Monitoring an electric power grid

Abstract

This document discloses a solution for a method of estimating short-circuit current at a given bus of an electric power grid, the grid comprising at least one transmission system and at least one active distribution grid According to an aspect, a method comprises: estimating (400) initial values and proportion for currents due to demand and production based on available, statistical information; refining estimates for the proportion of current due to demand and production using real-time measurement (404) of voltage and current of the given bus; and upon detection (406) of a voltage change event, estimating the short circuit current, short circuit capacity or short circuit ratio of the given bus and / or the local network based on the measured change in voltage and current.

Description

[0001]MONITORING^AN^ELECTRIC^POWER^GRID TECHNICAL FIELD The invention relates to measurement-based monitoring of an electric power grid and, for example, to determining the ability of the system to resist voltage changes. TECHNICAL BACKGROUND Since the standardisation of the frequency of alternating current (AC) electricity in large scale electric power grids in the mid-20th century around the globe, consumers of electricity have been able to enjoy a consistent and dependable service of electricity, ensuring safe and reproducible use of electrical appliances. Provision of such a reliable service may include monitoring the characteristics of an electric power grid and taking measures to anomalies detected in the grid. For example, system strength, or the ability of the system to resist voltage changes at any given location, is a parameter which has been monitored in electric power grids. Estimating the short circuit current or fault level in different nodes of a power system is an essential measure that can help system operators in voltage stability analysis, relay and protection setting, equipment sizing, and reliability analysis. In a traditional power system, it is assumed that the short circuit current of each node is not varying considerably in different working conditions; therefore, it is estimated from power system models using simulation software. However, modern power systems have more and more renewable energy sources, connected via an inverter to grids. The high penetration of these energy sources reduces the contribution of the transmission network in short- circuit current while it considerably increases the role of distribution grids. In modern power systems, distribution grids have changed from passive to active; they host a significant number of distributed generations, DG, and different loads controlled by power electronic converters. In these circumstances, the contribution of active distribution grids, ADG, which host embedded generators and modern loads, cannot be ignored in estimating short-circuit currents. A reliable measurement of the short circuit current or fault level is need for the modern power grids. BRIEF DESCRIPTION The invention is defined by the independent claims. Embodiments are defined in the dependent claims. According to an aspect, there is provided a method for estimating short- circuit current at a given bus of an electric power grid, the grid comprising at least one transmission system and at least one active distribution grid, the method comprising: estimating initial values for currents due to demand and production in the at least one active distribution grid based on available, statistical information; updating the initial values of currents due to demand and production in the at least one active distribution grid based on real-time measurements or information from corresponding grids; determining voltage and current of the given bus; and upon detection of a voltage change event, determining voltage and current of the given bus after the event; estimating short-circuit current coming from the at least one transmission system based on the voltage and current of the given bus before and after a detected voltage change event; estimating the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution grid based on the voltage and current of the given bus before and after a detected voltage change event; estimating the short-circuit current coming from the at least one active distribution grid based on the estimated shares and currents due to demand and production in the at least one active distribution grid; and estimating short-circuit current at the given bus by subtracting the short- circuit current coming from the at least one active distribution grid from the short- circuit current coming from the at least one transmission system; and upon detection of further voltage change events, updating the above current and share estimates and re-estimating the short-circuit current at the given bus. In an embodiment, two or more voltage change events are caused in the electric power grid. In an embodiment, the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution grid is estimated where ^^^^ ^ is the current due to demand in the at least one active distribution grid after a voltage change event, , ^^^^ ^ is the current due to demand in the at least one active distribution grid before a voltage change event, KM is share of motor originated current, KI the share of inverter-connected current and the share of static originated current KS is defined as KS = 1 - KM - KI. According to another aspect, there is provided an apparatus for monitoring an electric power grid (100), the apparatus comprising means (600, 602, 604, 610) for performing: estimating initial values for currents due to demand and production in the at least one active distribution grid based on available, statistical information; updating the initial values of currents due to demand and production in the at least one active distribution grid based on real-time measurements or information from corresponding grids; determining voltage and current of the given bus; and upon detection of a voltage change event, determining voltage and current of the given bus after the event; estimating short- circuit current coming from the at least one transmission system based on the voltage and current of the given bus before and after a detected voltage change