The Structure, Technical Parameters and Insulation Test of Converter Transformer

Introduction

The converter transformer serves as a bridge between AC system and DC system, the secondary side of which is connected with converter valves. How the converter valve works determines that high-order harmonic wave component and DC component exist in the converter valve, which will cause loss, noise and an increase in no-load current. On the other hand, the deviation in short circuit impedance of converter transformer also has an effect on harmonic wave component and DC component. The electrical stress and thermal stress converter transformer bears are far higher than AC transformer. Hence, the insulation problem of converter transformer is more prominent.

Discussions about the formulation of code for converter transformer are made through combining the experience in the operation of Genan DC system and actual condition of Three Gorges DC project. According to the results of electrical field calculation, present insulation test is evaluated and proposals for test conduction method and extra test items are put forward.

1. Structure and Rated Value

Theweight is the main factor in the selection of converter transformer, also including converter tie lines. It is easy and economic to use two-bridge 12-impulse tie lines. At present, this kind of tie line is widely adopted all over the world. Based on structures of converter transformer, three schemes are listed in Tab.1.

Tab.1 Structure of converter transformer

Obviously, as for scheme I, the capacity per set is too large; besides, it is impossible to transport them and the spare capability is also not economic; therefore, this scheme is dismissed.

The scheme II is technically and economically superior to scheme I. The valve bushings of all transformers can extend into the valve hall, greatly simplifying the leads and saving floor area. However, the weight per converter transformer amounts to 420t and the dimension is 3.7m × 10.5m × 5.6m. If the transformer is made abroad, it goes to Shanghai via sea transport and then to Three Gorges dock; at last 500-600t large vehicle is used to transport the transformer to the converter station. Of all modes of transportation, the biggest difficulty is road transport from Three Gorges harbor to the converter station. The vehicle unit totals between 500t and 600t, which requires “heavy-cargo road” (There are requirements for bearing, net weight, width, gradient and minimum radius of curvature etc). If the transformer is made at home, it is impossible to transport 420t converter transformer by means of rail transport. The workable mode is to combine road transport and water transport. The biggest difficulty for combined transport is road transport from three transformer factories to harbors. Three converter transformers at the Gezhouba station have ever been damaged during transport. Besides, the fault rate of converter transformer is higher than that of common transformers during the operation. Hence, scheme II is a high-risk one owing to the transport issue.

With regard to scheme III, the weight per set is 250t and the dimension is 3.3m × 9.5m × 4.7m, which is similar to the Gezhouba converter transformer (the weight is 245t and the dimension is 8.3m × 5.4m × 5.0m). Based on a lot of experience we gained from transporting the converter transformer to Gezhouba, we are able to transport the transformer made abroad. As for those made at home, the combination of road and water transportation can make it. Hence, scheme III is recommended.

When single-phase two-winding transformer is employed, there are outlets of 6 transformer valve sides will go into the valve hall of the polarity. The ideal method is to place 6 transformers at one side of valve hall and directly extend valve bushings into the valve hall, which requires that side (the longer side of oil tank) to not be long. Otherwise, three transformers will also cause complicated wiring and large floor area.  

The voltage of grid side of converter transformer is not related to rated voltage of output side, but depends on the system. The first choice for rated voltage of Three Gorges Power Station is 550kV, which is the same with converter transformer.

In order to calculate the rated voltage of output side of converter transformer, the capacity and tie line of converter transformer accords with two-bridge 12 impulse; the voltage of DC parent line is 500kV and 3KA for DC current. Then the rated voltage of output side of converter transformer is as follows:

Where:

DC voltage Udc= 500kV; α=15°

?=21°

Total capacity of each polarity converter transformer:

Sp=1.047×1.35×2Uvo·Idc=1773MVA

Rated capacity per set when adopting the 6 sets of single-phase two- winding transformer:

St = Sp/6 = 296MVA

2. DC Bias in the Winding

The slight imbalance in conducting and effect of earthing electrode current on converter station earth grid will cause DC bias current in the transformer windings. The DC bias caused by conducting imbalance has been listed in the DC project codes. However, the DC bias caused by the latter is lack of much attention.

