1 Overview
The neutral point of 35 ~ 6 kV transformer is not grounded or grounded through arc suppression coil, and it is totally insulated in structure. The end of transformer winding is protected by lightning arrester. When three-phase incoming waves come, the potential of neutral point may be increased by about twice of incoming wave voltage due to total reflection, which is very dangerous. However, according to practical operation experience, the neutral point can still operate safely without protection device. The reasons are as follows:
(1) The lightning current flowing through the end is generally below 2kA, so its residual voltage is reduced by about 2% compared with the predetermined residual voltage of 5kA;
(2) Most incoming waves come from far away, and the steepness is small;
(3) According to statistics, the probability of three-phase incoming waves is very small, only about 1%, only once every 15 years on average.
therefore, the overvoltage protection and insulation coordination of AC electrical equipment (DL/T62—1997) stipulates that neutral points of transformers in ungrounded, arc-suppression coil-grounded and public * * * resistance systems are generally not equipped with protective devices.
the p>11～22kV system is an effective grounding system, in which some neutral points are directly grounded, and at the same time, in order to limit the single-phase grounding current and meet the needs of relay protection, some transformers are not directly grounded. The transformer in this system is divided into two situations. One is that the neutral point is completely insulated, and the neutral point is generally not protected. Secondly, the neutral point is semi-insulated (this is true for all new transformers). Specifically, the neutral point of 11kV transformer is 35kV insulation level, while that of 22kV transformer is 11kV insulation level. The regulation stipulates that the neutral point protection of transformer in effective grounding system should generally adopt the protection mode of gap protection and lightning arrester protection in parallel.
2 neutral point protection gap and overvoltage protection
2.1 single-phase grounding overvoltage
When the single-phase grounding of an effective grounding system is used, the calculation of the neutral point potential of an ungrounded transformer is generally based on the fact that Xo/X1 is less than 3, but in fact, the Xo/X1 values of power grids and substations in different regions are quite different. The overvoltage level at the neutral point of the transformer is naturally different, so it is recommended in general articles to take the maximum voltage Umax according to the overvoltage value of 1,15 times and Xo/X1=3. For example, in a 1 1kV system, the maximum operating line voltage is 126kV, and the overvoltage calculation formula of the neutral point is:
uo = umax k/(k+2) where: k-.
XO-zero sequence impedance;
x1-positive sequence impedance.
when K=3, Uo=.6Umax, that is, the highest voltage steady-state value at the neutral point of 11kV main transformer is 43.6 in case of single-phase grounding fault.
if the circuit breaker on the grounded transformer side trips when the system is grounded in single phase, and the circuit breaker on the ungrounded transformer side refuses to operate, the system will form a partially ungrounded system. At this time, the neutral point overvoltage value is higher, which is approximately the phase voltage value. For example, the steady-state value of the neutral point potential in the 11kV transformer is 73 (the relay protection should act at this time).
2.2 lightning overvoltage
in thunderstorm season, the high-amplitude lightning wave that directly hits the substation or is transmitted to the power plant and substation along the line causes the neutral point potential of the transformer to rise, resulting in high lightning overvoltage, which endangers the safety of electrical equipment. The maximum lightning overvoltage at the neutral point of transformer mainly depends on the residual voltage of arrester at the entrance of transformer and the characteristics of transformer. General lightning overvoltage is calculated as follows:
Um=n/3(1+r)Us
where: n—— the phase number of intrusive lightning wave;
r—— oscillation attenuation coefficient of transformer, .5 for tangled winding and .8 for continuous winding;
u5-residual voltage on arrester at the entrance of transformer.
the effects of several forms of overvoltage on transformer insulation and protection devices are briefly described above, depending on the waveform, amplitude and duration of overvoltage. The standard lightning waveform is not necessarily derived from lightning, for example, when a single phase is grounded, a short wave front close to lightning overvoltage can be generated on the ungrounded phase.
2.3 protection function of discharge gap
The principle of using discharge gap protection is to connect a zero-sequence current transformer in the gap circuit in series, and use the discharge characteristics of the gap to discharge it in case of lightning overvoltage to protect neutral point insulation. After the system fails, the power frequency potential of the neutral point of the transformer rises to a certain value, and the zero sequence current protection acts to cut off the ungrounded transformer to avoid the fault operation of the neutral point grounding belt. Neutral zero sequence current protection firstly cuts off the power plant tie line on the low voltage side in a short time, and then trips the switches on each side of the transformer in a slightly longer time.
