3.     Energy Consumption Figures 

The recorded Maximum Demand , energy consumption and bill amount for last six months are given in the following table.
 
 

RMD, Energy Consumption and Production Level are correlated. RMD reached 100 kVA in high production months. The load at the beginning of the shift will be higher due to all the heater elements in the barrel and die heating systems remaining on for considerable period. Also, different types of input materials require different temperature settings for melting. Certain raw materials need about 145⁰ C setting whereas some others need 200⁰ C setting. This factor also will result in higher heater load on certain mornings some time during a month. And in a high production month both machine systems may be started simultaneously at the beginning of the shift along with the PP machine also due to higher demand placed on the production facilities for both HM and PP grades. The firm states that they switch on the second system only one or two hours after the first one. But it is likely that the personnel find it difficult to follow this rule strictly on all days of a high production month. It takes only one day to (i.e. just one 30-minute interval during which both machines get started simultaneously) upset the MD for that month.

4.    Reactive Compensation Available at the Factory.

The approved Electrical Diagram shows a 30kVAR Capacitor connected at the Main SwitchBoard. But this Study could not find all the units at the factory. Some Capacitor units were found to be connected and on metering them it was found that they were not drawing any current. All the capacitors installed are damaged and there is no capacitive compensation available in the system at present. This is seen to be the most important reason for the increased Maximum Demand level in the factory.

5.    Harmonic Measurements at the Factory

Detailed power and harmonic measurements were carried out at five points in the factory - (i) at the feeder supplying the HM Machine which included the 60HP HM Machine DC Drive , a few small HP Induction Motors , the field winding supply for the HM Machine DC Motor and all the heaters associated with HM-1 machine system. (ii) at the feeder supplying the 25HP DC Drive of the HM-2 Machine (iii) at the feeder supplying the auxiliary motors and the heaters of the PP Machine system (iv) at the feeder supplying the Printing Machine system and (v) at the main transformer outlet. There was no production of PP grade on the day of measurement and hence the harmonic analysis of current drawn by the 10 HP drive could not be performed.

5.1      HM-1 Machine Harmonic Analysis

The HM Machine load which consisted of the 60HP Thyristorised DC drive and its auxiliary motors along with the heaters was found to take a fluctuating and unbalanced current due to the single phase heaters getting switched off and on by thermostat control. However the 60 HP DC drive (plus the 1HP AC Cooling Motor, 3HP Blower Motor 4x0.5HP and 4 x 0.25HP small motors) was taking a more or less constant power throughout the day of study and it was about 18kW at about 0.55 lag power factor. The total load in the feeder varied from 18 kW to 25 kW with various heaters coming on and going off. And the power factor varied from 0.55 to 0.68 correspondingly.

The waveforms for the three line currents and the corresponding harmonic analysis showed that the Thyristorised DC drive was taking a current which was rich in even harmonics - i.e the most dominant harmonic in the current was at 100 Hz followed by 200 Hz component. The traces in the next page show the three voltage waveforms and three current waveforms under various conditions and their harmonic analysis. The voltage waveform with about 3% THD is seen to be more or less pure in waveshape. However the average THD in the line current is about 70% with the magnitude of second harmonic component hovering around 62% that of fundamental component. The presence of high even harmonic content suggested that the Thyristor converter used is of half-controlled type. This was verified by opening the drive cabinet and physically examining the power devices used. The converter used is a three phase half-controlled bridge - it uses three Thyristors and three diodes - and that explains the high THD level in current. Had it been fully-controlled (i.e using six thyristors) the THD figure would have been around 28% and the most dominant harmonic component would have been 250 Hz.It was also observed that the ac-dc converter does not use an isolation transformer i.e the Thyristor-Diode Bridge is straight across the three phase supply.
 

Harmonic Analysis of HM-1 System Voltages and Currents

5.1.1   Load Analysis of the 60HP DC Drive

Name plate rating - 45 kW,1500rpm

Armature - 440V , 112A

Field 220V,5.5A

Measured Load - 14kW approximately

Measured Armature Current - 36A

Armature Voltage - 14000 x 0.95/36 = 369 Volt

(95% efficiency was assumed for the Thyristor Converter)

Full load efficiency of armature - 100 x 45000/(440 x 112) = 91.3%

Field losses - 220 x 5.5 = 1.2kW

Total Losses at full load - 4.3kW + 1.2kW = 5.5kW

Usually DC Motors are designed such that their efficiency peak at around 80% load.At that point the fixed losses will be made equal to variable loss.On this basis the fixed losses are estimated to be 2.1kW and full load armature copper loss is estimated to be 3.4kW.Then the total losses at the running condition observed on the day of measurement will be given by

= 2.1 + (36/112)² x 3.4 = 2.45kW.

