A SAFE POWER SUPPLY SCHEME FOR EXPLORATORY DRILLING RIGS

I. G. Galeev

(Presented by the scientific seminar of the Department of Exploration Technology, MPI)

The use of electricity as the main form of energy for putting into operation the mechanisms and apparatus of exploratory drilling is increasing every year. In many geological exploration organizations, especially in detailed coal exploration, the only form of energy that drives all the mechanisms of a drilling rig is electricity. In this connection, problems of electrical safety become especially acute.

The conditions under which electricity is used at drilling rigs for geological exploration work have a number of distinctive features.

These features are characterized by:

a) the temporary nature of the operation of electrical equipment at a given site (the duration of operation of a drilling rig at one site usually ranges from several days to several months) and by frequent moves from one work site to another;

b) the presence of a large amount of moisture;

c) the operation of electrical equipment under conditions of insufficient protection from the effects of atmospheric phenomena.

These features adversely affect the provision of safety against electric shock for the operating personnel.

Along with complete safety, the power supply scheme of an exploratory drilling rig must ensure a certain reliability. Interruptions in the power supply of drilling rigs are mainly associated with reduced productivity and idle time for workers. They do not pose a danger to the life and health of the workers.

The principal device for protecting a person from electric shock at exploratory drilling rigs is protective grounding. Protection of people and electrical equipment from atmospheric electricity is carried out with the aid of lightning rods.

The main cause of electric shock to a person at drilling rigs is deterioration of the insulation condition. As a result, parts of the equipment that normally are not energized come under voltage. This leads to electric shock to a person who has touched the enclosure or the metal parts of electromechanical equipment.

The insulation currently used on electrical equipment ages over time and loses its insulating properties. Many grades of insulation, under the action of moisture and temperature changes, very quickly worsen their dielectric constant and dielectric loss angle.

Studies show, for example, that the dielectric constant and the dielectric loss angle of the insulation of cables of the GRShS type decrease to a considerable extent under the action of atmospheric factors [1]. As a result, the insulation resistance is greatly reduced. The wave resistance of cable insulation to alternating current is determined by the following expressions:

\[ Z = \sqrt{\frac{R + i\omega L}{G + i\omega C}}, \]

\[ G = \omega C \operatorname{tg}\delta, \]

where
\(R\) — active resistance of the insulation;
\(L\) — cable inductance;
\(G\) — insulation conductance;
\(C\) — insulation capacitance;
\(\delta\) — dielectric loss angle;
\(\omega\) — angular frequency.

The capacitance of a single-core cable is

\[ C = \frac{0.024\Sigma}{\lg \frac{r_2}{r_1}}, \]

\(\Sigma\) — dielectric constant,
\(r_1\) — radius of the conductor,
\(r_2\) — radius of the cable.

A consequence of deterioration of the insulation condition is an increase in leakage in the power-supply circuit of the drilling rig and an increase in the danger of electric shock to a person. In addition, deterioration of the insulation condition can lead to arc flashover of the insulation of the phases of electrical equipment and to fires. The insulation of current-carrying parts of electrical equipment loses its insulating properties for many other reasons as well, including heating during abnormal starting, operation on two phases, or overturning of the motor. In all these cases overcurrents arise, leading to overheating of current-carrying parts and deterioration of the insulation condition. Hence follows the possibility of electric shock to a person, the possibility of fires and explosions at the drilling rig in the event of gas release from boreholes.

Consequently, in order to create safe conditions for the power supply of exploratory drilling rigs, it is necessary to ensure the normal condition of insulation under all operating conditions, or it is necessary to ensure continuous monitoring of the insulation condition and instantaneous disconnection of the network in every case when the insulation resistance falls below the permissible standard. Since insulation that would preserve its properties within the standard under all conditions does not currently exist, this problem must be solved by means of continuous monitoring of the insulation condition and immediate disconnection of the damaged section of the electrical network.

At present, to disconnect a network with voltage up to 1000 V when its insulation is damaged in installations with an isolated neutral, protection is used that responds to zero-sequence voltage (asymmetry) or that operates on an auxiliary current, for example, a leakage relay of the type (RUV-0.4).

Both of these protections, in accordance with their characteristics, have certain shortcomings.

The main shortcomings of protection that responds to zero-sequence voltage are:

1) the lack of sensitivity of the protection to deterioration of the insulation condition simultaneously in all three phases;

2) the dependence of the precision of operation of the protection on the capacitance of the network relative to ground.

The main shortcoming of protection operating on an auxiliary current is the need for an auxiliary current source that ensures operation of the protection. This complicates the circuit and reduces reliability.

At present the most fully developed system is protection using a leakage relay of the RUV type.

The Safety Rules in coal and shale mines, § 410, require that people be protected from electric shock in underground conditions by means of protective earthing and leakage relays with automatic disconnection of the damaged network with voltage up to 700 V. For this purpose, in 380 and 660 V networks, leakage relays of the RUV-380 and RUV-660 types are used. For lighting networks and networks supplying hand tools, the RUV-127 is used.

The many years of operation of this protection system in the coal mines of the Soviet Union has fully justified itself.

The conditions of power supply for drilling rigs are in some respects similar to the operating conditions of electrical equipment in underground workings. It is therefore proposed that the protection of people from electric shock in the power supply of exploratory drilling rigs be carried out by means of relays of the RUV type.

In this case, the power-supply circuit for drilling rigs may be represented in the following form (Fig. 1).

Fig. 1

The protection circuit is implemented using standard electrical equipment.

Switching and protection of the transformers on the high-voltage side are carried out in the usual manner.

On the low-voltage side of the transformer, a feeder automatic circuit breaker of the AFV type is installed, or a standard automatic circuit breaker of type A equipped with a special tripping—

…with a tripping coil installed between the maximum-current relays on the automatic circuit-breaker panel.

A leakage relay is connected to the automatic circuit breaker, as shown in the diagram (Fig. 2).

Fig. 2

From the automatic circuit breaker, electric power is supplied in the usual manner to the magnetic station. If the insulation resistance of the network from the transformer to the current receivers falls below the standard, the leakage relay sends a signal to trip the automatic circuit breaker, and the supply of electricity to the drilling installation is stopped until the fault is eliminated. Thus continuous monitoring of the insulation resistance is ensured and, consequently, protection against electric shock to personnel.

The wiring diagram of the transformer substation according to the proposed scheme is shown in Fig. 2.

LITERATURE

1. I. G. Galeev. Changes in the tangent of the dielectric-loss angle and the dielectric permittivity of GRShS cable insulation under the action of moisture and time. Proceedings of Higher Educational Institutions, “Mining Journal,” 1962, No. 12.