Impact of Power Quality on T&D and Power Systems of Different Types

Published On: Jun 20, 2015


The electricity industry is essential for social & economic development and plays an important role in the development of an economy. In recent years, its key sectors, i.e. Transmission & Distribution (T&D), have faced a new set of challenges, largely stemming from a rapid increase in decentralized generation, inter-utility power transfer, de-regulation & privatization, and growing constraints on building new power systems. These constitute strong driving forces for more efficient use of existing T&D facilities, rather than building new ones as the need for transfer capacity in power systems increases.

Further, in the last decade, digitally driven customers across the globe have become more aware of PQ related disturbances at their installations. Flickering lamps are no longer accepted in society, nor are de-ratings or interruptions of industrial processes due to poor power quality. Disturbances such as voltage dips & fluctuations, flicker, harmonics, phase unbalance, etc. are all annoying as well as detrimental each in their own particular way, and, unless proper measures are taken, they will spread over the grid and become a nuisance to many.

Electrical power quality is a broad field and is a source of concern for various stakeholders like utilities, end users, facility management engineers, manufacturers and others. This blog attempts to highlight impact of poor PQ on discoms T&D network & power systems as well as on customers equipment/power systems.

IMPACT OF POOR PQ ON ASSETS OF UTILITY COMPANIES

Customer loads generate a considerable portion of power quality issues in today’s network and power systems. Due to large amount of PQ (EMI) emissions also from the customers’ sides, it is difficult for the network operator to maintain high voltage quality at a customer’s point of connection (POC). Moreover, a guaranteed high voltage quality supply requires large investments in the network. Among various PQ problems, mainly harmonics in the network often interact adversely with the network components and cause inconveniences to the network operators. The operations of power electronic devices produce harmonic currents that lead to additional harmonic power flow and increase network’s total apparent power demand while decreasing true power factor of the network. Large harmonic current can also cause overloading and extra power losses in the network components. In extreme (badly polluted power system) cases, it can lead to high thermal stresses and early ageing of the network devices.

Transformers, Cables, Switchgears, Protective Relays and Power-Factor Correction (PFC) capacitors are the major network components that mainly get affected by PQ disturbances and are briefly discussed briefly below:

Effects on Transformers: Presence of harmonic current increases the core losses, copper losses, and stray-flux losses in a transformer. These losses consist of ‘no load losses’ and ‘load losses’. No load loss is mainly affected by voltage harmonics, although the increase of this loss with harmonics is small. The load losses of a transformer vary with the square of load current and increase sharply at high harmonic frequencies.

Effects on Cables: Harmonic currents has two main effects on cables:

  • Additional ‘ohmic losses’ (I2R losses) in the line and neutral conductors of a cable because of increased RMS value of current due to harmonics. This causes increased operating temperatures in a cable.
  • Harmonic currents along with the grid impedances cause harmonic voltages across various parts of the network. This harmonic voltage increases the dielectric stresses on the cables and can shorten their useful lifetime.

The presence of harmonics in the cables influences conductor’s resistance and further increases its operating temperature. This eventually causes early ageing of the cables.

Effects on Switchgears: Switching operations in power networks are a common cause of transient disturbances. Depending on the network configuration and the characteristics of the switching condition, these transients can cause undesirable effects, not only on the switched load, but also on the entire network. As a consequence, the power supply can be drastically affected, as for example by nuisance protection operation due to high inrush currents, or under voltage due to transformer energization. Such disturbances can in turn aggravate interruption of power supply to certain loads or parts of the network. Hence, it is desirable to eliminate these potentially dangerous switching transients as far as possible.

Effects on Protective Relays: The variations in power frequency and the harmonic distortion can make a relay work incorrectly. Depending on the characteristics of the measured signal, there can be different types of harmonics namely quasi-stationary, fluctuating, rapidly changing and interharmonics. For protection relays, rapidly changing harmonics have the worst effect. High levels of harmonic in extreme cases can cause relay maloperation, which is mainly a consequence of measurement error of the peak value and/or of the waveform. Hence, it is recommended to make a continuous measure of the signal to detect any possible distortion.

Effects on PFC capacitors: Power-factor correction (PFC) capacitors are provided to draw currents with a leading phase angle to offset lagging currents drawn by the inductive loads such as an induction motor. In the presence of a non-linear load, the impedance of a PFC capacitor reduces as the frequency increases, while the source impedance is generally inductive which increases with the frequency. The presence of voltage harmonics in the power system increases the dielectric losses in the capacitors at high operating temperature and reduces the reliability.

In extreme situations, harmonics in the network can cause reduction of operational lifetime of a PFC capacitor. In electricity network, PFC capacitors are used to improve power factor of the network. However, with the capacitor and the stray inductance of the network components, a parallel resonant circuit can be formed. This causes very large (often localized) harmonic voltages and currents to flow, often leading to the catastrophic failure of the capacitor system. To reduce the chance of resonances in the network, tuned PFC capacitors can be used to filter harmonic components.

IMPACT OF POOR PQ FROM UTILITY ON ITS CUSTOMERS

Overall, more than 60% of power quality issues are generated by natural and unpredictable events. Some of these include faults, lightning surge propagation, Ferro-resonance, geo-magnetically induced currents (GICs) due to solar flares, etc. These events are considered to be utility related problems. Further, points of supply generation, transmission and distribution systems are other main sources of poor PQ related to electric utilities.

Point of Supply Generation: There are PQ problems originating at generating plants due to maintenance activity, planning, capacity and expansion constraints, scheduling, events leading to forced outages, and load transferring from one substation to another.

