Power Monitoring in the Modern Building and Harmonic Problems

7/30/2019 Power Monitoring in the Modern Building and Harmonic Problems

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5. 3. 1

POWER MONITORING IN THE MODERN BUILDING

AND HARMONIC PROBLEMS

Mr E. Conroy, Eta Projects Limited

AUTHOR BIOGRAPHICAL NOTES.

Eugene Conroy graduatedwith a Masters degree in Building Services from Brunel

University in 1995. He is a director of Eta Projects Limited, a small Consultancy practice

that specialises in power distribution systems with specific emphasis on power monitoring

and power related problems.

ABSTRACT

Extensive personal research has shown that the personal computer is responsible generating

the majority of harmonic related problems.

The problem of harmonic distortion caused by personal computers is a relatively new

phenomenon. It is becoming increasingly problematic in commercial buildings, especially

where large quantities of computers are present, particularly in dated buildings. The extent

of the problems caused by personal computers can now be quantified by the use of digital

power monitoring on distribution systems.

Current and voltage values are no longer adequate to understand the electrical load of abuilding. It is important to understand the complexity of the load. Therefore, a graphical

picture of the load can reveal more answers to apparently complex problems.

Some of the problems experienced have been solved with the aid of advanced power

analysers. Without the power monitoring systems, the problems would have remained

unsolved. With the present competitive market, power-monitoring systems prove to be a

cost-effective tool to carry out efficient management of buildings.

Studies have been carried out to establish the exact electrical characteristics of the PC and

the accumulative effect they have on distribution systems.

This paper is intended to be an overview of the problems presented by PC generated

harmonics with practical solutions to some of the problems.


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5. 3. 2

1.0 Power Monitoring

Power monitoring systems have evolved slowly over the years from electromechanical

devices to transducers and most recently to digital meters. Most manufacturers now

market a wide range of instruments to suit most application. Most power related problems

could be resolved by the use of digital analysers when used by trained persons. Two

examples of the benefits of a power monitoring systems are as follows:-

Example One

A UPS system was switching to battery for no apparent reason. The problem was quickly

identified as been due to disturbances on the main,s supply. (See figure No 1a)

Figure 1a ( Voltage disturbance on UPS supply.

Example Two

Computer equipment appeared to malfunction, with no apparent loss of supply. The power

monitoring equipment was interrogated and the problem was discovered to be a break in

the UPS output. This would be considered inconceivable. (See figure 1b) Figure 1b (

Voltage disturbance on UPS output.


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5. 3. 3

The general advantages of digital systems are as follows: -

a) Extreme accuracy.

b) Multiple measurement in a single meter.

c) Lower installation costs as less wiring.

d) Remote monitoring possible via comms link.

e) Minimum & Maximum logging.

f) Waveform recording of transient dip/surges. (See waveform No 1)

g) Waveform capture for harmonic analysis .

Due to the competitive market, digital maters are now relatively inexpensive. They can also

be retro-fitted to existing switchboards by using existing current and voltage transformers.


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5. 3. 4

2.0 Harmonic Distortion

It must be remembered that the majority of harmonics are load generated. Alternating

voltage and current is theoretical sinusoidal in shape. However in reality this is seldom true.

When a waveform is not sinusoidal in shape it is said to be complex. Refer to figure No 2

which is typical of the waveform for a building supporting a non linear load.

Figure 2 ( IT Load)

A complex waveform can be broken down mathematically by Fourier Analysis, which

proves that any periodic function can be expressed as a series of sine waves with varying

frequencies and amplitudes. Each frequency is a multiple of the fundamental 50hz

frequency.

The extent of the harmonic distortion depends on the frequency, amplitude and phase

relationship of the harmonic, relative to the fundamental.

3.1 Even harmonics, 2nd, 4th etc. give an asymmetrical resultant across the

positive and negative cycle.

3.2 Odd harmonics do not alter the symmetry of the resultant wave. (figure 2)

3.3 Harmonic distortion generates high crest factors.There is a G5/3 document, which sets out limits on the magnitude of harmonic that can be

reflected back at the point of common coupling. The limit for 3rd harmonic current is

34amps at 415volt and 5% for voltage at 415volts. Power monitoring surveys reveal that

these limits are regularly exceeded. It is expected that the limits will be strictly enforced in

the near future.


