Electrical Energy Auditing and Conservation in Plant Electrical Distribution Systems- An Article from Suresh Kumar K.S

[This article is an abridged version of a lecture note I prepared in 1994 and enhanced subsequently. The Government of Kerala State in India made Energy Audit mandatory for all HT Customers in 1993 and there were many Seminars and Workshops on Energy Auditing immediately after this at many locations in the State. The original version of this lecture was prepared for one such Workshop in 1994. A subsequently edited and enhanced version appears below. K.S.E.B in this article refers to the Power Utility in the State - Kerala State Electricity Board.]

1. Introduction

Energy Audit is the translation of energy conservation ideas into realities by blending technically feasible solutions with economic and other organizational constraints within a specified time frame. It attempts to balance the total input of energy with its use. The type of Energy Audit to be performed depends on:

the function and type of the Industry

the depth of final Energy Audit needed and

the potential and magnitude of cost reduction desired

The primary objective of Energy Audit is to determine ways to reduce energy consumption per unit of product or to lower operating costs. Electrical Energy Audit is but a subtask of the General Energy Auditing. However, an Electrical Energy Audit in isolation can be conducted, but the interactions between electrical energy use and other forms of energy use in the plant and the possible trade-offs between electrical energy and other forms of energy which may result in better overall energy efficiency in the plant may not emerge from such an Electrical Energy Audit. The aim of an exclusive Electrical Energy Audit will be to reduce Specific Electric Energy Consumption per unit of product output without increasing the specific energy (other than electrical energy) consumption per unit product output.

Minor or major modifications of process design of the plant will often result in dramatic improvements in the plant energy efficiency. The need for such modifications will be brought out by a comparison of Specific Energy Consumption of the plant with standard values pertaining to that particular product. However, such modifications in the plant processes fall under the purview of a General Energy Audit and an Electrical Energy Audit will not usually address this aspect of energy conservation.

Thus Electrical Energy Audit (EEA) assumes that the process design, material flow etc. of a plant cannot be significantly altered. It focuses almost exclusively on the Electrical System of the plant. Issues regarding process design will be examined in an EEA in so far as to their implication on reducing the load levels on electrical equipment and reducing losses in the Electrical System.

The intensity and the depth of the planned Electrical Energy Audit will depend on a comparison between the best Specific Energy Consumptionfigures of the product (using the same process design) achieved in the Industry on a world wide scale and the figure for the plant in question. Also, the Statutory/non-statutory bodies on energy conservation in the country may have laid down reference values for Specific Energy Consumption.

This article attempts an overview on Electrical Energy Audit of a medium scale industry assuming that the need for an intense and in-depth EEA exits in the plant. The coverage is by no means exhaustive, but is intended to be indicative of the general principles involved in EEA of such a plant.

2. The Aims of Electrical Energy Audit

The essential aim of EEA is to arrive at concrete project proposals with priority ordering and financial justifications, which if carried out, will result in reduction in Specific Energy Consumptionand to monitor the progress and performance of such projects undertaken.

Different steps are to be gone through before the EEA can reach the level of project proposals and recommendations. The first of these will be a Preliminary Audit.

3. The Preliminary Audit

3.1 The preliminary audit attempts to answer the following questions-

(i) How much electrical energy is actually used on a typical day per unit output ? And how does it compare with the target figure? (This figure is to be arrived at by calculating/measuring the output of the electrical equipment before the driven machinery).

(ii) How much is the company paying for its daily electrical consumption? Can it be brought down?

(iii) How much is the specific energy loss per unit output on a typical day? How are the losses distributed in the Electrical System?

3.2 Data Required to Answer the First Question

In general the output energy of all electrical equipment at the process end over 24-hour period will be needed. In the case of some equipment the input energy may directly furnish this data – e.g. heaters, lighting, air-conditioning and refrigeration etc. whereas in the case of equipment like motors a record of shaft speed over 24 hour period can be used to calculate the required data. Of course, the data taking operations can be considerably simplified by using the knowledge of plant personnel regarding type of duty on the equipment and load variations on the equipment and load variations on the equipment etc. An exhaustive data taking on all equipment may not be needed except when considerable energy loss is suspected in the plant. Judicious selection of important equipment and optimum use of available information can often simplify the data collection requirements to a manageable level.

