Power Factor Correction Manual (2000)

Power Factor Correction Manual Design

Site Analysis

Custom designed Automatic Power Factor Correction Units

Power Factor Profile service carried out on site.

Manufacture

Commissioning

Incorporation of Motorpol HEAVY CURRENT “SAFETY” capacitors.

Australia

New Zealand Head Office and Manufacturing Plants

Motorpol Australasia Ltd

Commissioning on site if desired

Sales Office

Motorpol Australia

17/101 Diana Drive, Glenfield Auckland New Zealand

39 Sierra Vista Boulevard Bilambil Heights NSW 2486 Australia

Tel: +64 9 444 3037 Fax: +64 9 444 3038

Tel: 1800 120 158(Within Aust.) Fax: 07 5590 7211(Within Aust.) E-mail: s a l e s @motorpol.com

WEB Page: www.motorpol.com

PFC42000

1

INDEX 1

Introduction to Power Factor Correction

1.1

Introduction

1.2

Causes of low power factor in an installation

3

1.3

Problems with low power factor

3

1.4

How to improve the system power factor

3

2

Factors Determining Degree of Correction

2.1

Economical considerations

4

2.2

Example of cost savings

4-5

3

Designing automatic power factor correction systems

3.1

Equipment

3.1.1

Capacitors

6-7

3.1.2

Switching equipment

8-9

3.2

Harmonics

9

3.2.1

Harmonic investigation

10

4

Method of correction

4.1

Individual correction

4.2

Group correction

11

4.2.1

Manual control of pf correction

11

4.2.2

Automatic control of pf unit

11

4.3

Motors

12

5

P.F. Correction Components

5.1

Motorpol "Safety" capacitors

13

5.2

Capacitor switching contactors

13

5.3

P.F. control relays

13

5.4

De-tuning reactors

14

5.5

Motorpol Power Factor Correction Modules

14

6

Calculations and Tables

15

Suggested Specification for Tenders

2

6

11

16

1

INTRODUCTION TO POWER FACTOR CORRECTION

1.1

Introduction To obtain the best possible economic advantage from electrical power, both the generating station and consumer's plant should be operated at the highest possible efficiency. To achieve this scenario it is essential to maintain/employ a good power factor throughout the system. Most A.C. electrical equipment draws from the supply apparent power in terms of kilovoltamperes (kVA). This value is in excess of the useful power, measured in kilowatts (kW), required to operate. The ratios of the quantities are:pf Where

=

kW/kVA pf

=

Power Factor

kVA

=

Apparent Power

kW

=

Useful Power

This is known as the power factor of the load and is dependent upon the type of equipment in use. A large proportion of electrical machinery used in industry has an inherently low power factor, which means that the generating stations have to generate much more current than is theoretically required. In addition transformers and cables must obviously carry this extra current. (often referred to as watt-less current.)

Power Triangle

kVA kvar

Ø1 Ø2

kW

To correct from CosØ1 to CosØ2 Kvar required = (TanØ1-TanØ2) x kW

2

1.2

Causes of low power factor in an installation. Apparent power derives largely from the inductive loads in the installation, the higher the inductive loads connected, the lower the power factor. Types of equipment that are likely to have a low power factor are:

1.3

a)

Induction motors

b)

Power Transformers and Voltage regulators

c)

Welding machines

d)

Electric furnaces

e)

Choke coils and magnetic systems

f)

Neon signs and discharge lamps

Problems with low power factor When the overall power factor of a generating station's load is low, the system is inefficient and the cost of electricity correspondingly high. To overcome this and at the same time to ensure that the generators, transformers and cables are not overloaded with watt-less current, the supply authorities often offer reduced terms to consumers whose power factor is high, or impose penalties for low power factor. By taking advantage of these special terms, reductions in power costs can be made. If the system power factor is improved and maintained, even if there is no such penalty, the factory cabling and supply equipment can be relieved of a considerable watt-less or reactive load, which will enable additional machinery to be installed without having to upgrade the existing switchboard facilities.

1.4

How to improve the system power factor. The use of static capacitors for the improvement of system power factor is the most economical solution for industry of today when considering the following factors: a)

Reliability of the equipment to be installed

b)

Probable life

c)

Capital cost

d)

Maintenance cost

e)

Running cost

f)

Space available

Capacitors have no moving parts, initial cost is low, upkeep costs are minimal, they are compact, reliable, highly efficient, and can be installed as individual unit, group or automatic methods of correction.