In fact the losses will be more than this due to armature voltage a.c ripple content. A figure of 3kW is likely to be more realistic.

Efficiency of the system under the observed running condition = 73.5 % including the losses in Thyristor Converter and Field Winding.

The torque output of the motor is only about one-third of its rated torque capability. It is possible that the load on this drive may vary with the raw material grade and thickness of the film being manufactured. However the plant personnel revealed that such load changes are only marginal. Even if this possibility is accounted the motor is likely to get loaded only up to 50%.The rating of 60HP is clearly not needed for this drive. In fact the Plant Equipment Supplier mentions that this motor must be a 30kW i.e 45HP machine. Using overrated motors lead to their inefficient operation under part loaded conditions.

The relation between the DC Output Voltage and the AC Input voltage in a three phase half-controlled bridge converter is given by V_DC = 292.4(1+cos(a) ) where 'a' is the firing angle in the converter.(AC input is assumed to be 420V in the above equation).The value of 'a' needed to get 440V (the rated voltage of DC Motor) will be around 60⁰ and will be lower when motor runs at speeds less than the rated value. And the equation for displacement power factor of such a converter is = cos(a /2). Thus the converter will run at a displacement power factor less than or equal to 0.87.And this kind of a converter will have about 80% THD in its ac side line current. Accounting for this also , the net power factor (which should include the fundamental frequency reactive component and harmonic components too) with which this converter will operate is 0.68 at the best (when motor is running at rated speed).Thus the power factor of operation of this DC drive is bound to be unsatisfactory and becomes more so when the drive speed is reduced.

5.2 HM-2 Machine Harmonic Analysis

The HM-2 Machine DC Motor Drive (a 25 HP Machine Drive) also suffered from same kind of problems. This system too has a half-controlled three phase Thyristor converter and produced a large amount of even harmonics. It was found to take 9kW at 0.42 lag power factor and the line currents had a THD of 85% (too high!).The 100Hz component was at 73% nearly. This drive is loaded to about 40% and employs a motor which is very old. The efficiency of this old DC motor is likely to be unusually low.The current waveforms and harmonic analysis of this drive is shown in the next page.

Harmonic analysis of HM-2 Drive Currents

5.3  Harmonic Analysis of the Entire Plant

The power and harmonics at the secondary output of the 250kVA transformer was studied for a few hours on 2-9-2002. Typical readings , waveforms and harmonic values etc observed at this point are given in the next page.

The Plant was found to take 87 Amp current at an average power factor of 0.6 consuming about 40kW power. The average Total Harmonic Distortion (THD) observed in three phases was about 60% , the 100Hz component being the most dominant. The phase angle of fundamental component of current with respect to the sinusoidal voltage was seen to be 45 - 50 degree at various measurement instants. This indicates that the displacement power factor is around 0.7 throughout the day.

The Maximum Demand observed on the measurement day was about 75kVA.However the MD is likely to climb at the beginning of the shift as explained earlier. The PP machine must also be contributing to MD. The design of DC Drive in 10HP drive is similar to the other drives and hence same power factor and THD levels can be assumed in that drive too. However the overall power factor, displacement power factor and THD will remain at the above mentioned values without much change. The maximum MD recorded at the Plant in the past was around 100kVA and the various components of this MD are shown analytically below.

5.4  Analysis of 100kVA Maximum Demand

Maximum Demand = 100kVA

Power factor = 0.6

THD = 60%

Active Power = 60kW

Line Current = 100000/(1.732 x 420) = 137.5A

Fundamental Current = 137.5 / (Ö (1² + 0.6²) = 117 A

Harmonic Current = Ö (137.5² - 117²) = 70.7A

Fundamental kVA = Ö 3 x 420 x 117 = 85kVA

 Fundamental Frequency Reactive Power = Ö (85² - 60²) = 60kVAr

This large amount of Harmonic Current makes it difficult to raise the plant power factor to unity by employing power factor correction capacitors alone. The maximum power factor and minimum MD that can be achieved are estimated below.

