Master's thesis theme:

Parameters basis of the automatic maximum current protection device of mining low-voltage electric installations with the veiw to the influence of the electrotechnical complexes parameters.

CONTENCE

1. Abstract
2. Process of short-circuit research
3. Conclusion
4. References

ABSTRACT

The subject of the research is mine section electric system. Low-voltage mine section electric system contains circuit breakers, magnetic contactors switches, asynchronous motors (AM) and cables [1].

The objective of this work is to discover the character and parameters of the short-circuit process in electrotechnical complex of a technological mine section with the view of influence of back electromotive force (EMF) of asynchronous motors.

The mathematical model structure of electrotechnical complex of technological mine section during asynchronous motors running-down after shutoff protection founded.

PROCESS OF SHORT-CIRCUIT RESEARCH

The electric diagram in Figure 1 shows possible energy flows to the fault point after electric system shutoff protection. On this figure AB is automatic circuit breaker; КМ is magnetic contactor switch; M is asynchronous motor.

Figure 1 - Possible energy flows to the fault point after electric system shutoff protection

Short-circuit process is represented by interchangeable conditions:

• short-circuit occurrence and current flow to the fault point from transformer;
• fault point in-feed from transformer and current flow the fault point from asynchronous motor stator of emergency connection (process continues to the electrical power shutoff from transformer);
• occurrence of equalizing current specified by back EMF AM after power supply shutoff protection;
• fault point in-feed only from AM emergency connection (process continues after contactors rupture of adjacent connections).

The electric diagram in Figure 2 shows equivalent electrotechnical complex with tree-phase fault between in-feed source and asynchronous motor [2].

Figure 2 - Equivalent electrotechnical complex with tree-phase fault between in-feed source and asynchronous motor

Equation of phase-voltage of short-circuited section:

(1)

where L_к = L_к,ф - М - - total phase inductance with regard to the influence of two other phases [2].

Solution of this equation:

(2)

where Z_к - short-circuit impedance; _к - current displacement angle in regard to the voltage in this circuit; - short-circuit switching-on phase; Т_a(u)=x_к / (r_к ) - short-circuit time constant – parameter of damping rate of short-circuit free current component exponent from transformer.

This equation describes processes in section from asynchronous motor to fault point:

iк(н) rн + Lн diк(н) / dt=0

(3)

where L_н = L_н,ф - М - - total phase inductance with regard to the influence of two other phases.

The solution of this equation with regard to the current:

iк(н) = ia(t=0) exp( - t / Ta(н) )

(4)

Current i_к(н) is free and damps with time constant:

Ta(н) = xн / ( rн )

(5)

Total current in the fault point on the interval from short circuit occurrence to shutoff protection by circuit breakers is:

(6)

The electric diagram in Figure 3 shows equivalent electrotechnical complex with the view of influence of back electromotive force of asynchronous motors.

Figure 3 – Equivalent chart of aggregate asynchronous motors

(7)

where р – number of magnetic flows pairs of asynchronous motors; i_s и i_r – current of stator and rotor; L₀ – main magnetic field inductance counting on asynchronous motors phase; А – phase “A” index.

CONCLUSION

The mathematical model structure of mine section electrotechnical system during asynchronous motors running-down after emergency shutoff protection was founded and analyzed. This model insures determination of motor back electromotive force effect on the short-circuit current in case of emergency.

REFERENCES

1. Риман Я.С. Защита подземных электрических установок угольных шахт. – М.: Недра, 1977.- 206 с.
2. Перехідні процеси в системах електропостачання: Підручник для вузів / Г.Г. Півняк, В.М.Винославський, А.Я. Рибалко, Л.І. Несен / За ред. академіка НАН України Г.Г. Півняка. – Дніпропетровськ: Національний гірничий університет, 2002.- 579 с.
3. Маренич К.М. Питання стійкості систем «тиристорний комутаційний апарат - асинхронний мотор» під час фазавого регулювання напруги / Теорія та моделі пристороїв вимірювальної техніки. Збірник наукових праць. – К.: Інститут електродинаміки АН України, 1993. – с.35-39.