surge protective device application common sense

- Aug 09, 2019-

With the continuous development of the economy and the rapid improvement of the level of modernization, under the guidance of industrialization driven by information technology, the application of various information equipment, electronic computers, precision instruments and data network equipment is becoming more and more widespread. It has low pressure resistance, high sensitivity and low anti-interference ability, so it is extremely vulnerable to lightning current pulses. Every year, it causes huge direct economic losses to human beings. The indirect damage caused by the damage of important equipment and the paralysis of the network is even more alarming. It has already attracted the attention of domestic related fields to strengthen the protection of such systems.

In recent years, the term "SPD" has been increasingly mentioned by people in professional research, product manufacturing and engineering design. As an important part of the lightning protection system, “SPD” has been widely used in various industries such as post and telecommunications, radio and television, financial securities, insurance, electric power, railways, transportation, airports, petrochemicals, and municipal construction. It is no exaggeration to say that wherever IT equipment is installed, there is a need to apply SPD.

So what kind of product is SPD? What are the features of SPD? How does SPD choose to apply? Here we set out to introduce you to some basic knowledge about SPD products in the most common language possible. I hope that readers who have not been exposed to SPD or who know little about SPD and want to master SPD knowledge and then use SPD products will benefit.

一、   What is SPD

SPD is the full name of the surge protective device. It is a transient overvoltage and snorkeling current (a transient wave of current, voltage or power transmitted along the line) that limits lightning strikes, intrusion waves, lightning induction and operating overvoltages. The device is characterized by a fast rise and then a slow rise. A port SPD is connected in parallel with the protected circuit to separate the input and output terminals, and a special series impedance is provided between the terminals; the two-port SPD has two sets of input and output terminals, and a special series impedance between the terminals; The voltage switch type SPD has high impedance when there is no surge, and can immediately become low impedance when there is a surge voltage. The commonly used components of the voltage switch type SPD are discharge gap, gas discharge tube, thyristor (silicon controlled rectifier) and Triac. This type of SPD is sometimes referred to as a "short-circuit SPD"; a voltage-limited SPD has a high impedance in the absence of a surge but its impedance will continue to decrease as the surge current and voltage rise. Commonly used nonlinear components are varistors and suppression diodes. These SPDs are sometimes referred to as “clamped SPDs”; composite SPDs are composed of voltage-switching components and voltage-limiting components, and their characteristics vary with the applied voltage. The characteristics can be expressed as voltage switch type, voltage limit type or both.

The SPD of the infinite current component is used in the information line with only one or several components for limiting the overvoltage, and the infinite current component; the SPD of the finite current component uses both the overvoltage limiting component and the limited current in the information line. element.

SPD voltage limiting components can be divided into voltage switching type and voltage limiting type.

SPD is subject to Class I, II, and III classification tests. SPDs are indoor, outdoor, accessible, and inaccessible (untouchable). Fixed installation and mobile.



二、Application of SPD to suppress types of abnormal overvoltage

A voltage exceeding the upper limit of the normal operating voltage specified by the design is generally referred to as "abnormal overvoltage" or "overvoltage". It is the working object of SPD. If there is no overvoltage, there is no survival value of SPD. Therefore, to understand SPD, you must have a basic understanding of overvoltage.

Overvoltage will cause direct damage to circuits and components in electrical or electronic devices. Such damage can be classified into the following four cases depending on its severity. .

1. Make the equipment and equipment work in a short time.

2, causing latent faults, resulting in reduced performance of the circuit and device, shortened life, early failure.

