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Surge Protection Design

Protection measures for the Electrical / Data / Signalling / Instrumentation systems

Sensitive electronic equipment often fails predominantly due to steady state error or transient error. Harmonics belongs to steady state error and surges are due to transient errors (lightning and switching surges). Harmonics measurement is simple and solution can be arrived easily. Surges are difficult to measure. Specialists in Electrical/Instrumentation/Communication and Information Technology are needed to address the problems due to transient error. Transient errors are of short duration. For example, switching surges are of few micro seconds and lightning surges are of tens to hundreds of microseconds. Hence, before any components used for steady state error can feel the surge, it would have already entered the sensitive equipment and damaged it.

SPD - Surge Protection Device operates in nano-seconds (one to few tens to few hundred) depending upon the type of non-linear device used in the Surge Protection Device. Examples: Surge diodes, MOV, Spark Gap or GDT.

When it comes to power line protection, which is fundamental, LPZ- Lightning Protection Zones need to be ascertained, based on which class of SPD can be determined. It is important to note that, there should be co-ordination among these SPD's installed in different zones so that the prime objective of protection of equipment and protection of SPD's are achieved. That is, SPD's need to be so selected that, in case of lightning or switching surges, both the equipment and Surge Protection Device should not fail and continue to operate for several surges depending upon the surge levels.

Surge Protection Device Installations

SPD's are voltage operated devices and the installation procedures are different from that of MCBs. Most of the electrical engineers and technicians are familiar with the MCB installation and hence use the same procedure to install the SPD's.

If the installers follow the installation instruction sheet provided with each SPD, then only the equipment and Surge Protection Device will be safe from lightning & switching surges. Short and straight length of wire shall be used to connect Surge Protection Device to Equi-potential Bonding Bar (EBB) and Surge Protection Device to Live and Surge Protection Device to Neutral wires.

IEC standard recommends a wire length of < 0.25m.
SPD's shall be placed physically near the EBB to achieve this requirement.
The distance between the EBB and earth pit can be much longer.
Only the continuity between the EBB and earth pit shall be ensured.

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Present Generation Electronic Equipment Failure and Remedy

Nowadays, it is a common phenomenon that, even after installing power conditioning protective devices like stabilizers, ups etc, and the protected equipment still fails or does not operate as expected, during thunderstorms and power outages. It is to be understood that various solutions are available, but to be implemented only after detailed analysis of the problem, because each of these protective devices addresses specific problems. For example Stabilizer or UPS or isolation transformer can’t be applied uniformly for all kinds of problems. Early day concerns were because of minimal utilization of electronic/electrical gadgets. To name a few-flicker in incandescent bulbs, overheating of motors due to harmonics etc.

Unfortunately, present day concerns are much more due to addition of equipment in day to day life for more comforts and security reasons like intelligent building management systems (including CCTV surveillance cameras), automatic on/off of lights/fans etc. Present day failures are attributed more towards transient voltage disturbances associated with lightning and power system switching. Many of the sensitive electronic appliances are sensitive to “voltage variation” that were not noticed in the past. Prior to the introduction of solid state devices, the only source of harmonics were from arc furnaces and HV DC transmission lines for which there is an easy but costly remedy to install a large, expensive filter between source and main power line of the user.

To add fuel to the fire, power and signal Engineers often differ in their perception of the problems and solutions. Added to this, there are difficulties in accessing need to apply power conditioning equipment viz., inability to quantify how much down time is power related and the reluctance of users to spend money on protective equipment. There are also plenty of examples where nuisance tripping caused more production loss rather than failure of equipment, because, present day electronic equipment tripping happens at very short time. For example, digital clock trip between 1 to 10 seconds. Microprocessor based PLCs trip between 1 to 20 cycles and VFDs trip between 1 cycle to few seconds.

Before deciding the kind of protective device, which is best suited for a given problem, it is necessary to understand the definitions and the minute differences between some of the most commonly used terms, which in general, may carry same meaning, when not looked in detail.

Some of the commonly used terms are defined below,

  • Over Voltage: It is a specific type of long duration variation – ex. RMS increase in AC voltage at power frequency for more than a minute.
    Surge Protection Design
  • Under Voltage: It is a specific type of long duration variation – ex. RMs decrease in AC voltage at power frequency for more than 1 minute.
    Surge Protector Design
  • Dip or sag: It is an RMS reduction in AC V, at power frequency for duration of 10 milli seconds to few seconds
    Surge Protector Design Guidelines
  • Swell: An increase in RMS voltage or current at power frequency for duration of 10 milli seconds to 1 minute
    Lightning Surge Protection Design
  • Transient: A sub-cycle disturbance in AC wave form that is evident by a sharp, brief discontinuity of waveform
    Surge Protection Device

