POWER QUALITY AND NEUTRAL CURRENT PROBLEMS FROM UNBALANCED

AND NON-LINEAR LOADS IN THREE-PHASE POWER SYSTEMS

Omer GUL Mehmet BAYRAK

Faculty of Electrical and Electronic Engineering Instute of Science and Technology
Istanbul Technical University, Maslak, Istanbul, 80626, Istanbul Technical University, Maslak, Istanbul, Turkey
Turkey ,Tel:+90 212 2856709, Fax:+90 212 2856700, 80626Tel:+90 212 2856709, Fax:+90 212 2856700
E-mail:gul@elk.itu.edu.tr

ABSTRACT errors, loss of memory, system lockout)of customer

equipment.”
An increasing percentage of building load consists of
electronic equipment supplied by switched-mode power Commercial and residential building applications, the
supplies, electronic and magnetic ballasts, on line UPS harmonic producing loads( adjustable-speed motor drives,
systems, adjustable-speed drives, harmonic distortion switched-mode power supplies, refrigerator, clothes dryer,
problems in commercial building distribution system is color television, laser printer, electronic and magnetic
becoming increasingly. Whenever three-phase, four-wire ballasts, on line UPS systems, and shunt capacitors for
power systems are used to supply power to computer power factor correction to reduce losses) are used single-
systems, office equipment and other similar sensitive phase which typically results in high current distortion and
electronic loads, the power system design should allow for a significant third-harmonic component commonly[1,2].
the possibility of high harmonic neutral current to avoid
potential problems. This paper discuss power quality There are many variation on the types of power quality
problems associated with high harmonic neutral currents problems that can occur, and the most significant power
including overloaded transformers, overloaded neutral quality issues are transients, long-duration voltage
conductor, voltage distortion, and common mode noise. variations (overvoltage, undervoltage, sustained
interruptions), short-duration Voltage Variations
(Interruption, Sags, Surge), voltage imbalance, power
INTRODUCTION frequency variations: Waveform distortion, noise[2].

In recent years, concern over the quality of electric power

has been increasing rapidly since poor electric power NEUTRAL CURRENT PROBLEMS
quality causes many problems for the affected loads, such IN THREE-PHASE POWER SYSTEMS
as malfunctions, short life time and so on. Today, load
equipment is more sensitive to power quality variations On three-phase power systems, neutral current is the vector
than equipment applied in the past. Many new load devices sum of three line-to-neutral currents. With balanced, three-
contain microprocessor-based controls and power phase, linear currents, which consist of sine waves spaced
electronic devices that are sensitive to many types of 120 electrical degrees apart, the sum at any instant in time
disturbances[1]. is zero, and so there is no neutral current. In most three-
phase power systems supplying single-phase loads, there
The term “power quality” has been used to describe the will be some phase current imbalance and some neutral
variation of the voltage, current and frequency on the current. Small neutral current resulting from slightly
power system. Most power system equipment has been unbalanced loads do not cause problems for typical
able to operate successfully with relatively wide variations building power distribution systems. There are conditions
of these three parameters. However, within the last five to where even perfectly balanced single-phase loads can result
ten years a large amount of sensitive electronic in significant neutral currents. Nonlinear loads, such as
equipment(Answering machines, Electronic cash registers, rectifiers and power supplies, have phase currents which
Electronic Clocks, Hospital equipment, Personal and are not sinusoidal. The vector sum of balanced,
Mainframe computers, Process control, Robotics and nonsinusoidal, three-phase currents does not necessarily
automation, Security Systems, Telecommunications) has equal to zero[3-7] .
been added to the power system which is not so tolerant of
these variations [2] . In three-phase circuits, the triple harmonic neutral
currents(third, sixth, ninth, etc.) add instead of cancel.
Power quality problem refers to “any power problem Being three times the fundamental power frequency and
manifested in voltage, current or frequency deviation that spaced in time by 120 electrical degrees based on the
results in the failure(equipment failure, power supply fundamental power frequency, the triple harmonic currents
problems, etc.), or misoperation (Automatic resets, data are in phase with each other, and so add in the neutral
circuit. High level of third order, zero sequence harmonics ON-SITE MEASUREMENTS OF VOLTAGE,
are principally the by-product of switch mode power CURRENT, POWER COMPONENTS IN
supply technology which is used in modern office INSTALLATIONS
equipment and lighting systems, and in virtually all other
low power electronic devices. High neutral currents in This paper deals with the neutral harmonic currents based
power systems can cause overloaded power feeders, problems in an electrical distribution system of a 6 floored
overloaded transformers, voltage distortion, and common building. There are a lot of offices in this building one of
mode noise[8-10]. which is a bank office. For this reason almost all the loads
in this building are harmonic sources. Buildings' power
Three-phase, four-wire building power feeders are often demand is supplied by a 34.5/0.4 kV 630 kVA Delta/Star
sized based on three current-carrying conductors in a (grounded) connected power transformer. Central
conduit. When the neutral conductor carries harmonic compensation is valid but no measures are taken for
currents, additional heat is generated and the capacity of harmonics.
the power feeder is reduced. With four current-carrying
conductors, the capacity of the power feeder must be
dreaded to 80% of the three-current-carrying-conductor. Main Distribution Sub-panel
Neutral conductors, can still be overloaded since the Board Board
neutral current can exceed the rated phase current. 0.4kV 0.4kV
1a
Because of the power supplies which produce the harmonic 34.5kV 1 1b
neutral currents have high peak-to-rms. current waveforms, 1c
the voltage waveform can become distorted. “Flat- 2 1d
tapping” of the waveform can result due to the impedance
of the power system at the harmonic current frequencies. 3
Since the power supplies use the peak voltage of the sine
wave to keep the capacitors at full charge, reduction in the
peak voltage appear as low voltage to the power supply,
even though the rms. of the voltage may be normal. The
wave form distortion can also cause additional heating in
motors and other magnetic devices which are operated 10
from the same(distorted) voltage source[4,6,7].

