PROBLEMS OF DETECTING SINGLE-PHASE GROUNDING IN LOW VOLTAGE NETWORKS

ABSTRACT

The article considers the main operating modes of neutrals in the power grids of Uzbekistan. A brief description of each mode in terms of emergency modes. In particular, the grounding modes in the phases of power grids caused by short-circuit currents are considered.

АННОТАЦИЯ

В статье рассмотрены основные режимы работы нейтралей в электрических сетях Узбекистана. Краткое описание каждого режима с точки зрения аварийных режимов. В частности, рассмотрены режимы заземления в фазах электрических сетей, вызванные токами короткого замыкания.

Keywords. Neutrals, capacity, fault currents, overcurrent, line voltages, complex branched networks, higher harmonic components.

Ключевые слова. Нейтрали, емкость, токи короткого замыкания, перегрузки по току, линейные напряжения, сложные разветвленные сети, высшие гармонические составляющие.

In our country, the following modes of operation of neutrals are traditionally used:

Neutrals are not grounded - they are connected directly to the grounding device. These networks include power lines up to 1 kV and above 110 kV.

Effectively connected to neutral ground - networks with grounding factor not exceeding 1.4. In three-phase networks, the grounding factor is the difference between the undamaged phase and the potentials between the ground and phase at the grounding point, or the potential difference between the ground and the other two phases before the short circuit at this point. As a rule, such networks are used at voltages of 110 kV and above, with the help of a disconnector it is possible to connect and disconnect the neutrals to limit the zero-sequence currents. Neutrally grounded networks - a generator or transformer is isolated from neutral ground or connected to the network using a measuring or signaling device with high resistance. Such networks include networks insulated from neutral ground on 6-35kV lines. Neutral ground compensated networks - networks connected by means of arc quenching reactors in order to limit the neutral grounding current [1,pp,13]. It is related to the phase capacity and the inductance of the arc quenching reactor, which is isolated or compensated from the neutral ground and separated from the ground. The networks are characterized by very small grounding currents. Thus, in 6-35 kV networks, the fault currents do not exceed 30–10 A, respectively [3,pp,28].

Single-phase ground fault protection is a typical type of damage to -6-35 kV networks. They account for 70-80% of line damage. It should be noted that currently the total length of 6-35 kV distribution networks in the country exceeds 100 thousand km. Almost 65% of the total length of 0.4-110 kV power transmission lines. Therefore, the issue of locating, diagnosing and timely disconnection of single-phase grounding is especially relevant. With a phase-to-earth fault, the line voltages do not change.

The currents in the phases change very insignificantly (by 10–30 A) and the consumer in these conditions maintains a normal operating mode, there are no strict requirements for the immediate disconnection of the damaged line. Also, the absence of requirements for disconnecting the line is explained by the fact that the sensitivity of the existing protections is not enough to ensure selective operation, and, as a rule, undamaged elements are subject to disconnection. However, the voltages of the undamaged phases with respect to earth increase[6,pp,86-88].

An intermittent arc is an open electric arc that periodically fades and reappears. In this case, over voltages up to 2.5-3 rated phase voltages occur on undamaged phases and lines of the network.

                                          (1.1)

The duration of such a high voltage is several tens of minutes, as there are no strict requirements for immediate shutdown of the damaged network through relay protection. In this case, there is a high probability of damage to the insulation of the undamaged phases and the occurrence of two-way grounding.

Therefore, it is necessary to identify the damaged element in the shortest possible time and notify the personnel on duty about the presence of an accident with the help of an alarm, so that they can find and eliminate the damage within a reasonable time (no more than two hours) [4,pp,107-108]. Otherwise, the line must be disconnected.

 The function of detecting the ground fault and the point of damage is carried out by means of special protection and by signaling to the duty officer. In some cases, protection can be provided by immediate disconnection in networks. According to Technical Operation Rules (TOR) requirements, such networks include 6-10 kV networks designed to supply peat and oil refining equipment as well as portable construction machinery. In the case of switching from a single-phase directional ground connection to a two-phase directional grounding, which may occur in such networks, there is a high probability of a step voltage that is dangerous to the human factor. In addition, the appearance of periodic arcs can cause fires, which can have serious consequences, especially in oil fields and peat mining areas. Therefore, the problem of creating high-frequency and selective protection devices that detect single-phase grounding is one of the main problems today [7,pp,75].

