table 2 Withstand voltage test data of voltage transformer
2) DC resistance and excitation inductance test data
The DC resistance test data of the voltage transformer is shown in Table 3. The primary winding of the voltage transformer is divided into three phases: A, B and C, which are connected in a star shape; The secondary winding is divided into A, B and C three phases, and each phase has four windings, which are 1x-1n, 2x-2n, 3x-3n, dx-dn (x=a, b, c), wherein the dx-dn winding adopts open triangle connection, and the rest of the windings adopt star connection. The inductive reactance of the primary winding of the voltage transformer is XL=260kW.
The DC resistance test data meets the requirements of the State Grid DL/T 596-2021 "Preventive Test Regulations for Power Equipment", the inductive reactance value is small, and the inductive reactance value of the voltage transformer of the same specification is at the MW level.
Table 3 DC resistance test data of voltage transformer
3) Excitation characteristics test data
The test data of excitation characteristics of voltage transformer are shown in Table 4. The excitation characteristics meet the requirements of 1.9Un (Un is the secondary rated voltage of 100V) without inflection point, but the rated capacity of the voltage transformer is 40V∙A, the limit capacity is 100V∙A, and the actual capacity is less than 1.9Un and 277.41V∙A.
Table 4 Test data of excitation characteristics of voltage transformer
3.3 Fault recording and retrieval
Figure 3 shows the fault recording on July 11, and the variation characteristics of the recording curves of the other two faults are the same. The fault recording data is shown in Table 5.
Fig.3 Recording of the fault on July 11
Table 5 Fault recording data
As can be seen from Table 5, before the 35kV PT burns out, the 3U0 gradually increases, from 4.66kV to 8.39kV, and the three-phase voltage unbalance gradually increases.
4 Failure Analysis
4.1 Principle of ferromagnetic resonance
1) 35kV voltage transformer ferromagnetic resonance principle
The PT ferromagnetic resonance curve is shown in Figure 4, when the system is operating normally, the voltage transformer is not saturated, the inductive reactance is greater than the capacitive reactance, and it works at point a, at this time, the loop is inductive, the capacitance and inductor voltage are low, and the loop current is small, which is the non-resonant working point. When the overvoltage saturation of a phase voltage transformer, the inductive reactance decreases, and the working point of the parallel circuit rises from point A to point B, and jumps to point C due to the instability of point B, that is, the stable ferromagnetic resonance point.
Fig.4. PT ferromagnetic resonance curve
2) The parameter range of ferromagnetic resonance
Referring to the domestic simulation tests of various harmonic oscillation conditions, with the increase of the ratio of system capacitive reactance to system inductive reactance XC/XL, the resonance of 1/2 harmonic, fundamental and 3rd harmonic occurs in turn, and the required resonant loop voltage EX also gradually increases, and the resonance range is XC/XL<0.01.
and 3) the excitation conditions of ferromagnetic resonance
There is a disturbance in the 35kV system of the power grid, such as the sudden closing of the voltage transformer, the sudden disappearance of the single-phase grounding fault in the power grid, the automatic extinguishing of the arc grounding, the disconnection of the line and the change of system current and voltage, etc.
4) The phenomenon of voltage and current after ferromagnetic resonance
Ferromagnetic resonance results in a neutral displacement voltage with an unbalanced voltage of 35kV. After the ferromagnetic resonance occurs, the three-phase winding of the voltage transformer is saturated with a certain phase core and the inductance becomes smaller due to the different voltages subjected to different voltages, the relative admittance is inductive, and the voltage transformer is low in the other two phases, which is capacitive, and the total impedance of each relative ground is not equal, and the greater the degree of impedance imbalance in the three relative grounds, the higher the Un, and the neutral point voltage is offset, and the calculation formula is shown in Eq. (1), and the neutral point voltage phasor offset is shown in Figure 5.
Equation (1)
Fig.5. Neutral voltage phasor shift
When the neutral displacement voltage increases, the ground voltage of the three-phase conductor is equal to the phasor sum of the power supply potential of each phase and the displacement voltage of the neutral point. The result of phasor superposition is a decrease in the single-phase voltage and an increase in the two-phase voltage. The magnitude of the voltage drop is related to the degree of unbalance Kt.
5) Hazards and phenomena of ferromagnetic resonance
Resonance is divided into three forms: power frequency, high frequency, and frequency division resonance. The amplitude of the power frequency and high-frequency ferromagnetic resonant overvoltage is generally high, up to more than 3 times the rated value, and the voltage amplitude during the initial transient process may be higher, endangering the insulation structure of electrical equipment.
The power frequency resonant overvoltage is similar to the phenomenon of single-phase grounding in the power grid, resulting in the phenomenon of "illusory grounding". Frequency division ferromagnetic resonance can lead to low-frequency swing of phase voltage, excitation inductance decrease, overvoltage is not high, generally below 2 times the rated value, but the inductance decline will make the excitation circuit seriously saturated, the excitation current increases sharply, and the current greatly exceeds the rated value, resulting in violent vibration of the iron core, so that the primary side fuse of the voltage transformer is overheated and burned.
