Required Limiting Breaking Capacities of Circuit Breakers at Thermal Power Plants
Abstract Generated abstract
This paper evaluates the maximum interrupting capacity required for generator-voltage bus-section circuit breakers at thermal power plants. Using two typical station bus schemes, the authors calculate three-phase short-circuit breaking power under assumptions including infinite higher-voltage system capacity and tie-transformer ratings of 50, 70, and 100 percent of generator capacity. The results show that required interrupting capacity is strongly affected by generator and coupling-transformer capacity, but only weakly by the number of bus sections. The study concludes that MG-10 circuit breakers rated up to 1800 MVA are inadequate for interrupting bus-tie short-circuit power in thermal power plants equipped with 100 MW generators.
Full Text
Required Maximum Interrupting Capacities of Circuit Breakers at Thermal Power Plants
I. I. Chalkin, V. I. Chalkina
(Presented by the scientific seminar of the Department of Electric Power Stations)
The circuit breakers for generator voltage produced by our plants have insufficient interrupting capacity. This is evident from the study presented below.
To determine the required interrupting capacity of a bus-section circuit breaker, two typical schemes were investigated, shown in Fig. 1. In the calculation it was assumed that the system capacity on the higher-voltage buses is equal to infinity, and the capacities of the tie transformers were taken as equal to 50, 70, and 100% of the generator capacity.
The turbogenerators have the following characteristics (Table 1). The reactors have the following data:
$S_p = 0.75 S_g$, and $x_p = 8\%$ in the first scheme; $S_p = 0.5 S_g$ and $x_p = 10\%$—in the second.
The calculations reduce to finding the interrupting capacity for a three-pole short circuit.
Fig. 1
Table 1
| Type | Power, MW | \(U_n\), kV | \(\cos\varphi_n\) | \(x''_d\) | \(S_n\), MVA |
|---|---|---|---|---|---|
| TV-60-2 | 60 | 10.5 | 0.8 | 0.132 | 75 |
| TV-100-2 | 100 | 10.5 | 0.85 | 0.183 | 118 |
\[ S_{\text{break}}=S_{t=0}=\frac{S_g}{x_{\text{calc}}}, \]
where \(x_{\text{calc}}\) is the calculated resistance of the equivalent circuit with respect to the short-circuit point.
Below is an example of determining the interrupting capacity for circuit 2 with four sections, for a three-phase short circuit at point \(K\).
Initial data:
\[
S_t=S_g=75\ \text{MVA};\quad S_p=0.5S_g=37.5\ \text{MVA};\quad x''_d=0.132;
\]
\[
x_t=0.105;\quad x_p=0.2.
\]
The calculated equivalent circuit is shown in Fig. 2.
Fig. 2
Table 2
| Circuit No. | Number of sections | Generator power, MW | Tie-transformer capacity \(0.5S_g\) | Tie-transformer capacity \(0.75S_g\) | Tie-transformer capacity \(S_g\) |
|---|---|---|---|---|---|
| Circuit No. 1 | 2 | 60 | 1310 | 1600 | 1720 |
| Circuit No. 1 | 2 | 100 | 1740 | 2100 | 2410 |
| Circuit No. 1 | 3 | 60 | 1315 | 1500 | 1680 |
| Circuit No. 1 | 3 | 100 | 1760 | 2060 | 2360 |
| Circuit No. 2 | 4 | 60 | 1430 | 1605 | 1800 |
| Circuit No. 2 | 4 | 100 | 1930 | 2210 | 2520 |
| Circuit No. 2 | 6 | 60 | 1450 | 1630 | 1950 |
| Circuit No. 2 | 6 | 100 | 1970 | 2270 | 2530 |
\[ x_1 = \frac{2x''_d \cdot 2x_t}{2x''_d + 2x_t} + x_p = 0.317, \]
\[ x_2 = \frac{x''_d \cdot x_1}{x''_d + x_1} + x_p = 0.293, \]
\[ x_3 = \frac{x''_d \cdot x_t}{x''_d + x_t} = 0.0585, \]
\[ x_p = \frac{x_3 \cdot 0.5x_2}{x_3 + 0.5x_2} = 0.0417, \]
\[ S_{t=0} = \frac{75}{0.0417} = 1800\ \text{MVA}. \]
The results of the investigations are given in Table 2, from which it is evident that the interrupting capacity depends very strongly on the capacity of the coupling transformers and generators, and is practically independent of the number of sections.
The MG-10 circuit breakers currently being produced, with the highest interrupting capacity of 1800 MVA, cannot ensure interruption of the short-circuit power in the circuit of the bus-tie circuit breaker at thermal power plants with 100 MW generators.
REFERENCES
- L. N. Bashidianov, V. I. Tarasov. Electrical equipment of power stations and substations.