3.3 Data Required to Answer the Second Question

Maximum VA Demand, Maximum Watt Demand, consumption figures of the whole plant, energy cost figures of the plant etc. for the preceding 6 months or one year will be sufficient data for this question. Daily VA and Watt load curve will help to assess the possible savings in payment to the utility.

3.4 Data Required to Answer the Third Question

The total electrical energy consumption per day and total actual energy use per day.

Power and Energy Meter readings at important subsections of the Electrical System.

Actual energy use in the important subsections of the Electrical System

3.5 Results of Preliminary Audit

The results of preliminary audit will be expressed in the form of Load Factor of the plant, plant power factor, average daily energy consumption and cost, estimates of losses in the system etc. The energy flow in the system will be displayed on a subprocess to subprocess basis in an energy flow diagram with electrical losses occurring in various subprocesses marked. Also, the energy flow will be indicated on a single line diagram of the electrical system with losses in various sections marked.

The results of preliminary audit set the parameters for the detailed audit. Also the depth required in the detailed audit is, to a large extent, decided by the results of the preliminary audit.

4. The Detailed Energy Audit

Almost all known energy conservation and energy management techniques aim at one or more of the following objectives.

Reduce the actual use of energy without much modification in the process design.

Reduce the payment to be made for energy.

Reduce electrical energy losses.

Detailed Energy Audit aims at identifying priority ordered, economically viable projects that will fulfil the above objectives. It will also arrive at recommendations regarding maintenance procedures, changes in operation, changes in sequence of operations, replacing inefficient equipment/process with energy efficient equipment/process etc. for fulfilling the energy efficiency objectives.

4.1 Reduction of Actual Use of Electrical Energy

The actual use of electrical energy has been calculated/measured at the output of electrical equipment in the preliminary audit. But the final value of electrical energy content in unit output will be less than this value due to losses in energy conversion process i.e. in the mechanical and chemical Systems in general. Improving the efficiency of energy conversion process will result in lower loading levels in electrical equipment and lower use of electrical energy. Reducing the pipe resistance by proper maintenance and correct sizing, implementing automatic level control in pumping systems, replacing throttling valve in pumping systems by variable speed a.c drives, replacing mechanical damper control and vane control in blower systems by adjustable speed a.c drives, using Smart Motor Controllers (SMC) on part loaded Induction Motors, reducing the leakage from compressed air systems, reducing air infiltration into air-conditioned spaces, providing false ceilings/window glazing/window curtains/automatic door closers/air curtains etc. in air-conditioned spaces ,relamping by CFLs and HID lamps etc. are some examples involving this concept.

In fact, this is the only context in which the EEA team pays attention to systems other than the Electrical System in the plant. EEA has to examine the major low efficiency energy conversion processes and arrive at ways to reduce the energy consumption in those processes. The possibility of using automatic controls in order to switch off electrical equipment when the process does not really need the energy and to adjust the efficient operation of electrical equipment against varying process load levels should be critically examined.

It is often possible to reset the process variables to new levels conducive for lower energy consumption after a critical examination of the process. For example, in a space heating application (or air-conditioning application) it may be possible to set the thermostat at a lower (or higher) temperature thereby reducing the electrical energy consumption. But such resetting of process variables will need a thorough study to ensure that the quality of product is not compromised.

The plant lighting system will also come under the scrutiny of EEA with a view towards finding out whether the existing illumination levels at various places are really needed for the task or purpose involved.

As far as electrical energy conservation is concerned, reducing the use of electrical energy is the primary action phase in the hierarchy of conservation strategies. At this point, EEA should identify the steps to be taken to reduce the use of electrical energy and draw up proposals for the financially viable projects aimed at this. The Company will have to allot top priority to these projects in general since they will have a direct impact on electrical energy consumption. The EEA proceeds to the next aspect of energy conservation and management with the figure for actual electrical energy use updated assuming that the projects aimed at reducing the energy use will be implemented.