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5. 3. 5

3.0 Personal ComputersThe personal computer is identified the most prominent source of harmonic distortion.

PCs cause power related problems due to the electronic power supply units which is a

Switch mode power supply unit (SMPS)

The switch mode power supply on computer equipment introduces harmonic distortion as it

draws current in short pulses. It can be observed that the current is pulsed at around the

peak of the voltage. This pulsing causes flat topping of the voltage. (See figure No 3 )

Figure No 3 ( Current Draw of Personal Computer)

Personal computers also generate high crest factors.


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5. 3. 6

4.0 High Crest Factors

Crest factor is defined as the ratio of peak current to rms. current. In a pure sine wave, the

crest factor is 1.4. Refer to the waveform below. On a non-linear load, the current crest

factor is pulsed much higher and values of 5.7 have been recorded.

Current with high crest factors can cause the following: -

4.1 Voltage Flat topping

Flat topping on the voltage waveform. (Refer to figure No 4 below) Electronic

equipment is susceptible to flat topping and can malfunction, as they rely on the

peak voltage to charge their power capacitor. As a result of this information and

other voltage disturbances captured, the facility managers attention was brought to

this transformer . This transformer subsequently failed.

Figure No 4 (Voltage Flat Topping)

4.3 Operation of breakers with low tolerance to transient currents

4.4` Inadvertent operation of peak acting breakers.

Peak toPeak Value

Peak ValueRms Value

Rms Value = 0.707 x Peak Value

Peak Value = 1.4 x Rms Value


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5. 3. 7

5.0 Neutral Currents

The most troublesome harmonic is the 3rd harmonic. Theoretically, on a balanced system,

the neutral should be zero. However, on a non-linear load, the individual 3rd harmonic in

each phase will return in the neutral. This is due to the summation of the in-phase 3rd

harmonics as shown in figure 2 .

This phenomena can result in a current flowing in the neutral, which may exceed the line

current. Excessive neutral currents in a system contribute to the following:-

a) Neutral to Earth voltages that create common-mode noise problems

b) Circulating currents to flow in transformer.

c) High voltage drop at loads

d) Failure of the neutral conductor.5.2 Manufacturers are now marketing a third harmonic filter that is inserted in the neutral line of

three phase systems. It is claimed, that up to 95% of the third harmonic component iseliminated. This value can not be substantiated at present against an actual IT load. Care

should be taken when specifying these filters. A harmonic analysis should be carried out

before and after the installation of such filters. The manufacturer should be committed to

achieving a benchmark reduction in harmonics.

Line 1 - N

Line 2 - N

Line 3 - N

3rd Harmonic - N

3rd Harmonic Summation


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5. 3. 8

As an example of the magnitude of 3rd harmonic currents, the three line and neutral currents

were recorded in a building as indicated in table 1 .

Table1 Neutral current measured on an actual non-linear load

Line > RedPhase YellowPhase Blue Phase Neutral

Amps> 1084amps 1131amps 947amps 1408amps

The system is relatively balanced at around 1100 amps per phase, but the neutral current is

recorded at 1400amps. This is mainly composed of the triple harmonics.

This is further validated by the waveform capture of the neutral current in figure No 5a and

5b which shows the 1500hz component.

Figure No 5A (Capture of Neutral Curent).

Figure No 5B (Harmonic Spectrum of Neutral Curent).


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5. 3. 9

6.0 Failure of Neutral Bus-Bar systems

Large third harmonic currents in the neutral conductor of rising bus-bar systems, may lead

to vibration of joints and cable terminations. If the neutral fails on a rising busbar system

serving single phase loads, Over Voltage can occur on each floor, as each single phase

board is connected across two phases as shown in figure No 6. Hence, failure of

equipment will occur. The most common problem is burnt out switch mode power supplies.

Red

phase Neutral

Yellow

phase

Blue

phaseLoad

2nd Floor

Load

1st Floor

Broken

Neutral

Rising Busbar

System

Figure No 6Rising Busbar with Failed

Neutral

Rising busbars should be checked at least annually to ensure the electrical integrity of all the

connections.


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5. 3. 10

7.0 Transformers

7.1 General

A transformer is classified by its rated power in kVA. (No load V x rated I x 3 )

However, load related losses occur in the transformer which are as follows:

7.1.1 Ohmic losses

The individual harmonic frequencies each contribute additional heating effect in a

transformer winding, i.e. (I2 r) losses. Therefore, power delivered by the transformer may

be greater than its rated power

7.1.2 Eddy current losses

Eddy currents are proportional to the square of the frequency. The additional heatgenerated can lead to instability in the core laminates.

These losses may not be included in the manufacturers load power/kVA rating of

transformers. Hence, it comes as a surprise when a transformer appears overheated when

apparently under loaded.

