Transient Over-voltages in Power System Causes, Types And Effect On Power Quality

Published On: May 10, 2019


The biggest problem with transients in the electrical networks is that very few recognize it as a problem since it exists for such short duration that it disappears even before one grip . Transients over voltage costs companies in many ways including direct damage to the power distribution systems, malfunctioning of sensitive electronic equipment, downtime and much more. The transient voltages can originate from inside the facility or emanate from the grid. With greater use of electronics and non-linear loads in every facility, it is important for facility managers and electrical network heads to go beyond the symptoms and understand the different types of transients and their effects on electrical systems.

INTRODUCTION

Electrical transients are observed at various levels within the electrical and computing systems. But the range of transient over voltages could be really vast – starting from the destructive lightning strike in instance of a thunderstorm, to the almost unnoticed static discharge from a human finger. Most of the transient overvoltage events could be traced to the inside of the facility. While the extreme ends of the over voltage range (lightning etc.) is a rare possibility the fluctuations in utility power are a major source of transients coming from outside of the electrical network.

Transient over voltages build a stress on the electrical network in several ways. It is observed that a few plants experience hundreds of transient events every hour with the voltage impulses exceeding several times the system voltage.
Typically, the end users are exposed to the symptoms and directly go after the solutions such as Surge protective devices (SPDs), to reduce the magnitude and duration of voltage transients.

This article focuses on developing an understanding of transients from various angles for the end users – its root causes, types, symptoms and solutions.

TRANSIENTS VOLTAGE CHARACTERISTICS

Transient voltage is often referred to as the most damaging power disturbance. And rightly so, given the number of direct and indirect ways in which it can cause damage to a healthy electrical network. Broadly, the transients are characterised as either impulsive or oscillatory.

Impulsive Transients

Observed as a sudden peak in the voltage or current levels in either positive or negative direction, the impulsive transients can be further classified as fast, medium or slow events, depending on the rise time from nominal to peak voltage.

Poor grounding is one of the biggest causes of impulsive transients. Other major causes include lightening, switching on of large number of inductive loads such as motors and Electro Static Discharge.

Oscillatory Transients

Characterised by a sudden change in voltage which is oscillating at the natural frequency. The oscillating currents have a tendency to decay to zero within the same cycle.

Oscillatory Transients are a result of switching-off a capacitive load, typically a capacitor bank.

TRANSIENT OVER VOLTAGE TYPES

Over voltage is said to have occurred in an electrical network when the voltage rises over 110% of the nominal rated voltage. Over voltages are a result of several factors. The over voltage also results in several adverse effects on the electrical network.

The transient over voltages can be classified as per the frequency content of the waveforms.

Types of over voltages

Transient over voltage events typically last from less than one microsecond and appear equally and in phase from each power conductor to ground

Type of Over voltage Root Causes Characteristics
Temporary Power frequency over voltage
(Low frequency)
Temporary over voltages caused due to the frequency changes during operation of non-linear loads in the electrical network
  • Electrical faults
  • Sudden increase/surge in the power drawn by the loads
  • Ferroresonance (a resonance phenomenon caused due to the nonlinear inductance and system capacitance)
Switching Over voltages
(medium frequency)
A temporary over voltage resulting from switching operations in devices such as capacitors, transformers etc. Switching of capacitor banks
Faults and voltage interruption
Energization/DE energization of transformers
Other
Lightning Over voltages
(high frequency)
A huge temporary surge in voltage due to a lightning stroke terminating at a phase conductor, shield conductor, any other part of a power system, or a nearby object (tree, etc.) Cloud to ground flash due to lightening in thunderstorm

As observed in the figure above, the magnitude of frequency of over voltages from devices or equipment from within the facility is relatively low, as compared to the other two types of over voltages. While the magnitude may range up to 50% of the nominal voltage, they are the slow and silent killers. Most modern over voltage protection devices may not be able to arrest these over voltages and lead to a slow damage of the equipment.

