T&D Network Planning Do Utilities Consider Power Quality?

Published On: Jun 04, 2014


Power Quality (PQ) issues are increasingly attracting attention in India. In our earlier blog – ‘Are Developing Economies at risk due to PQ issues and challenges’ , it is observed that PQ issues commonly occur during power utilization at customer end. Poor PQ impacts the electrical network components such as cables, transformers, etc. and creates excessive heating, overloading and early ageing of such components. Thus, although poor PQ and T&D losses are interrelated, systematic planned intervention can mitigate both risks.

One of the key objectives of electricity network planning, given the ever increasing non- linear load environment, is to determine the upgrade and expansion requirements of the network so as to ensure supply of quality and reliable power to end consumers. PQ escalations on the T&D systems are often complex, wide spread and expensive to mitigate, hence having PQ as a design consideration in planning and management could be overall rewarding.

This blog highlights the importance of considering PQ while designing and planning network improvements and expansions for power transmission and distribution utilities.

KEY ELECTRICAL NETWORK DESIGNS CONSIDERATIONS, CURRENT PRACTICES AND OPPORTUNITIES

Electricity network planning is highly customized across T&D systems. This is primarily because in each case the conditions of supply area, design, construction, load demand, geographic distribution, technical standards, practices, status of existing system, etc., have to be considered in the plan.

Generally, T&D systems are spread over large area and consist of various equipments like transformers, feeders, bus bars, circuit breakers, protective relays, isolators, protection system, etc. The performance of a network, while influenced by technical parameters like energy availability, equipment failure rate, tripping due to faults, etc. is mainly determined by inherent design characteristics like length of feeders, number of customers supplied per feeder, quality and reliability measures like System Average Interruption Duration Index (SAIDI), System Average Interruption Frequency Index (SAIFI), etc. Typically, utilities attempt to design and operate their infrastructure assets at least possible cost, with limited technology interventions and upgrades.

To meet with the growing power requirement, increasing demand for greater quality and reliability, lower O&M costs, asset management, demand side management, etc; utilities need to take all key design elements into consideration right from planning stage. These are influenced by capital investment decisions, and are made via network planning and design, and in turn have an impact on the inherent performance capability of a network. For any given network, with a set of possible maintenance interventions, there is a point beyond which only additional capital expenditure and not increased operational expenditure will result in improved performance.

KEY PQ ISSUES IN ELECTRICAL NETWORK SYSTEMS AND ITS SUGGESTED MEASURES FOR IMPROVING PERFORMANCE

The growing use of electronic loads in the electrical networks have increased concerns about Power Quality. Due to the imbalance in the network performance quality indices across T&D systems like voltage regulation, supply, service, etc. have been affected. Some of the PQ disturbances in the network are caused due to:

  • Natural Causes
    • Faults or lightning strikes on transmission lines or distribution feeders
    • Falling of trees or branches on feeders during stormy weather conditions
  • Transmission
    • Transformer energization or feeder capacitor switching
    • Equipment failure because of poor O&M
  • Non Linear Loads on Consumer Side
    • Power electronic loads (UPS, Adjustable Speed Drives (ASD), converters, etc.), arc furnaces and induction heating systems
    • Switching on or off of large loads, etc. 

Some of the physical manifestations of PQ issues on the electrical network systems are mentioned below:

PQ Phenomena

Specific PQ issue

Typical Causes

Suggested countermeasures

Transients

Impulsive

Lightning Strike, Transformer energization, Capacitor switching

Reduce the magnitude and incidence of switching transients,
Reduce the lightening surges entering the customer facility

Oscillatory

Line or Capacitor or load switching

Short Duration Voltage Variation

Sag & Swell

Ferro resonant transformers, 
single line-to-ground faults

Reduce the incidence rate, amount of variation or the duration of RMS variations

Interruption

Temporary (self-clearing) faults

Long Duration Voltage Variation

Under voltage

Switching on loads, capacitor deenergization

Reducing the load
changes, Reducing the source impedance, or decoupling the load from lighting circuits.

Over voltage

Switching off loads, capacitor energization

Sustained Interruptions

Faults

Limit the incidence rate and duration of sustained interruption

Voltage Imbalance

Single Phase loads, 
Single phasing condition

Improve voltage regulation and balance

 

Waveform Distortion

Harmonics

Adjustable speed drives and other non linear loads

Use active or passive filters to reduce harmonic voltage distortion

Notching

Power Electronic Converters

Observe proper routing of conductors, Retrofit drives that incorporate Silicon Controlled Rectifiers (SCRs)and diodes

DC Offset

Geo-magnetic disturbance, half wave rectification

Use DC link chokes

Voltage Flicker

 

Arc furnace, arc lamps

Dynamic compensation (Static VAR Compensator (SVC), SVC Light) should be applied

Table 1. PQ issue causes and countermeasures (Sources: 1. Power Quality & Custom Power & 3. On Development Planning of Electricity Distribution Networks)

Following measures should be taken by the utility operators to improve the electrical network systems:

