The invention relates to a method for improvement of earth-fault protection in compensated MV networks, used in order to prevent exposing people to

dangerous voltages and development of earth-faults to the phase-to-phase faults. More detailed the method is designed for MV network grounded through the Petersen-coil device and equipped with at least one of intelligent electronic devices lEDs as a protection device for power lines.

Background of the invention

The medium-voltage networks are in some countries grounded via Petersen-coil in order to compensate the network's capacitive currents during the earth-faults. The compensation limits the earth-fault currents which are not sufficient to maintain the arc and some faults are distinguishing without need to disconnect the customers from power supply.

The network is compensated via Petersen-coil installed at the grounding point of supplying transformer. The Petersen-coil controller is monitoring the network compensation level and in case of any change, the controller changes the

Petersen-coil inductance till the required compensation level is achieved. The Petersen-coil controller is a device that works independently of devices that are performing the earth-fault protection in the network (intelligent electronic devices lEDs).

One of the most challenging tasks in compensated medium-voltage networks is high-ohmic earth-fault detection. Traditionally, at each power line (cable or overhead line that connects the substation to the customers) is installed an IED that receives voltage and current measurements and starts earth-fault protection function when predefined residual voltage threshold UOTH is exceeded. The threshold value UOTH needs to be higher than network's natural (healthy state) residual voltage, which depends on power line asymmetries, state of power line circuit breakers, compensation degree etc. For this reason the residual voltage threshold (UOTH) is selected to be above the highest expected value of the network's healthy state residual voltage and it does not adjust according to the state of the network. In some cases, due to lack of knowledge of network's healthy state residual voltage, a high value of threshold is selected and used as a constant value for different medium-voltage networks. The consequence of high threshold values is that the earth-fault protection is able to detect only low-ohmic earth-faults, since for higher-ohmic earth-faults the residual voltage would not exceed the threshold UOTH.

The earth-fault protection would be more sensitive, which means to be able to detect higher-ohmic earth-faults, if the threshold is set above, but closer to the maximum possible residual voltage for a give network state. The Petersen-coil controller estimates the network's maximum residual voltage in each state, but this information is not available at lEDs, which are usually set at constant threshold.

Prior art

The prior art is mainly referring to the tuning techniques of Petersen-coil, which does not influence the earth-fault protection settings, since there is no

communication between the Petersen-coil controller and protection devices. From the patent EP0823057 there is known a method for the tuning of the arc

suppression coil. From the other patent application WO2014021773 there is known an another type of solutions which includes an additional device that injects currents into the grounding point of the network in order to detects the faulty power line and improve protection sensitivity. The drawback of this solution is that it requires additional hardware for current injection as well as measurements from all power lines.

From patent application WO 96/34293 there is known a process for monitoring a three-phase mains for a change in the tuning of the arc suppression coil in which the neutral point shift voltage arising at the arc suppression coil is continuously measured and compared with a tolerance range. The tuning of the arc suppression coil is the only purpose of the invention, hence the operation of earth-fault protection in the network is not considered nor improved

From patent application WO 2014/194941 there is known a method of detection of ground faults in energy supply networks with a compensated star point. The solution uses a grounding device which is temporarily connected to the star point of the energy supply network. A ground fault is detected if the residual voltage exceeds a specified threshold and the admittance variable exceeds a specified admittance threshold. In this invention earth-fault protection sensitivity depends on a specified residual voltage threshold which is not consider to be adjustable in real time for maximum protection sensitivity.

The summary of the invention

An essence of the invented method is that the method comprises the step of a determination a residual voltage threshold UOTHI for earth-fault protection function of lEDs of the MV network which is calculated according to the formula:

UOTHI = U0MAX1 + ΔΙΙ , where:

U0MAX1 represents an estimated maximum residual voltage value for a given network state,

ΔΙΙ represents a security margin in percentages selected by the user, and the method has a step of comparison of the residual voltage threshold UOTHI with a value of minimal residual voltage threshold UTH IN, defined by the user, and according to the result of the comparison a final threshold UFINAL for earth-fault protection in lED's (61... .6 _n ) is determining for the larger of two values UOTHI or UTHMIN.

Preferably a communication signal informing about the value of the final threshold UFINAL is sent via a communication network to the lEDs. Alternatively a communication signal informing about the value of maximum residual voltage UOMAXI is sent via a communication network to the lEDs where the final threshold UFINAL for earth-fault protection in lEDs is calculated.

Preferably the step of calculating the final threshold UFINAL for earth-fault protection in lEDs is performed in a central protection unit.

Preferably the security margins AU are recommended for different types of network and final selection of AU values is determined by the users.