event; estimating the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution grid based on the voltage and current of the given bus before and after a detected voltage change event; estimating the short-circuit current coming from the at least one active distribution grid based on the estimated shares and currents due to demand and production in the at least one active distribution grid; and estimating short-circuit current at the given bus by subtracting the short-circuit current coming from the at least one active distribution grid from the short-circuit current coming from the at least one transmission system; and upon detection of further voltage change events, updating the above current and share estimates and re-estimating the short-circuit current at the given bus. According to another aspect, there is provided a computer program product readable by a computer and comprising computer program instructions that, when executed by the computer cause execution of a computer process for estimating short-circuit current at a given bus of an electric power grid, the grid comprising at least one transmission system and at least one active distribution grid, the process comprising: estimating initial values for currents due to demand and production in the at least one active distribution grid based on available, statistical information; updating the initial values of currents due to demand and production in the at least one active distribution grid based on real-time measurements or information from corresponding grids; determining voltage and current of the given bus; and upon detection of a voltage change event, determining voltage and current of the given bus after the event; estimating short- circuit current coming from the at least one transmission system based on the voltage and current of the given bus before and after a detected voltage change event; estimating the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution grid based on the voltage and current of the given bus before and after a detected voltage change event; estimating the short-circuit current coming from the at least one active distribution grid based on the estimated shares and currents due to demand and production in the at least one active distribution grid; and estimating short-circuit current at the given bus by subtracting the short-circuit current coming from the at least one active distribution grid from the short-circuit current coming from the at least one transmission system; and upon detection of further voltage change events, updating the above current and share estimates and re-estimating the short-circuit current at the given bus. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which Figure 1 illustrates an example of an electric power grid to which embodiments of the invention may be applied; Figure 2 illustrates an example of a simple electric power grid in a schematic form; Figure 3 illustrates an equivalent of the typical power system; Figures 4A and 4B are flow diagrams illustrating some embodiments; Figure 5 is a block diagram illustrating an embodiment; and Figure 6 illustrates a block diagram of an apparatus according to an embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS The following embodiments are exemplary. Although the specification may refer to “an”, “one”, or “some” embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same embodiment(s), or that a particular feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Supply of electricity from providers such as power stations, to consumers, such as domestic households, offices, industry, etc. typically takes place via an electricity distribution network or electric power grid. Figure 1 shows an exemplary electric power grid 100, in which embodiments of the present invention may be implemented, comprising a transmission grid 102 and a distribution grid 104. The transmission grid 102 is connected to power generators, which may be power plants such as nuclear plants, hydroelectric power plants, wind generators or gas-fired plants, for example, from which it transmits large quantities of electrical energy at very high voltages (typically of the order of hundreds of kilovolts, kV), over powerlines such as overhead powerlines 110, to the distribution grid 104. The transmission grid 102 is linked to the distribution grid 104 via a transformer 112, which converts the electric supply to a lower voltage, typically of the order of 66kV, for distribution in the distribution grid 104. The distribution grid 104 is connected via substations 114, 116, 118 comprising further transformers for converting to still lower voltages to local networks which provide electric power to power consuming devices connected to the electric power grid. The local networks may include networks of domestic consumers, such as a city network 115 that supplies power to domestic appliances within private residences 132, 134 that draw a relatively small amount of power in the order of a few kW. The private residences may also use photovoltaic devices or other power generators 150 to provide power for consumption either by appliances at the residence and / or for provision of power to the grid. The local networks may also include industrial premises such as a factory 130, in which larger appliances operating in the industrial premises draw larger amounts of power in the order of several kW to MWs. The local networks may also include networks of smaller power generators such as wind farms 137 that provide power to the electric power grid. The local networks may further comprise energy storage devices 136 to store electric power locally. Such storage devices 136 may be used to compensate for a difference between supply and demand of electric power. Although, for conciseness, only one transmission grid 102 and one distribution grid 104 are illustrated in Figure 1, in practice a typical transmission grid 102 supplies power to multiple distribution grids 104 and one transmission grid 102 may also be interconnected to one or more other transmission grids 102. Electric power flows in the electric power grid as alternating current (AC), which flows at a system frequency, which may also be referred to as a grid frequency (typically in the range of 