One form of common mode noise in three-phase power

systems is the voltage difference between neutral and
ground. With high harmonic neutral currents, the
impedance of the neutral conductors at the harmonic Figure 1 Installation of building
frequencies can cause significant neutral conductor voltage 1)Bank Office
drops. The neutral conductor voltage drop appears as 2)on-line UPS
common mode noise to the computer system. The effect of 3) Other loads
these relatively low-frequency common-mode noise 4)1.office
voltages on the computer system is some What debatable, 5)2.office
yet computer vendor specifications typically call for loss 6)3.office
than 0.5-3V rms., neutral to ground, regardless of 7)4.office
frequency. 8)5.office
9)6.office
In order to develop an engineered solution to this power 10)7.office
quality problem, it is usually necessary to measure the
harmonics using a harmonics analyzer. These Three different measurements were done in buildings'
measurements will provide detailed information on the full electrical distribution system. First one is main distribution
spectrum of harmonic currents and voltages. It is best to board, second one is all sub-panels and the third one is the
start monitoring as close as possible to the sensitive first sub-panels' 1b feeder (bank office entrance floor).
equipment being affected by power quality variation. In
order to analyze power quality problems using First measurement(Table 1), all phase and neutral current
measurements, it is important to be able to correlate the harmonics of 1b feeder of first sub-panel supplying the
characteristics of a distortion with possible cause of bank offices' entrance floor was measured. Some important
distortion[11]. data are obtained for the analysis on harmonic based power
loses, overloading and power quality by determining the
bank offices' phase and neutral current harmonics.
Table 1,Magnitute and %THD of Phase, Neutral currents, power factor, active and reactive power measurement
1th. floor bank offýce (1a. feeder). devices. Also transformers' and other devices overload
conditions are searched.
A,B,C phases and neutral currents
Phase A Phase B Phase C Neutral
Mag THD Mag THD Mag THD Mag THD
dc 0,17 0,30 0,34 0,53 0,29 0,59 0,28 1,36
CONCLUSIONS
1 55,5 99,2 63,5 99,6 48,0 98,5 20,4 96,5
3 3,31 5,92 3,65 5,73 4,40 9,03 4,33 20,5 There-phase four-wire power systems are used to supply
5 3,85 6,88 2,58 4,05 5,15 10,5 0,74 3,5 power to computer systems, modern office equipment,
7 2,46 4,39 1,28 2,01 3,32 6,81 0,46 2,16 other similar electronic loads in commercial building, the
9 2,01 3,6 0,98 1,53 2,59 5,31 2,85 13,5 power system design should allow for the possibility of
11 1,76 3,15 0,91 1,42 1,93 3,96 0,38 1,77
13 1,50 2,68 0,72 1,13 1,38 2,82 0,19 0,92
high harmonic neutral current to avoid potential problems.
15 1,19 2,13 0,61 0,95 0,92 1,89 1,39 6,66 solution to the overloaded neutral current conductor
17 0,84 1,51 0,43 0,67 0,49 1,0 0,08 0,38 problem include the following
19 0,49 0,87 0,24 0,38 0,12 0,24 0,17 0,83
21 0,24 0,44 0,13 0,21 0,06 0,13 0,31 1,45 It is seen that harmonics must be taken into consideration
in planning of a building electrical installation.
Second measurement(Table 2) is to determine the neutral- Measurements in every sub-panel show that in spite of its
