Standards of Power Quality with reference to the Code of

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Standards of Power Quality with reference to the Code of

Transcript Of Standards of Power Quality with reference to the Code of

Standards of Power Quality with reference to the Code of Practice for Energy Efficiency of Electrical Installations
Ir. Martin WU Kwok-tin, Energy Efficiency Office, Electrical & Mechanical Services Department September 2003
Abstract
The Code of Practice for Energy Efficiency of Electrical Installations (Electrical Energy Code) developed under a dedicated Task Force of the Energy Advisory Committee (EnAC) was completed in May 1998. The Electrical Energy Code forms part of the comprehensive building energy codes established for Hong Kong. The Code sets out the minimum requirements to achieve energy efficiency design for electrical installations on distribution losses, utilisation losses, power quality, metering facilities, etc. The main objectives of the Electrical Energy Code are as follows:
• to enhance energy efficiency in electrical installation design for buildings; • to reduce losses, conserve energy, save money and minimise impact to local and global
environment; • to complement existing safety standards; and • to supplement requirements of other Energy Codes (e.g. A/C, Lighting and Lift &
Escalator) and help to improve power quality.
This paper focuses on the energy issues in relation to the power quality problems in the power distribution systems of buildings and describes the proposed standards and requirements of power quality as set out in the Electrical Energy Code.
1. Introduction
The Electrical Energy Code applies to all fixed electrical installations for all types of buildings except, emergency systems, small domestic houses, buildings with total installed capacity of 100A or less, single or three phases, at nominal low voltage, and buildings used solely for public utility services.
The general approaches used for the Electrical Energy Code are as follows:
a) To set out the minimum requirements for achieving energy efficient design of electrical installations in buildings without sacrificing the relevant safety and health regulations.
b) To minimise copper losses in the complete power distribution systems in buildings. c) To reduce equipment losses and energy wastage in the utilisation of electrical energy. d) To reduce all associate losses and inefficient use of electrical energy related to power
quality problems in buildings. e) To introduce appropriate metering and monitoring facilities to carry out future energy
audit and building management works.
As far as energy efficiency is concerned in a building power distribution system, the two dominant factors in power quality are its harmonic distortion and unbalanced distortion. Harmonic currents will generate additional heat in conductors due to skin and proximity
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effects, causing accelerated cable ageing and insulation breakdown. Unbalanced distortion in three-phase supply voltages will create negative sequence component causing additional power losses in conductors and motors. Both distortions will add undesirable currents and voltage drop in neutral conductors.

2. Harmonic Distortion

2.1 Requirements for Maximum Total Harmonic Distortion (THD) of Current

Clause 6.1 of the Code requires that the total harmonic distortion (THD) of current for any circuit should not exceed the appropriate figures in Table 6.1. According to the quantity and nature of the known non-linear equipment to be installed in the building, design calculations are required to demonstrate sufficient provision of appropriate harmonic reduction devices to restrict harmonic currents of the non-linear loads at the harmonic sources, such that the maximum THD of circuit currents, at rated load conditions, shall be limited to those figures as shown in Table 6.1 below.

Table 6.1: Maximum THD of current in percentage of fundamental

Circuit Current at Rated Load Condition ‘I’ at 380V/220V
I<40A 40A£I<400A 400A£I<800A 800A£I<2000A
I‡2000A

Maximum Total Harmonic Distortion (THD) of Current 20.0% 15.0% 12.0% 8.0% 5.0%

In case of motor circuits using Variable Speed Drives (VSD), group compensation at the submain panel or Motor Control Centre (MCC) is allowed, provided that the maximum allowable fifth harmonic current distortion at the VSD input terminals during operation within the variable speed range is less than 35%.

If the quantity and nature of non-linear equipment to be installed in the building cannot be assessed initially, appropriate harmonic reduction devices shall be provided at a later date after occupation.
2.2 Harmonic-Related Loss Mechanisms in Power Wiring

The problems associated with the present of harmonics on power distribution systems are not just the power quality problems but also affect the energy efficiency of the system. Typical problems include overheating transformers, motors, phase and neutral conductors, causing unacceptable neutral-to-earth voltage, voltage distortion, electromagnetic interference (EMI), capacitor bank failure, etc.

Many of the problems are related to the proliferation of non-linear loads such as variable speed motor drives, rectifiers for direct-current power supplies, electronic ballasts in energy efficient lighting and switch-mode power supplies in computers and other electronic office equipment.

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Figure 1 below shows a typical current waveform of a personal computer with a total current harmonic distortion of 130%.