4.2 Analysis of the causes of voltage transformer burnout
1) Firstly, according to the 35kV distribution network system parameter model in Section 1.2, the parameter values of the system components are estimated under the operation mode before and after the fault, and whether the 35kV system is in the resonance region.
During the burning of the voltage transformer, the 35kV system in Shunbei Zone 2 was not put into the arc suppression coil, and the voltage transformer and the system capacitor to the ground formed a series circuit. According to PT, the excitation inductance XL is 260kW, the system capacitive reactance XC is 2.7kW, and XC/XL=0.01, which just enters the resonance region.
2) Secondly, find out whether the power grid has resonant excitation conditions: check the operation and maintenance records of the operation and maintenance team of Shunbei Electric Power, before the voltage transformer burns out three times, the power grid has two single-phase grounding restorations, and one reverse operation, indicating that the 35kV system has disturbance conditions before the failure.
Combined with the fault recording of section 3.3, it can be seen that before the voltage transformer burns out, the 3U0 gradually increases, up to 10.83kV, and the three-phase voltage is unbalanced. The above phenomenon is in line with the characteristics of ferromagnetic resonant overvoltage saturation of voltage transformer, that is, the excitation saturation of 35kV voltage transformer, the inductive reactance decreases, and the neutral point voltage increases.
3) During the ferromagnetic resonance of the voltage transformer, the active power of the Shunbei 35kV line is 225kW, the reactive power is 326kvar, the current is 5.8A, the loop loss is small, the damping is small, and the resonance is easy to occur at no load and light load in the system.
Based on the above data and analysis, it can be concluded that the 35kV voltage transformer burned out in the substation of Shunbei District 2 because of ferromagnetic resonance, and the occurrence of ferromagnetic resonance is related to the system operation mode in the resonance zone, the system in the light load operation state and the resonance excitation conditions of the power grid.
In addition, the substation is a prefabricated cabin substation, and the 35kV switchgear is an SF6 inflatable cabinet, and the cabinet size is smaller than that of the vacuum circuit breaker cabinet. The 35kV voltage transformer is limited by the size of the cabinet and has a compact structure, and after the ferromagnetic resonance occurs, the heat is not easy to dissipate, resulting in the simultaneous burning of the voltage transformer and the fuse.
5. Ferromagnetic resonance control measures
According to the above analysis conclusions, the following measures have been taken:
1) First contact the ESC to change the grounding method of the neutral point of the 35kV system, and put in the 35kV arc suppression coil. Because the arc suppression coil is pre-compensated, the 35kV system changes from a series resonance loop to a voltage transformer winding in parallel with the system ground capacitor, and then forms a series circuit with the 35kV winding of the transformer and the arc suppression coil of the neutral point of the main transformer, which breaks the series resonance condition, and at the same time, when the system is disturbed by single-phase grounding, the capacitance current can be compensated in advance, the arc short circuit can be eliminated, and the resonant excitation condition can be eliminated.
2) Contact the manufacturer to replace the voltage transformer with good saturation characteristics, the impedance is MW level, the inflection point of the excitation characteristics is >1.9Un, and the volt-ampere limit output is >300V∙A;
3) Research and carry out the transformation of the 4PT connection of the voltage transformer, increase the loop reactance, and break the resonance condition of XC/XL<0.01.
4) Notify the reverse gate class to optimize the operation mode of the reverse gate, that is, when the bus connection is shorted, the PT will be withdrawn first, and then the PT will be put into the bus connection to prevent the excitation conditions from being met.
5) Contact with the ESC automation class, strengthen the system monitoring means, according to the phenomenon that the 35kV system will produce zero sequence voltage over-limit and phase voltage imbalance before the ferromagnetic resonance of the voltage transformer occurs, and set the telemetry over-limit alarm in the ESC system to achieve early detection and early intervention.
After taking the above treatment measures, after more than one year of operation inspection, the 35kV system of the substation in Shunbei District 2 has not been burned out by ferromagnetic resonance.
6 Conclusion
During the construction period of the oilfield power grid, the 35kV system or 10kV system of some substations builds new power lines every year, and the power grid parameters are in a state of change, which may enter the resonance zone, and the grounding mode is not grounded or grounded through the arc suppression coil, which is easy to excite the ferromagnetic resonance of the voltage transformer, such as the voltage transformer of the 35kV system of Tahe Oilfield Power Plant No. 1 is frequently fused at one time. Communicating with the local power grid of Kashgar and Aksu in Xinjiang, some power grids are prone to ferromagnetic resonance faults of voltage transformers at light loads.
Therefore, this problem should be considered in the substation design stage, the line parameters should be estimated, and preventive measures should be taken in advance, such as using capacitive voltage transformers, using mature 4PT to increase the inductive reactance, and adjusting the system parameters with 35kV bus installation capacitors.
The results of this work were published in the second issue of "Electrical Technology" in 2024, and the title of the paper is "Analysis of 35kV voltage transformer burnout failure of substation in Shunbei Oilfield Zone 2", and the authors are Wei Bin and Pu Jun.