50 or 60 Hz, depending on the country). The electric power grid operates at a synchronized frequency so that the frequency is substantially the same at each point of the grid. The electric power grid may include one or more direct current (DC) interconnects (not shown) that provide a DC connection between the electric power grid and other electric power grids. Typically, the DC interconnects connect to the high voltage transmission grid 102 of the electrical power grid. The DC interconnects provide a DC link between the various electric power grids, such that the electric power grid defines an area which operates at a given, synchronised, grid frequency that is not affected by changes in the grid frequency of other electric power grids. For example, the UK transmission grid is connected to the Synchronous Grid of Continental Europe via DC interconnects. In an embodiment, the electric power grid 100 may also include a measurement system in the form of measurement devices 120 to 129 at given measurement points of the grid arranged to measure the electric power grid. The measurement devices 120 to 129 may be configured to measure one or more electric parameters of the electric power grid at the given measurement points. At least some of the measurement devices 120 to 129 may be measurement points at the distribution grid 104, such as the measurement devices 122 to 129, but some of the measurement devices 120, 121 may be at measurement points at the transmission grid. The measurement device 120 is coupled directly to a high- voltage bus while the measurement device 121 is coupled to a lower-voltage-level bus of the transmission grid 102. A separate transformer 113 may be provided to transform the higher voltage level to the lower voltage level. As illustrated in Figure 1, the measurement devices may be coupled to various measurement points at various voltage levels of the electric power grid. For example, the measurement device 120 coupled to the transmission grid 102 may be configured to perform measurements at the very high voltage level of the transmission grid, e.g.132 kV. The measurement device 122 may be coupled to the distribution grid to perform measurement at a lower voltage level, e.g.11 or 33 kV. The measurement devices 124 to 129 may be coupled to the distribution grid at a still lower voltage level or levels such as 220 V, 400 V, and / or 11 kV. The voltage level at the measurement device 121 may also be the still lower voltage level such as 220 V, 400 V, or 11 kV. The reader is reminded that the actual voltage levels are merely exemplary, and different electric power grids may employ different voltage levels. Although, for the sake of simplicity, only a few measurement devices are illustrated in Figure 1, it will be understood that, in practice, a higher number of such measurement devices may be coupled to the electric power grid, at various voltage levels and / or at various measurement points such as at different sub- stations or sub-networks of the electric power grid. It should also be appreciated that some embodiments may employ the measurement devices on only a subset of the voltage levels of the electric power grid or the distribution network 104. Figure 2 illustrates an example of a simple electric power grid in a schematic form. As mentioned, a typical power system includes transmission grid 102 that have meshed interconnection and distribution grids 104 that have radial connections. Traditionally distribution grids were passive and did not produce energy. However, in modern power systems, we have active distribution grids (ADGs) that produce energy and contribute to the short-circuit current. Figure 2 shows a typical power system including the transmission system (upstream grid) 102 and one active distribution grid (ADG) 104 connected to a bus 200, where in this example a short-circuit 202 is happening. Without loss of generality, it may be assumed that the aim is to calculate the short circuit current in the bus 200. The short circuit current in the bus 200 is originated from the upstream grid 102 and the ADG 104 with embedded generations and dynamic loads. The example of a simple electric power grid comprises in the transmission system (upstream grid) 102 synchronous generators 204, inverter- based generators 206 and bus bars 208. The active distribution grid (ADG) 104 comprises synchronous generators 204, inverter-based generators 206, bus bars 208 and loads 210. In prior art, calculation of the short circuit capacity of the upstream grid is based on the Thevenin equivalent using the grid model. The Thevenin equivalent concept is a very well-known method where the upstream grid is represented with a voltage source V in a series connection with a resistance R. So far, the Thevenin equivalent is used to represent also the downstream grid. However, because of the modern active distribution grid comprise also embedded generators and dynamic loads, the Thevenin equivalent does not produce reliable results. As modern power systems have more generators connected via an inverter to the grid, the short- circuit current that comes from the upstream is decreasing while the part originating from ADG is increasing. Consequently, it is important to consider the impact of ADG in short-circuit calculation. In order to calculate the short-circuit current in the bus 200 an equivalent model for the downstream grid and the ADG is required, a model that represents the upstream grid better than Thevenin equivalent which is suitable for modelling the upstream grid. It is proposed to model ADG using aggregated model of demands and generations since distribution grids are not typically very long. Figure 3 illustrates the equivalent of the typical power system of Fig.1, from the perspective of the faulty bus 200 where upstream is modelled using a Thevenin equivalent and the downstream with an aggregated model of demands and generations. Figure 3 shows the Thevenin equivalent 300 of the upstream grid comprising voltage Vthand impedance Zthof the Thevenin equivalent. Itis the current coming from the upstream grid. Figure 3 further shows the equivalent 302 of the downstream grid comprising IDas demand current and IPas production current. In the equivalent model of Figure 