  • Flexible AC Transmission System (FACTS) devices are one such means of improving power quality. They increase the ability of transmission capacity of lines, and help control power flow over designated transmission, electronically and statically
  • Distribution Static Compensator (D-STATCOM) is used for voltage regulation, compensation of reactive power, correction of power factor and elimination of current harmonics. APFC at high density load centers and on line reactive compensation to use for Voltage profile improvement.
  • On-Load Tap Changer (OLTC) transformers are used between multiple voltage levels to regulate and maintain the voltage which is supplied to customers. The OLTC transformer equipped with automatic voltage control (AVC) is the most popular and effective control device. With Smart Grid concept, OLTC transformers will have a significant influence on the development of voltage regulation in distribution networks.
  • Automatic Voltage Regulators (AVRs) are units that regulate the voltage to ensure electronic units like rectifiers continue to operate during extreme mains voltage variations, without getting damaged. It is designed to automatically maintain a constant voltage level by regulation one or more AC/DC voltages and prevents damage of electronic units.
  • Feeder renovation activities like,
    • Installation of shunt capacitors or switched line capacitors or synchronous condensersInstallation of lower capacity distribution transformers near consumers premises
    • Re-conductoring of T&D lines according to loads
    • Undergrounding of distribution system in high density urban centers
    • Automated feeder control operation through auto reclosing, sectionalizing, fault detection and restoration system
    • Identifying weak areas in the distribution system and strengthening them
    • Putting monitoring and analysis system in place.

PQ ISSUE ESPECIALLY HARMONICS AFFECTS ELECTRICAL NETWORK ASSETS TOO

Today’s growing complexity and use of electronic equipment such as monitoring devices, power electronics such as variable speed drives, logic controllers, etc. leads to unbalance electric loads in the network. Harmonics, one of the growing PQ issue, are caused by the non-linearity of customer loads. Harmonics is a form of disturbance in electrical networks, which influences and affects the operations of assets like transformers, feeders, etc. It increases the total current through the asset, causes distortions in voltage waveform, and produce additional losses in feeders.

Some of the main issues caused by harmonics in the network systems are mentioned below:

  • Harmonic currents flow upstream from non-linear loads, through the impedance of cables and transformers and creates harmonic voltage distortion
  • Harmonic currents cause increased heating to distribution transformers due to iron and copper losses. This in turn reduces the life of transformers and its usability
  • Harmonic currents also create increased heating in electrical cables, leading to premature ageing and overstressing of the electrical insulation
  • Malfunction of communications and data processing equipment, as they rely on nearly perfect sinusoidal input
  • Nuisance tripping of protective devices, often dependent on periodic zero crossing of waveform, overheating of conductors (burn off, damage), etc.

Harmonic Prevention and Reduction

When designing an electrical system, it is very important to take into consideration as many precautions as necessary to minimize possible harmonic issues. This requires advanced planning and sometimes, additional capital.

  • Use of Active and Passive filters improves power factor thereby reducing high frequency harmonics and controlling output current
  • Use of capacitor banks or filters in local grid helps reduce reactive power demand or harmonics issues by reactive power compensation
  • Harmonic Mitigations Transformers (HMTs) have become a leading economical solution nowadays to improve the system reliability. When energized, they provide harmonic treatment and have excellent energy saving characteristics

UTILITIES MUST ADOPT HOLISTIC AND INTEGRATED APPROACH TOWARDS IMPROVING POWER QUALITY

It is observed that many PQ issues have origin in T&D network. Generally, PQ investments are made only when disturbances are viewed as high probability events affecting daily operations of utilities; instead of taking it into account right from the 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 installing capacitors and filters in the grids to compensate the reactive power consumption of assets like transformers. Installation of passive and active filters in grids will help reduce the harmonic voltage distortion, preventing excessive heating of electricity cables, motors and transformers.

One such recent story is that of New York Independent System Operator ( NYISO ) that designed the primary power control center to meet the modern grid reliability requirements and to strengthen the existing grid reliability. This facility also harnesses modern smart grid technology to manage the growing complexity of electric grid operations. It successfully deployed Phasor Measurement Units (PMUs) which improves grid operators’ ability to detect irregularities, predict problems and take corrective action to maintain reliability. It also included installation of capacitor banks to improve transmission system efficiency by reducing line losses.

CONCLUSION

With the increasing pace of modernization, power consumption has been increasing exponentially. The existing electrical network systems are unable to cater to the current demand of power quality, grid reliability and efficiency. It is clear that in order to meet the increasing power requirements of the society, it is important that the utilities recognize the need to overcome PQ issues by upgrading the network with available technology interventions and practices. This can be achieved through better planning & designing of grids, constant vigilance, effective monitoring and good maintenance. A properly planned and maintained electrical system including that at distribution level will avoid multiple PQ issues, thereby balancing quality power supply on continuous basis – every hour of the day, every day of the year.

Time is ripe that power transmission and distribution utilities take PQ into serious consideration while planning network interventions thus differentiating themselves in an increasingly open power market

REFERENCES

  1. Arindam Ghosh, Department of Electrical Engineering, IIT Kanpur, “ Power Quality and Custom Power.
  2. Haren Shah, Marketing Department, Meco Industries Pvt. Ltd., Mumbai, “ Harmonics- A Power Quality Problem.
  3. Viktoria Neimane, Royal Institute of Technology, Department of Electrical Engineering, “ On Development Planning of Electricity Distribution Networks.
  4. EATON, powering business worldwide, “ Energy Savings through Harmonic Mitigation.
  5. M. Ramadan Sayed, M. A. Moustafa Hassan, A. A. Hassan, “ Power System Quality Improvement Using Flexible AC Transmission Systems Based on Adaptive Neuro-Fuzzy Inference System.

Industrial Electronics, IEEE Transactions on (Volume:57 , Issue: 1

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