Types of ground protection

Below, we review the main types of protection used in networks with small grounding currents and highlight their main advantages and disadvantages. The non-selective general signaling device is installed on the busbars of 6-35 kV substations or generator busbars of power plants. This device is connected to voltage transformers of NAMI-10 type (voltage transformer is designed for anisotropic, oily, measuring) in an open triangular circuit (pic.1). Normally the voltage at neutral is ∆U = 0V, while at single-phase grounding ∆U= U_A + U_B + U_C = 3U₀. Uses RV-53 / 60 D voltage relay as protection relay. However, this type of protection is widely used due to its simplicity [9,pp,34-36] and high sensitivity.
 

Picture 1. Scheme of non-selective signaling device connection

Disadvantages: This protection is not selective, it is impossible to determine the point of single-phase grounding in a network of 6-35 kV. 

Residual overcurrent protection, non-directional

Depending on the distribution of the zero-sequence currents in the complex network, it is possible to determine the currents in the line short circuit and the external ground line.

Currents flow through intact lines due to the capacitances of the phases of these lines relative to the ground:

                                   (1)

 If the line is damaged, then a current will flow through it, which is the sum of the capacitive currents of all intact lines, depending on the total capacitance of these lines:

                                 (2)

here-   is the equivalent phase capacitance relative to earth of all intact lines.

In this regard, the residual current protection, which responds to steady-state currents, must be offset by the operation current from the capacitive currents of the protected line:

                                                   (3)

 In this case, there must be a sufficient number of outgoing lines providing the necessary sensitivity, so that the condition:

;

                                                   (4)

The fact is that in order to tune away from the transient inrush of capacitive current arising during various commutations, a detuning factor is introduced into formula (3), equal for protection without time delay K = 4-5 and for protection with time delay K = 2-2.5. Then, in the worst case, the sensitivity coefficient calculated by the formula,

                                                  (5)

may be less than the required value of K_cap ≥ 1.5, since it is known that the equivalent capacity and the equivalent capacitive current of the system are related to the capacity and capacitive current of the protected connection by the ratio.

;

                                    (6)

Therefore, in practice, there should be at least 8-10 outgoing lines powered by busbars to provide the required sensitivity. In addition, zero sequence current protection cannot provide selectivity in networks with a variable primary circuit, in which there is a periodic change in the capacitance of the phases relative to the ground [8,pp,135].

These networks include power supply systems for mobile construction sites, the length of which varies over time.

Picture 2. Scheme of zero sequence protection circuit

The relay, which responds to the zero sequence current, is connected to the single-transformer zero sequence current filter TTNP type TLMZ-1. Most often, the current relay RT-40 / 0.2 or more modern RTZ-51 acts as a KA relay.

To exclude false triggering from capacitive currents passing through the sheath of an undamaged cable at an external short circuit, the funnel of the cable is isolated from the ground, and the ground wire is passed through the hole of the TTNP. In this regard, the use of such current transformers is difficult in overhead lines due to the need to use cable inserts. In addition, the values of capacitive currents in air networks are small and certain difficulties arise with the stability of the operation of the RT-40 current relay at low currents[5,pp,24].

Directional residual overcurrent protection

Directional overcurrent protections that react to steady-state currents are used in cases where the number of outgoing lines is small and the required sensitivity cannot be provided. In directional protections, a power direction relay is additionally used, which is switched on for voltages and currents of zero sequence. This relay operates only in the positive direction of power from the busbars into the line, so there is no need to tune away from the current  according to condition (3), it flows in the other direction. In this case, it is enough to tune only from the unbalance current of the zero sequence filter.

The disadvantage of this type of protection is the need to use cable inserts, which significantly limits its use, as well as excessive operation of protection when an intermittent arc occurs and the lack of a clear method for determining the operation settings. Such protections are used in cases where the neutral is compensated and currents earth fault less than 5 A. In this case, the protection operation current is selected according to the condition of detuning from the transient earth fault current[5,pp,26].