4.2 Load Management

Examination of daily active and reactive load curves for a typical day at the Incomer will reveal the need for load management in the System. Load Management at primary level as envisaged here involves techniques aimed at improving the daily active power load factor and bringing it close to unity and techniques aimed at bringing down the daily average reactive load and improving the daily reactive load factor to unity. These measures will directly affect the Maximum VA demand of the plant and thereby effect savings in demand related cost of energy. Also, these measures effect a reduction in system losses too. But the extent of loss reduction achieved will depend on the exact method used to achieve active and reactive load curve smoothing. Reactive load reduction and smoothing is usually achieved by the use of switched capacitors and exact loss reduction achieved will depend on how the capacitor compensation is distributed in the system. This is where load management at the secondary level (i.e. at a deeper level into the system) becomes necessary.

The primary level daily load curve smoothing directly affects MD related costs. Secondary level load management is aimed at minimizing losses everywhere in the electrical distribution system equipment. The target energy savings set by the Company may decide the depth of secondary level load management into the system. For example, in the case of an Industry using voltages at 11kV, 3.3 kV and 415V levels, the secondary load management may penetrate up to the end of all 11 kV feeders or up to the end of all 3.3kV feeders or up to the end of all 415V main feeders or finally up to the end of all 415 V distribution feeders. Secondary load management at all voltage levels will lead to primary level load management. But load management at primary level does not necessarily result in secondary load management.

4.3 Data Needed for Secondary Load Management

Active and Reactive daily Load Curves for all 415V main feeders, 3.3kV feeders and 11 kV feeders.(In general at all the voltage levels employed in the plant).This may require submetering. If the metering facility is not there already, EEA has to execute a data collection operation.

Information on the loads that can be staggered and shifted to any time period during the day or to specified time periods during the day. This is a process related information and EEA has to collect this information through discussions with plant personnel.

4.4 Principles Employed in Secondary Load Management

A three-phase feeder carries a given amount of power with minimum feeder losses when all the three phases are balanced. Hence feeders with single-phase loads must be checked for balance and should be balanced if there is unbalanced loading. The balanced condition must be maintained throughout the day as far as possible.
A feeder that is to carry a given amount of energy over a day will do so with minimum energy losses if the daily load factor on the feeder is unity. This statement is true for any electrical equipment including transformers, motors etc.(If the equipment is off during some period of the day ,the load factor pertains to the period during which it is on).
A group of feeders emanating from a common bus and carrying a given amount of total energy over a day will do so with minimum energy loss if (a) all the feeders have daily load factor of unity and (b) all the feeders produce same IR drop (i.e. if the feeders are identical they must share the energy transport equally).
A group of transformers operating at same voltage level and transporting a given total amount of energy over a day will do so with minimum loss of energy if (a) the daily load factor at all the transformers is at unity and (b) VA loading of transformers is such that the product of loading factor (i.e load expressed as a fraction of transformer rating) and full load copper loss factor (i.e full load copper loss expressed as a fraction of transformer rating) is the same for all transformers. This assumes that all the transformers are ‘on’ all the time. If some transformers can be switched off it is possible to minimize energy losses still further.
The total capacitive compensation VArs at various periods of the day that should come into effect at the incomer point is decided by Maximum VA demand considerations. However, the distribution of capacitors is dictated by various other considerations.Prominent inductive loads at process end must be compensated right at the load terminal; at least partially. Typical example is the use of capacitors that are switched along with motors. But individual capacitors at the terminals of all the motors in the plant may not be needed and may not be economically advantageous. The remaining compensation may be carried out at the various buses at various voltage levels.
Capacitors at various buses at various voltage levels introduce flexibility into the problem of smoothing the load curves and balancing the load curves of various feeders terminating at a particular voltage level for optimum energy loss. The load curve smoothing on the various feeders can be carried out first by taking into account only the active part of the load powers. The resulting Var load curve (which may turn out to have a low load factor) on various feeders may be smoothed out by using fixed and switched capacitors of proper values at various buses. Also, the condition for minimum energy loss in all feeders put together may be adjusted simultaneously. The same strategy can be used to adjust the loading of transformers at a particular voltage level for minimum energy loss.
The general problem of load allocation to feeders, feeder allocation to buses, bus allocation to transformers etc and fixed/switched capacitor allocation at various buses before and after transformers, such that minimum energy loss conditions will prevail everywhere in the system throughout the day is a quite complex one. However, it may be formulated as a optimization problem and may be solved on a digital computer. The solution obtained thereby will have to be compared with the existing feeding arrangement in order to find out the additional switchgear installations, cable-laying etc. needed to implement it. System modifications needed to implement the optimum solution may turn out to be unacceptable due to various reasons like economic viability, time required to carry out such modifications, complexities involved in the modifications etc.
A better approach would be to identify competing sub-optimal solutions that do not involve any extensive structural changes in the Electrical System and to examine these alternatives on a cost-benefit basis. The EEA team should resort to approximate engineering calculations and good engineering judgement to arrive at three or four different feeding arrangements/capacitor allocation that will result in lower energy losses. Detailed calculations can then be employed to decide between them.