A close analysis of the above definitions reveal that transient is the odd man out from the above group. The time duration of over voltages, under voltages, dip or sag and swell is for a long period of time, whereas transient is a sub cycle disturbance usually in microseconds. While it is important to know the above terms by their definitions, it is also necessary to avoid ambiguous words like blackout; brownout; clean ground; clean power; dedicated ground; dirty ground; frame ground; dirty power; raw power; spike etc as they have varied history of usage depending upon the particular place of usage, for which, clear technical definition is not available. Disturbance in quality of power delivered can occur during normal operation of electric power system (ex. Switching power factor capacitor) or during abnormal events (ex. Lightning or clearing a feeder short circuit). During the olden days, when AC power became popular, so much energy was used to get the compressor started and the incoming Ac voltage was temporarily reduced (sag of dip) for which an easy remedy has been found our adding Power Factor correction capacitors. The advantages of solid state devices that made possible the modern day SMPS, Inverter – rectifier, high frequency induction heating and Variable Frequency Drives (VFDs) make these power convertors into harmonic current generators and additional sources of line voltage drops. Significant power line harmonics are being caused by many small, widely dispersed customer loads such as rectifiers, solid state control for adjustable speed motors and servos based on input from electronic sensors. Often, the mechanical aspects of processes, such as web tensioning, spindle acceleration, conveyor speed, extruder flow can’t tolerate variations caused by momentary voltage fluctuations. On the other hand, the operation of computers will be adversely affected by harmonics.

In practice, four power quality parameters; frequency, amplitude, waveform and symmetry serve as reference to classify voltage and power disturbances. Frequency variations are rare on utility connected systems but engine generator based distribution systems experience frequency variations due to load variations and equipment malfunctions. Amplitude variations occur in several forms- brief duration to steady state conditions. Voltage waveform variations occur when non-linear loads draw current that is not sinusoidal- a typical harmonic distortion. Dissymmetry or unbalance occurs when there is a neutral shift or cut. Also, there are disturbances that are not associated with power input of equipment. For example, ESD to equipment enclosure or cables radiated EMI, ground potential rise and operator errors.

Load switching is the common cause of surges in power wiring. Whenever a circuit containing capacitance and inductance is switched ON/OFF a transient disturbance occurs, because the current and voltage do not reach their final value instantaneously. The severity depends on relative power of the load being switched and short circuit current of the power system. If a lightning surge enters unprotected residence through power line, the large voltage causes sparks in house wiring or equipment. When such flashover occurs, they short circuit the power system and the resulting AC power are can start a fire. It is important to note that, lightning does not start the fire directly but causes a power fault and the power system itself does the damage. The damage caused by lightning is wide and varied – in case of metals, large current, heat the surface of metal by interaction between the air and the surface thereby heating the interior components. This is the reason why televisions go bad because of the lightning current entering through this antenna and via the cable and damages the TV. Non linear loads draw non-sinusoidal current from power system, even if power system voltage is a perfect sine wave. These current produce non sinusoidal voltage drop through system source impedance that distort the sine wave produced by power plant generator. A typical non-linear load is a DC power supply consisting of rectifier and capacitor input filter as used in computers drawing current only at the peak of voltage sine wave. This current has higher third harmonic content, which has created a new concern, that of insufficient size of neutral conductor in 3 phase system feeding power supplies.

Some of the common problems are described below

Surge related upset and failure mechanisms: Insulation breakdown, flashover, fracture, thermal and instantaneous peak power overloads, dV/dt&dI/dt limits being exceeded. For ex, thyristors or triacs responding to rate of voltage change can be turned on by a surge, resulting in failure of devices of hazardously energizing the load they control.

Power conversion equipment nuisance trip: Front and DC link where filter capacitor voltage can be boosted by a surge, resulting in premature tripping of down-stream inverter by on-board over current or over voltage protection schemes.

Data procession equipment malfunction: Data errors-not damage-cause by fast rate of voltage change which is due to capacitive coupling or fast rate of current change which is due to inductive coupling that reflect initial characteristics of surge event.

CFL fails prematurely due to insulation breakdown: Lamps may withstand short bust of additional heating caused by few micro seconds surge but fail when flashover occurs, triggering power frequency are that melts the filament or component junction. Finally, it should be borne in mind that, it is a grave mistake to protector swells by SPDs, as these are long duration power frequency swells where a Surge Protection Device is not designed to absorb large energy for long time. There are plenty of live examples that describe Surge Protection Device failures which are caused by swells rather than by large surges. So proper analysis either by in house Engineers or power quality consultants or specialists need to identify the problem with the proper tools and equipment and arrive at a solution by properly implementing the correct protective devices.

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