Manitoba HVDC Research Centre Design Practices for System

Manitoba HVDC Research Centre Design Practices for System Harmonics/Practical

cases

Speaker: Ravipudi SudhirDAR Engineering

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Definitions of Power Quality Phenomena

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The concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment (IEEE Std. 1159-1995).

Alternative definitions or interpretations of this concept power quality havebeen used in the industry.

Power Quality: Definition

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Power quality phenomena may have harmful impacts on power system components or customers’ appliances.

For example: resonance caused by harmonics, saturation of transformers caused by harmonics, impacts of over/undervoltage onsensitive loads, etc.

Power Quality: Analysis

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Relevant Standards

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A Few Standards

IEC 61000 (IEC 1000): Electromagnetic Compatibility (EMC)

Introduces various power quality phenomena, limits, monitoring techniques and measurement methods and mitigation guidelines.

IEEE Std. 1159: IEEE Recommended Practice for Monitoring Electric Power Quality

Defines the power quality terminology, explains impacts of poor power quality on utility and customer equipment, and introduces techniques for measuring electromagnetic phenomena in power systems.

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A Few Standards

IEEE Std. 519: IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

Addresses the problems involved in the harmonic control and reactive compensation of static power converters and provides recommended limits of disturbances to distribution system.

IEEE Std. 141: IEEE Recommended Practices for Electrical Power Distribution for Industrial Plants (IEEE Red Book)

Chapter 3 discusses the voltage considerations in an industrial plants and covers the concerned voltage quality issues.

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A Few Standards

IEEE Std. 1453: IEEE Recommended Practice for Measurement & Limits of Voltage Fluctuations & Associated Light Flicker on AC Power Systems

The voltage flicker phenomenon and its measurement is addressed in this standard. IEEE Std. 1453 adopts IEC 61000-4-15 as the guide for flicker measurement.

Also, this standard shall be used in conjunction with IEC 61000-4-15, IEC 61000-3-3, IEC 61000-3-5 andIEC 61000-3-7.

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References

IEEE Std. 519

IEC 61000

Power Quality Specification for Interconnection to Manitoba Hydro's Electrical System (PQS2000) Rev. 01

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Waveform Distortion

Example limits:

System voltage distortion(Manitoba Hydro’s Power Quality Specifications)

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HVDC System Advantages

Enables bulk power transmission over very long distances, with higher efficiency and lower electrical losses per 1,000 km.

Asynchronous grids

Long distance sub-sea cables

Better controllability

Low short circuit currents

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Manitoba HVDC System – Nelson River

BI-Pole I:±450kV, 895kMs, 1800 Amps

From Radisson Converter station near Gillam to Dorsey Converter station near Rosser

BI-Pole II:±500kV, 937kMs, 1800 Amps

From Henday Converter station near sundance to Dorsey Converter station near Rosser

To Improve reliability BI-Pole III is under construction

±500kV, 1384kMs, 3000 Amps

From Keevatinohk Converter station near Gillam to Riel Converter station near Winnipeg.

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Manitoba HVDC System

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Practical (Hands-on) Harmonic Issue Examples on HVDC System

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Case 1: 5th Harmonics from the AC system

• Voltage issues were reported on the Manitoba Hydro AC system near the Dorsey

HVDC Converter Station.

• Defective AC filter components were one of the areas suspected and investigated.

• Upon investigation, it was found that AC filters were providing MVAr’s in excess of that

specified as the voltage was at 1.05 p.u.

• The MVAr problem was resolved separately from the harmonic issue.

• However, site measurements highlighted a harmonic problem that had gone unnoticed.

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6 Pulse Bridge

6 Pulse Bridge

12 PulseInverter

#1 #3

#2

ACFilter

ACGrid

Problem identification:Excessive 5th harmonic currents were measured flowing in the AC filters. These harmonics can originate from either the inverter (DC) side or the AC system.

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Inverter (DC) side investigations:

• The converters are six pulse valve groups and thus generate 5th harmonic.

• The converters were first suspected as the source for 5th harmonic current.

o Wye – Delta converter transformer connection cancels the 5th harmonic

currents generated by each 6 pulse converter.

o HVDC control malfunctions can also result in incomplete cancellation, but the

controls were not the problem.

o However, the cancellation is not perfect when there are voltage unbalances

between phases and other non ideal system conditions.

