ratio is compared in the order of R1, R2, R3, R4 and the fault type is ascertained as given in Table 2.8.
Table 2.5 IEC gas ratios.
| Gas ratio | Value | Code |
|---|---|---|
| R2 = C2H2/C2H4 | R2 < 0.1 | 0 |
| 0.1≤ R2 ≤ 3 | 1 | |
| R2 > 3 | 2 | |
| R1 = CH4/H2 | R1 < 0.1 | 1 |
| 0.1≤ R1 ≤ 1 | 0 | |
| R1 > 1 | 2 | |
| R5 = C2H4/C2H6 | R5 < 1 | 0 |
| 1≤ R5 ≤ 3 | 1 | |
| R5 > 3 | 2 |
Table 2.6 Types of faults.
| No. | Type of fault | Code | ||
|---|---|---|---|---|
| R2 | R1 | R5 | ||
| 1 | No fault | 0 | 0 | 0 |
| 2 | Partial Discharge with low energy density | 0 | 1 | 0 |
| 3 | Partial Discharge with high energy density | 1 | 1 | 0 |
| 4 | Discharge (arc) with low energy | 1→2 | 0 | 1→2 |
| 5 | Discharge (arc) with high energy | 1 | 0 | 2 |
| 6 | Thermal faults of temperatures < 150 °C | 0 | 0 | 1 |
| 7 | Thermal faults of temperatures between 150 and 300 °C | 0 | 2 | 0 |
| 8 | Thermal faults of temperatures between 300 and 700 °C | 0 | 2 | 1 |
| 9 | Thermal faults of temperatures >700 °C | 0 | 2 | 2 |
Table 2.7 Doernenburg gas ratio method.
| Key gases | Minimum concentration L1 (ppm) |
|---|---|
| Hydrogen (H2) | 100 |
| Methane (CH4) | 120 |
| Carbon monoxide (CO) | 350 |
| Acetylene (C2H2) | 1 |
| Ethylene (C2H4) | 50 |
| Ethane (C2H6) | 65 |
Table 2.8 Types of faults by Doernenburg ratio method.
| No. | Type of fault | R1 | R2 | R3 | R4 |
|---|---|---|---|---|---|
| 1 | No fault | Conc. (H2 or CH4 or C2H2 or C2H4)>2L1 and Conc. (C2H6 and CO) <L1] or [Conc. (H2 or CH4 or C2H2 or C2H4)< 2L1] | |||
| 2 | Thermal decomposition | R1 > 1 | R2 < 0.75 | R3 < 0.3 | R4 > 0.4 |
| 3 | Low‐intensity partial discharge | R1 < 0.1 | R2 = ND | R3 < 0.3 | R4 > 0.4 |
| 4 | High‐intensity arcing | 0.1 < R1 < 1 | R2 > 0.75 | R3 > 0.3 | R4 < 0.4 |
2.6.1.3 Rogers Ratio Method
In this technique, three gas ratios R1, R2, and R5 are used for the explanation of the incipient faults in a transformer. The boundaries of the gas ratios suggesting a specific fault type is given in Table 2.9. Rogers method is not dependent on specific gas concentrations for the analysis to be valid. This technique is not always suitable as the accuracy is not very good in identifying faults.
2.6.1.4 Duval’s Triangle
The Duval Triangle uses three hydrocarbon gases only, namely methane (CH4), ethylene (C2H4), and acetylene (C2H2), to detect the fault types [75, 76]. The concentration of the gases is plotted in a triangular coordinate system. The types of faults detected by the Duval’s triangle method are given in Table