LV15317B - Medium voltage distribution network with distributed generation line ground fault protection method - Google Patents

Abstract

The invention pertains to the electric power engineering. When considering medium voltage distribution networks with distributed generation, there can be cases when due to a ground fault a power system connection line is disconnected only from one side leaving a network section with insufficient zero sequence current in order to completely disconnect the fault, which can result in damage to the network and risk to human lives. The proposed method tests the trip criteria of a directional ground fault protection and simultaneously determines the frequency and the rate of change of frequency of an analogue positive sequence voltage signal obtained with an analogue positive sequence filter. The change of voltage frequency can be expected because the active power balance of the separated system most probably would not be ensured. Using the rate of change of frequency as indicator of system link disconnection and zero sequence voltage magnitude as ground fault indicator the protection trip for the faulted line and distributed generators is issued after specified time delay. A time delay before reset of trip timer has been added to provide stable protection operation during intermittent ground faults. The proposed method can be used for creation of new medium voltage distribution line ground fault protection devices, and a skeleton diagram for its implementation is shown in fig.6.

Description

DISCLOSURE OF THE INVENTION The invention relates to the field of electrical power, in particular to methods of protecting medium voltage overhead and cable lines.

Analysis of the Prior Art Overhead lines with a nominal voltage of 20 kV are important for the supply of rural areas, and cable lines with a nominal voltage of 10 kV are the main type of power supply connections in cities. Medium-voltage networks, including the above-mentioned lines, have historically been operating in isolated neutral or compensated neutral mode [1], in which the capacitive current of the earth circuit is compensated by the inductive current of a neutral earthing reactor of neutral transformer. In compensated neutral mode, earth-leakage arcing and partial restoration of line insulation are possible, reducing the risk to human life and allowing the line to be temporarily retained at work. The disadvantages of the compensated neutral mode are the possibility of significant overvoltages due to line capacitance and reactor inductive current resonance and the difficulty of relay protection operation due to reduced earth leakage current. Nowadays, low-resistance neutral mode is also used, where the neutral transformer is neutral in the ground, by switching on the resistor in the auxiliary winding of the earthing reactor. In low-resistance neutral mode, the earth leakage current contains the active component and the total earth leakage current must be at least 1 kA according to current technical regulations [2]. It facilitates the operation of relay protection and eliminates the risk of overvoltages, but the installation of a low-resistance neutral transformer requires additional economic investment, which results in them being installed only in individual substations.

[003] For protection of medium voltage lines in the case of earth faults, the standard recommends the use of uninterrupted zero-current current protection or earth-fault current protection or directional earth-fault current protection (VZSA) [2].

[004] The ZSA-type protection operation is based on a zero-current current suppression with the trigger setting [3]. If a zero-current current exceeds the setting at a fixed time-delay (eg, electromechanical relays [4]) or a zero-current-dependent time delay (eg digital relays [5]), the protection provides a circuit breaker command. The ZSA-type protection standard [2] recommends that the mode use only low-impedance neutrals, because in isolated and compensated neutrals, the low-current neutral current can be comparable to the zero-current capacitance current of the intact lines, causing the NSA non-selective trip [3].

[005] The VZSA type protection is realized with the following set of steps [6]:

(i) the phase values and the currents with the zero-current filters produce the values of the zero-voltage and the current, (ii) the modulus of the zero-voltage and the current (equal to the amplitude or the effective value of the voltage and current) iii) protection defines direction of fault based on zero vector voltage or current mutual position or phase shift angle between the two vectors; iv) zero voltage and current complex value modules override settings and displaying "internal fault" on fault indicator (short circuit occurred) , after a fixed or zero-current-time delay protection, the circuit breaker trip command is given,

(v) In addition, a time delay shall be provided for resetting the timer which executes the time delay before the circuit breaker is tripped to provide protection in the event of an earth-fault intermittent circuit.

VZSA type protection is recommended for isolated and compensated neutral mode [2] because it is capable of providing selectivity in these modes but can also be used in low resistivity neutral mode. The disadvantage of the VZSA described is the lack of accuracy in the determination of the zero-current, since the phase-to-phase error (3-10% according to [1]) can become comparable to the earth-fault current, which can result in an incorrect protection trip or failure. Therefore, at least for cable lines, a cable zero-current transformer is used [2, 3], but this requires additional economic investment and its installation in small compact switchgear may be difficult.

