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______________________________________________________* E-mail: [email protected]

EXPERIMENTAL EVALUATION OF TRANSFERRED SURGES IN MV

TRANSFORMERS FROM HV/LV

HERMOSO B.*, AGUADO M., SENOSIAIN V., MARTNEZ CID P.M.UNIVERSIDAD PBLICA DE NAVARRA IBERDROLA

(Spain)

In MV lines lightning produces overvoltages by direct action on line conductors or by inducedovervoltages. In both cases, if the surge voltage resultant is lower than the line BIL, a surgewave is produced and travels by the line arriving, in many cases, to the MV/LV transformers.These transformers allow the path of the waves from the HV to the LV side by differentcoupling mechanisms (capacitive, oscillatory, electromagnetic), and finally the LVinstallations are the victims for this kind of overvoltages. According with the standard EN60071-2, Annexe E is easier to obtain the value of the transferred overvoltage in transformers,using a recurrent surge generator. From the results obtained in the tests describe above andconsidering the CIGRE model for coupling mechanism transformer simulation it has beenformulated a simple transformer model that it allows knowing the transferred surge at lowside.

Keywords:Lightning - Insulation Coordination-Overvoltages-Transformers

1. HV-LV TRANSFERRED OVERVOLTAGES

The overvoltages transmission in a distribution transformer from the HV to LV side, due to

the lightning, is a main point for the transformer design, and for the customer protection. Thevalue of the result transmitted overvoltages is given for the shape of the incident wave (fastfront or low front) and by the transformer structure (winding shapes, connections....) whose isresumed in capacitive and inductive values.

The different transmission modalities are indicated in the EN 600171-2, Annex E [1]:

- Electrostatic or capacitive transmission. This component appears the first and hisfrequency is into the range of MHz

- Oscillatory transmission. This component is due to the naturals oscillations generated bythe capacitance to ground and the inductance of transformers

- Electromagnetic transmission. This component, also called "normal transmission" isrelated with the transformation ratio, the inductance and the burden of transformer

21, rue d'Artois, F-75008 Parishttp://www.cigre.org CIGR

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The capacitive component appears the first; his frequency is in the range of MHz. and itsvalue is related with the windings capacities and the capacity between the windings andground.

After the capacitive component, appears the component transmitted by inductive way, whose

value depend of voltage distribution in the winding. The transformers work as in normalcondition.

The oscillatory component is dimmed and overlapped to the electromagnetic component.Normally is weak and is not very important except when it appears resonance phenomena.

Although Standard EN 60071-2 Appendix E gives the expressions to evaluate the differenttransmission components, this one recognizes the difficulty to applied them, due to the severalfactors that may appear and must been take into account. It recommends, as a more practicalmethod to measure the response with a recurrent generator.

2. TESTS WITH REGURRENT GENERATORThe tests have been achieved with a recurrent surge generator, output 100 volts wave 1,2/50s(Fig. 2), 35 Hz, applied on different distribution transformers (power, ratio, group connection,insulation). See data in table 1.

Table 1 Transformers characteristicsKVA HV/LV (kV) Group Insulation

25 0,945/0,4-0,230 Yy0 oil

25 13,8/0,38 Dyn11 oil25 13,2/0,38 Dyn11 oil25 13,8/0,4/0,132 Dz0-Dyn11 oil50 13,6/0,4 Yzn11 oil50 13,2/0,23 Dyn11 oil100 14,2-21/0,42-0,24 Yzn11 oil200 13,8/0,42-0,24 Dyn11 silicone250 13,86/0,42-0,24 Dyn11 oil400 13,2/0,42 Dyn11 dry630 13,86/0,42-0,24 Dyn11 oil630 13,86/0,42-0,24 Dyn11 silicone630 20/0,4 Ynd11 oil630 13,86/0,42 Dyn11 silicone

1250 30/3,19 Dyn11 oil

The voltage has been applied in the HV side between two phases, and the oscillatoryresponses obtained in the LV side has been registered as seen in figure 1 and figure 2 (100kVA-630 kVA) The values (frequency and amplitude as p.u. of voltage applied) are showedin table 2.

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Table 2 Test resultKVA input/ouput MHz p.u. KVA input/ouput MHz p.u.

25 AB-ca 0,164 0,0711 250 AB-ca 0,476 0,106BC-ca 0,172 0,0696 BC-ca 0,49 0,186CA-ca 0,175 0,0777 CA-ca 0,463 0,203




100 AB-ca 0,152 0,0425 630 AB-ca 0,58 0,0827

BC-ca 0,144 0,0077 BC-ca 0,556 0,0303CA-ca 0,135 0,03 CA-ca 0,606 0,0847250 AB-ca 0,458 0,0849 630 AB-ca 0,667 0,0784

BC-ca 0,465 0,181 BC-ca 0,6 0,157CA-ca 0,444 0,184 CA-ca 0,595 0,0849

1250 AB-ca 0,5 0,045BC-ca 0,5 0,035CA-ca 0,5 0,045

Input

Wave 1,2/50 sVc= 100 V

Oscillatory wavef=0,179 MHzVc=783mV=0,783V

Attenuation0,783/10=0,0783

Oscilatory wavef=0,172MHzVc=505mV=0,505V


Oscilatory wavef=0,179 MHzVc=777mV=0,777V


Fig. 1. Oscillatory response 100kVA; 14,2-21/0,42-0,242 kV; Yzn11; Oil

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4. SIMPLE TRANSFORMER MODEL

The model developed is based on Group III of CIGRE (figure 5) but with some modifications[2].

Fig. 5. CIGRE model, Group III

The reason of these modifications is connected with the objective of getting a very simplemodel, understanding simple as feasible, that is without taking so many measures.

According above indicated the model proposed is:

Fig. 6. Transformer model proposed

being

1R and 1L resistance and inductance of primary winding

2R and 2L resistance and inductance of secondary winding

MTC , BTC and MBC primary and secondary capacitance to ground and capacitance betweenprimary and secondary winding

tR and tL resistance and inductance of transformer grounding

1Rk , 1Lk , 2Lk coefficients of primary and secondary windings

The model in ATP is showing in the figure 7:

1Rk R1 1Lk L1 2Lk L2R2

W1 W2CMT CBT

CMB

Rt

Lt

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Fig.7. Model in ATP

5. CONCLUSIONS

This paper shows that the use of a recurrent surge generator to analyse the transferredovervoltages in distribution transformers is a very useful tool.

The transferred voltage coefficient for different distribution transformers is in the range of the10 % of applied voltage and the oscillatory frequency is in the range of 200 kHz fortransformers with rated power between 25-50 kVA and around 600 kHz. for a wide range oftransformers.

This paper presents too a simplified model to analyse the transferred voltages in three-phasedistribution transformers. The simplicity lies on the fact that we need only the constructivedata (given by manufacturer) and the capacitance to ground of each winding and betweenthem (feasible measurements).

6. ACKNOWLEDGEMENTS

The authors gracefully extent their thanks to IBERDROLA for provide us the necessarytechnical and economical support.

7. REFERENCES

[1] EN 60071-2 Insulation Coordination. Part 2: Application guide. 1997[2] CIGRE WG.33.02, Guidelines for representation of network elements when

calculating transients, October 1991

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