Current (A)

Fig. 1 Typical Harmonic Current of a PC with THDI=130% Irms=1.64A dpf=1 tpf=0.6

6

4

2

0

-2 0

0.005

0.01

0.015

0.02

-4

-6 Time (s)

Figure 2 shows a 3-phase 4-wire small power distribution system for a typical modern office floor with personal computers and other office machinery. It is noted that the system consists of large triplen harmonic currents and high neutral current.

Phase Current (A)

Fig.2 Distored Phase Currents (I1=100A, I3=50A, I5=30A & I7=15A) A Typical Modern Office Floor with PC's

300
200 100
0 0
-100 -200
-300

90 180 270 360 450 540 630 720 Phase Angle

Red Phase Yellow Phase Blue Phase Neutral

Another example is the installations of compact fluorescent luminaires with integrated electronic ballasts or linear fluorescent lamps with low-cost electronic ballasts (i.e. old design without effective harmonic filters). Figure 3 shows a typical waveform of one phase and neutral currents of such lighting installations.

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Fig. 3 Typical Phase & Neutral Harmonic Currents for Fluorescent Lighting with Electronic Ballasts

Current (A)

0

0.005

0.01

0.015

0.02

0.025

Red Phase Neutral

Time (s)
Figure 4 shows the current waveform of one phase of a typical variable speed drive system using pulse width modulation (PWM) type inverter. The total harmonic distortion is very high and is well over 80%.

Current (A)

Fig.4 Current Waveform of VSD loads I1=100A, I5=70A, I7=50A & I11=14A (Irms=133A THD=87%)

300 200
100
0 0
-100
-200 -300

0.005

0.01

0.015

0.02

Time (s)

Electronic equipment nowadays tends to be distributed in the building on various final circuits and socket outlets rather than centralised in one area as in a computer room where special power provisions (e.g. UPS system) are made. Most of the losses associated with harmonics are in the building wiring circuits. Harmonic distortion is serious at the terminals of the non-linear loads, but tends to be diluted when combined with linear loads at points upstream in the system.

The total harmonic distortion (THD) is defined by

�¥

(Ih ) 2

THD = h=2

(1)

I1

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where Ih is the rms current of the hth harmonic current, and I1 is the rms value of the fundamental current. A typical supply voltage waveform at a consumer’s metering point (or point of common coupling) normally doesn’t exceed 5% THD in Hong Kong but for some high-rise commercial buildings, the voltage THD exceeding 10% is not uncommon especially at those higher level floors fed with a common rising mains. The third harmonic is normally the most prominent component (zero sequence), resulting in high neutral current flow in the neutral conductors of a power distribution system. The adverse effects of high neutral current will be addition energy losses, overcurrent and additional voltage drop causing undesirable high neutral to earth voltage and low phase to neutral voltage.
For electronic appliances that are retrofitted to comply with the other energy codes and save energy, such as electronic ballasts, VSDs, VVVF lift drive system etc., an important point needs to be considered is how much of the energy savings must not be diminished by added harmonic losses in the power system.
2.2.1 Cables
The only cable power loss component is I2R, where I could be increased by the harmonic distortion, and the R value is determined by its dc value plus ac skin and proximity effects. The rms value including harmonic currents is defined by:

� ¥

I =

Ih2 =

I 12

+

I 22

+

I

2 3

+

.

..

.

..

.

(2)

h=1

Manipulating (1) and (2) yields the total rms current in

I = I1 1 + THD2

(3)

Equation (3) indicates that, without harmonics, the total rms current is simply the value of the fundamental component. For the above PC example, with 130% THD, the total current is nearly 64% higher than the fundamental current.

Taking into account the frequency-related effects, a ratio of ac to dc resistance, kc, can be defined as

k = Rac = 1 + y + y

(4)

c Rdc

s

p

Where ys is the resistance gain due to skin effect, and yp is the resistance gain due to proximity effect.

The resistance gain due to skin and proximity effects for multicore cables, as a function of frequency, conductor diameter and spacing of cores, can be assessed from the formula and information given in IEC287-1-1 “Current rating equations and calculation of losses".

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Consider three different sized cables: 10mm2, 150mm2 and 400mm2 4-core PVC/SWA/PVC
cables, typically used in a building power distribution system. Their ac/dc resistance ratios at
different frequencies can be calculated according to IEC287-1-1 shown in Fig. 5 below. Series 1, 2 and 3 indicate the variation of resistance with frequencies for 10mm2 cable, 150mm2 cable and 400mm2 cable respectively. It is noted that for smaller-sized cable, the effects of skin and proximity is small for the 3rd and 5th harmonics which are normally
dominated in the power distribution system of a building.