[007] VZSA techniques implemented in digital relay protection and automation (RAA) terminals are known [7]. These terminals provide VZSA with the ability to combine fixed and zero-current-dependent time delays before the circuit breaker is tripped to facilitate protection selectivity. The terminal introduces VZSA locking by monitoring transformer magnetization current jumps to prevent erroneous protection tripping, and the ability to rotate the zero-voltage voltage vector prior to detecting the phase-shift of the zero-current vector, which allows the network to adjust to neutral. For networks operating in isolated or compensated neutral mode, a sensitive VZSA is created which requires the use of a cable zero-current transformer with a transform factor of 1/5 A. The zero-current jet component is used to determine the sensitive VZSA depending on the neutral mode fault direction. (syrup method) or the active component of the zero-current current (co.s _Z method) [7]. There is also an intermittent arc protection, which continues to count down time even during power breaks, provided that their duration does not exceed the setting. The total duration of the intermittent arc triggered by this protection exceeds the total tripping time setting and a new electric arc has ignited at the instant of tripping. Unlike conventional VZSAs, intermittent arc protection compares the effective value of the zero-current current, including the highest harmonics up to 400 Hz, with the setting and does not determine the direction of the fault. The disadvantage of terminal VZSA is the ability to efficiently isolate faults in single-mains networks only if no additional neutral-forming transformers are installed in the network. The disadvantage of a sensitive VZSA is the need to install a sensitive cable zero-current current transformer, which requires additional economic investment, and which is difficult to install on small compact switchgear and natural measurement errors caused by asymmetry of network elements. The disadvantage of intermittent arc protection is to refrain from defining the direction of the damage, as it greatly facilitates the selectivity of the protection.

[008] There is known a technique implemented in a single-phase short circuit protection device for medium voltage networks [8]. This technique is accomplished by simultaneously performing a test of the normal VZSA actuation criteria and defining the direction of the fault using the zero harmonics current and voltage harmonics (150-3000 Hz). The attached defect path detector is used to record the insulating breakthroughs of the protected line to create a transition process. Based on the duration of power-free breaks, which are defined as the interval between insulation breaks, the protection records the emergence of an intermittent arc and provides a protection trigger lock command to prevent non-selective protection triggering. When the network is operating in isolated neutral mode, the protection is provided by the normal VZSA in case of permanent earthing, while in neutral neutral mode the protection is provided by either the normal VZSA or the directional detector of the highest harmonics. The technique implemented in the device [8] is characterized by the disadvantages of the RAA terminal [7], and the protection of said technique does not ensure the disconnection of the defective line and earth interruptions with intermittent arcing occur. The attached defect direction indicator is only effective in compensated neutral mode as described in the patent specification [8].

[009] The VZSA technique of medium voltage power transmission lines implemented in the RAA terminal is chosen as a prototype of the invention [9].

[010] A flowchart of the VZSA method as defined in the prototype is shown in Fig. 1 and includes the following sequential steps:

1. The symmetric component filters (1), (2), (3) and (4) derive zero-order voltage signals Uli, Ul2, Ul3 and phase current signals ki, Il2, кз from digitized and processed by random noise filters. complex values of voltage U °, zero current 7 °, return voltage U ² and return current I ² . The pretreatment of the input signals and the obtaining of the zero-current current and voltage complex values of the invention are similarly realized.

2. The module values of the zero values of the zero voltage U ° and the current 1 ° and the return voltage U ² and the current I ² are compared with the settings in blocks (5), (6), (7) and (8). Comparison of the complex values of zero voltage and current with the settings of the invention is similarly realized.

3. The magnetizing current jump of a nearby transformer activates the ENA MULT input [9], which increases the zero and back current settings in blocks (6) and (8) in proportion to the maximum possible phase current ki, Il2 and кз asymmetry. work. Similarly, correction of said zero-current current setting is used in the invention.

4. When a ground fault occurs, a significant zero-voltage voltage appears and the unit (5) is triggered and, if sufficient zero-current current is generated, the unit (6) also activates, and the two units together activate the logic "UN" element unit (9).