Transmission & Distribution (T&D) Systems: Similarly, PQ issues originating in T&D systems are due to galloping, lightning, insulator flashovers, voltage dips (due to faults or poor planning of network), interruptions (planned outages by utility), poor voltage regulations, inrush currents that are rich in harmonic components (resulting out of transformers energizing), slow voltage variations, electro-magnetic fields (EMFs), etc.

Above PQ issues have effects on customers and their equipment in some of the following ways:

  • Increased energy losses due to poor voltage or unbalanced system
  • Digital equipment like computers are the most affected devices due to momentary interruptions resulting in loss of data
  • Too many large loads, lightning, etc. on the same circuit can cause a transient impulse and damage electronic devices
  • Some other and important effects like electric shock, electrical faults causing fire, etc. are safety related risks associated due to poor PQ at premise/s

Note: A detailed PQ issue category, its typical causes and their potential impact on network equipment is shown below in Appendix section

BENEFITS OF MAINTAINING GOOD PQ ENVIRONMENT

A power quality disturbance causes significant economic consequences to customers and the network operators. With the increasing pace of modernization, power consumption is poised to increase exponentially. The existing electrical network systems are unable to cater to the current demand of power quality, grid reliability and efficiency. Therefore, utilities need to take all key design elements into consideration right from planning stage. Ensuring good power quality requires taking a stride towards good initial design and developing electrical T&D network grids in such a way so as to reduce power losses. This may be achieved through various ways, be it inducting automatic voltage control, automatic isolation of faulty section, automatic restoration of supply, installing capacitors and filters in the grids to compensate the reactive power demand by electromagneticassets like transformers, motors etc. Installation of passive and active filters in distribution substations will help reduce the harmonic voltage distortion, preventing excessive heating of electricity cables, motors and transformers.

At the same time, to avoid huge energy losses or demand constraint related to PQ, consumers also need to take action so as to addressthe PQ issues. Among the various mitigation steps, green field oversizing of power distribution equipment, selection of less PQ sensitive equipment can play an important role.Essential and sensitive equipment, such as PLCs, monitors, drives, vision systems and industrial computers must be safeguarded as well as appropriate Filters (active, passive, hybrid, etc.) and Surge Protective Devices could be used to mitigate PQ problems in an industry.

CONCLUSION

Thus, we note that Power Quality has become a matter of growing concern in recent years owing to a daily rise in use of non-linear sensitive loads. The fast growing technologies, sensitivity of automation and electronic devices, etc. is emerging as part of modern distribution system.
As an end user, we make significant investment in our equipment and appliances; hence it is our responsibility to protect those assets and ourselves against PQ related harm or even safety hazard arising out of this. Through regular inspections/preventive maintenance of the electrical installation, important steps like right planning and procurement of equipment besides overseeing implementation by a trained/certified/authorized electrician at premise can help customer protect themselves against PQ challenges to a large extent. Therefore, rather than things getting worse and the PQ issues getting aggravated, weak points in the electrical system should be identified and foreseen and basic initial steps must be undertaken.

APPENDIX

Following table gives a broad classification of the disturbances that may occur in a power system, its typical causes and their potential impact on network equipment.

Category

Causes

Impacts

Voltage dips

  • Local and remote faults
  • Inductive loading
  • Switch on/off large loads
  • Tripping of sensitive equipment
  • Resetting of control system
  • Motor stalling/ tripping

Voltage surges

  • Capacitor switching
  • Phase faults
  • Switch on/off large loads
  • Damage to insulation and windings
  • Damage to power supplies for electronic equipment

Overvoltage

  • Load switching
  • Capacitor switching
  • System voltage regulation
  • Problems with equipment that requires constant steady state voltage

Harmonics

  • Industrial furnaces
  • Non-linear loads
  • Transformers/ Generators
  • Rectifier equipment
  • Mal-operation of sensitive equipment and relays
  • Capacitor fuse or Capacitor failures
  • Telephone interference

Power Frequency Variation

  • Loss of generation
  • Extreme loading conditions
  • Motors run slower
  • De-tuning of harmonic filters

Voltage fluctuation

  • AC motor drives
  • Inter-harmonic current components
  • Welding and arc furnace
  • Flicker in fluorescent lamps
  • Flicker in incandescent lamps

Rapid Voltage Change

  • Motor starting
  • Transformer tap changing
  • Light flicker
  • Tripping of equipment

Voltage Imbalance

  • Unbalanced loads
  • Unbalanced Impedances
  • Over-heating in motors/ generators
  • Interruption of 3-phase operation

Short and Long Voltage Interruptions

  • Power system faults
  • Equipment failures
  • Control malfunctions
  • Circuit breaker tripping
  • Loss of supply to consumer equipment
  • Computer shutdowns
  • Motor tripping

Under-voltage

  • Heavy network loading
  • Loss of generation
  • Poor power factor
  • All equipment without back-up supply facilities

Transients

  • Lightning
  • Capacitive switching
  • Non-linear switching loads
  • System voltage regulation
  • Control system resetting
  • Damage to sensitive electronic

REFERENCES

  1. Economic Implications of Poor Power Quality – Norbert Edomah, Department of Engineering Management, University of Sunderlan, April 30 2012
  2. Consequences of Poor Power Quality – An Overview – Sharmistha Bhattacharyya and SjefCobben, Technical University of Eindhoven The Netherlands
  3. Economic Impact of Poor Power Quality on Industry – USAID-SARI/Energy Program, October 2003
  4. Experiences with improving Power Quality by Controlled Switching – Michael Stanek, ABB Switzerland Ltd, High Voltage Technology Switzerland, 7th May 2003
  5. Protective Relaying and Power Quality – T.W. Cease & Steven A. Kunsman
  6. Detailed overview of Power System Disturbances (Causes & Impacts) – Electrical Engineering Portal, April 17, 2015
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