2.4 protective function of lightning arrester
No matter whether it is a zinc oxide lightning arrester without gap or a common valve lightning arrester with gap, the same principle is to make the rated voltage of lightning arrester not lower than the temporary overvoltage of lightning arrester installation point. JB/T5894-91 "Guidelines for the Use of Alternating Current Seamless Metal Oxide Arresters" points out that the rated voltage of the neutral point arrester should not be lower than the highest phase voltage of the transformer when its neutral point is ungrounded in the neutral point effective grounding system (specifically, the rated voltage of the zinc oxide arrester is 6kV when the standard impact insulation level of the neutral point is 1.85 kV).
3 coordination of protection gap and volt-second characteristics of lightning arrester
3.1 basic requirements for coordination of volt-second characteristics of protection device
(1) In order to reliably protect electrical equipment, the protection device should meet the following basic requirements:
The impulse discharge voltage Ub(i) of the protection device should be lower than the impulse withstand voltage of the protected equipment. Take the transformer as an example, its impulse withstand voltage value is usually taken as its multiple chopper withstand voltage value Uid, so Ub(i) should meet the following requirements:
Ub(i)
(2) The discharge gap should have a flat volt-second characteristic curve and as high arc extinguishing capability as possible. Curve 1 in Figure 2 is the volt-second characteristic of insulation. The arrester and the protection gap should play a protective role, and the volt-second characteristic curve 2 of the discharge gap should always be lower than curve 1, and a certain interval should be left. Obviously, the flatter the volt-second characteristic of the discharge gap, the better. If the volt-second characteristic is steep, as shown in Figure 3, it may intersect with the volt-second characteristic of insulation, so that the equipment cannot be protected within a short discharge time range. At the same time, due to the dispersion of discharge, the gap and the volt-second characteristic of the protected equipment are actually in a banded range, so the upper envelope of the volt-second characteristic of the protected equipment is required to be lower than the lower envelope of the volt-second characteristic of the protected equipment, as shown in Figure 4.
3.2 discharge characteristics and volt-second characteristics of protection gap
breakdown characteristics of uniform electric field gap under steady-state voltage: strictly speaking, there is only one uniform field, that is, the electric field between infinite parallel plate electrodes, which is impossible in engineering. The parallel plate electrodes used in engineering generally adopt measures to eliminate the edge effect of the electrodes (for example, the edge of the plate electrode is bent into a circular arc with a relatively large radius of curvature, as the two electrodes of a high-voltage electrostatic voltmeter do). At this time, when the distance between the two parallel plate electrodes is h relative to the electrode size, the electric field between the two electrodes can be regarded as a uniform field. Because the two parallel plates of uniform field have the same shape and are arranged in parallel, there is no polarity effect and corona phenomenon in the discharge of air gap. Once the air gap is discharged, it will cause the breakdown of the whole air gap, so the breakdown voltage under the action of DC, power frequency AC and impulse discharge voltage is the same, and the dispersion of discharge is also small, and the breakdown voltage has nothing to do with the voltage action time. The breakdown characteristics of air gap in slightly inhomogeneous field are basically the same as those in uniform field. Its volt-second characteristics are shown in Figure 5.
in the extremely uneven electric field, the "bar-bar" gap and "bar-plate" gap have typical significance. The former has complete symmetry, while the latter has the largest incomplete symmetry. The air gap breakdown characteristics of other types of extremely uneven electric fields are between the two typical air gap breakdown characteristics. The conclusion drawn from the experiment is that the discharge of inhomogeneous field has obvious polarity effect, and with the increase of air gap length, the average breakdown field strength of air gap decreases obviously, that is, there is "saturation" phenomenon. Its volt-second characteristic is shown in Figure 5.
It can be seen from Figure 5 that the breakdown characteristics of the uniform electric field (that is, the breakdown characteristics under impulse voltage) are steep in a period of time before the island, that is to say, in t, where t1 is the voltage rise time, to is the statistical time delay, ta is the discharge development time, and tb is the sum of the above three parameters, which is the time required for discharge. Tb is less than to in value, so the discharge characteristics of the gap in a short time are related to the discharge development time. To discharge in this very short time, its volt-second characteristic curve is obtained.
