LIMITING CAPACITIVE GROUND-FAULT CURRENTS AS A MEASURE TO REDUCE ACCIDENTS IN MINE ELECTRICAL INSTALLATIONS

A. P. ZUBAREV

The power supply of coal mines hazardous with respect to gas and dust is carried out, as is known, by networks with isolated neutrals, at the most widespread primary voltage of 6 kV.

The basis for choosing this type of network was the condition that, in the event of accidental grounding of one of the phases in them, under certain parameters, no open electric arc arises. However, it is also well known that in such networks, when the capacitive ground-fault current is \(I_c > 5a\), the above emergency mode is already accompanied by the appearance of an electric arc of intermittent character. If, at values of \(I_c\) from 5a to 10a, this phenomenon is observed comparatively rarely, then at \(I_c > 10a\) it is no longer an exception and becomes almost inevitable.

In addition to its immediate danger for mines, an intermittent electric arc simultaneously excites resonant overvoltages in the network, reaching \(3.5U_{\text{ph}}\), which also cause breakdowns to the frame of the insulation of electrical equipment, with its subsequent failure; this is confirmed by a number of cases in the mines of Kuzbass.

Of particular danger here are secondary breakdowns to the frame in other phases, which already lead the networks to severe interphase short circuits.

Over the last 10–15 years, electrification has been widely applied in coal mining, and this circumstance has led to a great development of the electric-network system. At many large mines the total length of 6 kV electrically connected networks now reaches 10 km and more. It would be an obvious mistake not to assess this change, since a corresponding increase has occurred in the capacitive current \(I_c\), which is determined with sufficient accuracy by the known formula:

\[ I_c = \frac{U_l}{350}\left[35\sum l_k + \sum l_v\right], \]

where \(U_l\) is the line voltage of the network, kV,
and \(\sum l_k\) and \(\sum l_v\) are the lengths of the cable and overhead networks, km.

An especially serious situation is presented by supplying a group of closely located mines from a common 6 kV supply network (see Fig. 1). Such schemes already have a network length reaching 100 km and more.

Let us explain this by an example. Figure 1 shows a frequently encountered variant of the power supply of a group of mines.

Suppose that the total length of the electrically connected overhead and cable networks is represented by the data given in Table 1.

Table 1

The capacitive ground-fault current of such a group, at an average line voltage of 6.3 kV (taking into account losses in the mine networks), will be:

\[ I_c=\frac{6.3}{350}\,[35\cdot 44+116]\simeq 30\ a. \]

Fig. 1. Diagram of a typical power supply for a group of mines located near a district substation.

As can be seen, the power-supply scheme shown will undoubtedly contribute to the occurrence of arc overvoltages during ground faults of one of the phases, with all the ensuing consequences. It should be noted that in cable networks this phenomenon, caused by a primary breakdown of phase insulation, in turn rather easily causes secondary multiple breakdowns, both in the network and in electrical equipment. This occurs especially where, under production conditions, the insulation is subjected to everyday systematic damage and deterioration (mechanical impacts, moisture, dust, etc.).

A study of the existing power-supply schemes of a number of districts in the Kuznetsk Basin shows that the capacitive ground-fault current in most industrial groups exceeds 10 A, and this is already sufficient for serious accident consequences to arise.

The prospective development of the operating mines of the Kuznetsk Basin, as well as of some others, is proceeding mainly through the development of lower horizons. In many cases this circumstance will undoubtedly require an increase in the length of underground high-voltage cable networks. Supplying electric power to new workings through boreholes from surface sectional substations at low voltage has a known economic limit and, for the lower horizons, will not be profitable either in terms of electric-power losses or in terms of copper consumption in the cables.

Taking this tendency in the development of high-voltage mine cable networks into account, it should be considered entirely timely and necessary to implement a number of preventive measures to reduce capacitive ground-fault currents, as a means of preserving and protecting expensive high-voltage electrical equipment. It should be noted that the accidental failure of such electrical equipment, which as a rule has high power, in addition to causing direct damage to coal production, requires substantial expenditures by the mine for its restoration; moreover, the latter is not ensured with the required quality even under the conditions of the repair shops of combines, which for the most part possess only primitive technology.