3

2

FACTORS DETERMINING DEGREE OF CORRECTION

2.1

Economical considerations Power factor correction should always be regarded as an investment with two main opposing considerations. First the expenditure incurred in the overheads charged against the capacitor installation and, second the income brought about by the saving in the cost of electricity, together with the reduction of losses in the electrical system. The chief capacitor overheads are depreciation, interest on capital, electrical losses and maintenance costs. The last two items in most cases are covered by the saving on the losses on the electrical system (cables, transformer, etc). The financial benefits from installing power factor correction capacitors will naturally vary with different installations (supply authority tariffs, penalty charges etc) but, generally, capital expenditure on correcting equipment can be recovered in the first 18 to 30 months, and subsequent annual savings are thereafter clear profit.

2.2

Cost Saving Example Overview A sawmill's main switchboard currently employs a power factor correction unit that is deteriorating at an alarming rate, and is not compliant with current working practices. The existing capacitors may have PCB insulation, which has recently been banned from use in New Zealand. This means the plant is consuming more Reactive Units than necessary in the day to day running of this plant. This situation can only worsen as the existing capacitor bank deteriorates.

Costs The consumer is currently charged a

REACTIVE UNIT CHARGE of

5.46 cents

per kvar/h. If the existing power factor cabinet is not replaced, they could soon be charged to the order of 100kvar per hour. This figure could rise to as high as 300kvar per hour as the existing power factor equipment deteriorates further. The following table illustrates the cost of Reactive power, when charged at 5.46 cents per kvar per hour, assuming the plant is operated at a power factor of 0.95 or worse for a 12 hour period, 365 days per year.

Chargeable Reactive Power Consumed

$Cost per 12hr day

$Cost per 365 days

Payback Period based on 400 kvar unit installed (see budget price below)

50kvar/hr

$32

$11,957

16 months

100kvad/hr

$65

$23,914

8 months

300kvar/hr

$196

$71,744

2.5 months

4

Options •Ignore problems with existing cabinet, and pay reactive charges cost •Rebuild existing cabinet. •Replace existing cabinet with new unit.

Proposal Options 1 and 2 are not economically viable, as the cost of rebuilding the existing capacitor bank will be similar to the price of a new cabinet because of a high labour content in retro fitting new components to an existing structure. The existing Power Factor Correction equipment will also need to be taken out of service during the refit.

It is therefore proposed to replace the existing individual capacitor banks and contactors with a free standing self-contained power factor correction unit. The proposed unit as budgeted for below is sized for 400kvar to accommodate current needs, but allows space for future expansion.

Budget Cost to Implement above

1 only, MOTORPOL 400kvar power factor correction unit complete with 440 volt safety capacitors, Automatic power factor regulator, enclosed within a free standing cabinet.

$13,750 plus GST

1 only, installation of new unit and removal of existing plant.

$1,500 plus GST The above unit can be operational within four weeks of approval

5

3

DESIGN OF AUTOMATIC POWER FACTOR CORRECTION SYSTEMS

Each power factor correction scheme requires individual consideration and depends largely on the positioning of the capacitors in the system.

3.1

Equipment

3.1.1

Capacitors It is essential that the type of capacitors selected are suitable for the duty. Voltage rating should be 440V as a minimum, but where severe over voltage and/or harmonics are present, higher ratings such as 525V or 660V should be selected. Low cost capacitors are often

"dry type" which on failure, either as a result of a

fault, or at the end of their useful life, can burst their case and release large quantities of flammable gas, resulting in explosion and possibly a fire. To prevent fire hazard and other damages to the electrical installation "SAFETY" type capacitors should be selected. SAFETY type capacitors have an over pressure tear off fuse and are self healing. Example

Overpressure tear off fuse

At the end of service life, or due to inadmissible electrical or thermal overloads an overpressure builds up and causes an expansion of the cover. Expansion over a certain limit causes the tear-off of the internal fuses. The active capacitor elements are thus cut off from the source of supply. The pressure within the casing separates the breaking point so rapidly that no harmful arc can occur. (Refer Fig. 1)

6

Fig. 1 Details of Over Pressure Disconnection Self Healing Technology

At the end of service life, or due to inadmissible electrical or thermal overload, an insulation breakdown may occur. A breakdown causes a small arc which evaporates the metal layer around the point of breakdown and re-establishes the insulation at the place of perforation. After electric breakdown the capacitor can still be used.