4.5 Reduction of Losses in Electrical Motors

Minimization of losses in all the electrical equipment except the process end equipment would have been taken into account in the load management strategies arrived at by EEA so far. What remains is the reduction of losses in the equipment that form the loads on the Electrical System.

Induction Motors constitute 70 to 80% of electrical load and hence reduction of losses in these motors assumes special significance. It is assumed that EEA has already taken note of the various ways in which the loading levels of these motors can be minimized by better utilization of energy in the energy conversion process. This includes identification of motors that require automatic control for avoiding idling, motors which need to be fitted with variable speed controllers etc.

4.6 Data Needed for Loss Reduction in Motors

Rating of Motors and Nature of Load
Loading Data over a typical day; preferably in the form of load curve in the case of large motors.
Type and details of the controller provided for the motor.

4.7 Data Collection

The torque output of an Induction Motor is proportional to slip for a torque variation of 10% to 110% of rated value. Since the speed does not change much in this region the power output itself may be taken as proportional to slip in the range of 10% to 110% of rated HP output. Thus accurate measurement of speed of the Induction Motor and the System Frequency will permit determination of power output of the motor. Contact/non-contact type digital tachometers can be used for this purpose.
It is also possible to estimate the power output of the motor by measuring the kW and kVA input to the motor if certain assumptions regarding the full load efficiency of the motor can be permitted. However,the speed based method will be more accurate.
Power Measurement at the input terminals by using the power meter on the panel or clamp-on Energy Audit Meters will give the input power.
Losses can be estimated using these readings.

4.8 Factors Affecting the Induction Motor Performance

Voltage and Frequency – Operating the motor at other than rated voltage and frequency can result in reduced motor efficiency and adverse effects on power factor, break-away torque, starting current, running speed etc.
Unbalanced Voltage – Even a small degree of unbalance at the motor terminal voltages can result in large negative sequence currents in the motor. And the resistance of the rotor to negative sequence currents will be greater due to skin effect and deep bar effect. Thus small unbalance in voltage will cause large increase in motor losses and heating. Careful balancing of voltages everywhere is imperative from this point of view.
Loading Level– Motors are designed to operate with maximum efficiency at full load. At part load efficiency and power factor come down. Thus, for same power output, using an oversized motor will result in higher active power input and higher reactive power input into the motor compared to a properly sized one. Thus, part loading of Induction Motors (especially on a continuous duty basis) will increase the losses and the Maximum kVA demand of the plant.Motors maintain good efficiency in the range of 60%-100% of rated output. However, loading below 50% of rated load results in serious active and reactive losses. Replacing the oversized motor with a properly rated one or installing variable voltage controllers on the oversized motor will have to be resorted to when part loading of motors is observed.
Speed – For same HP rating motors with higher speed have higher efficiency at rated loads.
Duty Cycle – The losses in the Induction Motors depend on the type of duty on the motor. The duty cycle of the motor has to be obtained and suitability of the motor for the duty must be examined. For example, a continuous duty rated motor, if applied for an intermittent duty with frequent starting will have more losses than a high starting torque intermittent duty type motor.
High Efficiency Design - High Efficiency Design versions of Induction Motors with 20 to 30% higher costs are available in the market now. These motors use specially processed low loss steel core, longer stator and rotor and optimized precision airgap to minimize the magnetizing current and core losses and they use more copper/aluminum for reducing copper losses. The higher initial investment is often paid back in one to two years through loss reduction.