5. The protection continuously checks the phase shift of the zero-voltage voltage vector U ° and the zero-voltage current vector 1 ° in the block (11) to determine the fault direction. There are several methods to determine the direction of damage, including:

Method 5.1 with fixed zones defining the direction of damage, boundary values and the location of the zones (syrup and methods,

Method 5.2 with fixed zone boundary values and increased zero-current current setting as the phase shift approaches zone boundary values (Classic 80 and Classic 88 methods [9]),

Method 5.3 with a fixed zero-current current setting but with freely adjustable zone boundary values, and the ability to rotate the working zones relative to the coordinate center along the characteristic relay angle RCA CLT [9], which allows adaptation to the neutral operating mode. A similar method of defining the direction of damage is also used in the invention, but provides for a zero-turn current vector turning area at the turning point.

6. In the event that the block (9) and block (11) that the short-circuit is "internal fault" and activate their own output, the logic unit "UN" (13) activates the time-delay timer (16). ). The time delay timer, depending on the setting, implements a fixed time delay with the block (17) or a zero-delay current-dependent time delay with the block (18).

7. If the trip condition is maintained, the protection gives the circuit breaker trip command to the circuit breaker tripping coil YAT, which causes the circuit breaker to trip the short circuit and the trip trip conditions to disappear, resulting in the AtRES unit (15) the timer to return to the starting position via the RES input [9].

8. Conversely, if the zero-current module test in block (6) referred to in step 4 fails, but the zero-voltage module test in block (5) is completed, protection can be triggered based on the reverse-voltage and current criteria;

9. In turn, the protection trip referred to in step 8 is possible if the zero-voltage module test blocks (5), (7) and (8) are executed, which show significant resistance, zero-voltage and resistance current, and at the same time a phase shift test unit (12) for the vector U ² and the current vector I ² , which determines the fault direction using fault detection methods analogous to those described in step 5.

10. Upon completion of the tests described in step 9, blocks (10) and (12) activate a logic " UN " element unit (14) and provide a signal to the timer delay timer (16) to start the countdown. The time-delay timer, depending on the setting, implements a fixed time delay with the block (17) or a delay-dependent time delay with the block (18).

11. The next step in the protection is identical to that described in step 7.

12. When a short circuit with an intermittent arc appears, the operating conditions for block (13) or (14) alternate and disappear, so that time delays at AtRES before the timer (16) resets. The time delay unit (15) of the timer (16) resets to the rear of the incoming signal (when the signal goes from active to inactive) and if the AtRES electric circuit is able to re-ignite, the output of unit (13) or (14) the timer (16) resumes from the previous value. As a result, in the case of an often intermittent arc, the protection operates similarly to a stable short circuit but with a slight additional delay due to power outages. The time delay for resetting the timer is also contemplated in the invention.

[The inherent disadvantages of the Olll Prototype are the difficulty of simultaneously implementing selective and sensitive protection in isolated and compensated neutral mode because the earth leakage current is very low and close to the capacitance currents of intact lines [3]. Similar to a conventional VZSA, in the isolated and compensated neutral modes, the accuracy of the zero-current measurement may become comparable to the measurement current, which makes such measurements unreliable without the addition of a sensitive zero-current current transformer. The part of the prototype that uses the bias current and voltage may not work because in isolated neutral mode, the bias voltage module will be several times smaller than the direct and zero bias voltage modules during a single-phase short circuit. Given that VZSA may not operate in isolated or compensated neutral mode, there is a potential risk that dispersed generation sources will remain isolated in neutral with close consumers after disconnecting the high-voltage link that provided low-resistance neutral. In this case, without operating the VZSA on the dispersed generation side, the short circuit is still energized, endangering human lives and potentially causing damage to lines, distributed generation sources and other electrical equipment.

OBJECTIVE AND SUMMARY OF THE INVENTION The object of the invention is to provide a method that complements VZSA by enabling disconnection of distributed generation sources, effectively isolating a ground fault, in the case of power system disconnection when conventional VZSA lines do not operate on the source side. , avoiding the inherent disadvantages of a prototype.

1013] The objective pursued is achieved by using the following set of operations:

1. After initial processing with random noise and high harmonic filters, an analogue phase voltage signal is obtained from the analog phase voltage signals using an analogue direct line filter and the digital value of the zero-voltage voltage after converting the phase voltage analogue signals to an analog-to-digital converter component and null filters.