Conclusion: The DC converters were determined not to be the cause of the

excessive 5th harmonic in the AC filter.


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AC System investigations:

Isolating the DC link for measurements is not practical as the HVDC converters supply the bulk of the province's power.

However, a storm took out both HVDC transmission lines for 5 days. Line and theconverters were also out of service.

AC system harmonic and AC filter measurements during that time confirmed thatthe 5th harmonic problem was due to AC side background harmonics flowing intothe 5th harmonic AC Filter.


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Identified causes for AC side harmonics:

• Non linear load (Power Electronic/electronic devices) – Industrial, Commercial and Residential.

• Operating the system at 1.05 p.u. caused smaller transformers and reactors to

partially saturate and inject elevated levels for harmonics.

o Operating procedures were changed so that if the AC Filters were

overloaded, that a valve group would be blocked to restore 12 pules

operation or the AC voltage would be reduced to limit the saturation.

• AC System expansions that may have resulted in network harmonic resonance frequencies to shift closer to 5th harmonic.


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Studies and analysis to avoid harmonic issues:

• AC system network frequency scans cover the system conditions over the operating life time (planning period) of the converter station and AC system.

• AC filter design to have additional filtering capacity to cater for potential future network conditions and harmonics generated in the AC system.

• Increased grid code compliance for all customer loads to limit harmonic injection.

PSCAD/EMTDC was used to study and help resolve the AC system 5th harmonic issue.


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• Federal Regulations required Manitoba Hydro remove and destroy some 12 000

Existing Polychlorinated Bisphenols (PCB) oil filled capacitors in the AC and DC filters

for the Bipole 1 and Bipole 2 HVDC links.

• The old PCB filled capacitors had much higher losses compared to the new

replacement capacitors.

• As the AC Filters in Bipole 1 were high Q filters the resistors were required to be

replaced also to have a higher resistance to maintain the Q.

Case 2: Harmonic Resonances in Equipment

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• Q in a filter


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Resistors

• There are no IEEE/IEC standards covering filter resistors.

• Parts of other standards can be adapted and combined with technical specification.

• The resistors successfully passed the Factory Acceptance Tests (FAT).

• The AC filters were tuned by using a variable frequency generator and power amplifiers.

• The resistors have elements that could be added or removed to achieve thecorrect Q in the field.


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Resonance Issue

• Upon energization some of the AC Filters were generating an unacceptable acoustic noise well above 80 Db requiring hearing protection.

• The resistors being replaced produced very little acoustic noise.

• Upon investigation it was determined that the resistor enclosures were the sourceof the noise and the outside panels were amplifying the harmonics vibrations.

• Magnetic weights were attached to the panels, which dampened the noise.


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Mitigation /prevention

• The weight were only a temporary measure as the AC filters were required for service.

• The supplier studied the resonances issue then modified the enclosures in the field.

• The technical specifications for all AC filter equipment now include the study for resonances in the equipment itself as it is not required to test in the factory using harmonics.

Why compromise the power quality when we can mitigate or prevent such problems ahead of time.


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MHI Practices for Power Quality Analysis

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Stages of a power quality study

Definitions and examples of power quality phenomena

Analysis

Mitigation methods

Introducing relevant standards

Outline

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Stages of Power Quality Analysis

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Stages of Power Quality Analysis and Study

Field measurements: voltages and currents

This is typically performed prior to any design or new installation

Preferably it is carried out at different network conditions and seasonal load levels.

This provides the base line for the design and studies.

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Stages of Power Quality Analysis and Study

Various IEEE and IEC standards classify power quality phenomena and specify acceptable power quality limits.

Manitoba Hydro International uses those criteria as a basis.

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Definitions of Power Quality Phenomena

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The concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment (IEEE Std. 1159-1995).

Alternative definitions or interpretations of this concept power quality havebeen used in the industry.

Power Quality: Definition

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Power quality phenomena may have harmful impacts on power system components or customers’ appliances.

For example: resonance caused by harmonics, saturation of transformers caused by harmonics, impacts of over/undervoltage onsensitive loads, etc.