3, the short-circuit current ISCcan be calculated as ISC= It,SC– IADG,SC, (1) where ISCis the total short-circuit current, It,SCis the short-circuit current coming from the upstream grid and IADG,SCis the short-circuit current coming from the ADG. Here it is assumed without loss of generality, a solid fault (fault resistance Rf= 0), where we will face the maximum short-circuit current. However, the equations can be easily modified for different fault resistance Rf. The short-circuit current originating from upstream can be estimated (assuming Rf= 0) as ^ ^^,^^=^^^^^(2) where Vthand impedance Vthare the voltage and impedance of the Thevenin equivalent. In an embodiment, Vthcan be estimated by the voltage of the bus before ∆^ short-circuiting and Vthcan be estimated using ^ = ∆^ by measuring the upstream voltage and current before and after an event to detect voltage change. The short-circuit current originating from downstream or ADG can be estimated as IADG,SC = ^^,^^− ^^,^^. (3) Here ^^,^^is the short-circuit current originating from demand. It can be defined as the sum of current of static demand ^^,^,^^, the current of demand connected using power electronic inverter ^^,^,^^and the current of motor loads ^^,^,^^: ^^,^^= ^^,^,^^+ ^^,^,^^+ ^^,^,^^(4) ^^,^^is the short-circuit current originating from production in the ADG and it can be defined as the sum of current of synchronous generators ^^^,^^and the current of inverter-based productions ^^,^,^^: ^^,^^= ^^^,^^+ ^^,^,^^(5) Thus, the short-circuit current originating from downstream or ADG can be estimated as IADG,SC= ^^,^,^^+ ^^,^,^^+ ^^,^,^^− (^^^,^^+ ^^,^,^^). (6) Let us study the behaviour of demand when a short circuit occurs. The motor loads contribute the most in short-circuit current since they have kinetic energy stored in their rotors and when facing a very low voltage (short-circuit voltage) in their terminal, they act as a generator for a couple of cycles. Therefore, their short circuit current of motor loads ^^,^,^^can be estimated as follows: ^^,^,^^= −^^^^,^(7) where ^^,^is the nominal current of the motor and ^^is the motor contribution factor. ^^may be 0 to 6 depending on the motor starting and control technology. For example, for direct connection this factor could be up to 6, whereas in low inrush motors, it is reduced to 4. Motors with variable frequency drive have a contribution factor of about 2 and some more advanced controllers can even reduce this contribution to 0. Inverter-connected demand will not inject current in any short circuit. Many loads such as all electronic devices and many modern lightings are connected through power electronic interfaces. This type of demand continues to consume the same current during any voltage change unless the voltage goes very low, for example less than 50% of the nominal value. Therefore, their fault contribution can be as follows: ^^,^,^^= ^^^^,^(8) where ^^,^is the nominal current of the inverter-connected demand and ^^is their contribution factor. In a passive distribution grid, during a solid fault (Rf= 0), the voltage of the distribution grid will be almost zero in all places and therefore, ^^will be zero. However, in an ADG, where we have embedded generators, the voltage may be high enough in some buses to keep loads. In these cases, a factor between 0 to 0.5 for the aggregated demand may be more realistic. It may be noted that the direction of this contribution is against the motors or contribution of the embedded generators and therefore considering ^^= 0 will be the worst-case scenario. Static demand, such as conventional heaters, will not contribute to short-circuit current. Therefore, it can safely be assumed that ^^,^,^^= 0. (9) Let us next study the behaviour of the ADG short-circuit current originating from production. The main source of short-circuit current from ADG are the embedded (or distributed) productions in ADG. From the short-circuit perspective, the embedded generators can be divided into two major categories: synchronous generators (SG) and Inverter-connected productions. Some of the embedded generators in ADG may be connected to the grid through a synchronous generator (SG). An example of such embedded generator is a small hydropower. The SGs may contribute significantly to short-circuit current. In an embodiment, the short circuit current of SGs following: ^^^,^^= −^^^^^^ where ^^^is the nominal current of the SG and ^^^is the SG contribution factor. ^^^can depend on generator size and model but ^^^= 6 is a good initial estimation. Most of the renewable energy sources (RES), such as wind and PV, and storage systems are typically connected to the ADG via an inverter. Inverters have a limited short-circuit current capacity and therefore their controller limits the fault current. This limitation in transmission grids, where a large short-circuit current is desired, is about 20%. However, grid codes desire a lower short-circuit current in ADG and therefore this contribution is in the nominal range and even may reduce to 0 after a couple of cycles. The short circuit current of inverter- connected productions ^^,^,^^may be estimated by the following: ^^,^,^^= −^^^^^,^(11) where ^^,^n is the nominal current of the inverter-connected productions and ^^^is the inverters contribution factor. ^^^= 1 is a good initial estimation. Based on the above equations (5-9), the equation (4) for short-circuit current originating from ADG, can be written in following form: IADG,SC= ^^^^,^− ^^^^,^− ^^^^^^− ^^^^^,^(12) The above equation (10) comprises nominal currents ^^,^, ^^,^, ^^^and ^^,^. Let us next study how these nominal currents can be estimated. In an embodiment, initial estimation of the nominal production of embedded generators can be made using the available data of the system operator, such as the nominal capacity of embedded generators, historical measurement data, and weather information. This initial estimation can be further improved using the real-time measurement of current and voltage in the target bus. To estimate the nominal current of aggregated motors and inverter- connected demands, the voltage and current changes may be monitored. When the voltage changes from Voldto Vnew, the current of different types of loads changes