2. The live sequence voltage signal is used to determine the frequency of the network and the rate of change of frequency, which module compares with the setting.

3. At the same time, the value of the zero-voltage module is compared to the setting.

4. In the event of an earth leakage that could only be unlocked by line power system protection because the line-side dissipated generation had insufficient current to operate the VZSA, an independent part of the network would be generated which would expect the network frequency to increase or decreasing the frequency of the frequency change test. Given that the earth leakage circuit is still connected to the line source voltage, the zero-voltage voltage module will remain significant.

5. In the case described in the previous step, the test conditions of Steps 2 and 3 are met, and if they remain on after the delay time, the protection provides a command to disable the damaged line and distributed generation sources that would otherwise cause disconnected system damage.

6. In the case of an intermittent loop, the zero-voltage voltage criterion may not be met temporarily during non-current breaks, so there is a time delay before resuming the run-time delay from zero.

[014] The invention is explained by the following drawings:

1) Fig. a block diagram of a prototype-driven earth-fault current protection method is shown, wherein: a zero-voltage voltage filter unit (1); the zero-current power filter unit is (2); the return current filter unit is (3); the back-up voltage filter unit is (4); the zero-voltage voltage comparator unit is (5); the zero-current power module comparison unit is (6); the bias current module comparison unit is (7); the bias voltage module comparison unit is (8); the logic "UN" element unit is (9); the logic unit of the "UN" element is (10); a block for determining the offset current and voltage phase shift and fault direction (11); a block for detecting a bias current and a voltage phase shift and a fault direction (12); the block of logic "UN" element is (13); the block of logic "UN" element is (14); the time delay reset unit is (15); the time delay for running the timer unit is (16); the block for a fixed time delay in operation is (17); the current-dependent time delay actuation block is (18); Uli, Ul2 and Uu are complex values of phase voltages; Ile, Il2 and I13 are complex phase current values; U ° and 7 ° are the values of the complex zero-voltage and current; U ² and I ² are the values of the voltage and current of the complex return current; RCA CLT is the characteristic rotation angle of the relay; ENA MULT is the increase in the zero-current and counter-current settings of the transformer magnetisation current jumps; The YAT is a circuit breaker coil. Wide arrows denote data flow and narrow arrows denote control commands;

2) Fig. 2 a graph of the frequency change over time obtained from the scheme described in [10] in a one-phase short circuit simulation in the program [11] is shown: / is the frequency; t is time;

3) Fig. 3 the graph of the line-side distributed generation bus voltage symmetric component changes in the scheme described in [10] in a single phase short circuit simulation program [11] is shown: IU1I is a direct sequence voltage module; IU2I is a back-voltage module; IU0I is a zero-voltage module; IUI is a voltage module; t is time;

4) Fig. 4 a zero-resistance circuit of a low voltage neutral medium voltage line is shown, where: Rz is the active resistance of the low resistance neutral resistor; X1 is the inductive impedance of the arc-forming coil on the neutral forming transformer; a is the earthing distance in relative units; C® is the line zero capacitance; C ^ ₂ is the total capacitance of the zero sequence connected to the busbars (load capacities and downstream capacities) of the power switch Q2; Znt is the impedance of the neutral transformer; Q1 is the power switch on the power side of the line; Ζθ is the line impedance impedance; Q2 is the power switch on the line side of the distributed generation; is the zero-sequence electric motor at the short circuit; is a neutral sequence scheme; Z $ is the low-resistance current of the low-resistance neutral resistor; Ζθ is the neutral current of the circuit-breaker in the transformer neutral; and Z ^ ₂ are the zero-current currents flowing through the circuit breakers Q1 and Q2; Ζθ ₁ , Ζθ ₂ , Ζθ ₃ and Ζθ ₄ are the zero-order currents flowing through the line capacities; Ζθ1 and Ζθ ₂ are the zero-order currents flowing through the short-circuited line segments; Z ^ and Z ^ ₂ are the short-circuit currents from the sides of the power switches Q1 and Q2; Z ^ is the short-circuit total current;

5) Fig. 5 a graph of the change in the symmetric components of the line-side distributed generation currents obtained in the scheme described in [10] in a one-phase short circuit simulation program [11] is shown: UU is a direct sequence current module; 1121 is a return current module; 1101 is a zero-current module; / I / is the power module; t is time;

