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Power Quality Harmonics Audit (Practical Guide)

Five Steps for Succesful Power Quality Audit

Purpose of Harmonics Analysis?

Generally, the following are the objectives of performing harmonics measurement & analysis:

Power Quality Analysis

Doing harmonics audit for electric power systems allows us to:

  • Identify proliferation & impure components (harmonics, THD, swell, dips) in the power systems
  • Check current & voltage distortion in the system
  • Identify / pin-point harmonic distortion components damaging the system
  • Measure the extent & spread of distortion  
  • Measure the value / percentage of distortion
  • Measure the duration of harmonic distortion effect
  • Work out mitigation solution on the basis of analysis report

The following illustration shows a typical measurement of harmonics (THD) from a power quality analyzer:

Identify Heat Losses

One of the major issues of harmonics in power losses is in the form of heat which affect considerably electrical equipment. Thus harmonics audit allows us to:

  • Check whether electrical equipment like transformers , motors, generators, cables are overheated
  • Monitor & measure the duration of distortion effect on the machines & equipment
  • Work out energy savings due to mitigation

The following illustrations represent thermal images identifying overheated areas in a transformer and a motor due to harmonics!

Heat-losses-harmonics-analysis

Note: Clients requirements and any specific issue/problem/event in the power system defines the primary and secondary nature of the above objectives 

Step 1: Collection of Plant Information and Data

Once the purpose of the harmonics audit is clear, the next step is to collect basic preliminary information & data about the plant from the client. The best is to prepare a questionnaire about the plant and send it to the client.

The questionnaire should include the followings:

  1. Type of Plant (details about its’ batch, process, manufacturing and final product)
  2. SLD (Single Line Diagram of complete power system)
  3. Details of Load including motors, lighting, Variable Speed Drives, Rectifiers, Voltage Regulators, Heaters, Computers, IT Equipment etc.
  4. Details of Power Sources (utility, generators, turbines, renewable power sources etc.)
  5. Details of Earthing / Grounding system (number & location of earth pits; type of earthing system ; grounding conductor network inside & outside the plant; recent record of earth testing) 
  6. Data / Record / Information of Power Quality from the already installed Network Analyzers & energy meters, if any.
  7. Details / history of any specific issue/problem/event in the power system

Step 2: Preliminary Analysis of Collected Data and Strategy Preparation for Actual Harmonics Analysis (Audit)

This part of the activity has significant importance in setting the direction of the harmonics measurement procedure. A thorough & conclusive work on the data collected/received from the client identifies the problem & its causes.

This sets the course of actual harmonics audit in the plant. That is why it is said that accurate preliminary analysis means half of the job is done!

The preliminary analysis covers the following activities:

  1. Study of Data
  2. Making a simulation of the existing power system
  3. Identification of Potential Harmonics Areas & its probable causes

The work plan should consider the following points for a successful Harmonics audit:

  • How many loads to be analyzed
  • Planning of sequence & duration of sessions for conducting measurement in various load areas, sub-station, power house and utility
  • Making customized templates for final harmonics measurement

Step 3: Arrangement of Testing & Measurment Equipment for Harmonics Analysis

For a successful harmonic analysis, the team should be fully equipped with testing instruments & meters. All the below equipment must be available during the course of the audit, as complex problems in the power system require parallel testing by multiple instruments.

1. Power Analyzer / Data Logger

  • Flexible CTs are recommended for easy & safe measurements. For larger loads, CTs with multiple settings are suggested (see figure below)
  • If an analyzer is not available, one can get rental equipment as well
Power Analyzer with iFlex CTs (Fluke)

2. Thermal Imaging Camera with high-resolution features

  • It is used as supportive equipment with power analyzer to exactly pin-point the fault and the extent of damage (overheating effect) by harmonics.
thermal imaging of a motor
Thermal imaging of a motor (Fluke)

3. Temperature Gun

  • It is also used as supportive equipment with power analyzer to exactly pin-point the fault

4. Grounding/Earthing Tester

  • It is used as a supportive equipment to check the health of earthing. A weak or faulty earthing can lead to floating neutral which adds up third harmonics in an un-balanced power system and can contribute to noise & vibrations caused by the harmonics.

