Surge Protection Devices (SPDs) Surge Protection Devices (SPDs) have become increasingly important in recent years and has got wide presence across industries. Costly electronic equipment, which is sensitive to voltage peaks on the supply, is no longer found only in offices and factories, but in our homes as well. Apart from power lines, SPDs are important for data/communication lines also. The only exception is, fibre optic, Type A cable which is totally metal free.
Nowadays, highly- sensitive data processing, telecommunication and computer networks form the backbone of worldwide communications structures without which, no company can survive. Machines and production lines are monitored and controlled by electronic equipment programmed for specific purposes. SPDs are used for protection against surges from power lines, signal lines, feeder lines and other various line voltage surges, and can be used as class 1, 2 & 3 levels of protection.
A transient voltage surge is a random, high-energy, short-duration electrical disturbance with a rise time of only 1?s to 10?s. It’s a sub-cycle event, and in that respect it differs from an overvoltage condition, which requires a different approach to solve the problem. About 80% of the recorded transient voltage surge come from internal switching transients caused by turning motors, transformers, photocopiers, or other loads on and off. Externally generated surges, on the other hand, come from induced lightning, grid switching, or other facilities on the same distribution node. SPDs divert most of the transient energy away from the load, by creating an equipotentialisation between the connected lines as well as diverting the surge current via a low-impedance ground path.
Types of SPDs IEC 61643-11 defines three categories of surge protective devices for low-voltage electrical installations: Class 1 SPD The Class1 SPD is recommended at the Main Distribution board as It protects electrical installations against direct lightning strokes. It also equalizes the potential at all points when there is a high potential in the earth due to GPR- Ground Potential Rise. Class 1 SPD is characterized by 10/350 ?s wave. Eg: 3 phase 4 wire Line-neutral & neutral-earth Class 1 SPD
Class 2 SPD The Class 2 SPD is recommended at the sub- distribution board. It prevents the spreading of over voltages in the electrical installations and protects the loads. Class 2 SPD is characterized by 8/20 ?s wave. Eg: 3 phase 4 wire Class 2 SPD
Class 3 SPD These SPDs has a low discharge capacity. They must be installed as a supplement to Class 2 SPD and in the vicinity of sensitive loads. Class 3 SPD is characterized by a combination of wave 1.2/50?s and 8/20?s waves. Eg: 1 phase Class 3 SPD
SPDs use non-linear voltage limiting (or switching) components to clamp transient voltages to a safe level. SPDs are the most cost-effective power quality improvement device as:
Isolation Transformers, line filters and surge protection devices are three kinds of protective devices, finding extensive use in power system and very much involved in fixing power quality issues. Selection of the three devices for study is justified as there is a common myth in using these devices interchangeably without knowing its correct application.
Isolation Transformer Isolation transformers provide galvanic isolation and are used to protect against electric shock, or to transfer power between two circuits which must not be connected. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. An isolation transformer is designed to specifically address the problems associated with referencing its internal shields to ground. It is constructed with two isolated Faraday shields between the primary and secondary windings. When properly installed, the shield, which is closest to the primary winding, is connected to the common power supply ground and the shield closest to the secondary winding is connected to the shield of the circuit to be isolated.
The use of two shields in the construction of the isolation transformer diverts high frequency noise, which would normally be coupled across the transformer to the grounds of the circuit in which they occur. The two shields provide more effective isolation of the primary and secondary circuits by also isolating their grounds. The isolation transformer adds a third capacitance between the two Faraday shields, which may allow coupling of high frequency noise between the system grounds. However, increasing the separation between the two Faraday shields normally minimizes this third capacitance. Additionally, the dielectric effect of the shields plus the increased separation of the two windings significantly reduce the inter-capacitance between the windings. An equivalent circuit for an isolation transformer is presented in Figure 1.
Figure 1. Equivalent Circuit of an Isolation Transformer
Application 1) In electronics testing and servicing isolation transformer is a 1:1 (under load) power transformer used for safety. Without it, exposed live metal in a device under test is at a hazardous voltage relative to grounded objects such as a heating radiator or oscilloscope ground lead (a particular hazard with some old vacuum-tube equipment with live chassis). With the transformer, as there is no conductive connection between the transformer secondary and earth, there is no danger in touching a live part of the circuit while another part of the body is earthed.
2) Electrical isolation is considered to be particularly important in medical equipment, and special standards apply. Often the system must additionally be designed so that fault conditions do not interrupt power, but generate a warning.
3) Isolation transformers are also used for the power supply of devices not at ground potential.
Line Filters A line filter is the kind of electronic filter that is placed between an electronic equipment and a line external to it, to attenuate conducted radio frequencies (RFI), also known as electromagnetic interference (EMI) between the line and the equipment. In many applications, a Line Filter is used between the AC power line and the equipment.
Type of Line Filters 1. A Line Filter may be incorporated in a connector. For example:
2. A line filter may be mounted on a PCB
3. A line filter may be a stand-alone device, chassis mounted inside the equipment
4. A facility Line Filter is mounted inside a room or cabinet, at the point where the AC power comes from.
Characteristics of Line Filters 1. A line filter may be used to attenuate EMI in either direction.
2. The attenuation of Line filters is measured in 2 areas:
Applications of filters
Can we substitute isolation transformer for SPD? There is a common myth regarding isolation transformer that if isolation transformer is used, it will take care of the surge protection also. But the answer is not true. High frequency chargers utilizing high frequency transformers have the power semiconductors (MOSFETs / IGBTs) exposed to the AC line and thus are more susceptible to AC line disturbances and fluctuations.
Let’s say that the equipment being served by the isolation transformer is a control panel. HV switching transients, lightning strike, etc., may appear on the incoming control cables. Will the mains isolating transformer prevent these transients from getting into the rest of the LV installation? The answer is definitely not. The leading edge of transients contains high frequency components; there is a “parasitic” capacitance across any transformer, which effectively becomes short-circuited to the high frequencies involved (Zc=1/j?C), so the transient is propagated across the transformer.
Conclusion: Isolation transformers, line filters & SPDs find application depending upon the requirement. One can’t do the job of another. Hence depending upon the type of problems & failuresindividual or combination of either 2 or all the 3 protective devices have to be used to protect the installation from different kinds of disturbances.
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