Docstoc
EXCLUSIVE OFFER FOR DOCSTOC USERS
Try the all-new QuickBooks Online for FREE.  No credit card required.

PQ CASE HISTORIES

Document Sample
PQ CASE HISTORIES Powered By Docstoc
					                    Power Quality Case Histories
                                    N. G. Foster
                             Technical Services Manager
                               Power Quality Services
                             East Midlands Electricity plc.



        . .
Case Hlstonas Provide F             m
As the advantages of information technology and electronic control are increasingly
exploited by both manufkcturer and consumer, there has been, and will continue to be,
an increasing number of power quality concerns for electricity suppliers, equipment
manufacturers and customers. This situation has been recognised at East Midlands
Electricity plc. (EME), with the formation of a specialist technical team called Power
Quality Services (PQS). PQS’ main am is to deal with the growing concern of
                                      i
EME’s customers on power quality issues, by:

   Investigating and solving customer problems.
   Informing and advising customers on potential power quality problems, and how
   to avoid them.
   Improving EME’s response to customers by keeping its staff informed and up to
   date on power quality issues.

This paper presents various case histories of investigationsthat PQS have been
involved with over the past few years.

The most valuable tool for an engineer involved in investigating a power quality
problem is experience. The sheer breadth of the power quality field demands an
intuitive approach, which is based largely upon experience. At PQS engineers deal
specifically with power quality investigations, and are often called upon to apply the
lessons learnt in previous investigations. In order to share these experiences, a system
of circulating reports and newsletters is used within PQS and EME. The most
important investigations are included, as case studies, in a power quality training
pragramme developed at PQS, for use by the PQS team and other EME staff.




An industrial customer suspected that a conducted disturbance was causing the images
on various visual display units (VDUs), within the facility, to jitter or wobble. The
customer was sure it w s a supply problem, because it w s a new installation and the
                      a                                 a
wiring had passed all the required inspection tests, and the computer screens worked
perfectly at his home. From previous experiences of this type of problem, PQS
suspected the cause to be a radiated magnetic field. Site investigationsrevealed that
the live and neutral conductors feeding tbe ffowecent lamps, running down the centre
of the computer room, had been installed separately down either side of the length of
the room; coming together dong the centre ofthe room, at the lamps fittings. The
magnetic field, caused by the current flowing through the conductors, was strong
enough (1-2pTesla) to disturb the image on the VDUs. The answer was simply to run
both the live and neutral conductors together (general good practice); thereby,
virtually illiminating the magnetic field.

In another case concerning VDU jitter, a three year long battle between a Law frim
and a Radio Station was resolved in a matter of minutes of attending the site to cany
out a harmonics survey. The Law frnn leased several floors of their unused office
block to the Radio Station. Shortly after they moved in, the computer monitors i one
                                                                                 n
of the Law firm’s offices began to jitter, causing the operators to complain of
headaches. Then there began a three year battle between the two companies as to
what and who were responsible for the jitter. Various studies to check for radio
interference were solicited, along with checks on the computers and the building
wiring. As part of the ongoing electrical tests, EME were requested to carry out a
harmonics survey of the supply to the office block. It was
readily apparent, upon visiting the site, and from having observed the same




Using the VDU as a magnetic field strength indzator, the source of the field was
located to a cupboard at one end of the room. Inthe cupboard was the main intake
panel and distribution panels for the office block. It appeared that the increase in load
current used by the new occupants, had increased the ambient magnetic field strength
to a level that affected the monitors. In this case, the solution w& to move the VDUs
into another office. However, there are other solutions to this type of problem that
can be considered:

    Screen the VDUs with magnetic shields (commercially available).
e   Use alternative display technology such as plasma, liquid crystal, etc.
e   Specify VDUs for high magnetic field environment.
0   Screen the source of magnetic fields.
e   Plan building layout and occupancy to avoid sitting VDUs near magnetic sources.

The choice of solution depends very much on the individual circumstances
encountered. The most cost effective approach is prevention by good plarmhg.
    Casemtorv 2. F                                                             V
    Flicker is the term used for perceptible variations in illumination intensity fiom light
    sources (lamps). Certain levels of flicker are very annoying to customers. In order to
    avoid problems arising, careful consideration is given when customers (typically
    industrial) apply to connect flicker producing loads such as welders, arc furnaces, and
    rapidly varing motor loads. Most flicker complaints arise when small business and
    domestic customers connect welders and motors without seeking advice fiom their
    electricity supplier.
    A domestic customer was complaining about flicker, which was likely produced by
.   electrical equipment operating at a nearby farm. The farm had recently been
    converted to several small industrial units. A UIE Flicker meter was installed at the
    customer's premises to establish the validity of the complaint. A long term flicker
    severity level (Plt) in excess of 0.8 would indicate a valid complaint. A level of 1 Plt
    is the level at which 50% of observers would percive lamp (60w incandescent) flicker.
    The following results were obtained:



           I                          Mee8ured Flicker Levsl. 24 hour Period




                   12   +
               k    1 1




           I
                        Figure 1;Long Term Severity Level (PlQ Recommended
                                     maximum limit o 0.8 Plt
                                                     f

    The results established that the Ievels of flicker exceeded acceptable limits, and
    investigations were planned for monitoring the supply at the suspected industrial unit.
    However, several days before the monitoring was scheduled, the complainant rang to
    inform PQS that the owner of the industrial unit had been arrested by the police, and
    that the flicker had stopped. Apparently, the industrial unit was being used to process
    stolen cars; the flicker is caused by a welder.
        stow 3: Disturb=       lmd

Occasionally, the Company receives compfairrts fiom domestic customers of
running fast. The majority of these complaints are found to res
operation of triacs in the photocells of street lights. The failure
has been found to cause repetitive transients on the supply voltage of the local
distribution network (figures 3 and 4).




