This study, conducted in 1990, involved investigations in to recurrent failures of Delta-Delta transformer of a 72â Cold Rolling Mill's thyristor drive system, over a ...
Dr. Mohammed Safiuddin
Case Studies: Reactive Compensation and Harmonic Suppression Cold Rolling Mill-stand Drive System Transformer Failures Abstract: This study, conducted in 1990, involved investigations in to recurrent failures of Delta-Delta transformer of a 72” Cold Rolling Mill’s thyristor drive system, over a four year period, with special focus on harmonic distortions and their effects. The mill stand drive system consisted of two 1250 HP, 700 V, DC motors, powered and controlled by three-phase [6-pulse] thyristor dual converters, originally supplied by Westinghouse Electric. Since the main drive system’s thyristor power converters were also being upgraded at that time, the study spanned over a period of one year. With a brief failure analysis of the transformer in the first section, study of the harmonic distortions is documented in the second section. Study of harmonics had to be limited in scope because of unavailability of current transformers in each of the line feeding the two transformer primary windings. Based on the background of the prior failures, and the data collected, the study did not find strong evidence to conclude that excessive heating due to current harmonics contributed to the winding failures. The study did find that improper voltage ratings of the surge arrestors supplied by the transformer manufacturer, after the first failure of the high voltage windings, may not have been able to provide the desired protection against the voltage surge(s) responsible for the latest failure.
Transformer Failures: A. Historical Recap: The history of failures was gathered through individual interviews, and review of past records, as summarized below. 1. The unit that failed was manufactured by Company A and was installed in December of 1984. 2. The first failure of the transformer occurred sometime in July of 1987. The HV Center (B phase) coil had failed. The failure was attributed to excessive overvoltage. The unit was repaired by rewinding the failed coil only, and reinstalled with 18 KV lightening arrestors, Westinghouse Model # RX18NOBA99 with clamps, (12,470-14,400) nominal voltage, 60 KV BIL. 3. The second failure of the transformer occurred on November 16, 1988. However, this time the LV Center (B phase) coil had failed. Concurrently, the current sensor in the Y-Δ converter had also failed. The failure examination revealed that the temperature probe had pierced in to the winding. Company A had supplied the tube for the temperature probe as part of the transformer. However, the temperature probe was installed by the user [Company B]. The coil failure was attributed to improper installation of the temperature probe. Both HV and LV coils of phase B were rewound. 4. The latest failure of the transformer occurred on September 29, 1989. That time, the HV coil (C phase) had failed. Failure analysis reported by Company A indicated two failure points. One at a point between layers #3 and #4. The other at the ‘Finish’ end of the winding to ground (near the bottom) with accompanied fire damage. Close examination also indicated oxidation at the bottom end of the winding. Visits were arranged to examine the tear down of the failed “C-Phase” coil at the manufacturer’s plant on 25 October 1990 and to Company B’s facility for discussions on 7 November 1990. The
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Dr. Mohammed Safiuddin discussions held, at the Company B rolling mill facility, with personnel in the maintenance operation, and examination of the log, confirmed that the current sensor failure accompanied that second failure of the transformer (on 16 November 1988) and not at the time of the first failure in 1987. Information with respect to the distribution system, transformer test reports, main drive schematic diagrams, and the current harmonic spectrum recording taken by the mill personnel on 10/26/89 (with the brand X spare transformer) were gathered for analysis to determine possible cause of the latest failure. Plans to upgrade the converter equipment by replacement with Siemen’s units were also reviewed during that visit. Question was raised as to whether these failures were due to presence of excessive