5. Set of spare batteries (as a power backup)

6. Spare memory card (in case Analyzer memory gets filled with data)

7. Safety tools including electrical safety gloves, double-insulated tools, safety helmets, safety shoes, etc.

Step 4:  ON-SITE Harmonics Measurements

Harmonics’ measurement will be carried out as per the strategy worked out in the preliminary analysis of the collected data (step 2). The harmonic audit is conducted in stages in different locations & load areas of the complete plant and accordingly, the procedural methodology is adapted in this regard.

DETAILS OF AREAS & LOAD TO BE MEASURED:

  1. SUB-STATION (UTILITY SUPPLY)
    • Transformers (LV side)
    • Main outgoing feeders in the LT Panels
    • PFI Panels
    • Main outgoing feeders in bus-coupler panels
    • Main outgoing feeders of sync supply, if any
  2. POWER HOUSE (GENERATORS, TURBINES, SOLAR POWER ETC.)
    • Outgoing feeders of all individual power sources
    • Each outgoing feeder of the PDBs and DBs in the allied equipment like cooling towers, pumps etc. 
  3. PDBs (POWER DISTRIBUTION BOARDS) & DBs (DISTRIBUTION BOARDS) IN THE PLANT’s PROCESS & MANUFACTURING AREAS
    • Each outgoing feeder of the PDBs and DBs
    • Ideally, measurement shall be taken in all PDBs & DBs. However, in case of more than one similar rating of machines and fewer small loads up to 3 KW, measurements can be taken randomly (keeping in view the preliminary report and discussions with the client).
  4. UTILITYY AREA
    1. Each outgoing feeder of the PDBs and DBs
    2. In case of more than one similar rating of machines and fewer small loads up to 3 KW, measurements can be taken randomly (keeping in view the preliminary report and discussions with the client)

MEAUREMENT METHODOLOGY

This is a normal methodology for a standard plant. However, depending on the type of plant process, findings of preliminary report/analysis, and subsequent discussions with the plant staff, the methodology can be revised.

  1. Minimum of 30-minutes data logging of harmonics distortion (THD) in the sub-station and power house areas.
  2. In case the THD is high (i-e THD I is > 10% and THD V is > 5%) individual spectrum of the highest most harmonic order shall be measured as well. This practice shall be followed in all load locations.
  3. Data logging of 30-minutes to 24-hours duration shall be done in the PDBs and DBs of plant area & utility (data logging duration can vary in accordance with the plant type and client’s requirements). The following illustration shows data logging being carried out on a motor load.
Data logging being carried out on a motor load (Fluke)
  1. Thermal Imaging & Temperature measurement of bus-bars/busways, breaker terminals, fuses and main cables shall be done in parallel during the activity of harmonics data measurement in sub-stations and power house (* in case of abnormal results of thermal imaging, duration of data-logging can be extended).
  2. Thermal Imaging & Temperature measurement of breakers terminals & main cables in PDBs & DBs in plant area and utility. The following image shows thermal imaging on breaker terminals & cables.
Thermal imaging on breaker terminals & cables (Fluke)
  1. Thermal Imaging of motors, pumps, drives & allied machinery in the plant area.
  2. Grounding / Earthing testing of all earth pits.
  3. In the case where identification of heat losses is part of the agenda of harmonics analysis for energy savings, all of the above exercise will be supplemented with extended duration and multiple sessions of data logging on each feeder, scheduling it according to the process & batch of the plant.
  4. Make sure to transfer the data from the power analyzer memory to the computer system at the end of the day. This will keep enough space in the analyzer for the next day data logging
  5. Make sure to write down the name, number & location of each reading!

Step 5:   Report Preparation

The report can be prepared and written based on observations and data analysis by following these tips:

  • Transfer measured data from analyzer to the computer system
  • Give proper names to the files
  • Covert data to graphs & waveforms. Highlight THD and order of maximum harmonic distortion.
Bar Graph of orders of harmonic in a Ring section of Textile Spinning Unit
Bar Graph of orders of harmonic in a Ring section of Textile Spinning Unit
  • On the basis of the results, highlight load areas & load where distortion is high & abnormal.
  • Identify causes of distortion. Supplement it with the record of thermal images & temperature measurements.
  • For harmonics mitigation / reduction solution, it is recommended to refer the case to the suppliers of mitigation equipment.
The individual spectrum of 5th order of harmonic in a Ring section of Textile Spinning Unit
  • Whether the client goes for mitigation solution or not, always suggest to the client to install network analyzers and monitor power quality & energy consumption through the software.
Highlight Harmonic distortion of a motor (Fluke)

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Conclusion

In this article, we considered power quality audit, precisely harmonics analysis, and identifying anomalies in electrical power systems. The audit has been summarized in 5 steps with a lot of tips and advice to provide successful and professional reports to clients!