4.9 Motor Loading Analysis in Electrical Energy Audit

Classify the motors into various categories depending on type of loading viz. Continuous constant load duty, Continuous variable load duty, Intermittent duty with or without starting/electrical braking, short time duty etc. In each category, classify the motors into low HP, medium HP and high HP classes.
Short time duty motors of all ratings may not offer much in terms of possible loss reduction. However, they may be used for peak shaving applications.
Low HP motors may need only a cursory evaluation since the loss reduction achievable from them may not be enough to justify the effort and expense, especially in the first EEA. However, the final decision in this matter will depend on the number of such motors in the plant, their HP distribution, loading levels, the extent of loss reduction desired by the firm etc.
The adequacy of rating (under rating or over rating or wrong type of motor etc.)of the motors has to be paid close attention in the case of motors on intermittent duty with frequent starting/reversing/plugging etc. The starting/stopping control of these motors will have to be looked into from energy loss point of view.
Continuous constant load motors and continuous variable load motors offer possibilities of loss reduction. Their loading levels must be determined. If part loaded, EEA should come up with suggestions for loading them fully by transferring load from similar under loaded motors or for replacing the motor by one of suitable rating on an ‘interchange’ basis i.e. various underloaded motors in the plant must be relocated and reused with only a minimum number of motors being relegated to store room and only a minimum number of motors drawn from store room or for replacing the motor with a new high efficiency motor of suitable rating. Financial viability of the suggestions also must be examined by the EEA.

5. Energy Conservation in Standby DG Sets

Faced with power cuts and low power quality Industries in Kerala have been relying heavily bon the Standby DG Sets in the past decade. DG Sets are capable of generating about 3.8 units/lit of diesel if maintained properly and loaded economically. A well tuned and maintained Set can give about 3.2 units/lit to 3.8 units/lit over 30% to 80% of its rated load. Quite often it is sufficient to maintain the set as per the manufacturer’s recommendations in order to achieve this.

Two excerpts from Energy Audit Reports prepared by me (in connection with consultancy activities of Energy Audit Cell , NIT Calicut) are appended below to illustrate the methodology to be adopted in auditing the DG Set operation and to illustrate the possible energy savings through proper maintenance. The second excerpt brings out the importance of maintaining the Set frequency at rated value in the interest of energy conservation and plant productivity.

5.1 Excerpt from Audit Report for a Hospital

The Hospital under consideration receives Electric Power from K.S.E.B at 11 kV level and steps it down by a 250 kVA 11kV/433V OLTC Transformer to feed a connected load of 510kW+50H.P+15 kVAr. The Contract Demand is 140kVA. The tariff rate at the time of audit (July/August 1997) was Rs 118/-per kVA of M.D and Rs 1.325 per unit of energy consumption. The current R.M.D level is at around 70 kVA and it had touched 120kVA during 1994 and 1995.

The firm maintains a 125 kVA D.G. Set as stand by unit. The firm is located in an area that suffers from low voltage and generally unreliable power supply and as a consequence it is on its D.G. Set for about 4 hrs per day in the average at the time of audit due to scheduled power cuts , unscheduled power cuts, power quota, low voltage etc.

The average monthly electrical energy consumption during the three year period of April 1994 -March 1997 covered in the audit is about 40271 Units and 28% of it is Self Generated in the 125 kVA D.G.Set. The average monthly energy cost during the same period is about Rs. 0.75 Lakhs and Diesel Cost accounts for 51 % of it.

The average kW loading of the system during the day is about 31kW and the peak value measured during the audit was 60kW as measured on 10-07-1997.The daily consumption, as metered during the energy audit on 10-07-1997, was about 736 Units of energy and 325 units of reactive energy.

5.1.1 Analysis of DG Set Operation

The firm could supply only the purchase cost of Diesel on a monthly basis during the audit period. There was no data available on the diesel consumed in the DG Set and the unit generated in the set. The firm has not maintained a log of self generated energy and the diesel consumption.

The approximate amount of diesel consumed in the set was worked out from the diesel purchase cost data supplied by the firm. The average unit rate measured during the audit was used to convert the Diesel consumption data obtained thereby into Self Generated units data.

An efficiency run was performed on the 125 kVA set on 11-07-1997.The entire system was on this set during the run and the average kW loading was around 40 kW i.e. 32 % loading, during the test period. The test lasted for about two hours. The generated energy was metered by a Three phase Energy Analyser cum Harmonic Analyser.