differently: In motors, most of the time, the motors try to keep their output power fixed. Thus where is the motor nominal current after the event and ^ ^^^ ^,^is the motor nominal current before the event. Inverter-connected demands try to keep their output current fixed, therefore where ^^^^ ^,^is the nominal current of inverter connected demand after the event and ^ ^^^ ^,^is the current before the event. Static loads have a constant impedance; therefore their current will be changed proportiona ^l ^ to voltage, therefore ^^^ ^^^,^= ^^^ ^ ^^^^^^,^(15) where ^^^^ ^,^is the nominal current of static demands after the event and is the current before the As equation (4) indicates, the total demand current ID is the sum of motor, inverter- connected and static originated currents, each providing its share to the total current. The share of motor originated current may be denoted as KM, the share of inverter-connected current may be denoted as KI and the share of static originated current may be denoted as KS. Also, ^^,^= ^^^^(16) and ^^,^= ^^^^, (17) ^^,^= ^^^^. (18) As ^^= ^^,^+ ^^,^+ ^^,^and KM + KI + KS = 1 or KS = 1 - KM - KI, the total demand current ^^^^ ^ after the event can be as In Equation (19), there are two unknown factors, KMand KI, which can be estimated by having voltage and demand current before and after two events. We can also use a regression model and solve equation (16) for multiple events and find the best factors for different situations. It may be noted that Equation (19) can be rewritten also so with the unknown factors being KMand KSor KSand KI,^as one skilled in the art readily knows. The flowchart of Figure 4A illustrates an embodiment. In step 400, initial values and proportion for currents due to demand and production are estimated based on available, statistical information. In step 402, real-time measurement of voltage and current of the given bus are performed. In step 404, estimates for the proportion of current due to demand and production are refined based on the real-time measurements; and upon detection (406) of a voltage change event, in step 408, the short circuit current, short circuit capacity or short circuit ratio of the given bus and / or the local network are estimated based on the measured change in voltage and current. Let us study an embodiment of estimating short-circuit current at a given bus of an electric power grid, referring to Figures 4B and 5. The electric power grid comprises at least one transmission system and at least one active distribution grid. The proposed process may be considered to have three sections, estimation 500 of currents related to demands and productions, event analysis 502 and short circuit analysis 504. In step 430, initial values are estimated 506 for currents due to demand and production in the at least one active distribution grid based on available, statistical information. In an embodiment, initial estimation is based on 508 historical data, weather data, and operational data type and capacity of generation, for example. In an embodiment, real-time measurement is not available at this point. In step 432, the initial values of currents due to demand and production in the at least one active distribution grid are updated 510 based on real-time measurements 512 or information from corresponding grids. In step 434, voltage and current of the given bus is measured 514; A voltage change event is detected 406. In step 438, voltage and current of the given bus is determined 514 after the event. In step 440, short-circuit current coming from the at least one transmission system is determined 516 based on the voltage and current of the given bus before and after a detected voltage change event. In an embodiment, Theven equivalent voltage Vthand impedance Zthare determined or updated (in case of another voltage change) 518 and short-circuit current is calculated based on these values as following: The Theven equivalent Vthof the at least one transmission system ^^^^^^^^^voltage is calculated as ^^^= ^ , where Vold is voltage in the given bus before a detected voltage change event, Vnewis voltage in the given bus after the detected voltage change event. The Theven equivalent Zthof the at least one transmission system ^^^^^ ^^^^impedance is calculated as ^^^= ^^^^, where Iold is current in the given bus before a detected voltage change event, Inewis current in the given bus after the detected voltage change event. Finally, the short circuit current It,SCin the at least one transmission system is calculated as Vth / Zth. In step 442, the share of motor based, inverter-connected and static loads of currents due to demand KM, KI, KSin the at least one active distribution grid are estimated 520 based on the voltage and current of the given bus before and after a detected voltage change event. In an embodiment, these are determined by solving Equation (19). In an embodiment, the values for KM, KI, KSmay be given some initial values after one detected voltage change, and after more than one detected voltage change the values may be defined more accurately. In step 444, the short-circuit current coming from the at least one active distribution grid is estimated 522 based on the estimated shares and currents due to demand and production in the at least one active distribution grid. In an embodiment, the estimation is performed according to Equation (12), using initials estimates 524 for ^^, ^^, ^^^, ^^^. The values for ^^, ^^, ^^^, ^^^may be updated after more than one detected voltage change. After each detection 526 of a voltage change voltage and current measurements after the voltage change can be stored 528 in a log and values for ^^, ^^, ^^^, ^^^may be updated 530, using the new measurement recorded during the short- circuit. In an embodiment, after each short circuit event, the apparatus has one more record that can be used to improve the value of for ^^, ^^, ^^^, ^^^to reduce the error. In step 446, short-circuit current at the given bus is determined 532 by subtracting the short-circuit current coming from the at least one active distribution grid from the short-circuit current coming from the at least one transmission system, according to Equation (1). Upon detection 448 of further voltage change