The essence of a computer program product comprising computer-readable program code which when executed on a computer device being a Petersen-coil controller or a central protection unit or lEDs, which carried out the steps of improvement of earth-fault protection in compensated MV networks comprises the following steps: the step of measuring an input residual voltage signals Uo delivered to a Petersen-coil controller, the step of detecting a state of the network using a resonance curve R, being a function of a compensation level K, given by the user, and the input residual voltage signal Uo, the step of a determination a residual voltage threshold UOTHI for earth-fault protection function of lEDs of the network which is calculated according to the formula:

UOTHI = UOMAXI + AU , where:

UOMAXI represents an estimated maximum residual voltage value for a given network state,

AU represents a security margin in percentages selected by the user, the step of comparison of the residual voltage threshold UOTHI with a value of minimal residual voltage threshold UTHMIN, defined by the user, the step of determination a final threshold UFINAL for earth-fault protection in lED's for the larger of two values UOTHI or UTHMIN.

The advantage of the method according to the invention is to overcome the problem of insufficient earth-fault protection sensitivity for higher-ohmic earth- faults, due to constant residual voltage threshold. The present invention is based on real-time adjustment of residual voltage threshold, which is utilized via communication network between the Petersen-coil controller and lEDs. The basic functionality of Petersen-coil controller includes estimation of the maximum residual voltage in each network state. This information is shared with lEDs via a communication network in order to achieve a dynamic change of earth-fault protection thresholds and better sensitivity for high-ohmic faults.

The method according to the invention can be used in any type of compensated network with any type of power lines (overhead lines, cables) and different voltage levels (10kV, 20kV...)- It does not depend on the type of implemented earth-fault protection at lEDs, as long as it uses the residual voltage threshold as a start for the earth-fault protection function. It does not depend on the type of the main control technique of Petersen-coil, as long as it estimates the residual voltage in the resonant state of compensation. It only requires the communication network between the controller and lEDs and a module for real-time calculation of a residual voltage threshold UOTHI, which could be a part of Petersen-coil controller or lEDs.

Technical description

The method according to the invention is explained in an embodiment shown in the drawing, where: Fig.1 presents a single-line diagram of a distribution network compensated by a Petersen-coil,

Fig.2 presents a flowchart of the improved earth-fault protection method according to the first embodiment of the invention,

Fig.3 presents a flowchart of the improved earth-fault protection method according to the second embodiment of the invention,

Fig.4 presents a diagram of the change of the state of distribution network.

In Fig. 1 there is shown a single line diagram of a distribution network 1 that is supplied from an energy source 2, in the form of HV/MV transformer, electrically connected with a busbar 3 which is connected with at least one of 4i to 4 _n power lines, where n is a natural number. The power line further supplies energy to the end customers 5i ... .5 _n .. The solution is applicable to any number of sources, any number of power lines, switchgear type or busbar arrangement. The power lines are protected by intelligent electronic devices, lEDs 6i ...6 _n , but they could be protected by using one central protection device 6, what is indicated in the drawings by dashed line. The lEDs require measurements M6 of voltage and current signals from protected power line. The network 1 is grounded through the Petersen-coil device 7, which is controlled by Petersen-coil controller 8 in order to maintain the required level of network compensation. The grounding resistance 9 is connected in order to increase the resistive part of fault current and allows reliable detection of earth-faults. The controller 8 is a computer device with a processor which comprises some functional modules for control the Petersen-coil inductance device 7:

8i - the module for preprocessing the measured input signals M8. The required signal is an input residual voltage Uo, but depending on implemented functions it is adapted to receive additional signals from other parts of the network 1 , 82 - the module that controls the Petersen-coil 7 and grounding resistance 9 and is adapted to estimate a resonance curve being a function f(U ₀ , ) in order to determine and maintain the required network compensation level ;

83 - in the first embodiment of the invention, 83 is the module for real-time adjustment of a residual voltage threshold UOTH, for the earth-fault protection function implemented at lEDs. In the second embodiment of the invention the module 83 does not exist, what is presented in dotted line, since the residual voltage threshold U _O TH is adjusted at lEDs;

84 - the communication module that sends the information to lEDs via the communication network.

The control device 8 comprises also other modules what is nor presented in the drawing but the modules are implemented in the processor of the control computer device 8.

In the network 1 between the lEDs 61...6 _n and a controller 8 a communication network 10 is installed, which is connected with a controller communication module 84, implemented in the controller 8 and with communication modules 61 a ·■■ 6na, implemented in the electronic devices lEDs 61...6 _n ..