In addition, two phenomena can effect the correct operation of a transformer. These are:

7.1.3 Harmonic Flux

High frequencies harmonic flux which is superimposed onto the fundamental flux.Depending on the upstream impedances, these fluxes can increase the resulting peak value.

7.2 K-Rating of transformers

Transformers are now available which are rated for non-linear loads. These transformers

are given a rating prefixed with the letter K and hence are referred to as K-rated

transformers. These transformers are specifically designed to withstand a specified stress

as imposed by the harmonic distortion.

There is now a BS 7821 Part 4. 1995 for assessing the rating of transformers and thisshould be used for sizing transformers to carry non-linear loads. However, it is not user

friendly, as some of the parameter required is not easily identified. .


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5. 3. 11

7.5 Case Study

Transformers are failing and this is likely to be caused by the presence of harmonics. If the

load current is measured, it may appear that the transformer is operating within its rated

capacity. However , take for example an actual 1200kVA transformer and consider the

harmonic component measured as follows:-

Table 2 Load Recorded

1200kVA

Parameter Neutral Red Phase Yellow Phase Blue Phase

Amps 1408 1084 1131 947

Odd

Harmonics

% Harmonic

distortion

Even

Harmonics

% Harmonic

distortion

01 100 02 1.8

03 60.8 04 3.0

05 33.7 06 2.407 16.9 08 1.2

Total Harmonic Distortion 63.6% *

* Only the prominent harmonic are indicated in the table above.

These results were inserted into a K-Rating formulae provided by a major transformer

manufacturer. The results revealed that the actual rating of the transformer was only rated

at 1036 kVA at this particularload.

It is usual practice to allow for a 115% overload setting on the main LV breaker served

from a transformer. This is commonly used as the primary means of protecting the

transformer from an overload condition. This would indicate that this may not be suitable

where non-linear load are present, as the transformer could be operating in excess of its

overload capacity. This is of course assuming that there is no high temperature alarm/trip

fitted.


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5. 3. 12

8.0 ZIG-ZAG Transformer

These can be used to reduce the effects of harmonic distortion upstream of an installation

especially at the point of common coupling. The zig-zag transformer can be used upstream

which blocks the harmonic and dissipate them as heat. Field measurements carried out on

the primary and secondary winding of a zig-zag transformer is recorded on table 4

Table 4 Zig-ZAG Transformer

Transformer

(Secondary)

Transformer

(Primary)

Red

Phase

Yellow

Phase

Blue

Phase

Red

Phase

Yellow

Phase

Blue

Phase

Watts 36.1

kW

29.4

kW

26

kW

31.76kW 30.72kW 30.12k

W

Volt/Amp 35.5kVA 35.5 kVA 31.8

kVA

32.28

kVA

30.88

kVA

31.28

kVA

P.F .84 .83 .81 .97 .97 .97

Amps (rms) 151 151 135 135.6 130 129.6V.Crest Factor 1.28 1.28 1.29 1.33 1.31 1.35

A.Crest Factor 1.97 1.91 2.1 1.74 1.55 1.52

Fundamental 130 129 113 134.8 127.2 128.4

3rd Harmonic 70 70 68 5.84 7.36 5.36

The 3rd harmonics are virtually eliminated from the transformer primary.

However, zig-zag transformers block the harmonic by dissipating heat. Therefore, they

require considerable space and cooling requirements. This is rarely available in CommercialCity offices.


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5. 3. 13

9.0 System Protection

9.1 Inadvertent operation of Earth fault Protection

This is a common problem experienced in buildings. Two methods of applying earth fault

protection are :

9.1.1 Residual Type.

This type makes the vector sum of the phase and neutral currents.

Any imbalance between them is recorded as an earth fault and activates the trip circuit. This

type of protection should be avoided on non-linear loads.

If we take for example the recordings shown in table 1, the line currents are approximately

1200amps. The normal criteria for setting earth fault protection is to take a value of

between 20/30%. Now 30% of 1200amps = 300amps. However, the neutral current is

1400amps of mostly triplen harmonic current. Therefore, the earth fault protection on a

residual type earth fault protection system will operate inadvertently.

9.1.2 Source Ground Type.

This type operates directly from the signal of an external current transformer,

installed on the neutral-to-earth connection of the main source. If earth fault protection is

required, this method should be used.