Length of the Tank = 89.6 c.m

Diameter of the Tank = 61.4 c.m

Initial dip measurement = 46.5 c.m

Final dip measurement = 41.0 c.m

Duration of the test = 1.78 hrs

Energy recorded =70.24 Units

Diesel consumed =27.22 Litres

Unit Rate of this D.G. Set = 2.58 Units/Lit

[ Volume = l(2(r²-h₁²) x (h₁-h₂) + { (r²-h₂²) - (r²-h₂²) } x (h₁-h₂) )

where h₁= initial dip reading - radius

h₂= final dip reading - radius

r = radius

l = length of the tank ]

5.1.2 Comments

1. The kW loading in D.G. Set is about 32 % of its rating on the day of measurement .But it may go to about 70% if load restrictions are removed.

2. At about 32% loading, the measured unit rate of about 2.6 units/lit is low. A value of 3 to 3.2 units/lit can be obtained at this loading if the set is maintained properly. An overhauling and engine tuning is recommended for this DG Set.

5.1.3 An Estimate of Savings Possible by Good Set Maintenance.

A well-maintained and well-tuned DG Set should give about 3 Units per litre of Diesel at 32% loading. The unit rate at present is only about 2.58.

82,435 Litres of Diesel was used in this DG Set in the year 1996-97.The firm had been relying more on the DG Set for its power requirement over the last few years and this dependence is expected to go up in the current year.

A shift of unit rate from 2.58 Units/Lit to 3.0 Units/Lit will represent 14% savings in Diesel consumption. At the 1996-97 level this will be about 11,500 lit Diesel saved and Rs 115,000/- of money saved per year.

It is strongly recommended that the 125kVA set be overhauled and tuned. Also the periodic maintenance procedures laid down by the manufacturer may be strictly adhered to. Proper logbook should be maintained for this set. 5.2 Excerpt from Audit Report for an Indane (LPG) Bottling Plant

The Indane (LPG) Bottling Plant receives electrical power from K.S.E.B at 11 kV level with a Contract Demand of 300 kVA. The Connected Load of the Plant is 380 H.P + 41 kW + 150 kVAr. The Recorded Maximum Demand varies between 225 kVA and 250 kVA.

The firm maintains 2x250 kVA D.G. Sets and a 60 kVA D.G. Set as stand by units. The Plant is located in an area that suffers from low voltage and generally unreliable power supply and as a consequence the Plant is on its D.G. Sets for half its operating period in the average. The K.S.E.B ACB is set to trip at 330 V and the Plant is put on one of the 250 kVA D.G. Sets whenever K.S.E.B voltage goes below this value or K.S.E.B supply is not available. However the Plant is exempt from power cuts and Units restriction which other HT Consumers of the State are subjected to during periods of power shortage.

The Plant runs on a two-shift basis now (Since Feb 1997) and shift periods are 6 A.M to 2 P.M and 2 P.M to 10 P.M. The installed capacity for two-shift operation is 3000 MT of LPG bottled. This corresponds to about 28 Loads of LPG cylinders.

The average monthly electrical energy consumption is about 51881 Units and half of it is Self-Generated. The average monthly energy cost is about Rs. 1.17 Lakhs and Diesel Cost accounts for 58 % of it.

The average kW loading of the system during the 16 hr operating period is about 127 kW and the peak value measured during the audit was 150 kW. The corresponding kVA loading levels were about 180 kVA and 205 kVA. The average daily consumption is about 2033 Units of energy, half of which is Self-Generated.

There are 3 D.G. Sets in the Plant - Two 250 kVA units and a 60 kVA unit. The 60 kVA unit can be used to supply only the Light Switch Board. One of the two 250 kVA units and the 60 kVA unit can be operated at the same time if necessary. One of the two 250 kVA units is relatively new and is called the New D.G. Set and the other is referred to as the Old D.G. Set in this section.

Three horizontal mounted cylindrical tanks kept on a raised iron platform (approximately 4 m in height) deliver fuel to the three generators through a common piping system by gravity flow. One of the tanks is small in size and is rarely used. Normally diesel flows from two larger tanks to the D.G.Sets. Separate measurement of Diesel consumption in each set is not possible due to common fuel piping.