6) Fig. 6 a block diagram of an embodiment of the invention is shown, wherein: a block of random noise and higher harmonic filters is (1); the block of analogue-to-digital converters and orthogonal component filters is (2); the zero-voltage voltage filter unit is (3); the zero-current power filter unit is (4); the zero-voltage voltage comparator unit is (5); the zero-current power module comparison unit is (6); the block of logic "UN" element is (7); a block for detecting the offset current and voltage phase shift and fault direction (8); the logic "UN" element unit is (9); the time delay reset unit is (10); the fixed time delay timer unit is (11); the analogue order filter unit is (12); the frequency determining unit of the analog signal is (13); the frequency change detecting unit is (14); the module comparison unit during frequency change is (15); the logic "UN" element unit is (16); the time delay reset unit is (17); the fixed time delay timer unit is (18); the block of logical "OR" element is (19); uli, ul2 and иьз are the instantaneous values of the phase voltages; in, ii.2 and in are the instantaneous values of the phase currents; Uli, Ul2 and Ul3 are complex phase voltage values; Ill, Il2 ,; and In are complex phase current values; U ° and 1 ° are the values of the complex zero-voltage and current; kl ° is an increased zero-current current setting, capturing transformer magnetization current jumps; Фкон is the rotation angle of the zero-current current vector correction; The YAT is a circuit breaker coil. Wide arrows represent data flow, but narrow control commands.

DETAILED THEORETICAL DISCLOSURE OF THE INVENTION In the medium voltage networks of VZSA, the problems are caused by the negligible zero-current in isolated or compensated neutral networks and by earthing intermittent circuits. To overcome this problem, neutral transformers with a low-resistance, neutral-to-high-voltage substation (expressed in cable networks) are used, providing high zero-current current and fast protection tripping. Medium voltage networks have a single mode power supply. As a result, a short circuit can be isolated by performing a unilateral fault trip. A different situation occurs when a distributed generation source is connected to the medium voltage network, because after the protective trip line on the power system side, the fault remains connected to the power source if the VZSA does not operate on the distributed generation line side. It is important to note that the part of the network isolated from the short-circuit with the distributed generation source after isolated short-circuit operation is in isolated neutral mode and the measured zero-current current will be very low. For this reason, it is possible that the existing VZSA will not operate unless additional neutral transformer transformers are installed in the network, which requires significant economic investment. Given that the power supply side disconnected, the power generated and consumed in the isolated part of the grid will almost always be unbalanced, and that the zero-voltage voltage due to the earth fault will be significant, providing protection using the zero-voltage voltage module as a fault indicator and a stability failure indicator to disable short circuit and distributed generation sources before damage to equipment can occur.

[016] The instantaneous signals of the initial phase voltages and currents are processed by input analog filters, filtering out random measurement noises and higher harmonics. Further, the analog analog phase voltage signals are obtained from the processed analog phase voltage signals with the help of an analog direct order filter, since it additionally eliminates possible third harmonic disturbances which are similar to zero order. The analogue direct sequence voltage signal is further used to determine the voltage frequency and its change in time, or rate of change with a differentiating element. Frequency changes are expected as significant power system perturbations (system element disconnections, short circuits) change the power delivered to the system, causing the generator rotors to accelerate or decelerate depending on whether the generator drive capacity is greater or less than the new system consumed power. The said rotor motion is described by the differential equation [12] / 2 e (1) dt ² where: Tj is the generator inertia time constant, s; δ is the angle between the generator electromotive force vector and the synchronous rotating system voltage vector, °; t is the time, s; P _me hi ^r generator drive mechanical power, MW; P _maK is the maximum transmitted power in the power transmission network between the generator and the system, MW.

[017] In an unstable mode, the generator has an offset angle δ greater than 180 °, and the generator starts asynchronously with respect to the power system. In turn, when the power system connection is disconnected, the generator begins to determine the frequency of the network connected to it. According to equation (1), in the event of a power failure, an unbalanced system with distributed generators and loads will result in acceleration or deceleration of the generator rotors, but in this scenario it will also cause an increase or decrease in the voltage frequency, sometimes called . Frequency changes can be described by the equation [13]

Sf = -f _N (SP / P _N ) / K _f , (2) where: Δ / is the difference between the rated and actual voltage, Hz; is the rated frequency of the network, Hz; AP is the difference between generator active power and load active power, MW; P _N is the nominal power transmitted by the nominal generator or network, MW; Āiy- is the frequency change ratio in relative units, r..v.

