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Research and development of measures to improve the reliability of electrical networks in own needs of power stations

Content

Introduction

By the rules of electrical devices (RED) [1] operating mode of electric networks is set with voltage of 6 kV and 10 kV with isolated neutral or compensated neutral.

The most common type of damage in such networks – single-phase ground fault with intermittent arc, constituting more than 70 % according to [2]. Emerging with the multiplicity of the arc overvoltage up to (3–4)UP dangerous for electrical equipment, primarily for high–voltage electric motors, generators, cables and voltage transformers according to [2–5].

1. Relevance of the subject

The use of neutral grounding resistor allows to get rid of dangerous surges and increases the speed and selectivity of relay protection.

The occurrence of arc overvoltage mostly connected with intermittent arc and relatively small currents SPGF (single-phase ground fault), not exceeding 10 A. The value of the amplitude of the surge at the same time can reach 3.5–3.8 phase voltage UP. By increasing the current SPGF arc overvoltage are reduced. This is due to the fact that the arc has an even temper, and does not cut off at high currents. At currents SPGF from 10 to 20 A overvoltage does not exceed 3UP. At currents SPGF from 20 to 50 A overvoltage does not exceed 2,7UP.

The parameters of the transition process in the event of single–phase arcing fault in networks with isolated or grounded neutral through ASL (arc suppression lattice) determined by the capacity of the phases, the inductive resistance of the power source, transformer and ASL, as well as arc resistance. The main factors determining the maximum overvoltage at GF (ground fault) are: voltage on the emergency phase at the time of the initial ignition of the arc Ui, the moment of arc extinction and re-ignition voltage of the arc Uri.

2. The purpose of the research

The purpose of the research: the development of a mathematical model and investigation of transients at arc fault phase on the ground in distribution grids of 6–10 kV, that work with different modes of neutral grounding; analysis of the influence of neutral grounding modes on the parameters of arc overvoltage.

Object of the research: electromagnetic transients at ground faults in networks of 6–10 kV with different ways of grounding.

Subject of the research: the parameters of arc overvoltage to ground faults in networks of 6–10 kV with different ways of grounding.

Methods: during the research position of the electromagnetic field theory are used in electric circuit theory. To estimate the influence of neutral grounding modes on the parameters of arc overvoltage mathematical model of electromagnetic transients is applied. Model of electromagnetic transients in the electrical network is obtained using the method of the phase coordinates. To develop the network model structural simulation method is used.

3. Present state of research of arc overvoltage in networks of 6–10 kV and the foundation of research methods

3.1 High–resistive grounding of neutral network of 6 and 10 kV

The main purpose of high–resistive grounding of the neutral network is to limit the arc overvoltage and ferroresonance phenomena, while ensuring long-term operation of the network with the GF on search time and turning off the damaged connection with operational staff. Reducing of voltage on the neutral and surge suppressor with arc fault to ground is achieved by reducing the time constant of the discharge capacity of the working phases during the dead time tP via specially connected resistor RN (Fig. 1), which provides a decrease in flow resistance circuit residual current. Resistor RN connects to the network via the transformer with the connection scheme of windings Υ/Δ in one of two ways.

The first method – a resistor connected between the neutral point of the winding of high voltage TDM and circuit ground (Figure 1a.).

The second way – neutral high voltage winding CP (current protection) connected to ground, and the resistor is switched to the secondary winding of the transformer in open delta (Figure 1b.). Connection of the resistor is determined by the network structure and parameters of the installed equipment.

Connection scheme of the resistor to neutral of the network

Figure 1 – Connection scheme of the resistor to neutral of the network

To ensure the complete discharge of phase tanks during tP, which is equal to 0.008–0,010 s, the resistor is selected such that the active component of the ground fault current IR is equal to or greater than the capacitive component IC

pic_1

From this condition, the resistor for the circuit Fig. 1à, RN, Îm, calculated by the formula:

pic_2

and the resistance of the resistor to the circuit in Figure 1b, and the resistance of the resistor to the circuit in Figure 1b, RΔ, Îm, calculated as follows:

pic_3

where UHL – line voltage of the side of the higher voltage, V;
IC – capacitive current PTG, A;
K – CP transformation ratio, calculated according to the formula:

pic_4

where ULL – line voltage side of the transformer low–voltage, V.

Rated power of the transformer neutral grounding resistor and RN or RΔ, S, VA, calculated as follows:

pic_5

The value of the current flowing through the resistor SPGF mode for the circuit of Figure 1a, IR, A, is calculated according to the formula:

pic_6

The value of the current flowing through the resistor for the circuit of Figure 1b, IΔ, À, is calculated according to the formula:

pic_7

The current I(1) is in place SPGF geometric sum of the capacitive network and current of the active current generated by the earthing device. The current value I(1), À, is calculated according to the formula:

pic_8

and using the formula (1):

pic_9

By increasing the resistance of the resistor in comparison with the value calculated by formula (2), the voltage at the neutral point for the currentless pause is not reduced to zero but to a specific value ΔUN, which increases the level of arc overvoltage KP

The value of resistor calculated according to the formulas (2, 3), is redundant for power dissipation. More specifically, the resistance of the resistor in the neutral phase providing containers for the discharge time tP is calculated taking into account the active losses in the network based on the magnitude of the fault current to the ground and reducing the required level of arc overvoltage. The calculation is made on the specialized program for calculating electromagnetic transients in accordance with [13, 14].

