A3_301 CIGRE 2012

EGYPTIAN EXPERIENCE OF LIVE LINE WASHING SYSTEM

FOR 500KV OUTDOOR EQUIPMENT TO IMPROVE AVAILABILITY

N. Heggi*, D.El Arosi Egyptian Electricity Holding Company

Egypt

SUMMARY

It was a challenge for the Egyptian Electricity Holding Company (EEHC) to meet the continuous high growth rate of electricity demand, the peak demand increased from 3306MW to 23470MW during the period 1980 /2010, and energy generated from 20 TWH to 139 TWH in the same period. To meet this challenge, EEHC continuously adds annual achievements in the form of construction of power plants.

One of the big new power plants is the Nubaria power plant which was executed during the period 2005 – 2009. The plant includes three combined cycle modules with total capacity of (3x750) MW. The 500KV air insulated switchgear ( AIS ) of the power plant is made up of 9 identical bays for generator transformers which are interconnected to the 500kV network through two overhead transmission lines, each is single circuit and to the 220kV network through two identical three single phase 500/220 kV auto transformers plus one phase as spare.

As the External insulation of the 500, 220kV outdoor substations is exposed to salt- laden air and desert dust, high salt content accumulates on insulators at times of high humidity and fog conditions, which can cause insulator flashovers, hence an extensive substation insulator washing program was necessary. The system is used regularly for washing the insulators of the outdoor equipment under tension by spray jet water nozzles permanently mounted on the switchyard equipment.

This paper discusses the problem associated with the washing system of the 500kV outdoor current transformers (CTs) in the above mentioned power plant. Different processes were recommended as agreed between the manufacturers of the CTs and the live line washing system.

KEYWORDS

Current transformers (CTs) - Air insulated switchgear (AIS) - Live line insulator washing system (LLIWS) - Effective salt deposit density (ESDD). 1. INTRODUCTION

*E-mail: n_heggi@hotmail.com

21, rue d’Artois, F-75008 PARIS http : //www.cigre.org

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1. INTRODUCTION Methods of maintaining reliable operation of the equipment of the Nubaria power plant in a severe polluted environment is achieved by means of a fixed washing system designed to provide sufficient cleaning to avoid contamination flashover [1].

During operation of the 500 kV air insulated switchyard equipment, a failure of CT occurred on April 2nd 2009 immediately after completing the water wash using the live line insulator washing system (LLIWS ).

Again on October 26th 2009 another failure occurred after completing the live line insulator washing. The CTs were installed in august 2008, first energizing took place in November 2008 and permanent service started on January 6th 2009, while the commission of the live line washing system installation started on April 2nd 2009و that means on the same day of the failure. The CTs which failed in service passed the type and routine tests as per updated IEC standards [2].

The technical data of the 500 kV CTS [2] are:-

- Rated primary current : 2000 A

- Rated voltage phase to earth : 500/√3 kV

- Insulation level : 620 kV rms , 1550 kV peak , 1175 kV peak

- Creepage distance : 17500 mm

- Temperature range : 0/ + 45 ˚C

- Insulation medium : mineral oil impregnate

2. LIVE LINE INSULATOR WASHING SYSTEM

The system is used regularly and includes 12 spray jet water nozzles permanently mounted around the lower end of each CT insulator and permanently coupled by pipe work to a supply of demineralized water with a flow rate of 350 L/min, water stream should withstand A.C nominal voltage and over voltages [3].

The system has automatic and manual mode of operation using water with a conductivity value less than 50 microsiemens/cm. The nozzles operating pressure was 12 bar with washing periods of 30 seconds. All 12 nozzles around each CT start spraying simultaneously and not successively from the bottom to the top of the CT[4].

3. ACCIDENT SEQUENCE

3.1. First failure

On April 2nd 2009 switchyard CT T2 phase S connected to bus bar I, CTG 3B bay as per the general layout figure (1) failed immediately after the water wash using the live line insulator washing system.