KEY SOURCES OF TRANSIENTS

Among all the sources, the majority of transient voltage events in the electrical networks are caused by Switching. The electrical network could experience frequent switching from several sources including T&D lines, cables, transformers, generators, motors, capacitors, breakers etc. All these and other devices generate a ‘switching’ surge in the voltage. However, most prominent among all these is the surge created due to switching of capacitors and energising generators.

Lightening

Lightening is among the most visible external source of transient voltage. The transients are induced by lightening into the conductors, either due to direct lightning strike (rare occurrence), but mostly as a result of lightning strikes near the power line.

Studies suggest that even a cloud-to-cloud discharge can create a significantly higher transient voltage on the transmission lines. It should be noted that this transient is created without the lightning not actually striking or touching the electrical network.

Capacitor Switching

Capacitors are used to provide reactive power to ensure a unity power factor. One of the major drawbacks of capacitors is building up of oscillatory transients as they are switched on. There are two types of Capacitors – fixed and active. The former stays energised all the time while in the latter capacitor banks are switched on depending on the load levels and other parameters. The over voltage events are typically observed when these capacitors are switched on in view of an increase in load, especially during the start of the shift or day.

The transient over voltage events during capacitor switching lead to tripping of Adjustable Speed Drivers (ASDs), and malfunction of loads with electronic controls – automated valves, equipment on PLC or SCADA systems etc. The transient voltage causes a rise in the dc link voltage which leads to tripping of the drive off-line due to the over voltage. Also, these events are mostly sudden and will occur without any impact on the relatively non-sensitive loads such as lighting etc.

Transformer Energising

Inrush currents are commonly observed when Transformer is energised. The inrush currents create harmonics and if the electrical system has a resonance near the harmonic frequencies, dynamic over voltage conditions are created. This often results in failures of arrestors and damage to sensitive electronics in the electrical network. Further, this type of over voltage can significantly alter the life of capacitors in the electrical network.

Cycling motors in ACs, elevators

The cycling motors, typically used in ACs, elevators etc. cause Electrical Fast Transients (EFTs). While these transients have very little energy and are events of very short duration, they can interfere to disturb the operation of electronic loads. The cycling motors in elevators are observed to develop spikes up to 1 kV, while those in arc welding or large motor starters can cause spikes of even higher voltages.

Other key sources of Transient Voltages include:

  • Power electronic components
  • Electrostatic Discharge (ESD)
  • Copier and printer machines
  • Welding loads
  • Furnace, ovens, induction heaters etc.
  • Motor load start-up and turn-off
  • Faulty grounding, connections
  • Faults in circuit breakers

TRANSIENTS – TYPE OF EFFECTS ON NETWORK AND EQUIPMENT

Transient voltages can be classified and analysed in several ways, in addition to those discussed above. Starting from peak voltage to rate of rise, time or duration and frequency of the transient voltage, several factors help to understand the cause and effect of the transient on the electrical network and its equipment. The impact of transient voltage on equipment in the electrical network differs significantly. While some loads such as the ASDs are highly prone to the damage, other such as lighting loads are relatively less sensitive to voltage transients. However, broadly, the effects of transient voltages can be classified into three categories.

Small but unpredictable interruptions are observed typically when the transients are impacting the electronic controls and data transfer or storage. Typical symptoms include frequent shutdowns/tripping of electronic controllers or servers, instances of data corruption, improper operation of control units etc. The impact may vary depending on the ability of various devices and connecting mediums such as cables, EMI, grounding and earthing configuration etc. to withstand such transients.

Exposure to repeated transients can lead to slow damage of components, eventually, leading to failure. However, given their nature, such repeated transients are relatively easier to trace and arrest. Typical symptoms include failure of PLCs, Control Units, screens of HMI/Dashboard devices etc.

Failures from extreme transient events are clearly evident from its impact. The lightening with a direct strike to a phase conductor generally causes line flashover near the strike point. Along with an impulsive transient, it also causes a fault due to the voltage sags and interruptions that result from the lighting event. A strong foundation in terms of proper grounding along with an appropriate electrical network wide surge protection helps to minimise or prevent the impact of these transient events.