9.2 Inadvertent operation of Over-current Protection

9.2.1 Rms Devices

The protective device should be true Rms. acting and not peak acting as such devices

operate on the peak values of current. One example is the tripping of a 1200A rated

MCCB with an apparent load of 800A. The MCCB was of the peak acting type, which

would have an rms. rating of 1200/07 = 840amps. However, as the load crest factor was

2.1, the resulting peak current was 800 x 2 = 1600A which resulted in the inadvertent

tripping of the breaker.

9.2.2 Fuses

Fuses have been found to open sooner than expected when subjected to harmonic currents.

J.Brozek (1). None-linear current contains both harmonics and spikes. These subject the

fuse to two excessive heating conditions. The harmonic current is a continuous overload and

the fuse runs hotter than normal. The spike causes the I2t rating of the fuse to be exceeded

and as a result it will operate. This may be interpreted mistakenly as a fuse problem.


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5. 3. 14

9.3.3 Miniature Circuit Breakers

Individual PCs draw considerably transient in-rush currents of up to 40-50amps when

switched on and tripping of final sub-circuit breakers during power dips is a common

problem.

One solution is to ensure the load is distributed in small groups, and the rating of the

breaker sufficiently sized to suit the transient load. However, in a lot of cases the incorrect

type of breaker is fitted. To overcome the problem, type 4 or type D breakers should

ideally be utilised. The designer should ensure that the Maximum Earth loop impedance

limits of the breakers are not exceeded.

9.5 Cable ratings

Harmonics impose additional stress on cable conductors and insulation material. Unless

appropriate de-rating factors are applied, the cable may fail. There have been cases where

harmonic currents cause heating in conductors greater than expected for the RMS value of

current. Due to shielding of the inner conductor by the outer layer, current is concentrated

in the outer layer. This increases the effective resistance of the conductor which increases

with frequency and conductor size. This is known as the skin effect. In addition, the

magnetic field of the conductors distorts the current distribution in adjacent conductors and

is called the Proximity effect

One commonly overlooked aspect of cable sizing is that Multi-core cables are rated forthree loaded conductors only. As large neutral current can flow due to summation of the

triple harmonics, further de-rating of the cable is required.

Careful consideration should be given to sizing cables for non-linear loads and a sensible

de-rating applied. As a minimum, the cable should be sized for a neutral size of 2/1 against

the phase conductor


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5. 3. 15

10.0 Power Factor

Power Factor is meaningless when a non-linear load is present. Harmonics create errors

and incorrect phase angles are common due to the zero crossing error.

The product (V x I) is referred to as apparent power (measured volt-amperes)

The real power is measured in watts and is generally less than the apparent power. The

power factor of the AC circuit is defined as the ratio of:

Real power/apparent power = W/VA

In a complex waveform, the supply voltage remains sinusoidal in shape. However, the

current is non-sinusoidal in shape. Therefore the real power supplied by the system is the

average of the supply voltage and the current. Refer to figure 4 where a graph of 50hz

voltage v 50 and 150hz current is plotted

V X I

@ 150 hz

V X I

@ 50 hz

Figure 4

50hz Voltage x Currentat 50hz and 150hz.

When these expressions are evaluated, the following can be observed:

10.1 Only the in-phase component of the fundamental will contribute to thereal power. Harmonics do not.

10.2 The harmonic component increases the rms. value of the current and the

VA but not the power.

Therefore, the power factor is decreased (W/VA) if one or both of the following

conditions apply:

10.3 The phase angle between the fundamental current and the supply

voltage increases.


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5. 3. 16

10.4 The current contains harmonics.

Some common problems discovered on sites are as follows:-

10.5 Leading power factors recorded on loads.

This can be misleading, as shown by closer analysis of figure No 2. The distortion on the

current causes a momentary crossing of the zero line before the voltage. This records a

leading power factor, which is incorrect. It crosses above the zero line again only to fall

again after the voltage.

Unless this is picked up visibly on waveform capture, a leading power factor is assumed on

many loads. However, it should be stated that actual leading power factors have in fact

been recorded.

However, this phase angle is only significent if considering voltages and current at the same

frequency and harmonic components should not be included. Harmonic should not be

include as the harmonic component does not contribute to real power as shown on figure 4

above.

10.6 Zero crossing errors

Most companies utilise the zero crossing method to establish phase angles. Others utilise the

voltage peak to current peak method. Both have inherent flaws. Investigations on one site,

revealed that three manufacturers equipment recorded a leading power factor and anothertwo recorded a lagging power factor.