Fig. 2

7

3.1.2

Switching equipment Switching equipment for controlling and protection of capacitors is subjected to more onerous conditions than other equipment carrying other type of loads of similar kVA rating. The switching equipment should be derated to allow for maximum permissible overload due to a)

Increased voltage

b)

Increased frequency

c)

Non sinusoidal voltage supply (Harmonics)

and switching current transients likely to be encountered when capacitors are energised or de-energised. Switching capacitors in parallel to others being already energised, causes very high inrush current. As a consequence the life cycle of contactors and capacitors will be affected. Therefore to minimise the inrush current it is recommended that contactors with early make contacts (pre-contacts) in series with damping resistors be used. The current peaks are attenuated by resistor wires, during the making of the pre-contacts. If not controlled these high current peaks would cause contact bounce and weld the main contacts of the contactor. The pre-contacts of the capacitor switching type contactor are designed to open at a time at which the main contacts of the contactors are positively closed. Vertical: 250A/div

Vertical: 250A/div

Horizontal: 0.5ms/div

Horizontal: 0.5ms/div

Contactor make with Pre-contacts

Contactor make without Pre-contacts

Note that the current peaks without the pre-arcing contacts is approximately 4 times higher than the contactor with pre-arcing contact in series with damping resistors.

8

Vertical: 25A/div

Vertical: 25A/div

Horizontal: 10ms/div

Horizontal: 10ms/div

Contactor break with pre-contacts

Contactor break without pre-contacts

The pre-contacts has no influence on the breaking of the contactor therefore there is no difference between the two diagrams.

3.2

Harmonics Harmonics distortion is a common problem in modern industrial installations. It is often caused by high development and applications of power electronics, such as variable speed drives for motors, rectifiers and UPS systems. Harmonic distortion can cause overheating to cables and transformers and malfunction to communication equipment. Capacitors, themselves do not generate harmonics but they can either reduce or increase them, depending upon particular circumstances. The major sources of harmonics are in rotating machinery, transformers, rectifiers, arc furnaces, etc. By far the greatest cause in industrial systems is over-excited transformers and this is mainly the fifth and seventh harmonics, the third and its multiples are usually circulated in the delta-connected transformer windings. With normal voltage on a transformer the fifth harmonic is negligible, but as the primary voltage is increased the harmonic voltage increases, and for a 10% increase in primary voltage the fifth harmonic will be doubled. Transformers should, therefore, be operated as near as possible to rated voltage if harmonics are likely to occur. If there is a danger of harmonics or resonance occurring in the system it is recommended that the following measures be applied. a)

Adjust the transformer taps to approximately nominal voltage at low load.

b)

If capacitors are switched manually do not let the capacitor's kVAr rating exceed two-thirds of the transformer's kVA rating. If more capacitors are required they must be automatically switched.

c)

Relocate the capacitors away from the transformers.

9

3.2.1

Harmonic investigation It is recommended that an analysis of the connected load with a percentage of non-linear load to other load be undertaken. With less than 20% of the load consisting of rectifiers or converters of the 6-pulse type connected in the system generally no problem should be experienced. However one must consider, if the possibility exists, that under certain conditions any converters might be operated alone or almost alone with capacitors connected. This condition would certainly result in a very low damped resonance-condition, which would be critical, even when the total output on converters is less than 10% of the installed transformer size. To avoid failure of compensation, blocking reactors should always be installed if converter load of more that 20% of total load is possible in the future. If the possibility exists for a converter load to be switched on without other load blocking reactors are required regardless of the converter size. If the HV-system contains more than 3-4% voltage of a single harmonic, the possibility of a series resonance transformer/capacitor bank exists.