PROTECTION FROM TRANSIENT SURGES

  • Chokes are used to dampen the oscillatory transient. The chokes are now available as a standard feature on several ASDs or can be installed just ahead of these devices in the electrical network to prevent their tripping.
  • Surge Protection Devices (SPDs) are among the most popular solutions when it comes to arresting over voltages. The devices are widely used by end users, utilities, equipment manufacturers and even consumers to protect their facilities and homes. The modern SPD Technology is capable of suppression of not just the impulsive transients or voltage swells, but even other related independent high voltage conditions. Most electronic equipment have a built-in SPDs for protection from surges.
  • UPS and SPDs Combined with UPSs, SPDs have emerged as most effective method of protection against power disturbances, especially oscillatory transients. The combination is widely used to protect computer networks. However, it is observed that the combination of SPDs and UPS devices are at times ineffective in preventing the oscillatory transients with the power distribution system. The zero crossing switch and/or choke are more effective in protection of equipment with advanced but sensitive electronics in their controls.
  • Zero Crossing Switch is used to solve issues caused due to transients from Capacitor Switching. Zero crossing refers to the point when the sine wave reaches the zero level, before descending further on the negative cycle. A Zero crossing switch monitors the sine wave and ensures that the capacitor switching occurs closer to the sine wave’s zero crossing time.

Overview of key global standards for Surge Protection

  • BS 7671:2018 18th Edition of the IET Wiring Regulations covers protection against voltage and electromagnetic disturbances, including protection against transient overvoltages that arise from lightning or switching
  • BS EN 62305-2 – Enlists the requirements for high risk facilities such as nuclear facilities
  • BS EN 62305 (18th Edition of the IET Wiring Regulations) lists the standards for facilities with the risk of direct lightning strike to a structure or to the overhead lines connected with the structure
  • IEEE C62.41.1, Guides on the Surge Environment in Low-Voltage (1000 V and less) AC Power Circuits
  • IEEE C62.41.2, Recommends practices on characterization of Surges in Low-Voltage (1000 V and less) AC Power Circuits”
  • IEEE C62.47-1992 IEEE Guide on Electrostatic Discharge (ESD): Characterization of the ESD Environment
  • IEEE C62.48, Guide on Interactions Between Power System Disturbances and Surge-Protective Devices
  • IEEE C62.72, Guide for the Application of Surge-Protective Devices for Low-Voltage (1000 V or Less) AC Power Circuits

CONCLUSION

The damage from transient over voltage events are not always visible. Overvoltage can cause damage to components connected to the power supply
and lead to insulation failures, permanent damage to sensitive electronic components, over-heating etc. With these, there is a high chance for the diagnosis being misled or marked as completely unknown. Moreover, with no improvement in understanding the characteristics of the transients in the electrical network, the adverse outcomes can be considered as normal by the internal maintenance team.

With growing trend of miniaturisation and IoT, there is going to be more electronics in every device, then ever before. This not only means that transient voltage will be on the rise, but our devices will also be more susceptible to damages from the transients. An increase in transient voltage results in increased power disturbance, interruptions and therefore an adverse impact on productivity and product quality.

But as discussed earlier, to catch the culprits, that is the sources of voltage transients still remains a challenge. As the first step in ensuring healthy electrical network, Facility Managers, equipment designers and utilities must invest time and efforts, consistently, in discovering the voltage transient issues in their networks. Whether its lightning or configuration of equipment that is designed to improve the Power Quality or save money in penalties etc., the first step is to ensure the most appropriate application of protective devices.

REFERENCES

  1. Sources of Transient Voltages – ECM Web – https://www.ecmweb.com/contractor/looking-sources-transient-overvoltages
  2. Impulsive Transients – http://www.powerqualityworld.com/2011/05/impulsive-transients-power-system.html
  3. Transient over voltages – https://www.powerstandards.com/tutorials/transient-overvoltages/
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