Abstract: Lightning and other strong interference can cause serious damage to communication systems and the resulting consequences, making lightning protection essential. Lightning consists of high-energy low-frequency components and highly penetrating high-frequency components. The surge protector's grounding wire is not connected to the protective ground of the protected equipment, i.e., a separate lightning protection grounding is used. This will create a dangerous voltage between the protected wire and the equipment's protective ground during transients. The solution is to connect the surge protector's grounding to the equipment's protective ground. This article briefly introduces the grounding and equipment protection of surge protectors in power systems.
Keywords: Equipment protection; surge protector; communication system
I. Basic Principles of Lightning Protection
Lightning and other strong interference can cause serious damage to communication systems and the resulting consequences, making lightning protection essential. Lightning consists of high-energy low-frequency components and highly penetrating high-frequency components. Lightning primarily damages equipment in two ways: one is through direct conduction via metal pipelines or ground wires; the other is through surges generated by lightning electromagnetic pulses inducing these pulses in metal pipelines or ground wires via various coupling mechanisms. The vast majority of lightning damage is caused by this induction. For electronic information equipment, the main hazard comes from the coupled energy of lightning electromagnetic pulses, which generate transient surges through three channels: metal pipeline channels (such as water pipes, power lines, antenna feeders, signal lines, and aviation obstruction light leads); ground wire channels (ground backflash); and spatial channels (radiated energy from electromagnetic fields).
Surge damage to metal conduits and ground potential rise in grounding wires are the main causes of damage to electronic information systems. The most common form of damage is lightning strikes on power lines, making them a key focus for prevention. Because lightning strikes electronic information systems insidiously, lightning protection is a complex systemic project. The core of lightning protection is discharge and equalization.
1. Discharge involves releasing the energy of lightning and its electromagnetic pulse through the ground, and should adhere to the principle of hierarchy, meaning that excess energy should be discharged to the ground as far and as possible before being introduced into the communication system. Hierarchy means attenuating lightning energy in layers according to the established lightning protection zones. Lightning protection zones, also known as electromagnetic compatibility zones, divide the environment into several areas based on the varying intensities of lightning and its electromagnetic pulses felt by people, objects, and information systems: LPZOA zone: All objects in this zone may be directly struck by lightning, therefore all objects may conduct away all lightning current; the electromagnetic field in this zone is not attenuated. LPZOB zone: Objects in this zone are unlikely to be directly struck by lightning, but the electromagnetic field in this zone is not attenuated. LPZ1 zone: Objects in this zone are unlikely to be directly struck by lightning; the current flowing to each conductor is further reduced compared to LPZOB zone; the electromagnetic field attenuation and effectiveness depend on the overall shielding measures. If further reduction of the conducted current and electromagnetic field is required in subsequent lightning protection zones (such as LPZ2 zone), then a subsequent lightning protection zone should be introduced. The selection of the subsequent lightning protection zone should be based on the environmental requirements of the system to be protected and the specific conditions of the lightning protection zone. The higher the protection zone number, the lower the expected interference energy and voltage. In modern lightning protection technology, the establishment of lightning protection zones is of great significance, as it guides the implementation of technical measures such as shielding, grounding, and equipotential bonding.
2. Equipotential bonding aims to prevent damage caused by a potential difference in any part of the system. This means that the potentials of all conductive metal parts within the system and its surrounding environment remain essentially equal during transient events. This is fundamentally based on equipotential bonding. A potential compensation system, consisting of a reliable grounding system, metal conductors for equipotential bonding, and equipotential connectors (surge protectors), can rapidly establish an equipotential relationship between all conductive components, including active conductors, within the extremely short duration of a transient event. This complete potential compensation system can create an equipotential region in a very short time, which may have a potential difference of tens of kilovolts relative to distant locations. Crucially, within the area of the system to be protected, there should be no significant potential difference between all conductive components.
3. A lightning protection system consists of three parts, each with its own important function and none of them are interchangeable. External protection, composed of lightning rods, down conductors, and grounding electrodes, directly conducts most of the lightning energy into the ground for discharge. Transition protection, composed of proper shielding, grounding, and wiring, reduces or blocks induced currents introduced through various intrusion channels. Internal protection, composed of equipotential bonding and overvoltage protection, balances the system potential and limits overvoltage amplitude.
II. Function and Technical Parameters of Surge Arresters
Surge protectors, also known as equipotential bonding devices, overvoltage protectors, surge suppressors, surge absorbers, and lightning protection devices, are used for power line protection and are called power surge protectors. Given the current characteristics of lightning damage, surge protector-based solutions are the simplest and most economical lightning protection solution, especially in lightning protection upgrades. The main function of a surge protector is to maintain a consistent or limited potential across its terminals during transient events, transferring excess energy from active conductors.