The firm maintained proper records for the total Diesel consumed in the three sets. There are fully functional energy meters in the control panels of all the three D.G. Sets. However, the firm has not maintained a log of self-generated energy on a monthly basis in any of the sets. Though the K.S.E.B bills contain Self-Generated units, the value entered against that entry is grossly in error. Efficiency runs were performed on the D.G. Sets during the Energy Audit and the unit rate, i.e. the units generated for one litre of Diesel was measured. The firm ensures that the two D.G. Sets are run for equal periods over a month. Thus from the measured unit rates an average unit rate was arrived at by assuming equal periods of operation for the two 250 kVA sets. The 60 kVA is used sparingly and is used only when K.S.E.B is off, Plant is not running but lighting is needed. The average unit rate measured during the audit was used to convert the Diesel consumption data provided by the firm into Self Generated units data and these were the figures used in data analysis.

5.2.1 Efficiency Run Results for The Old 250 kVA D.G.Set

Two efficiency runs were performed on this set on 25-08-1997.The entire plant was on this set during the runs and the average kW loading was around 120 kW i.e 60 % loading, during the test periods. The first test lasted for two hours and the second test lasted for half an hour. Diesel was allowed to flow into the set from only one tank and dip measurements were recorded for that tank. The generated energy was metered by a Three phase Energy Analyser cum Harmonic Analyser.

Length of the Tank = 215 c.m

Diameter of the Tank = 78 c.m

Initial dip measurement = 50.4 c.m

Final dip measurement = 47 c.m

Duration of the test = 2 hrs

Energy recorded =217 Units

Diesel consumed =57 Litres

Unit Rate of this D.G. Set = 3.8 Units/Lit

[ Volume = l(2(r²-h₁²) x (h₁-h₂) + { (r²-h₂²) - (r²-h₂²) } x (h₁-h₂) )

where h₁= initial dip reading - radius

h₂= final dip reading - radius

r = radius

l = length of the tank ]

The second run of half hour duration confirmed the above value within measurement errors.

5.2.2 Efficiency Run Results for The New 250 kVA D.G. Set

Efficiency run lasting for about two hours was performed on this set on 26-08-1997.Diesel was allowed to flow from both tanks and simultaneous dip measurements were taken on both the tanks.

Initial dip reading on the Left Tank = 34.5 c.m

Final dip reading on the Left Tank = 31.0 c.m

Initial dip reading on the Middle Tank = 50.0 c.m Final dip reading on the Middle Tank = 49.0 c.m

Energy Generated = 221 Units

Diesel consumed from Left Tank = 57.5 Lit

Diesel consumed from Middle Tank = 16.1 Lit

Total Diesel = 73.6 Lit

Unit Rate of this D.G. Set = 3.0 Units/Lit

5.2.3 Comments

1. The kW loading in both D.G. Sets is about 60 % of their rating and
kVA loading in both is about 80 %.Hence both D.G. Sets get loaded
in the maximum efficiency range.

2. The Old 250 kVA set has expected efficiency of around 3.8 Units/Lit and is quite satisfactory.

3. The new D.G. Set with its 3 Units/Lit rate is not as efficient as the old one at the time of measurement. An unit rate of at least 3.4 Units/Lit is expected for a 250 kVA D.G. Set at 80% kVA loading. This D.G. Set has to be serviced and engine tuning has to be carried out to improve its efficiency level. It may be noted that it runs for about 4 hrs in a day on almost all days and improving its efficiency is important.

5.2.4 A Recommendation to Save Energy during D.G. Set operation

These two figures shown above depict the variation of kW and kVAr load variations in the motor control centres PMCC1 and PMCC2 when the system frequency changes in the range 46 Hz to 51 Hz.

During the audit measurements on 25-08-1997, it was observed that the old D.G. Set maintains a frequency of 50.1 Hz and regulates the value closely against load variations. But the new D.G. Set was found to maintain a frequency of 46.9 Hz when fully loaded and the variation in frequency with load is more in this set. The change over from the Old D.G. Set to New D.G. Set took place at 13:50 on that day and the system load immediately went up by about 20 % both in kW and kVAr. There was essentially no change in the plant operation or plant load during the forenoon and afternoon periods. The only difference was that Plant was on Old D.G. Set in the morning and it was on New D.G. Set in the afternoon. And the Old set was running at the right frequency whereas the new one was running at a low frequency of 46.9 Hz.