5th Harmonics Explained

Definition, causes, measurment and solutions

What is 5th harmonic?

In the ideal case, voltages and currents in AC power systems are pure sine waves with 50 Hz frequency (or 60Hz).

However, voltages (or currents) in the actual power system have additional components, called harmonics, whose frequencies are integral multiples of the power system frequency. Harmonics are multiples of the fundamental frequency and can therefore be expressed as 1f, 2f, 3f, 4f, 5f, etc.

Figure1 is a plot of harmonics (up to 50th order) measured from a power analyzer for an AC Power System of a beverages plant. The yellow bar represents the 5th harmonics.

 Figure 1: Harmonic content measured in AC power system of a Blow Moulding line of a beverages plant

Usually, power analyzers available in the market measure harmonics up to 50th order.

What is the frequency of 5th harmonic?

From above, you may conclude that the 5th harmonic for a 50 Hz power system is 250 Hz. The following illustration shows one cycle of a sinusoid with a peak amplitude of 1.00 (labeled as the fundamental). The other two waveforms shown in the figure are the third harmonic and the fifth harmonic.

5th harmonic explained

Note that the third harmonic completes three cycles during the one cycle of the fundamental and the fifth harmonic completes five cycles.

The 5th harmonic component is added to the fundamental one which yields a distorted signal as shown below:

Fifth Harmonic waveform

What causes 5th order harmonic?

Harmonics are generated due to non-linear loads as explained here. Most of the time, fifth order harmonics are present in distorted signals caused by non-linear loads such as:

  • Variable Frequency Drives (VFDs)
  • Power electronic converters such as rectifiers
  • LED lighting
  • Arcing devices
  • Uninterrupted Power Supplies (UPS)
  • Electric vehicle chargers, etc

Is 5th harmonic a negtaive or positive sequence?

To answer this question, we should define what we mean by positive and negative sequences.

In a balanced three-phase system, we have phase A is at 0 degrees, phase B is shifted by 120 degrees, and phase C by 240 degrees as shown below:

Three Phase System
  • If harmonics of a given order respect the same shifting: Harmonic of phase A, then harmonic of phase B, and after that harmonic of phase C, we say that the harmonic in question is a positive sequence.

The following image shows the 7th harmonic of each phase where they respect the same order: Harmonic of phase A, then harmonic of phase B, and after that harmonic of phase C (yellow, pink, and cyan). This means that the 7th harmonic is a positive sequence.

Positive sequence 7th harmonic

What about the 5th harmonic? is it a positive or a negative sequence? short answer: 5th harmonic is a negative sequence!

Indeed, the 5th harmonic is a negative sequence as shown in the following image where 3 phase harmonics do not respect the order: Harmonic of phase A, then harmonic of phase C, and after that harmonic of phase B (yellow, cyan, and pink).

How do you measure 5th order harmonic?

As we mentioned above, harmonics are measured by a Power Quality Analyzer which can measure up to 50th order of harmonics. It should be noted that some analyzers have the provision to measure harmonics up to 99th order.

Measurement is done in THD (total harmonic distortion) and in an individual order where the THD graph displays the contribution of the 5th harmonic to the total distortion in the power system.

How to eliminate 5th order harmonic?

Many solutions can be adopted to reduce or eliminate the 5th harmonic in the power system. To cite a few:

  • Using passive harmonic filters
  • Using active power filters
  • Using reactors for local loads
  • Adding isolation transformers
  • Using 12 pulse drive rectifiers instead of 6 pulse drive version.

Conclusion

In this short article, we highlighted the 5th harmonic in power systems. This includes its definition, why it is considered as a negative sequence, its causes, and how it can be measured. In the end, we presented some solutions to reduce (or even cancel) the 5th harmonic.

Further reading