Importance of lightning protection in distribution cable system

1. Importance of lightning protection

The probability of cable failure in North America is the highest in a few months in summer. Research shows that lightning impulse will shorten the service life of cables. Temperature and rainfall will also reach the highest value during this period. These factors will affect the probability of failure. It is recorded that water trees will reduce the lightning impulse level of extruded insulated cables and cause faults. In the past, the research on lightning protection of distribution system components mainly focused on overhead transformers. This is reasonable because the company that makes the transformer is also selling lightning arresters.

After the early paper insulated cable was produced, it has a high lightning impulse level, and this level can be maintained for more than 50 years. It has been widely used in underground cable insulation system for several months, but its strength has decreased significantly. It must be noted that the initial lightning impulse level of XLPE insulated cable is much higher than that of EPR insulated cable and paper insulated cable. The initial impact strength of EPR insulated cable is lower than that of other cables, and its impact level does not decline so fast. Over time, compared with paper insulated cables, the impact level of XLPE and EPR cables is closer to the basic impact insulation level (bil) of the system. Therefore, for these new cables, lightning protection is an important factor.

2. Surge protection (overvoltage protection)

2.1 protection margin

protection margin is defined as

Insulation withstand level lightning arrester protection level%$

another form of this equation is

Basic impulse insulation level of equipment arrester discharge voltage arrester lead discharge voltage%

for transformers, it is recommended that the minimum protection margin should exceed 20% of the basic impulse insulation level (bil).

2.2 rated voltage

the rated voltage of metal oxide varistor (MOV) arrester shall be determined according to its load cycle test. The load cycle test determines the maximum voltage allowed to be added to the arrester, and the arrester can release the rated current under this voltage. This voltage can also be considered as the voltage level at which the residual current can be cut off after surge discharge. For voltages above this level, overcurrent may not be cut off. The safe rated voltage of lightning arrester is determined by the maximum voltage of line to ground in case of unbalanced fault or conversion system grounding.

2.3 maximum voltage

the maximum voltage is calculated by multiplying the maximum line voltage of the system by the grounding coefficient set by the lightning arrester.

2.4 grounding coefficient

the grounding coefficient is defined as the ratio of the effective value of the highest power frequency voltage to the ground of the non fault phase to the effective value of the power frequency phase-to-phase voltage during normal operation of the phase, expressed in the form of percentage. Traditionally, when the grounding coefficient is less than 80%, the system is considered to be effectively grounded.

2.5 discharge

discharge refers to the starting process of protection when the impulse voltage reaches a certain value. When reaching a certain voltage level, the arc extends through the electrode of the equipment to form a discharge circuit to the ground. When the voltage is applied to the gap arrester, this process becomes somewhat uncertain, because the discharge on the simple gap structure is related to the wave front and subsequent impulse voltage.

the basic requirement for an appropriate discharge level is to give a consistent response to the slow rising waveform for the response speed of steep wavefront, such as lightning impulse. This waveform often appears in indirect lightning stroke and overvoltage generated by the system.

the discharge of lightning arrester cannot be confused with "flashover". Flashover refers to the external arc that may occur when the surface is contaminated.

2.6 surge discharge

surge discharge refers to the situation that the lightning arrester must pass through power frequency current and instantaneous impulse current. The overcurrent lasts until the arrester can extinguish the arc.

2.7 discharge voltage of insulation resistance

the insulation resistance (IR) discharge voltage of a lightning arrester is the product of discharge current and discharge path resistance or inductance. When the resistance is very low, the discharge current will be very high, and the discharge voltage of insulation resistance can reach or exceed the discharge voltage of arrester. The lead between the connecting wire and the grounding must be as short as possible. This is achieved by placing the arrester as close as possible to the cable terminal and always connecting the arrester closer to the incoming line than the terminal