practical power system harmonics, earthing and power quality - problems and solutions

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Practical Power System Harmonics, Earthing and Power Quality –

Problems and Solutions


Topics• Power utilities and deregulation• Power interruptions

Case study: Protecting critical processes from power failures

• Voltage variations Case Study: A heavy load on a weak branch of the utility system

• Problems caused by transients Case study: Transients causing malfunction of sensitive equipment

• Harmonics in power supply network Case study: Harmonics due to magnetic saturation of utility transformer

• Proactive utility actions


Deregulation and Its Effect• Utilities earlier used an end-to-end model (from generation to end user

distribution)

• Deregulation has given rise to utilities dealing with specific operational segments

• Electricity traded as a commodity

• Users have a choice between suppliers

• Imperatives for efficiency and cost reduction

• Full utilization of capacity before additions are taken up


Power Quality and Utilities• User expectations of interruption-free quality power

• Present day devices need quality power Equipment malfunction with poor quality electricity

• Interruptions cause production disruption and profitability Reliability is more important than ever

• Present day devices cause supply pollution (harmonics/reactive power/flicker)

• Disturbances because of poor quality may cause equipment failures due to operation near full capacity


Power Interruptions - Reasons• Equipment and line faults

• System instability leading to outages Tie tripping, Generator outage

• Deliberate tripping (usually automatically) to save a system from collapse For islanding to save part of a system Speeds up restoration

• Natural causes such as hurricane and floods Mitigation of effects by alerts and advance preparations


Avoiding Interruptions

• Auto - reclose operations (for self clearing of transient faults)

• Switching to redundant feeders

• Starting standby generating sources for meeting peak loads Possible with anticipated, gradual load increase Requires adequate capacity reserves Desirability of distributed generation by large consumers/ third parties


Distributed Generation - Advantages• Generation located close to the load

Not affected by transmission circuit problems

• Capable of being started and brought up to load faster

• Owned by industrial consumers and serve as standby sources

• Can run as base-load generators synchronized with the utility system

• Improve the voltage profile in remote end-of-line distribution circuits

• Usually use cleaner energy technologies


Typical Emergency Standby Scheme

• Not useful for uninterrupted supply due to ‘break-before-make’ operation


Redundant Generation Scheme


Voltage Variations

• Heavy loads on a weak distribution system

• Plant location decisions for social reasons /low cost (rural areas)

• Inadequate utility infrastructure

• Feeder strengthening-limited use

• Complaints of sag/flicker from other consumers in the same circuit


Industrial Load On A Weak System

• Problem Industry located at the tail end of the distribution system Flicker problems

• Analysis Heavy motor loads but reduced voltage starting Cannot cause objectionable sags during starting sags 500 HP motor with jogging feature Problems during jogging operation Also during step change of load torque (zero to rated torque)


Voltage Sags-Jogging / Step Load


Lamp Flicker IEEE Limits


Solution

• Thyristor switched capacitor (TSC)

• Capacitor value considering future expansion

• Switching algorithm to avoid capacitor switching transients

• Response time selected to reduce flicker-voltage

• 1 cycle response selected

• Significant reduction in flicker after TSC installation


TSC Schematic


Problems Caused By Transients

• Sensitive devices require pure sine wave input supply

• Mal operation of sensitive circuits due to transients

• Electromagnetic noise due to transients can result in data corruption/loss

• Lock up of computer based control/communication hardware

• Power quality event and power quality problems


Harmonics In Power Supply Network • Reasons for harmonic presence:

Injection of harmonic currents from non-linear equipment• Limits as per standards enforced by utilities

• Keeps distortion of voltage lower

• Utilities expected to limit harmonic voltage at service entrance

Magnetic saturation of transformer core• Not usual in well designed transformers

• Distortion of magnetizing current waveform

• Distortion of secondary output voltage waveform


Analysis• Saturation due to low frequency (50 Hz) operation

• Governed by equation

fNS

EB

RMS

44.4max

• Causes distortion of secondary output

• To avoid saturation, input voltage to be reduced by a factor of 1.2 (Ratio of frequencies)

• Not possible due to system voltage requirements


Secondary Voltage


Proactive Utility Actions

• Join hands with customers

• Encourage them to participate in creating new distributed generation capacity Mutually advantageous financial packages Clear rules for parallel operation of captive generation Captive generation capacity to be treated/controlled as a utility resource

• Educating the customers about the quality issues Guiding them in conducting studies Technical assistance to select the right solution Installing monitoring equipment at critical locations Sharing power quality data with customers


Summary Display With CBEMA Overlay


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practical power system harmonics, earthing and power quality - problems and solutions

Engineering

power quality problems

utility system problems

topics power utilities

transients case study

industrial load

transients sensitive

line faults system instability

weak system problem