[018] For the above assertion, simulations were performed with a circuit model depicting a real urban medium-voltage cable network described in [10], with the addition of two distributed generation source generators. The simulations were implemented in the program environment [11]. In the simulated network, a short circuit appeared 0.2 seconds after the start of the simulation and the VZSA lines on the power system side operated 2 s after the start of the simulation. Looking at Fig. 2, it can be seen that 2 s after the start of the simulation, the frequency decreases rapidly (generation in the distributed network was less than the consumption), confirming the claimed assumption of frequency change.

The earth-fault indicator of the invention is a zero-voltage voltage module because, despite its low zero-current current, the phase-voltage vector asymmetry will be significant and the zero-voltage voltage module will be close to the direct-sequence voltage module as observed.

Fig. 3 It shows that the values of the direct and zero voltage modules after the short circuit appear (0.2 s after the start of the simulation) are practically the same and close to the nominal voltage (10 kV), which allows safe voltage measurements and a high trip setting sequence voltage as follows:

U °> 0.5 -0.85i / ¹ , (3) wherein: U ° is a zero-voltage module, V; U ¹ is the voltage module of the direct sequence, V.

[019] To explain the cause of the possible non-strain VZSA non-conducting glue, a line zero-order substitution scheme is shown in which the low-resistance neutral mode is provided by a neutral transformer on the power system side (see Figure 4). The substitution diagram shows that, in the event of a short circuit, even before the circuit breaker Q1 of the line power system is tripped, the short circuit has split the circuit into two parts. Power line side of the zero-sequence current provides significant neutral-forming transformer neutral-on coil and arcing mazrezistīvas neutral resistor but through distributed generation line side power switch Q2 flows only capacitive current / g ₂ ⁿ⁰ more outgoing-feeders and the connected loads. The simulations carried out corroborated the assertion and showed that for the isolated part of the network the short-circuit current reached only 6 A (see Fig. 5), which is many times less than the load current (see Fig. 5 phase up to 0.2 seconds since of the simulation) and a current of 340 A flowing through the power switch Ql, providing a minimum total earth leakage current of 1 kA as defined in standard [2]. Accordingly, in order to provide sufficient protection sensitivity to distributed part of the network with distributed generation sources, it would be necessary to install at least one new neutral transformer or to use sensitive cable zero-current transformers for protection, which in both cases require relatively high investment. The invention would allow these investments to be replaced by a VZSA supplement. The method defined in the invention simultaneously implements the conventional VZSA, which allows it to be used in networks with sufficient zero-current to safely disable grounding.

[020] The part of the invention which uses frequency change rate and zero-voltage voltage module criteria for protection activation also provides a time delay before the protection resets to an intermittent arc, where a zero-voltage voltage test may temporarily fail during power breaks.

Embodiment of the Invention Embodiment A flowchart of the invention is shown in FIG. The method was implemented by executing the following set of operations in the following order:

1. The instantaneous phase voltage signals uli, ul2, ul3 and the instantaneous phase current signals il, IL2, IL3 are fed to a random noise and higher harmonic filter unit (1), where the noise and higher harmonics are filtered from the measurements.

2. The analog signals processed in block (1) are fed to an analog-to-digital converter and an orthogonal component filter unit (2), in which the analog signals are digitized and converted from instantaneous signals to complex phase voltages Uli, Ul2, Ul3 and phase currents values.

3. The measured set values obtained in block (2) are fed to the zero-voltage and current filter blocks (3) and (4), which determine the complex values of the zero-voltage and current from phase values.

4. The results obtained in blocks (3) and (4) are fed simultaneously to the zero-voltage and current-module comparison units (5) and (6), where the values of the complex zero-voltage and current modules are compared to the settings, and to the zero-current and voltage phase. an offset and fault detection unit (8) for determining whether a short circuit has occurred in the protected line based on the mutual arrangement of the zero-current and voltage vectors in the complex plane, and allowing the zero-current vector to rotate the correction angle value Фкон for various neutral modes.