current harmonics produced by the converters powering the main drive DC motors of the mill stand, and as to why the Company A unit had a history of failures but not the brand X spare unit. It was decided to re-examine the level of current harmonics after the replacement of the repaired unit and converter upgrade. B. Analysis and Conclusions The failure analysis first focused on determining whether all the three failures were related in any way to shortcomings in the design, manufacture and/or application of the transformer itself. Since the second failure (November 16, 1988) was confined to the LV windings and was associated with pierced temperature probe in the winding accompanied with a current sensor failure in one of the converters, it could be concluded that it was unrelated to the other two failures of the HV coils. Though details with respect to the analysis of the first failure, in July 1987, were not available, records indicated that the manufacturer [Company A] attributed the failure to a high voltage surge. They recommended and supplied lightening arrestor for installation at the HV side of the transformer. The 13.8 KV power distribution system appeared to be “non-effectively grounded”. The surge arrestors supplied by Company A were rated 18 KV. For ungrounded or non-effectively grounded systems, IEEE Red Book [ANSI/IEEE Std. 141] recommends a 15 KV arrestor. GE catalog and application guide for lightening arrestors also concurred with the standard for that distribution class. An 18 KV distribution arrestor has a maximum EFOW (Equivalent Front-ofWave] protection level of 67.2 KV. On the other hand, a 15 KV rated arrestor has an EFOW protective level of 56 KV. With the HV winding rated, and tested, for 60 KV BIL [Basic Impulse Level], the 18 KV rated arrestor could not provide the needed surge protection. The effectiveness of the 18 KV rated arrestor in protecting the HV winding was, therefore, questionable. Voltage, current and KVA ratings of the transformer were checked for the specified application to determine if regular overloading had contributed to general deterioration of the insulation resulting in eventual winding failure, since there had been transformer failures in the past attributed to excessive overloading. Following calculations were carried out [11/17/89]. Rated Motor Operating Requirements: 700 VDC, 1,425 Amps (997.7 KW) Motor terminal voltage at 1,750 Amps load current (assuming 5% IR droop): VT = (700-35) + 43 = 708 VDC At 150% current limit (2,625 Amps), and associated commutation drop in the converter bridge, the maximum DC output voltage has to be VT = CEMF + Icl Ra= (700-35) + 65 = 730 VDC
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Dr. Mohammed Safiuddin That is, 730 𝑉𝐷𝐶 = 𝐸!"# 1 −
𝑋! . 𝐼! (0.07). (1.842) = 𝐸!"# 1 − = 0.935 𝐸!"# 2 2 !"#
𝐸!"# = !.!"# ≅ 780 𝑉𝐷𝐶
Therefore,
Allowing for regulation and control margin at maximum DC output voltage and minimum AC input voltage, choose α = 30 0 ! !
𝐸! =
!
!"#!" ! ! !"# !"!
. 𝐸! . 𝐶𝑜𝑠 30! = 780 𝑉𝐷𝐶
= 385 𝑉𝐴𝐶; 𝐸!! = 3 385 = 667 𝑉𝐴𝐶
The transformer had 22 secondary turns for 698 VAC. A 21 turn secondary winding would produce
!" !!
698 𝑉𝐴𝐶 = 666.27 𝑉𝐴𝐶
Total transformer KVA needed: 3𝐸!! 𝐼! 𝐼! = 𝑓 𝜇 =
2 3
1 − 3 𝑓 𝜇
𝐼!"
2 + 𝐶𝑜𝑠 𝜇 𝑆𝑖𝑛𝜇 − (1 + 2𝐶𝑜𝑠 𝜇)𝜇 2𝜋(1 − 𝐶𝑜𝑠 𝜋)!
𝜇 = 𝐶𝑜𝑠 !! 𝐶𝑜𝑠 𝛼 − 𝑋! . 𝐼! − 𝛼 3 2 . 𝐸!! 𝐶𝑜𝑠 𝛼 = 708 𝑉𝐷𝐶 𝜋 708 𝜋 𝐶𝑜𝑠 𝛼 = = 0.7511; 𝛼 = 41.3! 3 2. 698 𝜇 = 𝐶𝑜𝑠 !! 0.7511 − 0.07. 1 − 41.3 = 5.77! = 0.1007 𝑟𝑎𝑑𝑖𝑎𝑛𝑠 𝑓 𝜇 =
2 + 𝐶𝑜𝑠 5.77! 𝑆𝑖𝑛 5.77! − 1 + 2𝐶𝑜𝑠 5.77! . 0.1007 = 0.0042725 2𝜋(1 − 𝐶𝑜𝑠 5.77! )! 𝐼! =
2 3
1 − 3 (0.0042725 . 1,750 𝐴 ≅ 1,420 𝐴𝑚𝑝𝑠
Each of the secondary windings of the transformer needed to supply 1,420 Amps to the respective drive system converters. The transformer had to be rated for 3 (698) 𝑥 1,420 𝑥 2 = 3,433.5 KVA. However, only 667 VAC was needed at the secondary terminals rather than 698 VAC. The actual transformer rating was 4,250 KVA. Therefore the Service Factor was about 1.24. The transformer had 24% overload capacity based on 1,750 Amps converter rating. Design calculations by the manufacturer [Company A] concluded that the LV winding temperature rise would be 93.5 0 C at its 4,250 KVA rating but it would be only 76.5 0 C when delivering the required 3,500 KVA. So, based on these calculations and analysis, insulation failure due to longterm overload operations had to be ruled out.