events, the short-circuit current at the given bus may be re-estimated 450 by updating the above current and share estimates. The proposed solution allows easily to estimate short-circuit current or total fault level and gives a system operator a tool for sizing their components and re-setting the relays and protection system. In addition, the proposed solution provides some insights into system reliability and voltage stability. Figure 6 illustrates an embodiment of an apparatus configured to carry out at least some of the above-described functions. The apparatus may comprise an electronic device comprising at least one processor or processing circuitry 600 and at least one memory 602. The apparatus may comprise a single computer or a computer system such as the cloud computing system described above. The apparatus may further comprise a communication circuitry 604 connected to the processing circuitry. The communication circuitry 604 may comprise hardware and software suitable for supporting one or more computer network protocols such as an Internet protocol (IP), Ethernet protocol, etc. The memory 602 may store a computer program (software) 606 comprising computer program code defining the functions of the processing circuitry 600. The computer program code may, when read and executed by the processing circuitry 600, cause the processing circuitry to execute the processes described above, for example the process of Figure 5, or any one of the embodiments as a computer-implemented process. The memory may further store a database 608 storing acquired measurement or estimated data, and static parameters of the electric power grid and log files. The apparatus may comprise a measurement data acquisition circuitry 610 connected to the processing circuitry. The measurement data acquisition circuitry may be configured to acquire measurement data from the measurement devices coupled to the electric power grid and to store the measurement data in the database. The processing circuitry acquire measurement data from the measurement data acquisition circuitry or the database and also static parameters of the electric power grid. The static parameters may comprise internal parameters of the electric power grid such as impedances in the electric power grid, power supply / consumption profile, topology, etc. That static parameters may comprise parameters external to the electric power grid such as weather profile, pricing tariffs, solar irradiation patterns, etc. As used in this application, the term ‘circuitry’ refers to all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and / or digital circuitry, and (b) combinations of circuits and software (and / or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processor(s) / software including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of ‘circuitry’ applies to all uses of this term in this application. As a further example, as used in this application, the term ‘circuitry’ would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and / or firmware. The term ‘circuitry’ would also cover, for example and if applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device. In an embodiment, at least some of the processes described in connection with the above figures may be carried out by an apparatus comprising corresponding means for carrying out at least some of the described processes. Some example means for carrying out the processes may include at least one of the following: detector, processor (including dual-core and multiple-core processors), digital signal processor, controller, receiver, transmitter, encoder, decoder, memory, RAM, ROM, software, firmware, display, user interface, display circuitry, user interface circuitry, user interface software, display software, circuit, antenna, antenna circuitry, and circuitry. In an embodiment, the at least one processor, the memory, and the computer program code form processing means or comprises one or more computer program code portions for carrying out one or more operations according to any one of the embodiments described in connection with the above figures. According to yet another embodiment, the apparatus carrying out the embodiments comprises a circuitry including at least one processor and at least one memory including computer program code. When activated, the circuitry causes the apparatus to perform at least some of the functionalities according to any one of the embodiments described in connection with above. The techniques and methods described herein may be implemented by various means. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a hardware implementation, the apparatus(es) of embodiments may be implemented within one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chip set (e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by processors. The memory unit may be implemented within the processor or externally to the processor. In the latter case, it can be communicatively coupled to the processor via various means, as is known in the art. Additionally, the components of the systems described herein may be rearranged and / or complemented by additional components in order to facilitate the achievements of the various aspects, etc., described with regard thereto, and they are not limited to the precise configurations set forth in the given figures, as will be appreciated by one skilled in the art. Embodiments as described may also be carried out in the form of a computer process defined by a computer program or portions thereof. Embodiments of the methods described in connection with the above figures may be carried out by executing at least one portion of a computer program comprising corresponding instructions. The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. For example, the computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. The computer program medium may be a non-transitory medium, for example. Coding of software for carrying out the embodiments as shown and described is well within the scope of a person of ordinary skill in the art. In an embodiment, a computer-readable medium comprises said computer program. Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly, and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.

