ichqp - distribution network capacitor resonance a case study

Distribution Network Capacitor Resonance A Case Study 1

Distribution Network Capacitor Resonance A Case StudyAuthors:Chris Halliday Frank Iannelli Dr Robert BarrDirector of Technical Servicesand Training

Power Quality Technician Director

Electrical Consulting andTraining Pty Ltd

Country Energy Electric Power ConsultingPty Ltd

This paper was presented at the ICHQP 2008 - 13th International Conference on Harmonics andQuality of Power, University of Wollongong 28th September 1st October 2008. It was alsopresented previously at the EESAs Energy NSW 2007....Balancing the New Energy EquationConference & Trade Exhibition in Sydney, Wednesday 12 to Friday 14 September 2007.

AbstractHarmonics are generally caused by customer non-linear loads interacting with network impedances.Capacitor banks used for voltage support can resonant and cause excessive levels of harmonics thatmay cause problems for customer equipment.

This case study looks at the problems associated with a capacitor bank in Country EnergysNarrandera Zone Substation, the levels of network harmonics at various locations, the effects oncustomer equipment and the solution implemented to overcome the problem.

1. IntroductionCountry Energys regional electricity network covers 95% of NSW. This network suppliesapproximately 870,000 customers and is supported by approximately 4,000 employees.

The distribution network consists of over 195,000 kilometres of subtransmission, high voltage andlow voltage distribution lines. There are approximately 1.4 million power poles for the overheadcomponent of this network. The distribution network is supplied and radiated out fromapproximately 340 zone substations via approximately 1360 distribution feeders. One of thesezone substations is located at Narrandera (see Map 1) and it is this zone substation that is the focusof this paper.

Harmonics are generated by customer non-linear loads interacting with network impedances.Harmonics in Australia are typically at low levels and generally cause no problems. At somelocations, harmonics can reach high levels due usually to a combination of local load and networkcircumstances. This case study is one of those rare situations.

Narrandera

Map 1: Showing NSWand Narrandera


Capacitors are used in banks by distribution companies, such as Country Energy, to correct poorpower factor, to provide voltage support and minimise losses. These capacitors can resonate at oraround the 5th or 7th harmonic if measures arent taken to prevent such resonance. Harmonicresonances can result in damage to capacitor banks and customer equipment as well as increasednetwork losses and maloperation of customer equipment.

Problems occurred at Narrandera in 2006 due to a relatively low level of customer generatedharmonic currents generated at the end of a rural line that was interacting with the capacitor bankat Narrandera Zone Substation (see Map 1 to show the location of Narrandera in relation to NSW).This paper looks at the effects on customers, the process taken to identify the problem and cause,the solution implemented and effectiveness of this solution.

2. Effects to CustomersOnly one customer supplied from the zone substation had reported problems. The customersinstallation is connected to the end of a lightly loaded radial rural feeder that is a combination ofHV 7/.064 copper conductor, 7/4.5 aluminium, 6/1/2.5 ACSR and 35 mm Aluminiumunderground cable: a total of 50.6 kms of powerline from transformer Tx 1 at Narrandera ZoneSubstation (see Sketch 1 for the single line diagram from the zone substation to the site).

Sketch 1: Single Line Diagram. Zone Sub to Site

The transformer supplying the site was a 3 phase 150 kVA padmount transformer, set on the11275/433 tap position and with a percentage impedance of 4.1%. The load was typically 30-35amps but at times getting up to 50 amps not a significant amount of load by any means.

The customer premise was a communication receiver station that comprised of racks of sensitiveelectronic communication equipment, switch mode power supplies and amplifiers. The site had abackup generator that was starting intermittently from an unknown cause since the site wascommissioned in 1999. This resulted in an enormous amount of unnecessary generator starts overthis time which was costing the customer a significant amount of money in terms of fuel cost, wearand tear on the generator and other equipment e.g. a changeover contactor had to be replaced.There was also the inconvenience and labour cost associated with topping up the generator fueland ensuring it was ready for use at all times. The customers consultant initially thought thatvoltage sags were causing the problem but monitoring proved that voltage sags did not cause anychange in the status of the generator.

In one seven day period alone the generator started unnecessarily 25 times and ran forapproximately 47 hours. The generator changeover sensing relay was set as follows:

Voltage Unbalance - 15% with a 10 sec delay;

Under Voltage - 85% with a 10 sec delay.

NarranderaZone Sub66/11kV

3.2 km7/4.5 AAC

22.8 km7/064 HDBC

11 km7/4.5 AAC

9 km6/1/2.5 ACSR

4.6 km35 mmal XLPE

150 kVA DistributionTransformer

Note: Numerous spurs and distribution transformers are connectedall the way along the feeder but are not shown for simplicity. Otherfeeders from the zone substation are also neglected


3. Identifying the Problem and CauseInitial investigations were focused at the customer who was having the problems. A GridsensePowermonic PM30 was installed at the distribution transformer. This locations was selected as itwas the point of supply and as voltage sags from the medium voltage network was suspected asthe cause of the problems.

The monitoring established that the supply voltage was within standards and the substation wasnot overloaded (see Graph 1).

Graph 1 Voltage and Current at the Distribution Transformer Supply the Receiver Site

The graphs showed that the generator was starting when Total Harmonic Distortion (THD) levelsexceeded approximately 9.8% (see Graph 2). This level is in excess of the required 8%compatibility level as set down by AS/NZS61000.3.6. The graphs also showed that the 5th and 7thharmonics were the most significant harmonics by far. The generator changeover relay appeared tobe confused by such high levels of THD and sensed the need to swap to the generator.

