PROTECTION AND MONITORING OF THE OPERATION OF HIGH-POWER SEMICONDUCTOR GERMANIUM AND SILICON DIODES

P. I. GOLOVACHEV, N. P. RYASHENTSEV, N. E. SHCHERBAKOV

(Presented by the Department of Mining Machines and Mine Transport)

Mercury and selenium rectifiers used for charging storage batteries, and other converters of alternating current into direct current currently used in mines, have substantial shortcomings:

1. The difficulty of automating traction and charging substations.
2. Low efficiency, and consequently excessive consumption of electric power.

At the Kuzbass mines “Im. 7 Noyabrya” of the LeninuGol trust and “Banakovskaya” of the Belovougol trust, traction substations were built using germanium diodes of the GVV-200 type. However, the imperfection of the protection system (series-connected diodes were shunted by lighting lamps rated at 75 or 25 W), as well as the absence of monitoring of diode cooling, led to frequent failures of the germanium diodes, which made it necessary to discontinue operation of the indicated substations.

Despite their high economy and ease of maintenance, traction substations using germanium diodes, because of the absence of reliable protection, did not come into wide use, and the substations in operation quickly failed.

At the Department of Mining Machines and Mine Transport of the Tomsk Polytechnic Institute, a charging substation (Fig. 1) was built using germanium diodes with automatic disconnection of the battery being charged. The charging substation consists of the electric motor of the fan 1, the blown block of germanium rectifiers (2) of the VG-50-80 type, regulating resistances 3, fusible fuses 4, start and stop buttons 5, contactor 6, ammeter 7, voltage relay 8 (serving for automatic disconnection of the charged battery), voltmeter 9, magnetic starter 10 (for starting the fan), air-flow relay 11, and the substation housing 12.

Depending on the rectified voltage and power, the diodes are connected for series or parallel operation, or in a mixed connection. When rectifiers are connected in series, it is necessary that the voltages across the series-connected elements be equal.

This is accomplished by selecting valves with identical characteristics and by connecting shunting resistors in parallel with the elements. The value of these resistors should be chosen so that it is directly proportional to the voltage across the elements and to the mismatch coefficient, and inversely proportional to the difference in the reverse currents of the elements.

Fig. 1. Converter substation for charging storage batteries.

For parallel operation it is necessary to select elements with equal voltage drops. The connecting leads of the individual branches must be equal in length, and the contact connections must be made reliably. Breakdown of one of the elements connected in series, as a rule, leads to rapid failure of the remaining elements. If an element burns out and the circuit of one branch is broken when the elements are connected in parallel, the other elements are overloaded or the waveform of the rectified current is distorted (in two- and three-phase rectifier circuits).

To monitor operation and protect the diodes, a combi-

[[unclear: beginning of word]] protection, which is shown in Fig. 2 and operates as follows.

To monitor breakdown of an element, the polarity of the voltage of battery \(B_1\) is applied opposite to the polarity of the diodes. In this case, when the diodes are in good condition, no current flows in the circuit \(B_1 = RP\text{-}7 = DR\text{-}1 = ShI\text{-}1 =\) diode \(= B_1\).

In the figure:
left block—“Circuit for monitoring diodes against short circuit”; “to the contactor coil for switching on the reserve”; \(B_1\), \(RP\text{-}7\), \(DR\text{-}1\), \(ShI\text{-}1\).
right block—“Circuit for monitoring diodes against phase interruption in the diode”; “to the contactor coil for switching on the reserve”; \(B_2\), \(ROF\), \(DR\text{-}2\), \(ShI\text{-}2\).

Fig. 2. Circuit for protecting germanium diodes against breakdown and phase interruption.

In the event of breakdown of an element in the indicated circuit, current begins to flow, relay \(RP\text{-}7\) operates and gives the corresponding pulse. Successive monitoring of all diodes is carried out by means of a sliding two-row contact \(ShI\text{-}1\) (of a step-type finder) and one relay \(RP\text{-}7\).

To monitor a break in the circuit in one of the parallel-operating branches, the polarity of battery \(B_2\) must correspond to the polarity of the diodes and, with the elements in good condition, current flows in the circuit \(B_2 = ROF = DR\text{-}2 = ShI_2 =\) diode \(= B_2\). In the event of a break in the circuit of the assembled elements, the flow of current in the coil of \(ROF\) (phase-interruption relay) ceases; the relay operates and gives the corresponding pulse for switching on the reserve.

Successive monitoring of each branch is carried out by means of the sliding contact \(ShI\text{-}2\) and one \(ROF\) relay. The speed of movement of the contact is determined by the release speed of the \(ROF\) relay and by the number of monitored elements.

The use of the indicated circuit and of a multi-row sliding contact makes it possible, with 2 relays, to monitor the operation of a practically unlimited number of semiconductor rectifiers. This significantly simplifies and reduces the cost of installing this type of protection, while the reliability of protection of semiconductor diodes is increased. In the event of multiple disconnection of the rectifier unit, the circuit automatically switches the battery supply to the reserve unit.

Conclusion

Charging substations assembled with germanium rectifiers quickly fail because of the absence of reliable protection.

The proposed protection of germanium rectifiers against breakdown and phase loss ensures reliable operation of the rectifiers and increases their service life.