Protections responsive to the transient current value of zero sequences

The most widespread are autonomous devices of directional wave protection and carbon dioxide charging station.

                                                   (7)

The device for single-phase ground faults consists of a starting element that responds to the appearance of zero sequence voltages, a current direction element, a power supply and an indicator relay. A feature of such protections is the short duration of the appearance of the transient earth fault current. This time is 0.1–0.2 seconds, therefore protections reacting to transient currents must have a trip latching device

The disadvantage of such protections is the complexity of the implementation and detection of the fact of excessive operation with intermittent arc faults, and the advantage is the ability to fix even a short circuit, which provides a preventive check of insulation and allows you to identify the places of its weakening in order to make timely repairs and prevent subsequent short circuits [7,pp,104].

Using high frequency superimposed current

This method assumes the presence of an high frequencies signal generator, which is usually connected to the phases of the monitored line through a capacitive divider. The generator sends a high frequency signal in phases and monitors the return time of the reflected signal. If there is no signal return, then one phase to ground has occurred, and the protection is triggered. In this case, the distance to the place of breakdown is fixed according to the resistance of the loop to which the generator is turned on - from the point of connection to the point of short circuit and again through the ground to the generator.

The application of this principle is difficult in complex branched networks that create a strong signal dissipation and require a high power generator.

Fixation of harmonic components in zero sequence current

When a phase is closed to ground, the signals, zero sequence currents are non-sinusoidal and in transient modes contain higher harmonic components: 100 Hz and above. By separating these components through frequency filters, it is possible to fix the earth fault with a very high sensitivity, which is especially important in networks with compensated neutral.

The disadvantage of such protections is excessive operation, which is a consequence of the wrong choice of protection settings, depending on a number of factors that change during operation. In addition, the process of finding a damaged connection takes a lot of time, therefore, according to [1,pp,102], it should be automated. Currently, devices have been developed that respond to harmonic components in the zero sequence current. These are devices of the USZ-2/2 (alarm devices) type - an individual device connected to a cable-type zero sequence current transformer and USZ-3M (alarm device) - a group device, alternately connected to a zero sequence current transformer of each connection.

The operation of the USZ-3M device is based on measuring the sum of the higher harmonic components in the earth fault current (from 150 to 650 Hz) [7,pp,64]. However, more than forty years of experience using this device has revealed a number of significant disadvantages:

1. non-selective operation in arc earth fault;
2. unsuitability of the USZ-3M device for use in complex networks with parallel lines;
3. impossibility of fixing short-term earth fault;
4. The need for personnel to go to the substation to carry out a large number of measurements in order to determine the damaged connection;

Picture 3. Diagram of the signaling device for single-phase ground faults, type USZ-3M

Currently used microprocessor devices SPAC 801-013, SPAC 801-113, LLC ABB Relay-Cheboksary [5,pp,30], Sirius, produced by NPF Radius [5,pp,32] and their foreign counterparts - SEPAM by Schneider Electric. These devices perform a relative measurement of higher harmonics, on the basis of which they are able to operate not only with a ground fault, but also with phase-to-phase faults. At the same time, all microprocessor devices of both domestic and foreign production have the following disadvantages:

a) high cost of devices;

b) complexity, with the choice of response settings due to the lack of an unambiguous methodology;

c) Thus, the indicated disadvantages of protections reacting to harmonic components in the zero sequence current create obstacles for their widespread use.

General summary

Thus, analyzing the available types of earth fault protection devices in networks with low earth fault currents, the following types of disadvantages inherent in these devices can be distinguished. This is nonselective protection work, difficulties in finding a damaged connection in branched networks, excessive triggering in the event of arc earth fault outside the protection zone, the complexity of the implementation of some types of protection, the lack of unambiguous methods for determining the response settings and the high cost of microprocessor terminals. Therefore, the most urgent in the current situation is the development of a device that is not associated with the use of transformer filters of zero sequence currents. A promising direction is the use of wavelet analysis algorithms for the selection of harmonic components of the current and voltage of the network , allowing it to be used to protect networks with both isolated and compensated neutrals, as well as to protect high-voltage electric motors.

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