5. Upon detecting the magnetization current jumps of a line-connected transformer, the zero-current current in setting unit (6) is automatically increased to a value kf greater than the maximum zero-current current produced by the phase current asymmetry during the transformer magnetization current jumps.

6. In the event that the earth fault results in a sufficiently high zero-voltage and amperage, blocks (5) and (6) are triggered by activating their outputs, which trigger the logic "UN" element block (7).

7. At the same time as the block (7) is triggered, the output of block (8) is activated to detect that the protected line is defective and the logic unit (9) of the logic "UN" is triggered.

8. Following the operations described in step 7, the fixed time delay timer unit (11) starts a time reference.

9. While the output of block (9) remains active at time interval At2, the output of block (11) is triggered, which causes the logic "OR" element block (19) to trip, which output signals to the circuit breaker YAT.

10. When the power switch is disengaged, the operating conditions for the blocks (5) and (6) disappear and after blocking the AtRES time delay for the timer reset, the unit (10) returns the block (11) to the output state via the RES input.

11. In the case of an earth-fault with an intermittent arc, the block (10) provides a continuation of the time reference from the previous value, provided that the power-off time does not exceed the time interval AtRES.

12. At the same time a set of phase-to-earth current protection operations described in the operations from the block (1) to the processed phase-voltage analog signals to a direct-sequence analog filter unit (12) which obtains an analog direct-sequence voltage signal.

The direct sequence voltage signal obtained at the output of unit (12) is fed to an analog signal frequency determination unit (13) which determines the frequency of the transmitted signal.

The frequency value determined in the block (13) is fed to the time change rate detection unit (14), which obtains the rate change rate module.

15. During the change of frequency, the value of the rate change module of the output of the module comparator block (15) from the output of the block (14) is compared with the setting.

16. In the case of an unbalanced part of the grid with distributed generation sources due to an earth fault, the voltage frequency will change after the release and the unit (15) will operate, activating its output.

17. In the situation where the earth leakage current of the earth fault called in operation 16 is insufficient to operate the unit (6), the zero voltage voltage will remain significant and the output of the unit (5) will be active as soon as the short circuit occurs.

18. When the units (5) and (15) are actuated, the trigger criterion for the logic " UN " element unit (16) is met and its output actuation initiates a time reference in the fixed time delay timer unit (18).

19. While the output of the block (16) remains active at the time interval Ati, the output of the block (18) activates, which causes the logic "OR" element block (19) to trip, which output signals the circuit breaker YAT.

20. When the power switch is disengaged, the operating conditions for the blocks (5) and (15) disappear and after blocking the AtRES time delay for the timer reset, the unit (17) returns the block (18) to the output state via the RES input.

21. In the case of an earth-fault with an intermittent arc, the block (17) provides a continuation of the time reference from the previous value, provided that the power-off time does not exceed the time interval AtRES.

[0221 The use of the mentioned network frequency change rate and zero-voltage voltage module criteria enables the implementation of line protection in the case of low-voltage medium-voltage networks with distributed generation sources, reducing the risk to human life and economic losses.

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Claims (1)

THE CLAIM 1. Method of Protection of Medium Voltage Power Transmission Lines in the Case of Earth Faults Using Zero Voltage Vectors and their Voltage Ratios and their Interference Vectors from Instantaneous Phase Voltage and Current Signals with Random Noise Filter, Higher Harmonic Filter, Analog-to-Digital Converter, Orthogonal Component Filter and Zero Filters to provide, after a fixed time delay, the disconnection of a defective line circuit breaker in networks without distributed generation sources, characterized in that: i. after processing with a random noise and high harmonic filter, obtains an analogue direct voltage signal from the phase voltage signals with an analogue direct sequence filter, ii. the direct-sequence voltage signal is used to determine the frequency of the network by supplying an analog signal to the frequency determination unit, iii. determining the rate of change of the obtained frequency in the differential block, iv. changing the frequency of the network with the speed and the zero-voltage voltage module simultaneously exceeding the set values after a fixed time-delay protection gives the command of tripping the circuit breakers of the defective line and the distributed generation sources, v. fixed time delay timers that provide time delay before the circuit breaker trip command is given, both for protection in networks with distributed sources and for protection in networks with non-distributed generation sources to return to the start position to provide protection in the event of an earthing intermittent loop.