Power Quality Analysis

Comprehensive Power Quality Analysis

Power Quality Analysis

Electric Power Quality Analysis for troubleshooting failure/ tripping due to voltage fluctuation/ sag. Preventive maintenance & stopping electronic failure.

Abstract: Large continuous process plants are increasingly resorting to use of non linear loads (NLL) such as DC drives, variable speed AC drives, UPS, increased electronic controls etc. to improve performance and productivity of their plants. The plant engineers are increasingly facing unfamiliar problems related to power quality (PQ) mainly problem of distortion of fundamental wave shape and associated effects.  

Harmonics are as old a phenomenon as the introduction of AC System. Mercury arc rectifiers (MAR) developed in 1920's and semiconductor controlled rectifiers (SCR) or thyristors developed in 1950's created harmonics, due to their nature of non-linear loads. However, use of non-linear load was less then (typically less than 5% in 1960), to have any significant impact. But it is now increasing rapidly thus making Power Quality a great concern among plant engineers and associated equipment manufacturers.

Magnified harmonics cause severe stress on the plant & machineries. Electronics are the first to fail being weakest among all, followed by fuse, capacitors, thyristor, CT, Motor etc.

This study represents summary of findings and recommendations of comprehensive analysis & data emerging out from Power Quality audit of a tyre manufacturing factory.  

The study revealed that most of the feeders had very high level of THD. Following are just example for 2 feeders from the study-Transformers TR-14 & TR-15 with major NLL and Capacitors ON/OFF.

Capacitors ON/OFF: Table 01 Example for high harmonics level at 2 feeders

Load with Major NLL                Without Capacitor   With Capacitor    REMARKS
                                                 THD-v    THD-i       THD-v    THD-i    
TR -14, 2.5MVA, 11/0.400kV     6.8%    42%          9.1%    106%      450kVAr at Panel -OFF/ ON
TR -15, 2.5MVA, 11/0.400kV     8.3%    77%          9.2%    126%      400kVAr at Panel -OFF/ ON

As THD-I and THD-v are much higher than IEEE norms, the study  recommended installing tuned harmonic filters at 400V secondary side for TR -14 and 15 (in Phase -I) to limit harmonic distortion of both THD-v & THD-i within regulatory norms, save energy substantially (typically 5%) and eliminate failure/nuisance tripping. It should help in enhancing product quality and overall profitability. Similarly, 2 sets of filters for 'Banbury' mixers and one filter for 450 kW squirrel cage induction motor feeder were recommended to resolve issues of high harmonics, abnormal behaviour during starting of motors through soft starter such as starting current as high as  9 times full load current, voltage dip of 23% and so on. The total cost of recommendation for phase I was INR 59,50,000 with an attractive simple pay back period of 10 months.

In Phase -II, it was recommended to install tuned harmonic filters in 7 more transformers where it was observed that the level of harmonics were high.

Industry Background:
Processing & Production: The tyre making process starts by mixing different varieties of rubber with process oils, carbon black, pigments, antioxidants, accelerators and other additives, each of which contributes certain properties to the compound. These ingredients are mixed in giant blenders (called banbury mixers) under tremendous heat and pressure. The ingredients are blended together into a hot, black gummy compound that will be milled. The cooled rubber takes several forms. Most often it is processed into carefully identified slabs that will be transported to breakdown mills. These mills feed the rubber between massive pairs of rollers, repeatedly feeding, mixing and blending to prepare the different compounds for the feed mills, where they are slit into strips and carried by conveyor belts to become sidewalls, treads or other parts of the tyre.

Indian Tyre Industry (Source: http://www.prlog.org/10083560-report-on-indian-tyre-industry.html): The Indian Tyre Industry produced 736 lakh units of tyres (11 lakh tonnes) garnering Rs. 19,000 crores in FY 07. MRF Ltd. was the market leader (22% market share) followed closely by Apollo Tyres Ltd. (21%). The other major players were JK Tyre & Industries Ltd (18%) and Ceat Ltd. (13%). The industry tonnage production registered a 5 year CAGR of 9.69% between FY 02-07.