Underground discharge is a crucial component in achieving equipotential bonding. Key technical parameters of surge protectors include: rated operating voltage, rated operating current, and the current-carrying capacity of specially approved series-parallel power surge protectors. Current-carrying capacity refers to the surge protector's ability to transfer lightning current, measured in kiloamperes (kA), and is related to the waveguide type. Functionally, surge protectors can be divided into those that protect against direct lightning strikes and those that protect against induced lightning strikes. Surge protectors that protect against direct lightning strikes are typically used for line protection where direct lightning strikes are possible, such as protection at the boundary between LPZOA and LPZ1 zones. Their current-carrying capacity is measured and represented using a 10/35μs current waveform. Surge protectors that protect against induced lightning strikes are typically used for line protection where direct lightning strikes are impossible, such as protection at the boundary between LPZOB and LPX1 zones, or at the boundary between LPZ1 zones. The response time of a surge protector is measured and represented by an 8/20μs current waveform. This time is related to the waveform characteristics and the residual voltage. The surge protector's voltage limiting capability for transient phenomena is related to the amplitude and waveform characteristics of the lightning current.
III. Selection of Surge Protectors
To achieve the desired protective effect from surge protectors, it is essential to "install the appropriate surge protector in the right place." The selection of surge protectors is of paramount importance.
1. The distribution of lightning current among various facilities entering the building is as follows: Approximately 50% of the lightning current is discharged to the ground through external lightning protection devices, while the remaining 50% is distributed within the metallic materials of the entire system. This assessment model is used to estimate the current-carrying capacity of surge protectors and the specifications of metallic conductors at the boundaries of LPAOA, LPZOB, and LPZ1 zones where equipotential bonding is implemented. The lightning current at this location is a 10/35μS current waveform. Regarding the distribution of lightning current among various metallic materials: the amplitude of the lightning current in each part depends on the impedance and inductive reactance of each distribution channel. Distribution channels refer to metallic materials that may receive lightning current, such as power lines, signal lines, water pipes, metal structures, and other grounding elements. Generally, the distribution can be roughly estimated based on their respective grounding resistance values. In cases where this is uncertain, it can be assumed that the grounding resistances are equal, i.e., the current is distributed equally among the metallic conduits.
2. When power lines are overhead and may be struck by direct lightning, the lightning current entering the building's protected area depends on the impedance and reactance of the external line, the surge protector's discharge branch, and the user-side line. If the impedances at both ends are the same, the power line receives half of the direct lightning current. In this case, a surge protector with direct lightning protection function must be used.
3. The subsequent evaluation model is used to assess the lightning current distribution at the boundary between protection zones after LPZ1. Since the user-side insulation impedance is much greater than the impedance of the surge protector discharge branch and the external lead-in line, the lightning current entering the subsequent protection zone will be reduced, and no special numerical estimation is required. Generally, the current-carrying capacity of the power surge protector used in the subsequent protection zone is required to be below 20kA (8/20μs), and surge protectors with high current-carrying capacity are not required. The selection of surge protectors for the subsequent protection zone should consider the energy distribution and voltage coordination between each stage. When many factors are difficult to determine, using series-parallel power surge protectors is a good choice. Series-parallel is a concept proposed based on the characteristics of many applications and protection range distinctions in modern lightning protection (relative to traditional parallel surge protectors). Its essence is an effective combination of multi-stage dischargers and filter technology through energy coordination and voltage distribution. Series-parallel lightning protection has the following characteristics and is widely used: It can be used not only in conventional applications but also in locations where the protection zone is difficult to distinguish. The voltage division and delay effect of the induced decoupling device under transient overvoltage helps to achieve energy coordination. Slowing down the rise rate of transient disturbances to achieve low residual voltage, long lifespan, and extremely fast response time.
4. The selection of other parameters for surge protectors depends on the level of the lightning protection zone where each protected object is located. Their operating voltage should be based on the rated voltage of all components installed in the lead-in circuit. For series-parallel surge protectors, their rated current should also be considered.
5. Other factors affecting lightning current distribution in power lines: A decrease in transformer terminal grounding resistance will increase the distributed current in the power lines. Increasing the length of power cables will reduce the distributed current in the power lines and achieve a balanced current distribution among major conductors. Excessively short cable lengths and excessively low neutral impedance will cause current imbalance, leading to differential-mode interference. Connecting multiple users in parallel with power cables will reduce the effective impedance, resulting in increased distributed current. In a network-like power supply configuration, temporary lightning current mainly flows into the power lines, which is why most lightning losses occur at the power lines.