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Typical steps for analyzing power quality issues: Frequency scanning: The frequency-dependant impedance

of the system, observed from the desired bus is obtained;

Field measurement: Measurement records are used for further reference and analyses;

Detailed simulation: Effects of the existing power quality issues are studied through electromagnetic transients-type simulation.

PSCAD™/EMTDC™ is the preferred tool to perform those simulations


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Categories of Power System Electromagnetic Phenomena

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Transients

Impulsive transientsFigure fromIEEE Std. 1159-1995

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Transients

Oscillatory transientsFigures fromIEEE Std. 1159-1995

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Short-Duration Variations

InterruptionFigures fromIEEE Std. 1159-1995

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SagFigures fromIEEE Std. 1159-1995

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SwellFigures fromIEEE Std. 1159-1995

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Long-Duration Variations

Overvoltages

Can occur due to switching off a large load, switching on capacitor banks, incorrect transformer tap settings, …

Undervoltages

Can be the consequence of overloaded circuits, switching on a large load, switching off capacitor banks, …

Sustained interruptions

Applies to decreased system voltage to zero for more than a minute.

Usually have permanent nature and require manual restoration.

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Long-Duration Variations

Voltage unbalance (imbalance)

Is defined as the ratio of the negative or zero sequence

component of the voltage to the positive sequence component.

Also is estimated as:Voltage imbalance% = (max. deviation from average voltage/average voltage) x100

Figure fromIEEE Std. 1159-1995

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Waveform Distortion

Steady-state deviation of voltage and/or current waveform from ideal power-frequency sine wave.

Primary types of waveform distortion:DC offset

Harmonics

Interharmonics

Notching

Noise

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Waveform Distortion

DC offset

Presence of dc component in voltage or current Can

be a result of geomagnetic disturbances, half-wave

rectification, …

Direct current in ac systems can have destructive effects by increasing saturation in transformers and machines, imposing additional stress on insulations, …

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Waveform Distortion

Harmonics

Harmonics are sinusoidal voltage or currents having frequencies that are integer multiples of the fundamental frequency.

Harmonics are superimposed on fundamental waveforms of voltage or current and cause waveform distortion.

Harmonics are mainly due to nonlinear characteristics of devices in power systems.

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Waveform Distortion

Harmonics

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Waveform Distortion

Interharmonics

They are sinusoidal components whose frequencies arenot integer multiples of the power frequency.

Main sources:Power electronics, induction motors, arcing devices;

IEEE Std. 1159 also classifies power-line carrier signals under this category.

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Waveform Distortion

Some criteria to assess harmonic distortion(According to IEEE Std. 519-1992)

All harmonics and interharmonics up to the 50th shall beconsidered.

Distortion Factor (Harmonic Factor)

Total Harmonic Distortion (THD)

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Waveform Distortion

Some criteria to assess harmonic distortion(According to IEEE Std. 519-1992)

All harmonics and interharmonics up to the 50th shall be considered.

Total Demand Distortion (TDD)

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Waveform Distortion

Example limits:

System voltage distortion(Manitoba Hydro’s Power Quality Specifications)

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Waveform Distortion

Notching

Ma Po

in source:wer electronic devices

Voltage notching falls between transients and

harmonic distortion.

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Waveform Distortion

Noise

Unwanted electrical signal with broadband spectral content lower than 200kHz, that cannot be classified as transients or harmonic distortions, and are superimposed upon the power system voltage or current.

Noise may appear in phase conductors, neutral conductors or signal lines.

Main sources: Power electronic devices, arcing equipment etc.

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Waveform Distortion

Voltage fluctuations

o The magnitude of voltage fluctuations does not normally exceed the voltage ranges of 0.95-1.05pu.

o IEC 1000-3-3 defines various types of voltagefluctuation

o Voltage fluctuation may result in flicker

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Flicker

The effect of voltage fluctuation on the illumination of lighting equipment.

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Flicker

Indices for quantifying the flicker phenomenon

Short-term flicker evaluation (Pst)(As defined in IEC 61000-4-15)

Pst is a measure of severity, based on an observation period Tst=10min.

Pst can be obtained by any of the following methods:Direct measurement (using the IEC flickermeter)

Analytical calculations

Simulation

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Flicker

Indices for quantifying the flicker phenomenon

Long-term flicker evaluation (Pst)(As defined in IEC 61000-4-15)

Psti : consecutive Pst readings

Manitoba Hydro’s guidelines include 12 Pst readings for calculating Plt .

