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.
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.
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:
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, |
Oscillatory |
Line or Capacitor or load switching |
||
Short Duration Voltage Variation |
Sag & Swell |
Ferro resonant transformers, |
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 |
Over voltage |
Switching off loads, capacitor energization |
||
Sustained Interruptions |
Faults |
Limit the incidence rate and duration of sustained interruption |
|
Voltage Imbalance |
Single Phase loads, |
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:
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 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.
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.
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
Industrial Electronics, IEEE Transactions on (Volume:57 , Issue: 1
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