Truck & Bus tyre category (accounting for 57% of the tonnage production) recorded a 5 year CAGR of 7.85% (a rate slower than that of the industry) while Light Commercial Vehicle (LCV), Motorcycle and Car tyre categories grew at 15%, 16% and 14% respectively (at rates faster than that of the industry).Off the road (OTR) tyres (customized tyres which fetch a higher margin compared to other tyres) category is growing at a fast pace. The OTR tyre category registered a 5 year CAGR of over 20% in the last five years.

The exports from the country clocked a CAGR of 13% in unit terms and 18% in value terms in the period FY 0207. Most of these tyres that are exported are of cross ply design.

With radialisation catching up in some of these markets, the manufacturers will need to graduate to radial tyres so as to protect their share in the export market. Radialisation of tyres is still minimal in India. Only the car tyre market has moved to radial tyres (95%) but in all other categories cross ply tyres are still preferred. Poor road conditions, overloading in trucks, higher initial cost of radial tyres and poor awareness levels in tyre users are the main reasons for the non transition of the domestic market to radial tyres.

However, going ahead, radialisation in truck & bus tyres may increase due to government's focus on infrastructure development. Research expects the tyre industry to register a tonnage growth of 910% in the next five years (FY 0712).

The truck & bus and LCV tyre category are expected to register a CAGR of 8% and 14% respectively (FY 0712). As per the annual report for the year 2007-2008 of a leading company, the energy Consumption was 671.57 KWH/MT in 2007-08.

System details

Power Source
1.    GEB TR I/C -1, 10MVA, 66/11kV
2.    GEB TR I/C -1, 10MVA, 66/11kV
3.    DG-1, 4.0MW, 11kV
4.    DG-2, 4.0MW, 11kV
5.    GT-1, 5.0MW, 11kV

Banbury Mixtures -General Details - Cycle time & Energy Intensity: The plant have old DC Drive system for BNB-1 to 4 installed during 90's and new ABB VFD system for BNB-105 & 106 installed during recent times. Their power consumption depends upon product mix and cycle time. BNB-1 has shortest 1.7 minute cycle time for final mix and have highest power consumption. BNB-2 to 4 has longest 3.3 minute cycle for master mix & consumes least power. BNB-105 (master mix) and BNB-106 (final mix) have 2.3 minute cycle time & have its own pattern of power consumptions.     

Consumption Details - Energy pie - Table 02 Energy consumption details

Plant Details                        kW-ave.    Process    Banbury    Utility    Remarks
Banbury & Aux                        
BNB-1, BIAS                       650                             650         
BNB-2, BIAS                       150                            150         
BNB-3, BIAS                       110                             110         
BNB-4, BIAS                       120                            120         
BNB-5, PCR                        340                            340         
BNB-6, PCR                        240                            240         
TR-3, BNB-1+2 Aux          1104                           1104         
TR-11, BNB-3+4 Aux          603                             603         
TR-16, BNB-5+6 Aux          590                            413             177    
TR-4, BIAS                         860           860              
TR-5, BIAS                         746           746              
TR-6, BIAS                         657           657              
TR-7, BIAS                         317           317              
TR-13, BIAS+30%PCR      710           710              
TR-14, PCR                        270          270              
TR-15, PCR                        440          440              
TR-8, Utility                       1114                                             1114    
TR-9, Utility                         673                                             673    
TR-18, PCR+Project           330          198                             132    
TR-19, PCR+Utility             910           637                             273    
TR-20, PCR                                         250              
Total Energy (Ave.)       10934         5085         3730         2369    
%-Consumption                               46.5%      34.1%       21.7%    

Energy Pie Chart -Based on 24hrs average running, total plant load was 10934kW-average, comprising of Banbury & it auxiliary, Process and Utility section for both BIAS and PCR tyres. This pie chart (as in the caption picture above) is based on the running condition that prevailed during the audit.

