Distribution Network Capacitor Resonance A Case Study

Transcription

1 Distribution Network Capacitor Resonance A Case Study Authors: Chris Halliday Frank Iannelli Dr Robert Barr Director of Technical Services Power Quality Technician Director and Training Electrical Consulting and Training Pty Ltd Country Energy Electric Power Consulting Pty Ltd This paper was presented at the Energy NSW Balancing the New Energy Equation Conference & Trade Exhibition in Sydney, Wednesday 12 to Friday 14 September Abstract Harmonics 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 that may cause problems for customer equipment. This case study looks at the problems associated with a capacitor bank in Country Energy s Narrandera Zone Substation, the levels of network harmonics at various locations, the effects on customer equipment and the solution implemented to overcome the problem. 1. Introduction Country Energy s regional electricity network covers 95% of NSW. This network supplies approximately 870,000 customers and is supported by approximately 4,000 employees. The distribution network consists of over 195,000 kilometres of subtransmission, high voltage and low voltage distribution lines. There are approximately 1.4 million power poles for the overhead component of this network. The distribution network is supplied and radiated out from approximately 340 zone substations via approximately 1360 distribution feeders. One of these zone substations is located at Narrandera (see Map 1) and it is this zone substation that is the focus of this paper. Map 1: Showing NSW and Narrandera Narrandera 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 some locations, harmonics can reach high levels due usually to a combination of local load and network circumstances. This case study is one of those rare situations. Distribution Network Capacitor Resonance A Case Study 1

2 Capacitors are used in banks by distribution companies, such as Country Energy, to correct poor power factor, to provide voltage support and minimise losses. These capacitors can resonate at or around the 5 th or 7 th harmonic if measures aren t taken to prevent such resonance. Harmonic resonances can result in damage to capacitor banks and customer equipment as well as increased network losses and maloperation of customer equipment. Problems occurred at Narrandera in 2006 due to a relatively low level of customer generated harmonic currents generated at the end of a rural line that was interacting with the capacitor bank at 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 Customers Only one customer supplied from the zone substation had reported problems. The customers installation is connected to the end of a lightly loaded radial rural feeder that is a combination of HV 7/.064 copper conductor, 7/4.5 aluminium, 6/1/2.5 ACSR and 35 mm Aluminium underground cable: a total of 50.6 kms of powerline from transformer Tx 1 at Narrandera Zone Substation (see Sketch 1 for the single line diagram from the zone substation to the site). Sketch 1: Single Line Diagram. Zone Sub to Site Narrandera Zone Sub 66/11kV 3.2 km 7/4.5 AAC 22.8 km 7/064 HDBC 11 km 7/4.5 AAC 9 km 6/1/2.5 ACSR Note: Numerous spurs and distribution transformers are connected all the way along the feeder but are not shown for simplicity. Other feeders from the zone substation are also neglected 4.6 km 35 mm² al XLPE 150 kva Distribution Transformer The transformer supplying the site was a 3 phase 150 kva padmount transformer, set on the 11275/433 tap position and with a percentage impedance of 4.1%. The load was typically amps 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 sensitive electronic communication equipment, switch mode power supplies and amplifiers. The site had a backup generator that was starting intermittently from an unknown cause since the site was commissioned in This resulted in an enormous amount of unnecessary generator starts over this time which was costing the customer a significant amount of money in terms of fuel cost, wear and 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 fuel and ensuring it was ready for use at all times. The customer s consultant initially thought that voltage sags were causing the problem but monitoring proved that voltage sags did not cause any change in the status of the generator. In one seven day period alone the generator started unnecessarily 25 times and ran for approximately 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. Distribution Network Capacitor Resonance A Case Study 2

3 3. Identifying the Problem and Cause Initial investigations were focused at the customer who was having the problems. A Gridsense Powermonic PM30 was installed at the distribution transformer. This locations was selected as it was the point of supply and as voltage sags from the medium voltage network was suspected as the cause of the problems. The monitoring established that the supply voltage was within standards and the substation was not 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) levels exceeded approximately 9.8% (see Graph 2). This level is in excess of the required 8% compatibility level as set down by AS/NZS The graphs also showed that the 5 th and 7 th harmonics were the most significant harmonics by far. The generator changeover relay appeared to be 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 the customer s load was contributing significantly to the problem. The generator changeover relay would sense when the harmonic distortion dropped to below 9% voltage THD and the generator would be stopped and mains supply would return. The generator would cycle in and out unless the THD remained below about 9.8%. Graph 2 Voltage THD percentage at the customer s premise Generator cutting in and out Distribution Network Capacitor Resonance A Case Study 3