The distance from Three Gorges converter station to grounding electrode is about 20-30km and the soil resistivity is quite low. So, the cases like Ben-more or Tiansheng Bridge is not likely to happen there. To date, there has been no report on Gezhouba converter station. The research and calculation should be specially carried out to study how large the DC bias is. The idea that allowable DC bias current per single-phase transformer is 1.5A (1 time rated excitation current) is suitable.

The DC bias will lead to an increase in no-load loss. The intensity depends on the excitation property of iron core and steel sheet, and is also related to the working flux density. Although the increase in no-load loss does not serve as loss acception standard, it must be added to the total loss of temperature-rise test to calculate the influence of DC bias on the rising of temperature.

The DC bias can also cause the noise increase. Hence, the effect of DC bias should be taken into account when working out the level of the noise.

3. Allowance of Short Circuit Impedance

The short circuit impedance of converter transformer is a part of commutation impedance. When converter commutation fails, the secondary side of converter transformer short-circuits. The faulty current is mainly limited by short circuit impedance of transformer. The converter transformer is more likely to have this kind of fault than common AC transformers, so short circuit impedance should be a little higher. However, too large short circuit impedance will cause arc angle to be too big. As a result, the power factor of the whole equipment is quite low. Generally speaking, the short circuit impedance of converter transformer should be 15%. However, the difference between windings, phases and transformer short circuit impedance should remain little. If there is a great discrepancy among commutation time of all valves, great non-characteristic harmonic waves will generate at the AC side, resulting in some problems including over-current and over-voltage. Therefore, the allowance of short circuit impedance for AC transformer is ±10% while that of converter transformer is ±3.75%. The impedance difference among all transformers is ±2%.

4. Load Loss and Harmonic Wave Loss

The feature of converter transformer load loss is that harmonic wave component in the winding load current causes high additional loss. The load loss can be divided into two parts: resistance loss and additional loss. The resistance loss is caused by DC resistance of winding and additional loss includes winding lines and eddy-current loss. Because the frequency of harmonic wave is high, additional loss of harmonic wave is higher than that of fundamental wave.

The harmonic wave loss of converter transformer can be obtained through the following formula:

Px = Hf (Ps - I2R)

Where:

Rx -- total loss of harmonic wave current

Ps -- short circuit loss by injecting rated fundamental wave current

I -- rated fundamental wave current, corresponding to rated load or continuous over-load

R -- DC resistance of winding (80°)

Hf -- harmonic wave loss factor

By doing so, the total load loss of transformer:

Pc = Ps + Px = Ps + Hf (Ps - I2R)

5. Basic Insulation Level

According to the standards IEC 71.GB311.1-83 and SD 326-89, basic insulation level of converter transformer 500kV grid side has two choices: BIL/BSL for 1425kV/1050kV & 1550kV/1175kV. The 1550kV/1175kV is recommended given that the electrical load, thermal load and mechanical load converter transformer withstand are higher than common AC transformer and its insulation degradation is faster.

The basic insulation level of other windings shall be selected in accordance with calculation results of insulation match and design requirements. The basic switching level should be 0.83 times basic impulse level. All test voltages to be calculated should be close to IEC standards.

6. Insulation Test

The insulation test of converter transformer includes power frequency test, impulse test and DC test. The two former tests are the same as AC transformer’s while the DC test is to check the capability of withstanding the DC power stress. The converter transformer bears higher DC electrical stress during the service and system reversal. For insulation structure of oil paper, distributive steady state of DC electrical field is determined by the resistivity of insulation; when transient state, the capacitive distribution gradually changes to impedance distribution, which relies on the time constant of insulation, namely, the resistance and capacitance. Different from the capacitance, the resistivity of insulation material is a variable and can be affected by many factors, including temperature, humidity, electrical strength and voltage time etc. As a consequence of that, the DC test has great uncertainty.

The safety margin of insulation varies greatly according to test condition and different resistivity. Therefore, appropriate test condition, method and monitoring method should be selected to ensure sufficient safety margin.

