Practical Power System Harmonics, Earthing and Power Quality - Problems and Solutions

Practical Power System Harmonics,
Earthing and Power Quality –
Problems and Solutions
www.idc-online.com/slideshare Technology TTrraaiinniinngg tthhaatt WWoorrkkss

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
B ERMS
S N f
· · ·
=
4.44 max
• 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

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