3. Permanent damage to the circuit or device.

4. Lead to fire, electric shock and other safety accidents.

Abnormal overvoltages may be foreign, or they may be self-generated inside the device or device. The intrusion path of external intrusion voltage can be conducted through wires, circuits, and pipes; it can also be intruded by electrostatic induction or electromagnetic induction. The occurrence of overvoltage may be regular and periodic. But more are random. Therefore, in most cases, it is difficult to accurately grasp it. Abnormal overvoltage can be divided into lightning overvoltage, operating overvoltage, static electricity and transient overvoltage according to its different causes. The following is a classification introduction:

1) Lightning overvoltage: When Thundercloud directly discharges equipment and devices, the device device is subjected to “direct lightning overvoltage”. This happens with less probability, and the commonly used lightning overvoltage refers to “induced overvoltage”. ". When a lightning strike discharges to a certain point on the ground, a transient voltage of a certain magnitude is usually generated in the wires and conductors within 1.5 km of the square. The main mechanism for generating such a surge voltage is as follows:

①Thundercloud discharges objects near the ground or discharges in nearby clouds, and the generated electromagnetic fields generate induced voltages in the line conductors of the power supply system.

②The ground current generated by the discharge between the cloud and the ground. Coupled to the common ground impedance of the ground grid, a voltage difference is created across the length and width of the ground grid.

③If the lightning strike occurs, the arrester on the primary side of the transformer acts, and the voltage on the primary side drops rapidly. This rapid drop is transmitted to the secondary side through the capacitive coupling of the transformer. The superposition is on the voltage coupled through the normal transformer to form a secondary side surge voltage.

④Lightning directly hits the high-voltage primary side line, injecting a large current into the primary side line, which flows through the grounding resistance or the impact resistance of the primary side conductor. High voltages are generated, and this high voltage on the primary side can be coupled to the normal transformer through capacitive coupling and appears in the low voltage AC power line.

⑤The lightning strikes directly on the secondary side line, and the extremely large current and the extremely high voltage generated by this current far exceed the capability of the device itself and the protection device connected to the secondary side line.

In order to simulate lightning shock, the international standard "1.2/50" voltage wave is a standard lightning voltage wave (the wavefront time is 1.2μs, and the wave tail drops to half-peak time is 50μs.) "10/350" current wave is radius Conducted attenuated lightning current waves; "8/20" current waves are induced-induced induced lightning current waves. The characteristics of the lightning shock wave are short in duration but high in peak value.

2)Operation overvoltage refers to the overvoltage generated in the system circuit, circuit to ground and both ends of the switch when the circuit breaker, isolation switch, relay, thyristor switch, etc. are switched on and off in the circuit.

The reason for generating the overvoltage is because the circuit and the components in it have inductance and capacitance, the magnetic energy stored in the inductor and the electrostatic field energy stored in the capacitor, the energy conversion generated during the sudden change of the circuit state, and the transition oscillation. In the process, an overvoltage occurs due to oscillation.

The duration of the operating overvoltage is longer than the lightning overvoltage, shorter than the transient overvoltage, between hundreds of microseconds and 100 mS, and the attenuation is fast.

3)It is well known that in the dry weather, the friction between the human body and the clothes will charge the human body. When the charged person comes into contact with the electronic products, it will discharge the electronic products (such as mobile phones). This is a typical electrostatic discharge, static electricity. The discharge is characterized by a high voltage, but the time is very short, in the nanosecond range.

The voltage of the simulated contact electrostatic discharge specified in IEC61000-4-2 is from 2kv to 8kv, and the corresponding current peak is (7.5 to 30)A.

4) Transient overvoltage refers to the phase-to-ground or phase-to-phase voltage rise when the power system has a ground fault, cut off the load or resonance, and it is characterized by a long duration (0.1S to 60S, Related to the way the system is protected). The amplitude of the transient overvoltage varies with the grounding method of the power supply system. For systems with large grounding resistance, the transient overvoltage multiple is large.

In addition, in practice, it is also possible to encounter a so-called "wrong power" accident, that is, a device designed for a 110V power supply is erroneously connected to a 220V system, or a device designed for a 220V power supply is erroneously added with a 380V voltage, etc. The overvoltage caused by this is not only the peak value of the access voltage, but also the oscillating voltage of the transition process.