Claims

CLAIMS 1. A method for estimating short-circuit current at a given bus of an electric power grid, the method comprising: estimating (400) initial values and proportion for currents due to demand and production based on available, statistical information; perform real-time measurement (402) of voltage and current of the given bus; refining (404) estimates for the proportion of current due to demand and production based on the real-time measurements; and upon detection (406) of a voltage change event, estimating (408) the short circuit current, short circuit capacity or short circuit ratio of the given bus and / or the local network based on the measured change in voltage and current.

2. The method of claim 1 further comprising: estimating (410) short-circuit current coming from the at least one transmission system based on the voltage and current of the given bus before and after a detected voltage change event; estimating (412) the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution grid based on the voltage and current of the given bus before and after a detected voltage change event; estimating (414) the short-circuit current coming from the at least one active distribution grid based on the estimated shares and currents due to demand and production in the at least one active distribution grid; and estimating (416) short-circuit current at the given bus by subtracting the short-circuit current coming from the at least one active distribution grid from the short-circuit current coming from the at least one transmission system; and upon detection (418) of further voltage change events, updating (420) the above current and share estimates and re- estimating the short-circuit current at the given bus.

3. The method of claim 1 or 2, further comprising: causing two or more voltage change events in the electric power grid.

4. The method of any preceding claim, wherein the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution is estimatedwhere ^^^^ ^ is the current due to demand in the at least one active distribution grid after a voltage change event, , ^^^^ ^ is the current due to demand in the at least one active distribution grid before a voltage change event, KM is share of motor originated current, KI the share of inverter-connected current and the share of static originated current KS is defined as KS = 1 - KM - KI.