A method according to the first embodiment of the invention is realized in the following steps, according to the Fig. 2:

Step S1

In the step S1 the measured input signals M8 are acquired, preprocessed and prepared in the controller 8 for usage in the next steps. It is required to measure at least a residual voltage Uo, (what could be measured at 7 or at other places in network 1) and estimate the magnitude of the residual voltage Uo.

Step S2

In the step S2 the state of the network is examined in the controller 8. The measured signals M8 are compared to predefined thresholds, given by the user or on the basis of the previous measured values of M8, in order to detect if the state of the network 1 has changed. The detection of the state of the network is well known for skill in the art so there is no need to explain such process in detail. If the state of the network has changed (YES), then the next steps are executed, otherwise the steps S1 and S2 are executed again.

Step S3

In the step S3 the controller 8 controls a Petersen-coil inductance in the device 7 and changes its value in order to maintain a required compensation level K, given by the user. In Fig. 4 is shown an example of this process. There are two

resonance curves R1 and R2 as a functions f(U ₀ , K). The network 1 changed the state in step S2 in which the operating point was Oi indicated on the resonance curve Ri. The maximum value on the curve R1 was indicated as UO AX and it was determined for the compensation level K having value of 100%, while for earth-fault protection a residual voltage threshold was indicated as UOTH. In the new state the network operating point is O2, on the resonance curve R2, and the compensation level K having a value X is different from the required 95%. The controller 8 changes the Petersen-coil inductance in the device 7, in order to maintain a required compensation level K as 95% and the operating point moves from O2 to the point 03 0η the resonance curve R2. In that process the resonant curve R2, which represents the residual voltage Uo as a function of compensation level K of the network, is calculated in module 82. For the resonance curve R2, a new value of a maximum residual voltage UOMAXI for a new state of the network is determined. The residual voltage is given in form of relative values to the nominal phase voltage.

Step S4

In the step S4 a residual voltage threshold UOTHI for a given network state is determined according to the formula:

UOTHI = UOMAXI + AU, where: - Uo Axi represents the estimated maximum residual voltage value for a given network state,

- ΔΙΙ represents the security margin in order to distinguish the healthy and the fault state of the network.

The ΔΙΙ depends on many factors and it is given in form of recommendations for different types of networks. The user choose the set of values and limits

appropriate for their network and local regulations. The AU value is result of deep investigation of distribution networks. It depends on type of power lines (cables, overhead lines), the overhead line geometry, the level of asymmetry and its share in the network.

Step S5

In the step S5 the residual voltage threshold UOTHI is compared to a minimal allowed voltage value UTHMIN defined by the user. If UOTHI < UTHMIN then the next step S6 is performed. If UOTHI ½ UTHMIN, what means that the residual voltage threshold UOTHI exceeds, or it is equal to the minimum required value UTHMIN, and it is used as a final residual voltage threshold UFINAL= UOTHI .

Step S6

In the step S6 the final residual voltage threshold UFINAL for earth-fault protection in lED's (61....6n) is set to the minimum value allowed by the user UTHMIN:

UFINAL= UTHMIN. Step S7

In the step S7 the communication module 8 ₄ is sending an information about the final value of residual voltage threshold UFINAL to the communication modules

6i _a ..6na via communication network 10. The lEDs 6i ..6 _n are taking UFINAL as a new threshold for starting the earth-fault protection. The earth-fault protection sensitivity is improved, since it is adjusted to the current network state, which leads to detection of higher-ohmic earth-faults. The measured data M6 of voltage and current signals from protected power lines are received in the lEDs. If the residual voltage Uo in the network 1 exceed the value of residual voltage threshold UFINAL, the earth-fault protection function implemented in lEDs are executed and faulty power line is disconnected from the network 1 .

A method according to the second embodiment of the invention is realized in the following steps, according to the Fig. 3:

Step S1 is the same as in the first embodiment of the invention.

Step S2 is the same as in the first embodiment of the invention.

Step S3 is the same as in the first embodiment of the invention.

Step S4*

In the step S4* the communication module 84 is sending an information about the new value of a maximum residual voltage UO AXI to the communication modules 6ia..6na via communication network 10.

The main difference between two embodiments of the invention is that the optimal threshold is determined in lEDs (6i..6 _n or 6), instead in the controller 8.

Step S5* is the same as the S5 in the first embodiment of the invention, but it is performed in the lEDs (6i..6 _n or 6).

Step S6* is the same as the S6 in the first embodiment of the invention, but it is performed in the lEDs (6i ..6 _n or 6).

Step S7*

The lEDs 61..6 _N are taking UFINAL as a new threshold for starting the earth-fault protection. The earth-fault protection sensitivity is improved, since it is adjusted to the current network state, which leads to detection of higher-ohmic earth-faults.