ground voltages which is problem for sensitive electronic expense, compensation systems designed as a filter in
devices. And it is also to determine the neutral currents, every sub-panel is the best solution for energy loses,
which causes neutral-ground voltages and overload overloading and power quality. At least a filter can be used
conditions of neutral line for the triplen harmonics on the neutral line to reduce the
neutral-ground voltage, overloading of neutral line and
power loses on neutral line. If there is no compensation
Table 2 Magnitute and%THD of Neutral currents and system in every sub-panel, central compensation system
Neutr-ground voltages in sub-panels must be designed as a filter. So that, transformer and other
Sub-panel
devices should not be overloaded and measurement faults
Neutral Currents Neutral-Ground Voltages can be reduced. Also buildings' 34.5 KV distribution
No Magnitute %THD Magnitute(volts) systems' adverse effects to power quality should be
Ampers eliminated to satisfy the standards[12].
1 63,13 27,90 8,7
2 - - -
3 18,2 23 2,9
One proposed method of dealing with the potentially high
4 19,8 61,5 3,1 neutral currents involves using full-sized neutral wiring and
6 12,9 49,6 1,82 monitoring the neutral current. All neutral components,
7 23,1 40,7 3,6 including neutral terminals, and neutral busburs, should be
8 36,2 64,3 5,4 sized for the additional neutral current. The loading on the
9 16,5 51 2,4
10 24,2 43,5 3,63
three phases should be kept as balanced as possible. In this
way, neutral current in excess of the triple harmonic current
is minimized. A separate neutral conductor for each phase
Table 3 Measurements of main Distribution panel. in the three-phase circuit that serves single-phase nonlinear
loads should be run, increasing neutral conductor rating,
630kVA, 36/0,4kV,Delta/star(g) Transformer double neutral conductor,
Currents
A B C N The loading of the transformer with nonlinear loads
RMS 765,6 654,8 853,4 210,7 requires additional considerations. Most transformers are
Peak 1316,8 995,3 1424 363,2 designed for linear loads with a peak current of 1.414 times
rms. current. With nonlinear loads, the maximum loading
DC Offset -0,3 -0,28 -0,31 -0,27
of transformer should be reduced to less than nameplate
Crest 1,64 1,52 1,67 1,72
capacity tip to avoid overheating the transformer and avoid
THD Rms 33,7 28,2 36,9 66,99 causing excessive output voltage distortion. It should be
THD Fund 42,5 34,7 46,2 90,23 noted that K-factor transformers are now available, with
K Factor 3,25 2,92 3,49 5,39 winding and a magnetic structure which is specifically
Active Power 347kW designed to reduce the heating effects of harmonic load
Appearen Powert 526kVA current distortion. zig zag transformer on the load side of
Nonactive Power 395kVAR the affected neutral conductor.
Total PF 0,66
DPF 0,92 Parallel connected third harmonic filter on the load side of
the affected neutral conductor. Series filter to block third
Third measurement(Table 3) is aimed to determine the harmonic current in the
correctness of measurements of various current, voltage,
ACKNOWLEDGMENT
The authors thank Istanbul Technical University(ITU)
Institute of Science and Technology for the financial
support.

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