In short, the cause of abnormal overvoltage is complicated, and the duration and the strength of voltage and current vary greatly. Therefore, protecting abnormal overvoltage is sometimes a complicated and difficult task. Generally, the varistor is a "transient" overvoltage protector that protects against static electricity for a short duration, lightning overvoltage and overvoltage. For transient overvoltages with long duration, only fuses can be used, and open circuits can be used. Devices such as devices to protect.

三、   Definition of key terms for SPD products

There are many terms in SPD products, among which the following terms are more commonly used.

1Nominal discharge current (In: The current peak flowing through the SPD with an 8/20 waveform. SPD classification for Class II tests and pretreatment tests for SPDs for Class I and Class II tests

2Maximum discharge current Imax: The SPD has a peak value of 8/20 waveform current, and its value is determined by the procedure of the II-stage operational load. Imax is greater than In.

3The inrush current Iimp: determined by the current peak value Ipeak and the charge amount Q, and the test should be carried out according to the procedure of the action load test, and is an SPD classification test for the class I test.

4Maximum continuous operating voltage Uc: Maximum rms rms or dc voltage that is permanently applied to the SPD.

5Voltage Protection Level (Up: A performance parameter that characterizes the voltage between the SPD limit terminals. The value can be selected from a list of preferred values that should be greater than the highest value of the limit voltage.

6Limit Voltage: The maximum voltage peak measured between SPD terminals when a surge voltage of a specified waveform and amplitude is applied.

7Residual voltage: The voltage peak between the terminals when the discharge current flows through the SPD.

8SPD Disengager: The device required to disconnect the SPD from the power system when the SPD fails.

9Continuous running current: The sum of the current flowing into the SPD protection element and the current flowing into the internal circuit connected in parallel with the SPD plus Uc.

10Freewheeling (If): The power frequency current supplied by the power supply through the SPD when the SPD discharge operation is just finished.

11Insertion loss: At a specific frequency, the SPD insertion loss connected to a given power supply system refers to the voltage ratio after the insertion of the SPD before and after the SPD is inserted immediately after the insertion point, and the SPD for the information line. In other words, the insertion loss is the ratio of the power of the transmission system before and after the SPD is connected to the transmission system. Expressed in dB (decibel).

12Bit Error Rate (BER): The ratio of the number of incorrectly transmitted bits to the total number of transmitted bits in an information transmission system at a given time.

13SPD frequency range (fG: The SPD connected to the signal line can generate energy loss after the access line, which is specified to be 3dB insertion loss, and the starting frequency to the cutoff frequency is the frequency range for the signal line SPD.

14SPD data transmission rate (bPS): The SPD used for the information line should not affect the upper limit data transmission rate transmitted by the system after accessing the network system, and is represented by the transmission bit value bPS within 1 s, that is, bPS/S.

15Return Loss AR): The energy ratio at which the forward wave produces a reflection at the SPD insertion point under high frequency operating conditions. It is a parameter that measures the degree of impedance matching between the SPD and the protected system. AR is a modulus of the inverse of the reflection coefficient in decibels (dB). When the impedance can be determined, the AR can be determined from the following formul

a:AR=20×lgMOD[(Z1+Z2)/( Z1- Z2)]

In the middleZ1The characteristic impedance or source impedance of the transmission line before the impedance discontinuity.

       Z2The characteristic impedance after the discontinuity or the load impedance measured from the junction between the source and the load. MOD is the calculation of the impedance mode.

16、Near-end crosstalk NEXT: The direction of propagation of the crosstalk in the interfering channel is opposite to the direction of propagation of the current in the interfering channel. The interfering channel port that continues the near-end crosstalk typically approaches or coincides with the energizing end of the interfering channel.

17Class I classification test: The test sample was subjected to a nominal discharge current In, a 1.2/50 μs surge voltage and a maximum inrush current Iimp. The waveform of Iimp was 10/350 μs.

18Class II classification test: The test was conducted on the nominal discharge current In, 1.2/50 μs impulse voltage and maximum discharge current Imax. The waveform of Imax is 8/20 μs.

19Class III classification test: The test sample was subjected to a mixed wave (1.2/50 μs, 8/20 μs) test.