10.7 UPS Specification

UPS systems and generators are designed to operate at a power factor of 0.8 . If the load

power factor is above or below this figure, derating is required. Designers should obtain a

harmonic spectrum of the load if possible and issue this to the UPS and Generator

manufacturer at design stage. UPS systems for non-linear loads should be specified to

operate at a minimum crest factor of 3:1.

10.8 Resonance

When a capacitor bank is connected to a distribution system, resonance can occur under

certain conditions. Reactance varies with frequency and a condition can be reached where

the capacitive reactance and the system reactance are equal. This condition is referred to

as selective resonance. If resonance occurs at, or near to the frequency of one of the

harmonics generated, large currents may flow into the capacitor bank and failure can occur.


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5. 3. 17

11.0 Screen Flicker on Computer Monitors

One commonly reported problem is one of screen flicker on computer monitors. This is

often suspected to be harmonic related. However, site surveys have revealed that magnetic

interference is the sole cause. Electronic equipment depends on a clean reliable supply to

function correctly. Harmonic distortion can, in some cases, cause screen flicker as in

addition to producing a distorted waveform, the higher frequency currents can increase the

magnetic flux around the current carrying cables.

Computer monitors are basically a Cathode Ray Tube. These use a magnetic field to

produce an image on the monitor screen. If an external magnetic field is introduced near the

monitor, it will affect the magnetic field of the Cathode Ray Tube and interference will

occur. The level of interference will vary, depending on the strength of the external field and

the quality and construction of the monitor.

A magnetic field of 0.5 - 1.0T (micro Testla.) will produce screen interference. This will

distort the image produced on the screen of a typical commercial unit. The external field,

combined with the refresh rate of the monitor combine to produce distortion of the screen

image.

Screen interference can cause discomfort and generally affcet the well being of the

operator.

11.1 Sources of Interference.

Most electrical equipment produces some degree of magnetic interference. Examples ofsuch equipment vary from fans, photocopiers, power supplies to mobile /modem /answer

phones and fluorescent lighting.

However the biggest source of interference is produced by power lines and power

distribution equipment. In a number of buildings, the main incoming services from the

supply authority were to blame. Levels of up to 8 T have been measured in offices spaces

above local authority sub-stations and incoming power cables. These are generally single

core cable. Therefore, consideration should be given to the proximity of supply authority

incoming services and single core cable in buildings where interference is experienced. In

one other case, levels of 50 T were recorded. This was traced to a trapped neutral in aunder-floor trunking system. Again, this supports the case for the use of appropriate power

monitoring systems.

11.2 Eliminating interference.

It is very difficult to totally eliminate magnetic interference unless the equipment affected is

totally enclosed by a screen. This is rarely possible. Special shields can be purchased to fit

around the monitor. These are expensive and problems can be experienced with staff who

understandably will query the reason to shield their monitor from an unknown magnetic field

with no provision put in place to protect them.


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5. 3. 18

The optimum solution is to move the source of the problem from the affected area.

However, this is sometimes not practical after a building has been occupied. Therefore,

careful consideration should be given to the presence of power cables when evaluating a

building for commercial purposes.

11.3 Health and Safety

The current evidence of any health risk from exposure to EMF is inconclusive, with no link

established. The general statement is form the supply authority is, If any such risk does in

fact exist, it is very small indeed. We are not in a position to support this argument. We

are of the opinion, that if proximity to magnetic interference can be avoided, it should be.


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12.0 Solutions

12.1 Digital meters should be specified on all new installations andre-furbishment.

12.2 The nameplate power ratings of computer equipment should be ignored.

Independent information should be obtained regarding power rating of

computer equipment by direct measurement.

12.3 The incoming switch mode power supply on equipment should be improved to reduce

harmonic distortion.

12.4 Provide if possible, separate neutral for each phase of a rising bus-bar system.

12.5 Consideration should be given to Over voltage protection/monitoring on rising busbars in

high rise commercial buildings..

12.6 Consideration should be given to the installation dedicated 3rd harmonic filters on the

neutral line.

12.7 Zig-Zag transformers should be installed as a last resort.12.8 Type D miniature breakers should be installed on final circuits serving comouter equipment.

12.9 The presence of single core power cables and sub-stations should be identified in buildingsand their proximity to computer equipment.

12.10 A load profile including harmonic analysis should be carried out on power transformerserving non-linear loads and the appropriate de-rating factor applied, to ensure they are

operating within their rated capacity

Bibliography

1.0

J.P. Brozek. The effects of harmonics on over-current protection devices. Proc.1990 1A51990 PP1965-1967

Power Monitoring in the Modern Building and Harmonic Problems

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