IV. Installation of surge protectors
1. Power lines should implement multi-level protection. Multi-level protection uses each lightning protection zone as a hierarchy to progressively reduce lightning energy (energy distribution), ensuring that the limiting voltages at each level are coordinated, ultimately limiting the overvoltage value within the equipment's insulation strength (voltage coordination). Multi-level protection becomes necessary in the following situations: failure of a certain level of surge protector or failure of a certain circuit of the surge protector; the residual voltage of the surge protector does not match the equipment's insulation strength; and when the cable is long within a building.
2. Cable protection in almost all cases should be divided into at least two levels, and the same level of surge protector may also contain multiple levels of protection (such as series-parallel surge protectors). To achieve effective protection, appropriate surge protectors can be installed at the interfaces of each surge protection zone. Surge protectors can be for a single electronic device, a space containing multiple electronic devices, or all conductors passing through a surge protection zone that is usually spatially shielded. Surge protectors are connected at the interfaces of the surge protection zones. In addition, the protection range of surge protectors is limited. Generally, the protective effect will deteriorate when the distance between the surge protector and the equipment line exceeds 10m. This is because there is a oscillating voltage caused by reflection on the cable between the surge protector and the equipment to be protected, and the amplitude of this voltage is proportional to the line length and load impedance.
3. In multi-stage protection using surge protectors, if energy distribution is not carefully managed, more lightning energy may be introduced into the protected area. This necessitates that surge protectors be selected according to the aforementioned evaluation model. Generally, surge protectors have the characteristic that the higher the lightning current, the higher the residual voltage. After energy distribution, the lightning current flowing through the final stage surge protector is extremely small, which is beneficial for voltage limiting. Note that it is dangerous to select a surge protector with a low response voltage as the final stage protection without considering voltage matching.
The key to achieving energy distribution and voltage coordination lies in utilizing the inductive reactance of the cable itself between the two surge protectors. The cable's inductive reactance has a certain effect on impeding buried current and dividing voltage, allowing more lightning current to be distributed to the upstream stage for discharge. Generally, the cable length between the two surge protectors is required to be around 15m, suitable for situations where the protective ground wire is laid close to other cables or within the same cable. The length of branch lines on the cable affects the required cable length; when the protective ground wire and the protected cable are a certain distance apart (>1m), a cable length greater than 5m is sufficient. In situations where using the cable itself as a decoupling measure is not suitable, such as when the interfaces of the two surge protection zones are close or the cable length is short, specialized decoupling devices can be used, in which case there are no distance requirements.
4. Decoupling devices are an important measure to achieve energy distribution and voltage matching. The following materials can be used as decoupling devices: cables, inductors, and resistors. Series-parallel surge protectors are a type of surge protector combination that considers energy distribution and voltage matching, using filters as decoupling devices, and are suitable for various applications.
5. In certain extreme cases, installing surge protectors may actually increase the possibility of equipment damage and must be avoided; such situations must be prevented. A surge protector may protect several lines, but if one of the protectors fails or its response is too slow, common-mode interference may be converted into differential-mode interference, damaging the equipment. This necessitates the implementation of multi-level protection and careful maintenance of surge protectors. Installing surge protectors arbitrarily without considering lightning protection zones, energy coordination, and voltage distribution—for example, installing only one surge protector at the front end of the equipment—will result in a strong lightning current being attracted to the front end of the equipment due to the lack of upstream protection, causing the residual voltage of the surge protector to exceed the insulation strength of the equipment. This requires surge protectors to be installed according to a hierarchical principle.
6. In other cases, incorrect installation will render the equipment ineffectively protected. Excessively long surge protector connection lines can cause problems. When the surge protector is operating, the voltage on the connection line caused by inductive reactance will be extremely high, resulting in a dangerous voltage applied to the equipment. This problem is even more pronounced in the application of final-stage surge protectors. The solution is to use short connection lines, or at least two separate connection lines to distribute the magnetic field strength and reduce voltage drop. Simply thickening a single connection line is ineffective. If necessary, the wiring of the protected conductor can be modified to bring it closer to the equipotential bonding busbar (grounding point) to reduce the connection line length.
The output lines, input lines, and grounding wires of surge protectors are laid close together and side-by-side. This situation has a significant impact on series-parallel surge protectors. When the output lines (protected lines), input lines (unprotected lines), and grounding wires of a series-parallel power surge protector are laid close together, transient surges will be induced in the output lines. Although their intensity is less than before, they can still be dangerous. The solution is to separate the input lines and grounding wires from the output lines or lay them perpendicularly, minimizing the length of parallel laying and increasing the distance between them.
The surge protector's grounding wire is not connected to the protective ground of the protected equipment, meaning a separate surge protection grounding is used. This will create a dangerous voltage between the protected wire and the equipment's protective ground during transients. The solution is to connect the surge protector's grounding wire to the equipment's protective ground.
For details, please click: A Brief Discussion on the Grounding and Equipment Protection of Surge Protectors in Power Systems