3.2 Low–resistive grounding of neutral network of 6 and 10 kV

The main purpose of low–resistive neutral grounding resistor network is fast tripping SPGF relay protection and maximum coverage of the windings of electrical machines (motors, generators, transformers) protection of SPGF. It also provides surge suppression and ferroresonance phenomena.

Low–resistive grounded neutral network is carried out using a special TDM grounding transformer neutral connection with the scheme Υ/Δ, windings, as shown in Fig. 1a. Resistor RN is turned on between zero point HL winding and ground loop.

Resistor is chosen the least, on the basis of two conditions:

Selective shutdown can be achieved by connecting the neutral resistor networks with resistance, calculated by the formula:

pic_10

where IS.Z. ÌÀX – maximum tripping current at SPGF.

Chosen by these conditions resistor typically creates the active current significantly exceeding capacitive. If the capacitive current is much less active IÑ << IR, the current SPGF can be calculated by the formula:

pic_11

where UN – linear voltage.

At SPGF in the winding connected in star, SPGF current IZ, A and considering (11) is calculated according to the formula:

pic_12

where W – the number of loops of the stator winding of the clamp circuit to the point, percent of the total number of loops of the faulty phase.

For winding connected to a triangle, the smallest fault current at the midpoint of the housing IG, windings, A, is calculated according to the formula:

pic_13

At SPGF in the windings of high–voltage motors to prevent burn–active steel stator fast tripping of the motor protection against ground faults should be provided.

The number of windings of the protected SPGF W, %, calculated as follows:

pic_14

where ISZ – tripping current from SPGF, A.
IR – current grounding resistors, A.

We can enhance protection by increasing the active current resistor, or reducing the tripping current within acceptable values, the calculated sensitivity factor protection.

Depending on the method of selection of the grounding resistor and the current value of SPGF grounding transformer for low-resistance grounding resistor network neutral and resistor must be calculated either on a short-term or the long-term work in SPGF mode, during which shall be no excess of normalized temperature parameters.

Tripping current connections from SPGF ISZ, A, calculated by the formula:

pic_15

where KH – reliability coefficient taken equal to 1.2;
KB – coefficient taking into account the capacitive inrush current;
IC – capacitive current RCT (residual current transformer) protected connection with the SPGF section on indoor switchgear – 6 kV and 10 kV.

In the event of an extended mode of SPGF (eg, in case of failure in the protection), the protection of the residual OPR (overcurrent protection residual) delayed effect on the OPR switch off, thereby transferring network with isolated neutral mode. If the OPR switch is switched off, this protection may act to trip the circuit breaker and the input section switch (if enabled).

3.4 Combined neutral ground in networks with 6 kV and 10 kV voltage

The resistance value of the resistor R*N, Îm, allowing to eliminate the beats are selected based on the ratio:

pic_17

where ΔI – current mismatch compensating reactor, A.

Determined by the formula (17) of the resistor R*N, resistance value connected in parallel coils, leads to a complete elimination of the beat after the arc extinction and reduction of surge in repeated breakdowns to UÌÀX ≈ 2,4UP. However, most of all, the power of the resistor is redundant.

Clarification of the resistor values can reduce overvoltage to a predetermined value, it is carried out by means of calculation, taking into account all the influencing factors on the specialized programs set forth in [13, 14]. In this case, the resistor parameters are calculated for:

At connection in parallel with the resistance of the resistor ASL:

pic_18

maximum surge does not exceed the level UÌÀX ≈ 2,6UP.

4. Mathematical model for the study of transients in networks of TPS

For the analysis of transients in networks own needs of TPP at arc short circuits on the ground is taken as the basis of power supply circuit TPS shown in Figure 2.

Scheme of electricity supply their own needs TPP

Figure 2 – Scheme of electricity supply of TPS own needs (Thermal power stations)

In drawing up the scheme of substitution, given the relatively small length of the cable connections for the auxiliary power conditions (0,5 km), we can accept for all the elements focused on network settings. We will also consider the network in the research as a linear, i.e we neglect the saturation of the individual elements. Based on the above Figure 3 shows the equivalent circuit of the test network, adopted as the basis of a mathematical model.

Equivalent Circuit own needs <nobr>6–10 kV</nobr>(Animation: 5 frames, 10 cycles of repetition, 215 kilobytes)

Figure 3 – Equivalent Circuit own needs 6–10 kV
(Animation: 5 frames, 10 cycles of repetition, 152 kilobytes)

Conclusions

The main results are as follows:

1. The main reasons of the high damage of electrical equipment in medium voltage class are arc–voltage, resulting in intermittent nature of the arc at the site of the breakdown phase insulation to the ground.

2. The problem of improving the reliability of distribution networks operation of 6–10 kV consists of a whole range of tasks, effective solution which can be found for each particular network individually, taking into account specific features of it based on the combined use of relay protection, improving the neutral grounding mode, the application stops series arresters with different thresholds and limitations of the system quickly and automatically bypass the faulty phase.

3. An effective solution to the problem of improving the reliability of distribution networks operation of 6–10 kV can be found with big number of scientific and experimental research.

*This master's work is not complete yet. The full text and materials can be obtained from the author or his research manager in July 2017.

List of sources

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