All of the foregoing confirms the need for a serious survey of the existing electrical-network facilities at mines, aimed at studying and determining the magnitudes of capacitive ground-fault currents. First of all, this should be carried out by the combines in those trusts where cases of breakdown of high-voltage electrical equipment and cable networks have already been recorded.

Although the survey and analysis of the existing electrical-network facilities of coal trusts and mines does not present any particular difficulty, it nevertheless requires observance of a certain procedure, namely:

Using the refined current diagrams and plans, the lengths and types of individual sections of the networks at each mine are determined separately; the data obtained are entered in summary statements, from which the values of the corresponding capacitive ground-fault currents are determined by the formula given above.

Measures to combat resonant overvoltages in existing 6-kV mine networks may be divided into two basic types: prevention of operational deterioration of network insulation, and limitation of capacitive ground-fault currents.

Measures of the first type should include, for example:

1) introduction at mines of systematic kenotron testing of all high-voltage electrical equipment and cable networks on the surface, and, where possible, underground as well;

2) installation at the central substation of sensitive relay protection against single-phase ground faults.

For measures of the second type, the following may be recommended:

1) “sectionalizing” the electrical networks;

2) arranging mine power supply according to the principle of a “deep” 35-kV input;

3) installation of arc-suppression coils of the ZROM-6 type in neutral circuits, with their subsequent grounding.

Let us consider all this in more detail.

The question of preventive insulation tests of high-voltage electrical equipment and cable networks by high-voltage direct current ...

voltage requires no special explanation, since it is sufficiently well covered in the technical literature and precisely regulated by the PTEEPT [Rules for the Technical Operation of Electrical Installations of Industrial Enterprises] of 1947/1951. Underestimation of this issue by mine personnel deserves only censure.

The nature and procedure for “sectionalizing” electrical networks of great length are determined from the specific data of a given existing power-supply scheme for the group of mines being examined. In particular, in radial and trunk schemes of power supply for mining enterprises of categories I and II, the indicated “sectionalizing” can be carried out by means of separate operation of the power transformers of the district substation and separation of the loads on the busbars of the substations of industrial enterprises into independent sections (see Fig. 2). By this operation one can achieve a sharp reduction in capacitive currents. At the same time, as the reactance increases, short-circuit currents will also be reduced.

Fig. 2. Scheme of “sectionalizing” in trunk power supply of mines.

In a ring network (see Fig. 1), “sectionalizing” can be carried out in two variants. As is clear from the diagram of the first variant, “sectionalizing” is performed by dividing the entire group of mine substations into two parts, which are connected to different sections of the 6 kV distribution busbars of the district substation (see Fig. 3).

Fig. 3. Scheme of “sectionalizing” according to variant I in ring power supply of mines.

In the second variant (see Fig. 4), the ring circuit of the power supply of the entire unit remains unchanged, but the “sectioning” into two subgroups is provided by installing two transformers T with \(K_{tr}=1\). This circuit has the positive feature that, in addition to “sectioning,” it makes it possible to supply normal or increased voltage to consumers at remote substations. Its negative feature consists in the need to install additional equipment, which at the same time will also be an inductive load. The latter, as is known, will somewhat increase losses in the network and in some cases will cause a partial decrease in \(\cos\varphi\) of the given group of enterprises. However, taking into account the comparatively easy possibility of compensating \(\cos\varphi\) under district conditions, and the insignificance of the “specific weight” of the additional losses from the transformers, the “sectioning” circuit according to variant II is quite permissible in operation.

Fig. 4. “Sectioning” circuit according to variant II
with ring power supply of mines.

... especially when supplied from a powerful power system, when at the same time a reduction in short-circuit currents is also required.

The question of arranging relay protection against single-phase ground faults in 6 kV networks with an isolated neutral has now been solved with the aid of high-sensitivity current relays, manufactured since 1952 by the “Energo-pribor” plant.^1)

Undoubtedly, this protection must be widely introduced, since it also ensures complete selectivity of operation. In some cases it is possible to use the insulation-monitoring circuit available at each substation for automatic “sectioning” of networks. Such a relay-protection circuit (Fig. 5), using voltage relays, can provide automatic “sectioning” of the networks shown in Figs. 2 and 3; for this purpose it is only necessary to replace the sectional disconnectors with oil circuit breakers where required.