10

4

4.1

METHOD OF CORRECTION

Individual correction On large installations, applying individual correction to motors, or in the case of kVA maximum demand tariffs, on certain motors, known to be operated at the time of the maximum demand, is much more economical. If the no load magnetising current is not known, a maximum FL pf of 0.95 lagging should not be exceeded. The advantages of this method are: No additional switchgear is required as the capacitor is connected directly across the motor terminals and, therefore, switched in with the load by the motor starter. (refer section 4.3)

4.2

Group correction In some cases it may not be economical to individually correct each piece of equipment in an industrial installation with high diversity factor, group pf correction should be employed. This is the case where the power factor correction unit is connected normally directly into the bus of the main distribution board. The unit can either be manually controlled or automatically controlled via a microprocessor-based controller.

4.2.1

Manual control of pf correction When the capacitors are manually operated there is always a possibility of them being left on during the lighter load periods. If all the capacitors are in circuit continuously, the compensating kvar remains the same and hence when the load is light, the pf becomes heavily leading causing serious overvoltage and consequently causing damage to motors and capacitors themselves.

4.2.2

Automatic control of pf correction In large installations automatic pf control is an ideal method of obtaining the full electrical and financial benefits from a capacitor installation. Optimum power factor is achieved under all conditions and there is no possibility of human error or equipment being left on or off of the supply system. Advantage With Automatic control, optimal matching of the capacitor output to the specific requirements for reactive power; this makes sure that the specified pf is maintained in an economical way.

11

12

5 5.1

P.F. Correction Components

MOTORPOL "SAFETY" P.F.C. CAPACITORS •Motorpol

HEAVY DUTY "SAFETY" type - no risk of explosion or fire

•High tolerance to harmonic distortion •Fully comply to latest standards IEC 831-1 and 831-2 including destruction test •Non PCB liquid impregnated, very low loss

CAPACITOR MODULES

Complete with discharge resistors, cable lugs and terminal cover.

5.2

CAPACITOR SWITCHING CONTACTORS

complete with Dual Step Surge Limiting device

Note: These contactors are designed specifically for switching capacitor banks and are fitted with early make contacts switching damping resistors parallel to the maincontacts which significantly reduces inrush peaks.

5.3

MOTORPOL P.F. CONTROL RELAYS

13

5.4

MOTORPOL DE-TUNING REACTORS Capacitors can be overloaded in networks distorted by harmonics. This situation is even more critical in case of resonance. In case of resonance the capacitor current or voltage can be a few times as high as the nominal voltage. Therefore, special precautions (e.g. series filter circuit reactors) should be taken in networks distorted by harmonics.

Reference

Description

PS-12.5KVAR-3P

3 Phase Reactor-12.5kvar

PS-25.0KVAR-3P

3 Phase Reactor-25.0kvar

PS-50.0KVAR-3P

3 Phase Reactor-50.0kvar

5.5

MOTORPOL POWER FACTOR CORRECTION MODULES

For the convenience of the switchboard builder and OEM customer, Motorpol manufactures a range of easy to install modules containing capacitors, contactors, fuse disconnects and busbars. Various sizes are available both with and without De-tuning reactors. Specific details and pricing information can be provided on request.

Module without De-tuning Reactors

Module with De-tuning Reactors 14

6

CALCULATIONS

Calculation and selection of required capacitor rating Qc = P * (tan Ø1-tan Ø2) Qc = required capacitor output (kvar) P

= real power (kW)

Ø1 = phase angle of actual power factor Ø2 = phase angle of target power factor

The table below (Table 2) shows the values for typical power factors according to the Formula ,tan Ø1-tan Ø2”

The required capacitor output may be calculated as follows: • Select the factor k (matching point of actual and target power factor) • Calculate the required capacitor rating with the formula:

Qc = k * P Example:

Result:

actual power factor = 0.74

Qc = k * P = 0.58 * 35kW = 20.3kvar

target power factor = 0.95 real power = P = 35Kw capacitor output = Qc

15

Suggested specification for tenders Specification for Low Voltage Power Factor Correction Capacitors. The specified capacitor units shall be suitable for operation on a ……..V, …….Hz ……… phase system and shall be of MKP-type (Metallized Polypropylene Film). The three winding elements shall be enclosed in a common cylindrical aluminium casing to form a true three-phase capacitor. The capacitor should be vacuum impregnated with a biodegradable non-toxic vegetable based impregnating oil with a fire point above 315°C. The dielectric shall be a low loss (