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Flicker

IEC flickermeter(As introduced in IEC 61000-4-15)

Block 5Block 4Block 3Block 2Block 1

Pst

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Flicker

Emission Level

The maximum allowable Pst contribution available to a customer connecting a load, assuming there is zero background flicker.

Planning LevelThe maximum allowable Pst and Plt levels used by the utility for planning purposes and is used to control the cumulative impact of all fluctuating loads connected to the system.

Manitoba Hydro’s planning levels:Pst < 1.0 and Plt < 0.8 for 99% of the time.

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Flicker

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Power Quality Analysis Procedure

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Simulation and analysis

Scope of study depends on the problem to be solved(e.g. harmonic distortion, flicker, etc.).

The network around the point of interest is represented in PSCAD™/EMTDC™.

Depending on the power quality category, the components affecting or causing the power quality issue must be included in the model.

For example, in case of flicker caused by an arc furnace, the arc furnace has to be implemented in detail in PSCAD™/EMTDC™.

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Transformer saturation

Frequency dependence of transmission lines and cables

Correct values for shunt reactors and capacitors

Power electronic devices

With such details, simulations will determine potential power quality issues.

In case of post-event studies, it will be possible to reproduce the recorded field measurements, thus allowing for further analyses.

It is important that the simulation model includes thecorrect and critical parameters to obtain acceptable results.For example:

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Developing mitigation methods will be based on the results obtained from simulation studies.

For example:

Reconfiguring the system

Designing filters

Developing appropriate control strategies for power electronic devices (e.g. FACTS apparatus, etc.)

Modifications to equipment

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Mitigation Methods

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Harmonic Mitigation

Filters:

It is often required to minimize the harmonic contents of the current drawn from the utility grid. Filters provide the harmonic needs of such loads.

Grid

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Harmonic Mitigation

wn

Filters:

Passive filters, comprising passive R, L and C elements.Used in applicationswhere the harmonic order is kno and does not change.

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Harmonic Mitigation

Filters:

Active filters, employing power electronic converters.Used in applications where the harmonic content of the voltage/current varies dynamically and randomly.

Active filter

(Shunt A.F. is shown)

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Flicker Mitigation

Requires voltage supporting equipment with quick response.

FACTS devices, such as Static VAr Compensators (SVC) and Static Compensators (STATCOM) at transmission level;

SVC

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Flicker Mitigation

Requires voltage supporting equipment with quick response.

Voltage support apparatus such as distribution-level SVCs, Distribution STATCOMs (DSTATCOMs) and Dynamic Voltage Restorers (DVR) at distribution level .

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Relevant Standards

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A Few Standards

IEC 61000 (IEC 1000): Electromagnetic Compatibility (EMC)

Introduces various power quality phenomena, limits, monitoring techniques and measurement methods and mitigation guidelines.

IEEE Std. 1159: IEEE Recommended Practice for Monitoring Electric Power Quality

Defines the power quality terminology, explains impacts of poor power quality on utility and customer equipment, and introduces techniques for measuring electromagnetic phenomena in power systems.

77

A Few Standards

IEEE Std. 519: IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

Addresses the problems involved in the harmonic control and reactive compensation of static power converters and provides recommended limits of disturbances to distribution system.

IEEE Std. 141: IEEE Recommended Practices for Electrical Power Distribution for Industrial Plants (IEEE Red Book)

Chapter 3 discusses the voltage considerations in an industrial plants and covers the concerned voltage quality issues.

78

A Few Standards

IEEE Std. 1453: IEEE Recommended Practice for Measurement & Limits of Voltage Fluctuations & Associated Light Flicker on AC Power Systems

The voltage flicker phenomenon and its measurement is addressed in this standard. IEEE Std. 1453 adopts IEC 61000-4-15 as the guide for flicker measurement.

Also, this standard shall be used in conjunction with IEC 61000-4-15, IEC 61000-3-3, IEC 61000-3-5 andIEC 61000-3-7.

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References

IEEE Std. 519

IEC 61000

Power Quality Specification for Interconnection to Manitoba Hydro's Electrical System (PQS2000) Rev. 01

Documents

Manitoba HVDC Research Centre Design Practices for System