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Dr. Mohammed Safiuddin The brand X transformer unit, originally supplied by Westinghouse Electric, was rated for only 2,830 KVA but had a specified temperature rise of only 800 C and an overload SF of 1.3. This did explain as to why that unit could be temporarily installed and used for this application without any problems. A closer examination of that unit also revealed that the HV windings were of “Pancake Coil” construction with insulated and dipped windings. Though an initial attempt was made to study the presence of harmonics in the transformer windings and its possible contribution to the transformer failure, a detailed study was deferred till after the replacement of converter equipment. Since “C-phase” coils had not been touched during the two previous repairs, the latest failure for the HV winding could only be attributed to HV surge. Improper selection of the 18 KV rated lightening arrestor probably could not protect the coil.
Harmonic Distortion and Power Factor Study: The failed transformer was repaired and reinstalled in January of 1990, followed by installation of Siemen’s thyristor power converters and control systems during the spring of 1990. Because of lack of current transformers on the primary and secondary sides of each of the transformers, the scope of study had to be limited. In contrast to the Westinghouse system, no current transformers in the AC line feeders to the power converters were provided in the Siemen’s system. No changes had been implemented on the 13.8 KV side of the power system feeding the Y-Δ and ΔΔ transformers of the main drive power converters. Both 13.8 KV primary windings were fed from a common disconnect with a single set of CTs for overcurrent protection relays. No PTs for voltage monitoring of the feeder bus existed. However, a separate 3-phase, 15 KVA, 13.8 KV/480 V, Δ-Δ, transformer for thyristor gating and control systems was available and was used for monitoring of voltage harmonics and power factor. Harmonic spectrums and waveforms for voltages and currents at the 13.8 KV feeder were collected in June 1990 after the equipment upgrade was completed. However, the current waveforms appeared to resemble measurement noise rather than the classical six-step waveforms typical of three-phase full-wave thyristor bridges. A plant visit was, therefore, undertaken on November 7 & 8, 1990 to investigate in to the current measurement problems and to gather additional data. It was discovered that, because of incorrect wiring diagram information of overcurrent relay and CT inter-connections, what was considered to be secondary circuit of the CT turned out to be the neutral circuit of the over-current relay (50N). Two of the overcurrent relays monitored the two line currents, while the third one monitored the neutral current for any unbalance load conditions as shown in Figures 1a and 1b. However, the actual wiring of the relays and CTs was found to be correct. Only the wiring diagram was incorrect and misleading. Also, the “C” clamp current probe [Tektronix Model # P6021] used in the earlier monitoring of the current waveforms at the CT secondaries was found to have a very limited bandwidth with respect to frequencies below 60 Hz, as shown in Figures 2a and 2b. Therefore, all current waveform measurements made earlier were determined to be unusable for any analysis.
(a)
(b)
Figure 1: Overcurrent Relay 50N Connections- (a) Actual Connections (b) Incorrect Diagram
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Dr. Mohammed Safiuddin
Figure 2: Current Waveforms-- Line current with wrong probe (left) Line Current with proper probe (right) The harmonic spectrum and associated voltage and current waveforms for the 13.8 KV feeder at Line 2 [L2], as recorded, are presented in Figures 3 and 4. The data captured from the harmonic spectrums of voltage and current waveforms were converted to per-unit values and are tabulated in Tables I and II. The analysis of the data indicated a THD [Total Harmonic Distortion] in the line current of 11.43% of the operating current, which was at 50% of the rated load and at 50% operating speed. However, the THD in voltage waveform was almost negligible at
