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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 aim is to deal with the growing concern of 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 was a supply problem, because it was a new installation and the 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

[IEE IEE Colloquium on Issues in Power Quality - Coventry, UK (28 Nov. 1995)] IEE Colloquium on Issues in Power Quality - Power quality case histories

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Page 1: [IEE IEE Colloquium on Issues in Power Quality - Coventry, UK (28 Nov. 1995)] IEE Colloquium on Issues in Power Quality - Power quality case histories

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 aim is to deal with the growing concern of 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 investigations that 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 was a supply problem, because it was a new installation and the 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 investigations revealed that

Page 2: [IEE IEE Colloquium on Issues in Power Quality - Coventry, UK (28 Nov. 1995)] IEE Colloquium on Issues in Power Quality - Power quality case histories

the live and neutral conductors feeding tbe ff owecent 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 firm 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 in one 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. In the 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.

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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:

.

Mee8ured Flicker Levsl. 24 hour Period I 12 +

k 1 1

I Figure 1 ; Long Term Severity Level (PlQ Recommended

maximum limit of 0.8 Plt

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.

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stow 3: Disturb= l m d

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.

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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 was with 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 was remote from the customer's house. Later, a local farmer 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.

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After the transformer was replaced, it was noted that the transformer was making a high pitched noise, unlike the typical 1 OO& “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.”

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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 of 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 “Assignment Of Costs And Responsibilities For Power Quality In A Competitive Power Market Environment”

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