In the radial power-supply circuit (Fig. 2), this measure must be implemented at all substations of the group of mines under consideration, including the district substation.

In the ring power-supply circuit according to variant I (Fig. 3), a sectional oil circuit breaker will be required only at the district substation.

^1) See the journal Elektricheskie stantsii, No. 11 for 1951, article by S. Ya. Gelfand, I. N. Popov, and G. V. Subbotin, “Sensitive Current Protections Against Ground Faults and Experience in Their Operation”; Elektrichestvo, No. 3 for 1952, article by I. N. Popov, “Sensitive Protection Against Ground Faults with a Cable Current Transformer.”

...the relay-protection circuit shown (Fig. 5) operates when the potentials in any two phases rise relative to “ground.”

This protection is easiest to implement when a five-limb measuring voltage transformer with a compensation winding is available at the substation. In this case only one voltage relay and, correspondingly, one time relay are required. The latter is installed in each circuit to prevent false operation in the event of short-duration ground faults. In view of the need to monitor the state of insulation in networks with an isolated neutral, the installation of a “group” of either three single-phase voltage transformers or one five-limb transformer will in all cases not constitute additional equipment. The installation of a set of voltage and time relays will not involve major expense for mines and other enterprises. It should be assumed that this measure for preventing accidents in high-voltage electrical installations due to arc overvoltages will be quite rational.

Fig. 5. Circuit of an automatic “sectionalizing” device.

The transition to mine power supply according to the principle of a “deep” 35 kV input will, in many cases, hardly meet with objections from enterprise personnel as to its expediency.

A “deep” 35 kV input fundamentally changes the state of industrial network nodes. In this case each mine already has only its own 6 kV electrical installation, not electrically connected with others. This will undoubtedly lead to the desired sharp reduction of capacitive currents to values that are of low danger or are incapable of causing intermittent arcs to appear, and consequently also of the overvoltages that give rise to accidents. At the same time, the “deep” input will make it possible to sharply reduce electrical-energy losses, the consumption of nonferrous metals, and to ensure normal operating voltage on the buses of mine central substations. It should be noted that the voltage of the primary supply networks of 35 kV in no way eliminates the possibility of applying the principle of “sectionalizing” in the secondary 6 kV electric-power distribution networks at mines, should the need arise.

Finally, regarding the use of arc-suppression coils of the ZROM type. The use of this means, which provides quite reliable compensation of capacitive currents, has fairly broad practice in the USSR even in 6–10 kV networks. In operation, the latter, with \(I_c \leq 20a\), present significantly less danger than mine networks with a neutral grounded through an arc-suppression coil of the ZROM type. The refusal of the Ministry of the Coal Industry to use this protection system is more “traditional” than justified.

CONCLUSIONS

1. The electrical-network facilities of coal mines, at their present stage of development, require urgent inspection and study in order to determine the magnitudes of capacitive ground-fault currents.

2. In order to reduce accident rates, it is necessary to introduce systematic kenotron testing of electrical equipment and of the 6 kV cable networks of surface installations.

3. It is necessary, by every possible means, to reduce capacitive ground-fault currents in 6–10 kV networks by “sectionalizing” the latter or by arranging a “deep” 35 kV feed into the mines. As preventive measures against emergency overvoltages, it is advisable to install highly sensitive current relay protection against single-phase ground faults and to employ automatic “sectionalizing” of power-supply circuits by means of insulation-monitoring schemes. The operating experience with ZROM arc-suppression coils should also be used.

The implementation of the above measures will be one of the factors contributing to increased coal production and to the profitability of enterprises in the coal industry.

REFERENCES

1. L. I. Sirotinskii. Overvoltages and Protection Against Overvoltages in Electrical Installations, ONTI, 1937.

2. A. I. Kuznetsov. Safety Engineering in Electrical Installations, GEI, 1952.

3. L. N. Baptidanov and V. I. Tarasov. Electrical Equipment of Electric Power Stations and Substations, GEI, 1952. Safety Rules in Coal and Shale Mines. Ugletekhizdat, 1951.