6.1 Test voltage, withstand duration and monitoring method for DC voltage test

For test voltage, both Institute of Electrical and Electronics Engineers (IEEE) and International Council on Large Electric systems (CIGRE) have calculation formulas. The two formulas are basically the same, but the precision of results obtained by means of CIGRE formula is 5% or so higher than the other. Now CIGRE formula is applied in most of DC system converter transformer tests. In the calculation formula, Ud& UVO should adopt the maximum operating voltage since the transformer is likely to work under maximum operating voltage. 

According to calculation results, DC field distribution also has a transient process, that is, from capacitive distribution to impedance distribution to steady distribution. As for paper and plate insulation, it takes about 500-1200s to reach steady distribution, even 3600s in some cases. In other words, 1h of withstand duration is not enough for some insulation and should be extended to 2h.

The quantity of partial discharge is measured in the last 10min of test to determine whether the test passes or not. Based on the experience of Genan DC system converter transformer test, insulation fault often arises between seconds and several minutes after the DC voltage rise or the voltage polarity reverses. Besides, the fault is often accompanied by high-amplitude partial discharge and audible discharge sound. Therefore, partial discharge signal and sound signal should be monitored during the whole test according to the method stipulated in IEC 270. The acceptance standard is still determined by the quantity of discharge impulse within last 10 min. 

6.2 Method to raise the voltage and monitor for polarity-reversal test

The polarity reversal test is used for evaluating the capability of transformer insulation against polarity reversal field. In actual operation, there are two kinds of polarity reversals: -U→+U and +U→-U. At DC voltage, uneven electrical field has polarity effect. Which reversal test is more strict with insulation is related to the design and production of transformer. The IEEE suggests adopting double polarity reversal test. Besides, the Tianguang DC project has adopted this kind of test. The double polarity reversal test is also recommended for Three Gorges project. The time should be extended from 60min to 90min and from 30min to 45min.

Both partial discharge sound and electrical signal should be recorded during the whole test. Whether the test passes or not is still determined by the quantity of discharge impulse within last 10 min. CIGRE calculation formula is used to calculate the test voltage.

7. Additional Test Items

The CIGRE puts forward some additional test items, including high temperature DC voltage test, AC + DC test and 1h AC applied voltage test. In the aspect of oil assessment, the two former tests are not as good as the AC applied voltage test. Moreover, both tests are quite complicated. The AC applied voltage test is as good as polarity-reversal test in terms of oil assessment. Hence, the AC applied voltage test should be added as a supplement to DC voltage test and polarity-reversal test. Meanwhile, the partial discharge is measured. But, it should be pointed out that the property of electrical field is different from polarity reversal.

For test method and test standard, you can refer to the IEC76 standard on long-time inductance voltage test and partial discharge test. The test voltage should be 1.5 times working voltage, namely: 

8. Valve Side Bushing Test

The insulation structure outside the bushings directly affects the electrical field distribution of bushings at DC voltage. Although bushings simulate the insulation structure, the electrical stress bushings withstand during the converter transformer test is likely to be higher owning to the change of resistivity, which is confirmed by many failures of DC tests at Genan project. Therefore, there should remain certain margin for bushings tested lonely. It is suggested that test voltage be raised to 15% of corresponding test voltage to ensure the safety.

9. Conclusions

(1) It is suggested that single-phase two-winding converter transformer of 296MVA, 550kV be applied for Three Gorges Project.

(2) When you are working out the code of transformer, it is suggested that DC bias and an increase in loss, noise and no-load current be taken into account. Also, the impact of high-order harmonic wave should be considered.

(3) The basic insulation level of converter transformer should be higher.

(4) The DC voltage test and polarity reversal test are important tests for assessing converter transformer insulation. The following are supplements to test methods:

a. The withstand duration of DC voltage test should be extended to 2h;

b. The polarity reversal test adopts double reversal; the voltage-raising time before reversal and staying time after reversal should extend to 1.5h; 45min for the last reversal and 30 for staying time after zero.

(5) The AC applied voltage test should serve as a supplement to DC voltage test and polarity-reversal test in the aspect of oil insulation assessment.