四、Selection and application of low voltage distribution system SPD

The current lightning protection engineering technology has entered a new era, and it must be implemented considering various physical characteristics and functions of lightning. The electrostatic field changes, magnetic field changes and electromagnetic radiation generated by powerful lightning cause damage to many microelectronic devices within a certain range. The former lightning protection concept was limited to one-dimensional channels along the lightning current or lightning voltage wave transmission, but it was different after the 1980s, and the lightning caused damage in the three-dimensional space around the one-dimensional channel. Therefore, lightning protection engineering must be fortified from three-dimensional space. This is a new kind of thinking. It was originally thought that a lightning strike was introduced into the earth by a lightning rod to achieve the lightning protection needs, and there would be no thunderstorms. However, there are still many units in the area of several kilometers with lightning disasters and even serious losses. This is because today's various electrical and electronic products have to work in a heavily polluted electromagnetic environment. This pollution comes from lightning, electrical system operation and failure, and static electricity. The abnormally high voltages they generate are particularly damaging to information technology products. Because a large number of integrated circuits (ICs) are used in such products, the size of the components, the spacing between the wires is only a few microns, or even less than 1 μm. Therefore, the surge voltages acting on them can be damaged by being slightly higher than the design value. The damage caused by such damage is mainly not in the equipment itself, but in the indirect loss caused by the stoppage or work of these information technology equipment. For example, the power board of a bank computer is damaged by the lightning-induced voltage. The replacement of the damaged component may be only about 100 yuan, but the loss caused by the shutdown or loss of memory data cannot be calculated. Therefore, for modern electronic equipment, especially information technology products, protection against abnormal overvoltage is a problem that must be seriously solved and cannot be sloppy. Simply and sloppyly placing the SPD device in various lines does not mean that it is the optimal protection solution. Only the correct selection, configuration, and installation can make the SPD achieve the desired effect, and the protection solution can be successfully implemented.

The prerequisite for the normal functioning of the SPD is to ground the surge current caused by the surge voltage in the shortest way through the equipotential system. The power line, the signal line, the metal pipe, etc. are all equipotentially connected by an equipotential bonding conductor or a surge protector, and the respective local equipotential bonding rows are connected to each other. And finally connected to the main equipotential bonding row. According to the induction principle, the higher the inductance, the higher the voltage generated by the current in the circuit. V=L·di/dt. The inductance is mainly related to the length of the wire, and has little to do with the cross section of the wire. Therefore, the wire should be as short as possible, as far as possible in the area to be protected, all conductive parts can be considered to have nearly equal potential, so there is no significant potential difference.

1Determination of the level of the SPD configuration scheme

(1)The maximum acceptable annual average number of lightning strikes that determine system equipment damage. Nc

Nc=5.8×10-3/(C1+C2+C3+C4+C5)    Times/year   (1)

The values of C1, C2, C3, C4 and C5 are shown in Table 4.1.

Note: Some experts believe that Nc should be 5.8×10-2/( C1+C2+C3+C4+C5), which is more suitable for China's national conditions and the acceptability of the owners.

(2)According to the regional thunderstorm day Td, determine the regional lightning strike frequency Ng

Ng=0.24Td1.3/Kyear             (2)

LPZ0A area: All objects in the area may be subjected to direct lightning strikes and lead to all lightning currents; the electromagnetic field strength in this area is not attenuated.

LPZ0B area: It is unlikely that each object in the area will be directly struck by lightning current corresponding to the radius of the selected ball, but the electromagnetic field strength in this area is not attenuated.

LPZ1 zone: It is impossible for each object in the zone to be directly struck by lightning. The current flowing through each conductor is smaller than the LPZ0B zone; the electromagnetic field strength in this zone may be attenuated, depending on the shielding measures.

LPZn+1 follow-up lightning protection zone: When it is necessary to further reduce the inflow current and electromagnetic field strength, the subsequent lightning protection zone should be added, and the requirements of the subsequent lightning protection zone should be selected according to the environmental zone required by the object to be protected.