3.4 Coordination between protection gap and volt-second characteristics of lightning arrester
(1) Requirements for discharge gap: First, for power frequency, according to the requirements of system operation, when Xo/X1 value is less than 3, the discharge gap should not act when single-phase grounding occurs, and the discharge voltage should be greater than 43.6kV (effective value, peak voltage is 61.7kV); When the system forms a partially ungrounded system, the neutral point overvoltage value is higher at this time, and its value is approximately the phase voltage value. For example, the steady-state value of neutral point potential in a 11kV transformer is 73kV, and the single-phase grounding gap should act to start relay protection to remove the fault, that is, the discharge voltage in the discharge gap should be less than 73kV (effective value, peak voltage is 13.2kV); Second, the gap should not act under lightning overvoltage and transient overvoltage of single-phase grounding of the system. The breakdown voltage of the gap is very large.
3.3 discharge characteristics of lightning arrester
In the current transformer neutral point protection, MOA is the mainstream lightning arrester. MOA valve plate has excellent nonlinear volt-ampere characteristics; It has no spark gap, and once the applied voltage starts to rise, the valve plate immediately begins to absorb the energy of overvoltage to inhibit the development of overvoltage; There is no gap discharge delay, so it has good impact response characteristics. No afterflow, light operating duty, and repeated protection; Only the overvoltage energy is absorbed, but not the aftercurrent energy, so the operating duty is light. Parameters of lightning arrester currently used in 11kV (taking the lightning arrester produced by Fushun Haiyue Electric Manufacturing Co., Ltd. as an example).
(2) Requirements for lightning arresters: First, lightning arresters should not operate under power frequency overvoltage and operating overvoltage, but should operate under lightning and system single-phase grounding transient overvoltage; Second, the discharge voltage and residual voltage of lightning arrester should be less than 153kV (the operating wave strength of transformer insulation is 75.5×√2×1.4=153kV); Third, the power frequency discharge voltage and arc extinguishing voltage of lightning arrester should be greater than 73kV (the effective value of gap control voltage, with a peak value of 13.2kV).
(3) Coordination requirements between the discharge gap and the arrester (when power frequency overvoltage and high frequency overvoltage appear one after another, the arrester will act first and then intermittently to ensure the normal operation of the arrester, so there is no possibility of the arrester explosion):
First, the arc extinguishing voltage of the arrester should be higher than the highest power frequency discharge voltage of the gap, so that the arrester will not explode without extinguishing the arc under the protection of the gap; Secondly, the impulse discharge voltage of the arrester is low, which ensures the arrester to act under high-frequency transient overvoltage, and avoids the discharge gap action when a single-phase grounding fault occurs in normal system operation, resulting in zero-sequence current components and misoperation of gap zero-sequence current; Third, the highest power frequency discharge voltage in the gap should be lower than the lowest phase voltage, so as to ensure that asymmetric faults such as single-phase grounding in ungrounded systems can be removed; Fourth, the Xo/x1 value of the power system should be less than 3 during normal operation. When the Xo/x1 value is greater than 3, the discharge gap should act when the single-phase grounding occurs in the operating system.
(4) The minimum discharge voltage of arrester should be 13.2kV, the minimum discharge voltage of protection gap should be greater than 61.7kV and the maximum discharge voltage should be less than 13.2kV..
when t is less than to, it is the key to match between arrester and gap. We have solved the problem of matching between them by using the characteristics of discharge delay (usually tens of milliseconds) of gap discharge and no discharge delay of metal oxide arrester.
4 Conclusion
(1) The discharge characteristics of gas change with the uniformity of electric field. The breakdown voltage of gas in uniform electric field is stable and the overall volt-second characteristics are flat, but there is a problem of discharge delay in a short time.
(2) MOA valve plate of MOA has excellent nonlinear volt-ampere characteristics; It has no spark gap, and once the applied voltage starts to rise, the valve plate immediately begins to absorb the energy of overvoltage to inhibit the development of overvoltage; There is no gap discharge delay, so it has good impact response characteristics.
(3) It is of great practical significance to reasonably apply the matching curve of the protection gap and the volt-second characteristic of the lightning arrester, and verify it under experimental conditions, so that they can discharge under their respective specified conditions and then play their respective roles.