The fundamental current of the Plant if all the fundamental reactive kVAR is 
cancelled by using 60kVAr of Capacitors = 60000/(1.732 x 420) = 82.3A

Harmonic Current = 70.7A

Total Current = Ö (82.3² + 70.7²) = 108.5A

Then , the possible MD = Ö 3 x 420 x 108.5 = 78.9 kVA

However, if only 30kVAr Capacitor is used;

The fundamental current of the Plant if all the fundamental reactive kVAR is 
cancelled by using 30kVAr of Capacitors = Ö (60² + 30²) /(1.732 x 420) = 92.2A

Harmonic Analysis at the Transformer Secondary

Harmonic Current = 70.7A

Total Current = Ö (92.2² + 70.7²) = 116.2A

Then , the possible MD = Ö 3 x 420 x 116.2

= 84.5 kVA

This is satisfactory considering that the minimum chargeable MD at the Plant is 83kVA.

6.    Proposals for Maximum Demand Control

One of the following proposals can be implemented to control the maximum demand of the Plant. The second proposal will result in energy savings and reduced maintenance of drives.

6.1  Install a fixed 30kVAr High Quality Capacitor at the Main Switch Board

The electrical diagram of the Plant shows that a 30kVAr capacitor is connected at the Main Switch Board. But only a 18kVAr unit was found there during the measurement. And it was found that this unit is damaged and is not providing any kVAr to the system. Hence this damaged unit may be replaced by a high quality 30kVAr Capacitor unit. Units made by Saha-Sprague , Bangalore or ABB will be preferred.

There is a chance of harmonic resonance when power factor correction capacitors are connected in a system with non-linear loads drawing harmonic currents. In this case the resonant frequency of the system is estimated to be close to 690Hz (for a 30kVAr capacitor and 4.41% impedance transformer).This is somewhat close to the 13^th and 14^th harmonics of supply frequency and can cause harmonic resonance and amplification if there is large 13^th and 14^th harmonic content in current drawn by the Plant. However the Plant draws mostly even harmonics only and the current component at 14^th harmonic is around 1% only and the chances for resonant amplification and overvoltage is remote; though it can not be ruled out.

The cost of capacitor is likely to be between Rs. 1500 to Rs. 2000 per kVAr. This Capacitor will help the Plant maintain its Maximum Demand close to 83kVA even when all the Plant Loads are operational.

6.2  Replace HM-1 Drive by a 45HP Variable Speed AC Drive and HM-2 Drive by a 25HP Variable Speed AC Drive.

The above proposal does not solve the problem at its root. Rather it is liking creating a problem and then solving at by extra investment.

[Note- HM-1 motor is very much underloaded.60HP capacity is needed here.45HP is sufficient. In fact the manufacturer's specification states that it should be a 30kW motor i.e a 45HP motor]

The essential problem is that the Drive Manufacturer has used a half-controlled three phase bridge to carry out ac-dc conversion in the drives. This bridge uses three thyristors and three diodes instead of six thyristors as in the superior 6-pulse fully-controlled bridge. Thereby, the drive manufacturer saves a little on semiconductor cost. Also the control of half-controlled bridge is less complex and easier to design. The flip side is that the half-controlled bridge will always have poor displacement power factor and it will draw large amounts of harmonic currents from the line and that too at even harmonic frequencies starting right from 100Hz onwards. That makes it almost impossible to filter out using passive tuned LC Harmonic Filters. Of course Active Harmonic Filters can do the job well; but they come at a high cost. The net result is that the Plant is stuck with a ac-dc converter which runs at 0.5-0.6 power factor and has a THD close to 80% with 100Hz component accounting for most of it. Thus the low displacement power factor and very high harmonic content directly contribute to the maximum demand.

Half-bridge converter also brings in another problem in connection with deployment of power factor correction capacitors. Refer to the harmonic equivalent circuit in the figure in the next page. The power factor correction capacitor and the transformer inductance will form a parallel resonant structure as far as the harmonic currents are concerned. At the parallel resonant frequency the combination will present a very high impedance to the currents. If the Plant load is drawing a harmonic current at or near this resonance frequency value high voltage at that frequency will be generated across the capacitor and the Plant leading to equipment and capacitor damage.

This problem is usually solved by adding a so-called 'detuning reactor' in series with the capacitor. The value of this reactor is chosen such that the parallel resonant frequency is shifted to some frequency well below the lowest harmonic that is expected in the system. But that will work only if the lowest harmonic is well away from fundamental frequency. In the case of half-controlled bridges the most dominant harmonic is the next one to fundamental i.e the second harmonic. One just can not find a detuned parallel resonance frequency without interfering seriously with the 50 Hz behaviour itself.(This is why a Power Factor Correction Capacitor without detuning reactor was recommended for this plant even though there is a certain degree of risk involved in that.)