4 It was not easy to tong the medium voltage network to determine where the major harmonic current sources were originating from so an analysis of customer loads connected to the network was 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 this customer was supplied by the other Zone Substation transformer, Tx 2. Spot readings of THD levels were taken at this site and found to be very low and it was concluded that this site was not contributing greatly to the problem. It has since been concluded that the largest contributing harmonic load from Tx1 was in fact the customer with problems (this can often be the case). 4. The Identified Problem Narrandera zone has 2 x 10MVA 66/11 kv transformers operating in a split bus arrangement in the peak load periods during summer and winter. This zone substation had a capacitor bank (see Photo 1) that was used to improve power factor on Tx 1 which supplies part of Narrandera township and some of the surrounding rural network. Graphs taken at the zone substation showed the Voltage THD at times to about 5.7% and with low levels of current THD and individual harmonics. The voltage THD levels at the zone substation are much less than those recorded at the receiver site. It was decided that the old capacitor bank may be causing a Photo 1 Old Capacitor Bank resonant affect and it was to be disconnected from the network for a short period. This was to occur between 9 am 4 th September to 4 pm on the 5 th September Monitoring at the receiver site was to be undertaken to monitor the change in the harmonic distortion (see Graph 3). Unfortunately the current to the capacitor bank was not logged at any stage to prove that resonance was or wasn t occurring. Also logs at the zone substation were not 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 Voltage distortion reduced while capacitor bank was switched out. The generator did not cut in or out during this period. Distribution Network Capacitor Resonance A Case Study 4

5 Total harmonic voltage distortion at the receiver site when the capacitor bank was disconnected had reduced by approx 4 % and the operations of the generator had stopped due to the voltage THD 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 site and the old capacitor bank. The capacitor bank was then disconnected from supply for an extended period to assist the customer. No further reports of the generator cutting in and out were received through this period. It is noted from the graphs taken at the receiver site that when the load on the feeder is high at midnight to 3 am due to hot water load (resistive load) and during the day period from 8 am to 6 pm that the total harmonic distortion was somewhat lower than at lightly loaded times. Dugan, et al [1] states that as little as 10% resistive loading can have a significant beneficial impact on peak impedance and this is illustrated by the results of the graphs for this site. 5. Modelling the Network A model was set up in Excel to investigate what was occurring. A simplified sketch of this model is provided at Sketch 2. The numerous parallel paths of connected spurs and distribution transformers 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 that there was a resonance in the low order harmonics around the 5 th harmonic (see Graph 4). The graph produced from the model also supports that a reduction in harmonics was to be expected when the capacitor bank was taken out of service. Graph 4 Modelled Harmonic Impedances at Zone Substation 11 kv Busbar 70 PU Harmonic Impedance with P.F. Correction Capacitors without P.F. Correction Capacitors Harmonic Distribution Network Capacitor Resonance A Case Study 5

6 6. Solution Implemented The capacitor bank at Narrandera Zone Substation was planned to be replaced in the near future due to its PCB dielectric. The capacitor bank was left out of service for a period until it was replaced some months later. The new capacitor bank was installed with blocking inductors to prevent resonance (see Sketch 3). Sketch 3 New Capacitor Bank Single Line Diagram (Courtesy of ABB) Harmonic filtering was also recommended for the receiver site to assist in reducing harmonic levels generated. 7. Solution Effectiveness Levels of THD were measured again once the new capacitor bank was installed. This showed voltage THD levels at the zone substation were maintained at similar levels as without the old capacitor bank at approximately 3% and at the receiver site at approximately 7.6%. This is a significant improvement and the customer is not having further problems. Voltage THD levels are still nearing the compatibility limits allowed by the standards. Future upgrading of the network will help to overcome this issue and/or emission limits may need to be applied to customer. 8. Conclusions 1. 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 related problems. 3. Excessive harmonic voltages caused by capacitor resonances or other reasons can cause sensitive customer equipment to malfunction. Investigation techniques can always be improved and problems like this provide a learning opportunity for all. 9. Acknowledgements Brendon McPhillips, Kris McCanna, Peter Day and others from Country Energy for information and assistance provided in compiling this paper. 10. References [1] Dugan, etal Electrical Power Systems Quality, Second Edition. McGraw Hill. Distribution Network Capacitor Resonance A Case Study 6