The weather conditions were fine, sunny day, no wind and no rain. The failure exploded the CT and scattered the broken ceramic insulation portions at high velocity to about 40 m around the CT which caused insulator damage to adjacent equipment (C.B, D.S, CT, C.V.T) . The CT internal oil caught fire and the smoke from the fire contaminated other HV insulators. The CT explosion

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caused a line to ground fault on phase S and the protection relays operated correctly and isolated the fault as follows:

1. After 100 ms, busbar 1 protection disconnected all breakers on the faulted busbar.

2. The fault was still fed from busbar Π through the second breaker which required the unit overall protection (delayed backup protection) to intervene, after another 100 ms to trip the unit. CTG 3A and STG 3X (steam unit) bays were not affected by the accident.

Figure 1. first accident

3.2. Second failure

On October 26 th 2009 switchyard CT T4 phase R connected to busbar Π, CTG 3B bay shown in figure (2) failed immediately after the live line washing process , an explosion was heard and subject CT caught fire.

The weather was fine, sunny day, no wind and no rain. The failure exploded the CT and scattered the broken ceramic insulation portions at high velocity which caused insulator damage, figure (4) to adjacent equipment (C.B, D.S, and I.V.T). The CT explosion caused a line to ground fault on phase R and the unit overall, protection intervened immediately and tripped the unit after 60 ms. The fault caused a voltage dip on other unit's switchgear, which caused LV load centers to trip and consequently the tripping of STGS. CTG 3A and STG 3 x bays were not affected by the accident.

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4. PROPOSED PLAN TO DETERMINE ROOT CAUSE ANALYSIS

Speculation suggests that the failure mechanism is triggered by either: a. possible CT manufacturing defect b. Live line insulating washing system (LLIWS) design problem.

The plan to assess the above two possible root causes was as follows:

1. Perform a tan delta test on all unfailed ones to confirm the CTs insulation integrity and that they are safe for operation without using the LLIWS. The results show that no significant differences have been found with respect to the initial measurements.

2. Perform an internal insulation oil sample analysis on all unfailed CTs using equipment and instructions from the CT manufacturer to investigate the existence of water ingress inside the CTs inner tubes. The results did not show remarkable deviations from the standard values.

3. Concerning The LLIWS, safety and performance investigated taking into account the influencing parameters such as voltage applied, nozzle, conductor distance, water resistivity, water pressure and diameter and shape of nozzle orifice[5], in addition tests were carried out to check the efficiency of the fixed live line insulator washing system fitted on the CTs at 500KV switchyard and determine the optimum nozzle arrangement, and also determine if the washing efficiency is improved by extending the washing period from 30 seconds to 35 and 40 seconds.

Figure 2. second accident

The tests were carried out on six separate current transformers within a selected, wash zone with each CT being provided with different nozzle arrangements and flow rates as follows:

• Nozzle arrangement 1: Existing nozzle arrangement (total flow rate 350 L/min) • Nozzle arrangement 2: Coverage as existing nozzles but increased flow rate for bottom

and middle nozzles (total flow rate 420 L/min).

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• Nozzle arrangement 3: Coverage and flow rate increased for all nozzles (total flow rate 400 L/min).

• Nozzle arrangement 4: Nozzles to cover length reduced from 3 to 2 but located on 6 sides of insulator instead of 4 (total flow rate 400 L/min).

• Nozzle arrangement 5: Nozzle coverage and flow rate increased, nozzle arranged on 6 sides of insulator instead of 4(total flow rate 400 L/min).

• Nozzle arrangement 6: Nozzles to cover length reduced from 3 to 2 still located on 4 sides of insulator (total flow rate 400 L/min).

The insulators to be tested was pre – polluted with a solution consisting of a mix of saturated salt solution and fine powdered clay[5]. A standard slurry was adopted for all tests consisting of 50 g of clay per litre of saturated salt solution to provide a pollution coating within the range of 0.03 to 0.06 mg/cm2 in each case, the effective salt deposit density (ESDD) on the insulator was determined before and after washing to find the efficiency of washing systems using “Hewitts tables”[6 ], where:

ESDD (mg/cm2) = weight of salt (mg) / service area of insulator shed (cm2)

The insulator was washed for a period of 30 seconds and repeated for washing period of 35& 40 seconds to determine if there is any benefit from longer washing periods, the flow regulating valves adjusted for each spraying to provide nozzle operating pressure of 12 bar.