Rac/Rdc

Fig. 5 Cable ac/dc resistance ratios as a function of harmonic frequencies

3 2.5
2 1.5
1 0.5
0 0

400mm2 150mm2
10mm2

5

10

15

20

25

Harmonic Number

Series1

Series2

Series3

2.2.2 Transformers

Most of the distribution transformers in Hong Kong are provided by the two power supply companies and all these transformer losses are therefore absorbed by the power companies. Harmonics produce extra losses in transformers and these costs could not be recovered from their consumers. Both CLP and HEC have been considering to specify requirements that the consumer must comply with in order to limit the magnitudes of harmonic distortion at the consumer’s metering point.

Transformer loss components include no-load (PNL) and load-related loss (PLL). The load loss, as a function of load current, can be divided into I2R (PR) loss and stray losses. The stray losses are caused by eddy-currents that produce stray electromagnetic flux in the windings, core, core clamps, magnetic shield and other parts of the transformer. For harmonic-rich currents, the eddy-current loss (PEC) in the windings is the most dominant loss component.

PLoss = PNL + PR + PEC

(5)

For non-linear load currents, the total rms current can be obtained by (2) and (3), and the power loss can be obtained by the sum of the squares of the fundamental and harmonic currents, as shown in (6)

� ¥

PR = Ih2 Rh

(6)

h= 1

The winding eddy current loss in transformers increases proportional to the square of the

product of harmonic current and its corresponding frequency. Given the winding eddy current

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loss at the fundamental frequency as PEC1, the approximate total eddy current losses including harmonic frequency components can be calculated by

� ¥

PEC = PEC1 I h2h2

(7)

h=1

2.3 Other Electrical Equipment in the Building

Other equipment that may be affected by harmonics includes protective devices, computers, motors, capacitors, reactors, relays, metering instrument, emergency generators, etc. The major harmonic effects to these equipment include performance degradation, increased losses and heating, reduced life, and possible resonance. For motor and relays, the primary loss mechanism is the negative sequence harmonic voltage (e.g. 5th and 11th order) that is present at the terminals of the equipment.

3. Unbalanced Distortion

3.1 Requirement for Unbalanced Distortion

Clause 6.2 of the Electrical Energy Code requests that all single-phase loads, especially those with non-linear characteristics, in an electrical installation with a three-phase supply should be evenly and reasonably distributed among the phases. Such provisions are required to be demonstrated in the design for all three-phase 4-wire circuits exceeding 100A with singlephase loads.

The maximum unbalanced single-phase load distribution, in term of percentage current unbalance shall not exceed 10%. The percentage current unbalance can be determined by the following expression:

Iu = (Id · 100) / Ia

(8)

Where Iu = percentage current unbalance Id = maximum current deviation from the average current Ia = average current among three phases

3.2 Effect on Neutral Conductors

The connection of single-phase loads of different characteristics and power consumption to the three-phase power supply system will result in unequal currents flowing in the threephase power circuits and unbalanced phase voltages at the power supply point, i.e. unbalanced distortion.

The adverse effects of unbalanced distortion on the power distribution system include: i) addition power losses and voltage drop in the neutral conductors ii) causing unbalanced 3-phase voltages in the power distribution system iii) reduced forward operating torque and overheating of induction motors iv) excessive electromagnetic interference (EMI) to sensitive equipment in buildings v) additional error in power system measurement

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All single-phase loads are potential sources of unbalanced distortion. They should be carefully planned at design stage for balancing, even though the random connection and operation of large number of small rating single-phase loads on the final circuits will tend to cancel their unbalance distortion effect in the main and sub-main circuits.
A 10% unbalanced phase current in a 3-phase 4-wire power distribution system with an average phase current of 100A (Fig. 6) would produce a neutral current of about 17A and increase the total copper loss by about 1%. The combination effect of 10% unbalanced and 30% THD phase currents (Fig. 7) on the same circuit would produce a neutral current almost the same magnitude as the phase current resulting in much higher losses in a 3-phase 4-wire power distribution system.

Phase Current (A)

Fig.6 Unbalanced Phase Currents (110A,100A & 90A) (10% unbalanced)

200

150 100
50 0
-50 0 -100

90 180 270 360 450 540 630 720 810

Red Phase Yellow Phase Blue Phase Neutral

-150 -200

Phase Angle

Phase Current (A)

Fig. 7 Unbalanced & Distorted Phase Currents (110A, 100A & 90A with I3=30A) (10% unbalanced & 30% THD)

250 200 150 100
50 0
-50 0 -100 -150 -200 -250

90 180 270 360 450 540 630 720 810 Phase Angle

Red Phase Yellow Phase Blue Phase Neutral

3.3 Effect on AC Motor Operation
Voltage level variation and unbalanced voltage caused by unbalanced distortion of singlephase loads are some of the voltage deviations which can affect motor operating cost and reliability. The published 3-phase induction motor characteristics are based on perfect