The audit also noted that though the PMCC1 and PMCC2 were taking more power when on New D.G. Set, the LPG Compressors, Air Compressors and LPG Pumps were not able to develop the rated pressures and there were complaints from filling section regarding the low air pressure. Also, the bottle transfer section had to adjust the flow rate to a lower value to develop the right pressure differential. Discussion with Plant Engineers revealed that this is a regular feature whenever the plant is on the new D.G. Set and sometimes these problems crop up when the system is on K.S.E.B too.

All these problems regarding insufficient pressure on compressors and pumps and slowing down of conveyors in the filling section etc. are due to the low frequency of the new D.G. Set. And at the same time all the motors will draw more reactive current(because of lower frequency and saturation of iron) and active power losses in them will be more (due to higher flux level and higher current level). Thus lower frequency makes the motors less efficient and creates operational problems in the system. The compressors and pumps will develop the right pressure only if they run at the rated speed.

Examine the figures in fig 4.9 and fig 4.10.The increase in active and reactive powers taken by the motors in PMCC1 1 and PMCC2 at low frequency is clearly evident. It may also be seen that if the New D.G. Set is run at 50 Hz always there will be a minimum reduction of 25 kW load from these two motor control boards. This amounts to a saving of 100 Units per day since this D.G. Set runs for 4 hrs per day in the average. The measured unit rate of this D.G. Set is 3 units/lit. Thus the saving in Diesel is about 33.33Lit per day, about 12000 Lit per year and the corresponding financial saving is Rs. 1.32 Lakhs.

Hence it is recommended that all the D. G. Sets be run always at 50 Hz frequency. The operator may be instructed to adjust the frequency of the set once in a half hour if needed. Digital frequency meters may be installed on the 250 kVA D.G. Sets to facilitate easy monitoring of the frequency. The investment required would be about Rs.2000/- and payback is almost immediate.

6. The Electrical Energy Audit Report

The Electrical Energy Audit Report in general should be organized as follows.

I The company, products, the processes, flow diagram of process, performance figures of the plant.

The performance figures of the Electrical System of the plant as a whole and comparison of these figures with achievable ones.

The electrical energy flows (over a typical day) displayed in process flow diagrams with electrical losses occurring at various points marked.An example is shown below.

The electrical energy flows (over a typical day) displayed in one-line diagram of the Electrical System with electrical losses occurring at various sections marked.See an example below.

The scope and depth of energy conservation program envisaged by the EEA for the company as a result of preliminary audit.

II Findings of the EEA as to the causes of over use of electrical energy at process end (including lighting and air-conditioning)

Listing of projects (with priority ordering based on ROI analysis) recommended by EEA in order to reduce the process end use of electrical energy.

Time frame for these projects.

Estimates on reduction in energy consumption and energy cost that will result from implementing these projects (i.e. impact analysis).

III Findings of EEA as to the cause of low load factor (both active and reactive load factors) at various points in the Electrical System and unacceptably high reactive loading (if any) at various points.

Findings of EEA as to the causes and degree of phase unbalances, harmonics, loading unbalances (from optimal loss point of view) in feeders and transformers etc.

The various feeding arrangements and load staggering arrangements arrived at by EEA along with the projects to be executed in order to implement them (installing additional breakers, switches, bus couplers, relocating transformers, replacing transformers, automatic switching ON/OFF of transformers, capacitor installations, additional cable laying etc.)

Comparison of various alternatives in terms of resulting loss reduction, cost incurred in implementing, time needed for implementation, reliability aspects etc.

The recommended scheme.

IV Motor Loading Analysis.

Recommendations to replace/interchange/replace with a new high efficiency motor etc. with financial viability analysis.

Recommendations to improve losses in other electrical equipment if any (eg. Lighting, heating etc.)

The report also should contain an abstract that summarizes the present energy position and a list of various projects with time, cost and benefit included.

In the case of subsequent Energy Audits, the report should also cover the progress of projects taken up as a result of earlier EEA reports and their impact on Specific Energy Consumption and cost of production.

7. Conclusion

A general framework for carrying out Electrical Energy Audit of a medium scale industry has been outlined in this article. Important general principles that aid the analysis of audit data also have been briefly touched upon.