From the carried tests it was found that:

1. Washing of the bottom shed is more efficient than those of the middle and top sheds of the insulator.

2. Washing efficiency is better for a washing period for 40 seconds than 35&30 seconds respectively.

3. Leakage current measurements were recorded using measuring circuit shown in fig.(3).

4. No change in all cases in the measured values.

Figure 3. Measuring circuit for leakage current

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5. CAUSE OF THE FAILURES

Tests and excessive investigations have led to the conclusion that no internal discharge occurred and that the failures were not induced by the weakness of the insulation system of the CTs, which indicates that the failure was induced by a heavy external flashover under power frequency which damaged the porcelain until complete destruction followed by an explosion due to the ignition of the insulation oil.

The CTs are designed with a special shed profile and an extended arcing distance up to 4.2 meter and a creepage distance of 17.5 meters which make them suitable for a washing process in service, but it’s extremely important to proceed in a special way with the washing process. Washing the whole insulator simultaneously from bottom to top seems to support the inception of surface discharge leading to complete flashovers of the sparking distance; washing should always start at the bottom and proceeds upward successively.

6. SOLUTION

The live line washing system manufacturer did not provide any technical accepted reason for the CTs accident. While the CTs manufacturer recommendations can be summarized as follows:-

1- Stop LLWS in CTs and perform dead washing only for CTs 2- Dead washing can be performed using automatic washing 3- Line washing for all 500 kV equipment except CTs is permitted in normal sequence

followed by dead washing for CTs.

Accordingly the following modifications were done: As the CTs are included in the same wash zone of the circuit breakers and due to high flow rate required for these zones washing is controlled by two 150 mm control valves operating in parallel, it’s recommended to change the piping layout such that one 150 mm control valve is connected to the C.Bs only and the other 150 mm control value is connected to the CTs only. The control valve for the C.Bs still be included in the automatic washing sequence, but the control valve for the CTs would be excluded from the automatic washing sequence and only be operated under manual control. The CTs shall be washed manually in the dead condition prior to automatic live washing i.e. the automatic washing sequence interlocked with the CT control values.

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Figure 4. The CT lower porcelain part was completely destroyed

7. CONCLUSION - Field experience of AIS CTs with rated voltage 500 kV installed in Egypt has been quite wide and generally satisfactory, but field experience of 500 kV AIS CTs with live line washing system is still more limited and mostly confined to dead tank design circuit breaker with CTs built in, where the performance has been generally satisfactory with LLIWS. On the contrary two failures have been observed immediately after live line washing process of outdoor CTs.

- Field investigation was carried out to obtain information on the causes of failure in particular visual inspection of failed CTs and field diagnostic measurements.

- For 500 kV CTs with vertical insulators it’s extremely important to proceed in a special way with the washing process. Washing the whole insulator simultaneously from bottom to top seems to support the inception of surface discharges leading to complete flashovers of the sparking distance, washing should always start at the bottom and proceeds upward successively.

BIBLIOGRAPHY

[1] ANSI/IEEE, Std 957(2005) IEEE Guide for cleaning insulators (2005) [2] IEC60044-1Edition 1.2 (2003-02-13) Instrument transformers - Part 1: Current transformers

[3] CIGRE WG33.07 and TF 33.07.09 “Dielectric Strength of external insulation systems under live working “ CIGRE 1994– paper 33.306.

[4] [V. Sklenicka: "Hot line insulator washing", ICOLIM 92, Keszthely, Hungaria 1992, paper No. 37-1

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[5] IEC 60507 Edition 2.0 (1991-04-17) Artificial pollution tests on high-voltage insulators to be used on a.c. systems.

[6] Hewitt, G. F: Tables of the resistivity of aqueous sodium chloride solutions / Harwell, Berkshire, England : Scientific Administration Office, Atomic Energy Research Establishment, 1960