"    To identify power quality & harmonics distortion present in the power system and assess the reasons for frequent failure of electronic components, capacitors, overheating/spurious tripping of switchgear due to  large usage of non-linear loads (NLL) like DC drives, VFD, UPS etc.
"    To recommend suitable correction measures for the issues of PQ after analysis.

Methodology: A detailed PQ audit consisting of various measurements was planned. Study analysis included taking various electrical related measurements using sophisticated data logger as per the requirements of relevant national & International standards namely IEEE. Refer Annexure - 1 for harmonic distortion limits as per IEEE-519,1992. Measurements were taken of relevant parameters, which are listed in following tables and diagrams/graphs.

Process: A detailed PQ audit involving measurements of various parameters at different locations of the plant was undertaken.

PQ study OBSERVATIONS and ANALYSIS: Measurements-plant power & harmonic levels are given in the table below:   

Harmonic Analysis Report for Tyre Industry

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Power Quality Analysis Report for Tyre Industry

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Observations & Recommendations

Harmonics Distortions & Power Quality (PQ) Issues  
Capacitors - the cause of harmonic magnification

To improve efficiency of low PF systems, the simplest solution is to add capacitors to correct inductive load. However, such approach creates serious operation problem if high level of harmonics are present in the system.

Any combination of capacitance and inductance forms a circuit, which is tuned to what is called as "resonance frequency". It is the frequency at which both capacitive & inductive reactance equals.

If significant harmonic energy is present due to usage of nonlinear load (NLL) like dc drives, vfd, ups etc. and its frequency matches with that of the resonant circuit formed by the PF correcting capacitor and system inductance, it will excite the current into oscillation. A very large amount of harmonic current will flow magnifying harmonic voltage. Its effect can blow fuses, damage components and reflect in excessively high harmonic level into the entire facility. In addition, since the high oscillation current in the resonant circuit is wasted and no useful work is done, it will drop pf.
Following are just example for TR-14 & TR-15 with major NLL and Capacitors ON. With such magnified harmonics it causes severe stress on plant & machineries. Electronics are the first to fail being weakest among all, followed by fuse, capacitors itself, Thyristor, CT, Motor etc.  

Typical measurement graphs of TR-14 and TR-15 only are given below.

Power Quality measurements of Tyre a Industry

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Banbury -DC System - Harmonic Performance & Energy Efficiency

All measurement were taken at common 11kV factory's PCC Bus, thus all voltage and Voltage harmonic data should reflect to summation of plant's figure and not for individual BNB, which if required were measured at BNB working voltage level, which was 750V for DC system.

The voltage harmonic THD-v exceeded regulatory norm of 5% at 11kV bus, which reflected ambient harmonic conditions, which was harmful to plant & machineries. The recommended measures contained in this report should also help to bring down this ambient THD-v distortion.

The current harmonic THD-i exceeded regulatory norm and was in the order of max 50% at 11kV bus for all BNB - 1 to 4 drives with major 11th, 13th & 23rd harmonics. However THD-i at 750V side was expected to be few times its HT value with 5th & 7th as major harmonics. Such a system caused severe stress for sensitive drives & electronics and usually handled with de-rating which caused substantial energy loss and increased initial project cost.

Banbury -AC System - Harmonic Performance & Energy Efficiency

The voltage harmonic THD-v exceeded regulatory norm of 5% at 11kV bus, which reflected ambient harmonic conditions, which was harmful to plant & machineries.

The current harmonic THD-i exceeded regulatory norm and was in the order of max 80% & 90% at 11kV bus for BNB - 105 & 106 with major 3rd, 5th & 11th harmonics which were unexpected and highly undesirable. In-addition THD-i at 1.9kV side was expected to be few times its HT value. Such a system caused severe stress for sensitive drives & electronics and usually handled with de-rating which cause substantial energy loss and increase initial project cost. Looking into undesirable harmonic performance this AC system would be less efficient and troublesome. Its harmonic trend graph (long duration/ 12hrs & close-up view) indicated mostly 3rd and 5th & 7th.     