A ac-dc converter with six thyristors i.e a six-pulse full-controlled bridge will usually run with a power factor of 0.85 and a THD close to 30%.Its dominant harmonic will be at 250Hz i.e the fifth and it will have only odd and non-triplen harmonics like 5^th ,7^th ,11^th ,13^th ,17^th etc. That will make it easy to detune the resonant frequency. In fact it will be possible to arrange a series resonant frequency for L and C close to 5^th harmonic and to shift the parallel resonance frequency to a frequency lower than 5^th harmonic thereby providing harmonic filtering along with avoidance of harmonic resonance. And, with a much lower THD in such a converter the Maximum Demand would have been well under control.

Theoretically it is possible even now to convert these half-controlled bridges into fully-controlled bridges. But that will call for a major re-engineering of the Drives and addition of three phase delta-star transformers at the input of each drive. The cost involved will make the replacement of the existing drives by Variable Speed AC Drives look more attractive.

A highly simplified schematic diagram of a Variable Speed AC Drive using PWM (Pulse Width Modulated) Inverter is shown below. All details regarding control and protection are omitted in the diagram.

Three phase mains is first converted into DC by the six-diode uncontrolled six-pulse rectifier with dc side smoothing. The DC voltage generated across the capacitor is constant (if the mains voltage is constant).The PWM Inverter using IGBT Power Devices run off this DC supply and generate a variable frequency - variable amplitude three phase AC voltage across the motor. The speed of the motor is controlled by controlling the frequency of Inverter. Corresponding voltage magnitude control is also done by the PWM Inverter.

The diode rectifier in this case runs at close to unity displacement power factor (displacement power factor is the cosine of the angle between the voltage sine wave and the fundamental component of current wave).It will draw harmonic current too. But the THD will be close to 32% and the current drawn will contain only non-triplen odd harmonics. This means that the first (and the most dominant) harmonic is the 5^th.In general this level of THD does not affect the maximum demand much and can be left unfiltered. Of course 32% THD is not tolerated by International Standards. However a customer is under no statutory obligation to control the harmonic current he draws from the grid at present.

The expected maximum demand at the plant if the two HM drives are replaced by the above described Variable Speed AC Drives is estimated below.

Maximum Estimated Power Requirement of HM-1 and HM-2 = 30kW

(23kW was measured, 30kW is taken to represent maximum requirement with change of raw material and production speed)

Displacement Power Factor = 1

50Hz Current = 30,000/(1.732x420) = 41 A

@32% THD total harmonic current = 13 A

Total Average requirement from heaters and other small motors contributing to MD = 30kW at almost unity pf

Total 50Hz Current = 82 A

Actual RMS Line Current = Ö (82² + 13² ) = 83 A

THD of this current = 100x13/82 = 16%

Expected MD = 1.732x420x83/1000 = 60 kVA

The PP drive harmonics were not accounted in this calculation. But PP drive is a small unit and used very infrequently. It may not add more than two or three kVA to MD.In any case if PP drive causes a problem to MD it will be possible to switch off some other load like pump motors when PP drive is used.

Thus the maximum estimate of MD with AC drives comes to 60kVA.This means that the firm can get its Contract Demand revised to 80kVA and pay MD Charges for 60kVA only. It may take consistent recording of MD below 60kVA for a few months before the Utility will agree to such a revision of Contract Demand.

Note that there is no need for a power factor correction capacitor in this case. In fact power factor correction capacitors should not be used when there are six-pulse rectifier systems in the plant (as in the case of Variable Speed AC Drives) unless they are installed in the form of harmonic filters.

DC Motors tend to be inefficient compared to current day Induction Motors. Also they call for increased maintenance. Part load operation of DC motors further increase their inefficiency. Variable speed AC drives optimize the motor voltage to optimize the losses during part load operation. Thus, increased efficiency of Induction Motors along with optimized running under part load conditions will make the proposed AC drives in this plant consume about 15% less energy compared to what the present old DC drives consume. The average monthly energy consumption is about 15,000 of which about 50% come from HM machines. Thus there will b e 1125 units savings per month representing about Rs 33,000/- savings per year.

When the Contract Demand revision comes into effect the minimum chargeable MD will go down from 83kVA to 60kVA and this will result in an annual savings of around Rs. 75,000/-.Thus a minimum savings of Rs. 1 Lakh per year can be expected from replacing the HM DC drives by Variable Speed AC Drives. In fact the estimation of energy savings carried out here is quite conservative. The firm is well advised to contact a Variable Speed Drive supplier for a detailed measurement and study as a precondition to equipment supply.

Variable Speed AC drive makes the 30kVAr capacitor proposed in the earlier section unnecessary. The investment saved thereby should be factored into economic analysis of efficacy of this proposal. Also the resale value of the existing DC motors and Drives will also help to make the proposed replacement more attractive.