SPD security performance requirements of the MOV

Author: Fei Zi Hao, Zhu Ying Fu, Source-Guizhou Flyboat Electronic Co.,Ltd

1. Preface

With the GB18802.1-2002, GB50057, YD/T1235, GA173 series of standards such as the promulgation and implementation of promotion, lightning protection laws and regulations around the increasingly sound and healthy, Surge Protector (SPD) the application of more and more popular around the production plant springing up one after another first emerged, sometimes bustling, mine industry, a bustling, the prospects seem to be illuminated. But at the same fire caused due to SPD time to time instances of fire, due to SPD commonly used in the more important occasions or equipment of mine safety protection, accident caused a huge loss, a number of manufacturers and users are paying for this expensive the cost of the individual regions or even refuse to use or trade caused by the user to disable the adverse effects of SPD. To provide users with lightning at the same time security protection, if not effectively solve the issue of mine safety product itself, and then a low residual pressure, and then a large flow passage, and then a small leakage flow Dengjun meaningless. The following restrictions on the application of the most popular pressure-type SPD’s own security and the use of the core functional elements-MOV performance requirements and related detection methods to talk about some of the author’s own analysis and views, fallacious advanced hesitate to look to peer-correction

2. Pressure limiting type SPD Fire Cause Analysis

Compliant pressure-type SPD largely limited by the MOV chips, hot from the protection agency, connect copper fittings, fire-retardant packaging shell composition. Clearly, SPD chip on fire with the MOV closely related cause of the fire to ascertain the SPD, it is necessary to understand the work and failure MOV Principle

2.1. Principle of work and failure MOV:

Power systems up and running, the lightning-chip (MOV) showed high resistance state, does not affect the power grid to work; when the lines caused by lightning or surge over voltage operation, MOV conduction velocity will nanosecond response, quickly diarrhea on the ground over-current release, the magnitude of over-voltage surge protection devices have been limited to affordable levels below permitted the use of electrical equipment that scourge. Over-voltage surge diarrhea after release, MOV normal work resumed high-impedance state

If the short-wave surge of energy too, more than mine chip can withstand the limit MOV easily lead to fragmentation

If the system of sustained over-voltage of electricity supply failure, more than mine chip can withstand the limit, would result in MOV sustained fever, at this time if it still can not be effectively cut off the circuit, will further lead to MOV is hot melt breakdown; Once the MOV by hot-melt breakdown, the circuit is equivalent to one with a current-limiting resistor arc discharge circuit [1], can easily cause arcing of packaging materials, such as burning fire. Most of the SPD are triggered fire
   
   There is another situation: a strong lightning surge and normal-frequency voltage combined effects of: continuing the strong energy of lightning surge caused by the rapid heating MOV (or the ambient temperature is too high, for example, sealed in a metal box within the sunlight exposure), if the MOV action voltage and temperature characteristics of negative temperature coefficient, it may result in action-voltage (U1mA) dropped drastically at this time the normal power frequency voltage of the MOV is equivalent to imposing a very high sustained over-voltage, resulting in a fire MOV breakdown.

2.2. Domestic low-voltage power supply system easy to occur in several over-voltage [1] and its impact on the MOV

2.2.1. Over-voltage single-phase ground

National electricity supply system, the most common. When the power supply system in a relatively short-circuit, the other two-phase to ground voltage (220V) that represents an increase of line voltage (380V). This will lead to a low value of Uc elected SPD common-mode failure in a relatively short period of time. As long as the over-voltage duration is not long, MOV’s good quality and U1mA ≥ 510V, this is usually able to withstand over-voltage caused by SPD or just tripping action, will not cause other serious consequences

2.2.2. A total of coupled transfer of high and low over-voltage

As the high-voltage and low voltage distribution transformers share a ground, when the high-pressure side through the PE failure occurs when the ground, there will be a very high over-voltage amplitude, through the sharing of land transferred to the low pressure system coupled to make low-voltage power supply system in the amplitude the transfer of high over-voltage, over-voltage amplitude of such high and long-lasting, easy to heat in seconds within the SPD and the movement away from institutions have not yet even before the thermal breakdown MOV failure, could easily lead to arcing, fire, etc.

2.2.3. Over-voltage zero-loss

Low-voltage power supply system of the neutral wire disconnected for various reasons after the loss of zero that generate over-voltage. At this point phase voltage output into a line voltage output of each phase voltage is chosen by a different load distribution decisions. Such cases are sometimes over-voltage up to 700V or higher, so even if the SPD of the maximum allowable operating voltage Uc elected to 420V (U1mA = 680V ±), using differential-mode in the MOV connected SPD will also be a direct hit within a few seconds to wear, heat from the body too late to move, could easily lead to arcing, fire, etc.