TR-20 400kW Squirrel Cage Induction Motor (SQIM) - Performance during Starting with soft starter   

Trouble-Shooting - 65uS Inrush Signature Analysis

All measurements were taken at common 660V outgoing feeder for 400kW SQIM Soft Starter. It was reported that squirrel case AL winding bar gave away during starting twice and after its strengthening with CU bar, stator winding was failing during starting.  

Stating current for a motor depends upon the instant of voltage waveform during which the start takes place and loading on the motor. During audit we have taken totally 3-starts at different times with motor coupled condition, but without any load in the cracker mill drive.

Normal start behavior was with starting current about 5-times full load current of the motor, which is 365A and voltage dip about 11%. Abnormal start behavior was with starting current about 9-times full load current of the motor, which is 365A & voltage dip about 23%. In-addition THD-v shot-up to over 10%. Due to All such high fluctuations, compounding each other and motor was getting a magnetic run-away type of situation during starting causing oscillation for both voltage & current, due to which the motor had a history of failures.

PQ solution recommended

It was  recommended to install tuned harmonic filters at 400V TR SEC side for TR -14 and 15 in Phase -I, to limit harmonic distortion both THD-v & THD-i within regulatory norms, save energy substantially (typically 5%) and eliminate failure/nuisance tripping.

In-addition harmonics were found to be high in the following TR, for which it was recommended to install tuned harmonic filters in Phase -II.
o    All TR in PCR Plants -TR-16 and TR-18-totalling 2 Nos.
o    BIAS Plant-    TR-3, 5, 6, 7 & 11-totalling 5 Nos.

In Banbury - DC System, it was recommended to install tuned harmonic filters at 750V side that should limit harmonic distortion both THD-v & THD-i within regulatory norms, save energy (typically 10%) and eliminate failure/nuisance tripping.

In TR-20 400kW squirrel cage induction motor, in order to restrict all the 3-fluctuations in the soft starter, within some manageable level while starting, It was recommended to install a tuned L-C circuit that should work like an electrical fly-wheel opposing any change of voltage that in turn oppose rise of current. This will in turn minimise voltage spikes, thus helping starting current to remain stable.  

Once the L-C tuned circuit was installed, motor should have fewer fluctuations from frequent starts that were required for this cracker mill application, helping to run it in cooler condition. Because of cooler run and fluctuation controls, it was expected on average should save 5% motor energy.

Table 04: Summary of Recommendations - Phase -I is given in the table below:

Summary of                                                No. of      Cost -                     Savings-                 Simple Payback
Recommendations                                     Filters      INR Lakhs              INR Lakhs               - In Months

Banbury -1 at 750V TR SEC
TR-1, 2.4mva, 11/0.750 -0.750kV              2-Sets     2x8.70=17.40         28.55                       7-Months

400kW SQIM at 660V I/C of Soft Starter
TR-20, 1.25mva, 11/0.660kV                      1-Set      6.70                         5.10                        16-Months
TR-14 & 15 at 380V TR SEC
TR 2.2mva, 11/0.380kV                              2-Sets    2x17.70=35.40        40.02                       11-Months
    2x 17.70 =

Total cost and Payback Period                   5-Sets    59. 50                       73.67                      10 -Months

The recommendations are under active consideration by the management of the tyre industry studied. The solutions are well within the acceptable levels as the benefits and simple pay back period indicate that they more than justified.

About the Author and the company
Author        :     Undeclared
Company    :    Reputed Global Tyre Manufacturing Company in India

Case Study 17. Year: 2009.

The sole responsibility for the content of this document lies with the authors. It does not represent the opinion of the European Communities. The European Commission is not responsible for any use that may be made of the information contained therein.

APQI is supported by Asia Invest Europe Aid Co-operation Office and Leonardo Energy.

Europe Asia Power Quality Initiative

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