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balanced voltages between phases. Overheating (additional loss) and reduction in output torque are serious ill effects caused by operation of induction motors on unbalanced voltages. The magnitude of these ill effects is directly related to the degree of voltage unbalance.
The adverse effects of unbalanced voltage on 3-phase induction motor operation come from the fact that the unbalanced voltage break down into the positive sequence component and the opposing negative sequence component.
The positive sequence component produces the wanted positive torque. This torque is generally of less magnitude than the normal torque output from a balanced voltage supply and with somewhat higher than normal motor losses, because the positive sequence voltage is usually lower than rated voltage. The negative sequence component produces a negative torque which is not required. All the motor power that produces this torque goes directly into the loss that must be absorbed by the motor. By increasing the amount of unbalanced voltage, the positive sequence voltage decreases and the negative sequence voltage increases. Both of these changes are detrimental to the successful operation of motor. Positive (E+ve) and negative (E-ve) sequence voltages can be calculated by the symmetrical components relationship as (9) and (10)
E +ve = 31 ( E R + aEY + a 2 E B ) (9)
E -ve = 13 ( E R + a 2 EY + aE B ) (10)
Where ER, EY and EB are the original unbalanced voltages for red, yellow and blue phases and a=-1/2+jY3/2.
The application of negative sequence voltage to the terminal of a 3-phase machine produces a flux which rotates in the opposite direction to that produced by positive sequence voltage. Thus, at synchronous speed, voltages and currents are induced in the rotor at twice the line frequency. The application of negative sequence voltage can therefore affect torque, stator and rotor copper losses, rotor iron losses and consequently machine overheating. It is interested to note that harmonic voltages of the 5th, 11th, 17th, etc. order are also negative sequence and would produce similar adverse effect as unbalanced voltages.
4. Requirements for Metering & Monitoring Facilities
The last category of requirement of the Electrical Energy Code is the metering and monitoring of various electrical characteristics and energy consumption to facilitate future work on energy audit and building management.
The first requirement is that all main circuits exceeding 400A rating must be incorporated with meters or metering facilities to measure all voltages, currents including neutral, power factor, maximum demand in kVA and energy consumption in kWh.
The other requirement is that all sub-main distribution and dedicated feeder circuits exceeding 200A rating must be complete with meters or metering facilities to measure phase and neutral currents and energy consumption.
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The advanced power monitoring instrument available nowadays can be used for metering, power quality analysis, energy management and supervisory control for power distribution systems. The instrument can be linked into the building management system of the building as one element in an energy management network. Selection for applying the most beneficial tariff system could also be analysed by the instrument from the logged data of energy consumption and load profile of the building.

5. Methodology for Energy Efficient Design of Circuits with Harmonics

5.1 Conventional Cable Sizing Procedure

The majority of electrical engineers base their design and specification calculations on the assumption that the voltages and currents in power distribution systems are sinusoidal and perfectly balanced.

For conventional cable sizing method, the relationship among circuit design current (Ib), nominal rating of protective device (In) and effective current-carrying capacity of conductor (Iz ) for an electrical circuit can be expressed as follows:

Co-ordination among Ib, In & Iz : Ib £ In £ Iz

Calculated minimum tabulated value of current : It(min) =In · C1 · C1 · C1

a

g

i

Effective current-carrying capacity : Iz = It x Ca x Cg x Ci

where

It = the value of current tabulated in Appendix 4 of BS7671:1992 - The Regulations for Electrical Installations
Ca = Correction factor for ambient temperature Cg = Correction factor for grouping Ci = Correction factor for thermal insulation

This conventional cable sizing technique needs co-ordination among the design current, nominal rating of a protective device and the effective current-carrying capacity of conductors of a circuit. All relevant correction factors, such as grouping, ambient temperature, thermal insulation, etc. should then be applied to determine an appropriate cable sizing of a circuit to fulfil the safety requirement.

Although the correction factors applied in cable sizing have the effect of using larger cable size and eventually reduce the copper loss, the actual calculation work to fulfil both safety and energy requirements are very tedious. In order to simplify the calculation work, a second approach based on the maximum conductor resistance could be used instead.

5.2 Energy Efficient Method for Cable Sizing

This energy efficiency method for cable sizing requires the calculation of the maximum allowable conductor resistance based on the maximum copper loss requirement as stipulated in the code.

For a 3-phase 4-wire circuit (assuming balanced, linear or non-linear): Active power transmitted via the circuit conductors, P = 3U LI1 cosq

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PhaseDistortionCurrentsLossesBuildings