3. The performance requirements of the MOV and associated detection method

3.1. The performance requirements of the MOV

Obviously, as a transient over-voltage surge protector, we must attach importance to SPD’s protective effect, use residual down, pass flow of a large MOV chips;, but at the same time we should pay attention to the safety of the product itself SPD to prevent the situation in the absence of lightning strikes The SPD emerged under the self-ignition and the resulting fire caused fire, otherwise it is impossible for a user account for, the development of the industry inevitably have a negative impact. As the SPD’s core functional elements, the following four aspects of the MOV is necessary to carry out assessment

a. 8/20μS Flow capacity and residual voltage: A nominal discharge current and the maximum discharge current assessment. Commonly used 34S for SPD-type MOV, so long as to reach 20kA × 20 times +40 kA × 2 times, U20kA/U1mA ≈ 2.5 can be, and should not be excessive in the pursuit of individual indicators

b. High temperature stability: with actions voltage U1mA within a certain range of temperature characteristics and coefficients, leakage current high temperature stability.

c. a temporary over-voltage tolerance features: This indicator should be based on the actual situation under examination, on a temporary over-voltage MOV Sealing-off point to cut the fat melting capacity of the circuit.

d. moist environment tolerance: this national standard and IEC standards have been defined, it will not go into.

3.2. Detection of high temperature stability

　　A stable high-temperature evaluation chip moves at a higher temperature stability of voltage and leakage current. MOV aging failure rate of charge to its load are closely related to the rate charged is calculated as q =; Clearly, the negative temperature characteristics of the MOV, the high temperature U1mA reduced the rate charged increases, will lead to increased leakage current ; while the increase of leakage current in turn lead to further heat MOV to form a vicious circle, making MOV expire soon, it must assess heat load increases the stability of the next U1mA, IL. According to the actual situation, assessment 120 ℃ ~ 160 ℃ under U1mA, IL stability are necessary.

In the following three indicators to assess temperature stability

a. voltage temperature coefficient τ = =- 0.05 ~ +0.05% / ℃; recommended as 0 ~ 0.05% / ℃. U1mA (130 ℃) to 130 ℃ to impose 75% U1mA DC load, when the value of the insulation 120min prevail.

　　b. leakage current IL trends: 130 ℃ under a constant imposed 75% U1mA (25 ℃) DC voltage, the current flows through the varistor stable or showed a decrease trend.

　　c. 130 ℃ under a constant imposed by 1.1-fold UC (AC RMS), continued to 7h, cooled after 1h recovery measurement, leakage current IL remains stable and the rate of change of ≤ 100%, limited voltage Ur, varistor voltage U1mA rate of change of ≤ 5% .

In general, the positive temperature coefficient MOV better high temperature stability.
   High-temperature stability test table

Must be noted that the impact of the bear 8/20μS more important after the high temperature stability.

3.3.Temporary over-voltage tolerance characteristic of detection methods

　　Temporary over-voltage is due to power failure caused by sustained over-voltage, as a transient over-voltage surge protection device SPD which is powerless. When the system occurs when a temporary over-voltage requirements of either SPD is not action, able to withstand (the amplitude is small); either quickly out to cut off the flow through the SPD of the circuit (magnitude higher), no fire. In short to ensure their own safety, not because of reasons SPD caused fires. The SPD, taking into account differences in plant product mix, technological level of its black sheep, we believe that using the following method MOV, on a temporary over-voltage from the capacity assessment under the more appropriate: The two leads to electrodes with the 63A solder (183 ℃) and the chip Silver After a good connection of welding wires, wires and chips 200g imposed outside the tension between the imposition of 1.1 times the U1mA of the AC voltage (RMS), tin solder at melting, wires, or electrodes cut off the circuit with the chip disengagement. Chip does not appear to puncture or other visual qualified visible mechanical damage. The process should be soft pad beneath the chip off when you throw things in order to prevent breakage. Voltage is applied in a manner that the power supply fault simulation of the real situation: sudden appearance.

Temporary over-voltage tolerance characteristic checklists

同Similarly, the assessment 8/20μS to withstand the impact of the temporary over-voltage after the tolerance is even more important

4. Conclusion

　　As a transient over-voltage surge protection devices, SPD not only attach importance to its protective effect (requiring low levels of residual voltage, the larger-pass flow), but also should pay attention to their own security. Or less the same protective effect in the case, should give priority to the safety performance of a good product. MOV performance requirements of the comprehensive consideration to the protective effect, security, performance, life characteristics, etc., not sacrifice too many other ways to pursue a particular aspect.

This article cites Mr. Guo Yaping ‘s Discussion of the temporary overvoltage ,hereby we are grateful.

References

[1]: Guo Yaping, etc.: Zinc oxide varistors for low voltage power supply system of the cause of the fire and solve the problem; Eleventh Annual Meeting of pressure-sensitive cross-strait the first technical seminar-cum-special issue,P25.