THD reduces by approximately 4% when the generator cuts in as shown by Graph 2 and so thecustomers load was contributing significantly to the problem. The generator changeover relaywould sense when the harmonic distortion dropped to below 9% voltage THD and the generatorwould be stopped and mains supply would return. The generator would cycle in and out unless theTHD remained below about 9.8%.

Graph 2 Voltage THD percentage at the customers premise

It was not easy to tong the medium voltage network to determine where the major harmoniccurrent sources were originating from so an analysis of customer loads connected to the networkwas carried out to determine those customers likely to have been generating significant harmonics.The only identified major harmonic producing load was a large plant in Narrandera township and

Generator cutting in and out


this customer was supplied by the other Zone Substation transformer, Tx 2. Spot readings of THDlevels were taken at this site and found to be very low and it was concluded that this site was notcontributing greatly to the problem. It has since been concluded that the largest contributingharmonic load from Tx1 was in fact the customer with problems (this can often be the case).

4. The Identified ProblemNarrandera zone has 2 x 10MVA 66/11 kV transformersoperating in a split bus arrangement in the peak loadperiods during summer and winter. This zone substationhad a capacitor bank (see Photo 1) that was used toimprove power factor on Tx 1 which supplies part ofNarrandera township and some of the surrounding ruralnetwork.

Graphs taken at the zone substation showed the VoltageTHD at times to about 5.7% and with low levels of currentTHD and individual harmonics. The voltage THD levels atthe zone substation are much less than those recorded at thereceiver site.It was decided that the old capacitor bank may be causing aresonant affect and it was to be disconnected from thenetwork for a short period. This was to occur between 9 am 4th September to 4 pm on the 5thSeptember 2006. Monitoring at the receiver site was to be undertaken to monitor the change in theharmonic distortion (see Graph 3). Unfortunately the current to the capacitor bank was not loggedat any stage to prove that resonance was or wasnt occurring. Also logs at the zone substation werenot taken when the capacitor bank was switched out to see the effect at that location.

Graph 3 THD at the Receiver site while Capacitor Bank Switched out

Total harmonic voltage distortion at the receiver site when the capacitor bank was disconnectedhad reduced by approx 4 % and the operations of the generator had stopped due to the voltageTHD at the site staying below 9 %. THD levels at the zone substation reduced from peaks of 6%down to approximately 3-4% at all times.

A link was therefore established between the cause of the generator operations at the receiver siteand the old capacitor bank. The capacitor bank was then disconnected from supply for an extendedperiod to assist the customer. No further reports of the generator cutting in and out were receivedthrough this period.

Voltage distortion reduced whilecapacitor bank was switched out.The generator did not cut in or outduring this period.

Photo 1 Old Capacitor Bank


It is noted from the graphs taken at the receiver site that when the load on the feeder is high atmidnight to 3 am due to hot water load (resistive load) and during the day period from 8 am to 6pm that the total harmonic distortion was somewhat lower than at lightly loaded times. Dugan, etal [1] states that as little as 10% resistive loading can have a significant beneficial impact on peakimpedance and this is illustrated by the results of the graphs for this site.

5. Modelling the NetworkA model was set up in Excel to investigate what was occurring. A simplified sketch of this modelis provided at Sketch 2. The numerous parallel paths of connected spurs and distributiontransformers had not been included in the sketch for simplicity.

Sketch 2 Simplified drawing of model developed in Excel

This model showed consistency with the practical measured results. The model confirmed thatthere was a resonance in the low order harmonics around the 5th harmonic (see Graph 4). Thegraph produced from the model also supports that a reduction in harmonics was to be expectedwhen the capacitor bank was taken out of service.

Graph 4 Modelled Harmonic Impedances at Zone Substation 11 kV Busbar

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Harmonic

PUH

arm

onic

Impe

danc

e

with P.F. CorrectionCapacitorswithout P.F. CorrectionCapacitors


6. Solution ImplementedThe capacitor bank at Narrandera Zone Substation was planned to be replaced in the near futuredue to its PCB dielectric. The capacitor bank was left out of service for a period until it wasreplaced some months later. The new capacitor bank was installed with blocking inductors toprevent resonance (see Sketch 3).

Sketch 3 New Capacitor BankSingle Line Diagram (Courtesyof ABB)

Harmonic filtering was also recommended for the receiver site to assist in reducing harmoniclevels generated.

7. Solution EffectivenessLevels of THD were measured again once the new capacitor bank was installed. This showedvoltage THD levels at the zone substation were maintained at similar levels as without the oldcapacitor bank at approximately 3% and at the receiver site at approximately 7.6%. This is asignificant improvement and the customer is not having further problems.

Voltage THD levels are still nearing the compatibility limits allowed by the standards. Futureupgrading of the network will help to overcome this issue and/or emission limits may need to beapplied to customer.

8. Conclusions1. Electricity distributors need to be aware of the potential adverse harmonic impacts of shunt

capacitor bank resonances.

2. Modelling over the harmonic spectrum is an important part of avoiding harmonic relatedproblems.

3. Excessive harmonic voltages caused by capacitor resonances or other reasons can causesensitive customer equipment to malfunction.

Investigation techniques can always be improved and problems like this provide a learningopportunity for all.

9. AcknowledgementsBrendon McPhillips, Kris McCanna, Peter Day and others from Country Energy for informationand assistance provided in compiling this paper.

10. References[1] Dugan, etal. 2002. Electrical Power Systems Quality, Second Edition. McGraw Hill.

ichqp - distribution network capacitor resonance a case study

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