The initial transient (in figure 2) occurred at the time the street lamp was switched on.
The transients that follow are a result of the triac switching on and off rapidly, causing
a fast rate of change in dv/dt across the power factor correction capacitor, located on
the load side of the triac. This causes a large impulse of current to be drawn through
the impedance of the supply cables. This current impulse causes an associated high
frequency oscillatory voltage transient to appear across the impedance of the supply
cables. It is thought that the clocks, connected to the same phase, advance in time by
the introduction of extra zero crossing points. Some clocks use zero crossings ofthe
supply voltage as a time base, i.e. 100 zero crossings (50Hz)    equal 1 second. Once a
triac has gone into this particular failure mode, any number of transients can be
present on each half cycle or both half cycles, from several milliseconds up to 24
hours per day. The random nature of the disturbance can make it very difficult to
locate the offending lamp, especially on large housing estates. Figure 3 shows how the
nature of the disturbance has changed several minutes after the street lamp has been
switched on.




                                        7/4
   Audible hum from the street lamp column.
   Street lamp on 24 hours a day.
   Street lamp not illuminating.
   Equipment containing magnetic circuits, such as microwave ovens and fluorescent
   lamps, produce an audible hum.
   Remote controlled television sets change channels spuriously.
   Picture interference on television sets.
   Interference on long wave radio channels.
   Digital clocks advancing in time.
   Spurious tripping of RCD operated circuit breakers.

Circumstantial evidence is also available that relates component failures in computer
power supplies to disturbances caused by faulty street lamps.

Since the "fast clock" phenomena is well understood throughout EME, and all such
customer enquiries are routed to the Local Authority's Street Lighting department, it
     ih
was wt some surprise that a complaint of clocks running fast was passed through to
PQS. After initial questioning, it was apparent that the enquiry had come fiom a
customer in a rural area, which had no street lighting.

The customer's supply was fed from a SOkVA pole mounted transformer which fed
approximately ten properties. High frequency transients (Figure 4) detected on the
voltage waveform. were, apparently, responsible for the clocks running fast.
Disconnecting the household supply from the network did not remove the disturbance,
indicating that the cause w s remote from the customer's house. Later, a local farmer
                            a
informed the engineer that the pole mounted transformer had been hit by lightning and
was making "crackling and popping" sounds, and that the clocks had been operating
correctly prior to this event. The damaged transformer was replaced, but this action
did not solve the problem.



                                       7/5
After the transformer was replaced, it was noted that the transformer was making a
high pitched noise, unlike the typical 1OO& “hum”. Following this, all the
customers were warned of a supply interruption, and the transformer LV fuses were
drawn. When the “Blue”phase fuse was removed, the high pitched noise abated. An
investigation of the customer premises, fed &om the “Blue” phase, determined that the
disturbance was caused by a refrigerator with an electronic “energy saving” device
installed at the socket. When the “energy saving” device was removed, the high
frequency disturbance ceased. It was concluded that the lightning strike in the
previous week may have caused a failure in the device. Furthermore, a high
frequency resonance condition on the radial distribution feeder may have contributed
to the severity of the disturbance. Examination of the “energy saving” device
revealed the main component to be a triac operating in the more conventional phase
controlled mode, as opposed to the triac found in a street light photocell, and which
operates as an “on/ofY switch. The triac had gone into the same failure mode as those
found in faulty photocells.


Case Historv 4: Sensitive Load Equbment

A customer discovered an audible “hum” whenever he used his hi-fi system.
However, when the system was taken to a fiend’s home, the “hum” was not
When the audiophile contacted the manufacturer of the hi-fi system, he was told that
the amplifier was sensitive to dc OB the supply voltage.

PQS were asked to investigate the customer’s complaint of dc on
Because of the unusual nature of the complaint, a site investigation was p
During the site survey, dc voltage levels at the house were found to vary as
appliances (especially a hair dryer) were connected, and reached
0.5V.The audible “hum” on the hi-fi system was found to be bare1
the music was between song tracks. An isolation transformer was t
installed, which solved the problem. However, the audiophile was unwilling to
purchase one to “solve a problem with the supply.”
The PQS engineer contacted the hi-fi equipment manufacturer, who again expressed
                                                                                    ’
concem about the level of dc voltage present. The manufacturer was told that the
level of dc voltage was considered normal, and was asked why the equipment was not          .
immune to such a minor disturbance. The manufacturer replied that the problem was
due to an intemal transformer which saturated with dc voltage, and that newer units
were being manufactured with better immunity characteristics. The manufacturer
agreed to have the transformer replaced, to solve the problem. The manufacturer said
“Itk really not much of a problem, we only get 3-5 calls per week on this now!!”




As can be seen fiom these few case histories, power quality is a diverse subject.
Although great efforts are being made by electricity suppliers to provide power that is
fit for its purpose, and by manufacturers to produce equipment that is less sensitive to,
and creates less power disturbances, power quality problems will inevitably continue.
To be successfurin resolving power quality problems requires a thorough technical
knowledge of the electrical principles involved, but it is equally important to build up
practical experience in identifying and solving power quality problems. This can be
done through a combination of actual site investigations, and by learning from other
peoples experiences.




1.David Mueller, Eamon Delaney, Nigel Foster, PQA94-Amsterdam “Both Sides o   f
the Meter: An Electric Utility’s Response to Customer Power Quality Concerns”      D-
1.05
2.Nigel Foster, Security and Quality of the Electricity Supply-Ibedrola, Bilbao, 1995
              f
“Assignment O Costs And Responsibilities For Power Quality In A Competitive
Power Market Environment”




                                       7/7

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:8
posted:2/19/2012
language:
pages:7