5. The method of any preceding claim, further comprising: determining a Thevenin equivalent Vth of the at least one transmission system voltage as ^^^^^^^^^^^^= ^ , where Vold is voltage in the given bus before a detected voltage change event, Vnewis voltage in the given bus after the detected voltage change event, and by determining a Thevenin equivalent Zthof the at least one transmission system impedance ^^^^^ ^^^^as ^^^=, where Iold is current in the given bus before a detected voltage change event, Inewis current in the given bus after the detected voltage change event, and determining the short circuit current It,SCin the at least one transmission system as Vth / Zth.

6. The method of any preceding claim, wherein the short-circuit current IADC,SCcoming from the at least one active distribution grid is estimated from IADG,SC= ^^^^,^− ^^^^,^− ^^^^^^− ^^^^^,^where ^^,^, ^^,^, ^^^are the motor based, inverter-connected and static load currents due to demand and ^^,^is current due production, and ^^, ^^, ^^^, ^^^are corresponding contribution factors of the currents.

7. The method of claim 6, further comprising assigning initial values for the contribution factors based on hardware used in the at least one active distribution grid;updating the values for the contribution factors based on short circuit currents measured after one or more detected voltage change events.

8. An apparatus for monitoring an electric power grid (100), the apparatus comprising means (600, 602, 604, 610) for performing: estimating (400) initial values and proportion for currents due to demand and production based on available, statistical information; refining estimates for the proportion of current due to demand and production using real-time measurement (404) of voltage and current of the given bus; and upon detection (406) of a voltage change event, estimating the short circuit current, short circuit capacity or short circuit ratio of the given bus and / or the local network based on the measured change in voltage and current.

9. The apparatus of claim 8, comprising means for updating the initial values of currents due to demand and production in the at least one active distribution grid based on real-time measurements or information from corresponding grids; determining voltage and current of the given bus; and upon detection of a voltage change event, determining voltage and current of the given bus after the event; estimating short-circuit current coming from the at least one transmission system based on the voltage and current of the given bus before and after a detected voltage change event; estimating the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distribution grid based on the voltage and current of the given bus before and after a detected voltage change event; estimating the short-circuit current coming from the at least one active distribution grid based on the estimated shares and currents due to demand and production in the at least one active distribution grid; and estimating short-circuit current at the given bus by subtracting the short-circuit current coming from the at least one active distribution grid from the short-circuit current coming from the at least one transmission system; and upon detection of further voltage change events,updating the above current and share estimates and re-estimating the short-circuit current at the given bus.

10. The apparatus of claim 9, comprising means for estimating the share of motor based, inverter-connected and static loads of currents due to demand in the at least one active distributionwhere ^^^^ ^ is the current due to demand in the at least one active distribution grid after a voltage change event, , ^^^^ ^ is the current due to demand in the at least one active distribution grid before a voltage change event, KM is share of motor originated current, KI the share of inverter-connected current and the share of static originated current KS is defined as KS = 1 - KM - KI.

11. The apparatus of claim 9, further comprising: means for determining a Thevenin equivalent Vth of the at least one transmission system voltage as ^^^^^^^^^^^^= ^ , where Vold is voltage in the given bus before a detected voltage change event, Vnewis voltage in the given bus after the detected voltage change event, and by determining a Thevenin equivalent Zthof the at least one transmission system impedance ^^^^^ ^^^^as ^^^=, where Iold is current in the given bus before a detected voltage change event, Inewis current in the given bus after the detected voltage change event, and determining the short circuit current It,SCin the at least one transmission system as Vth / Zth.

12. The apparatus of any preceding claim 9 to 11, further comprising means for estimating the short-circuit current IADC,SCcoming from the at least one active distribution grid utilising IADG,SC= ^^^^,^− ^^^^,^− ^^^^^^− ^^^^^,^where ^^,^, ^^,^, ^^^are the motor based, inverter-connected and static load currents due to demand and ^^,^is current due production, and ^^, ^^, ^^^, ^^^are corresponding contribution factors of the currents.

13. The apparatus of claim 12, further comprising means for assigning initial values for the contribution factors based on hardware used in the at least one active distribution grid, and means for updating the values for the contribution factors based on short circuit currents measured after one or more detected voltage change events.

14. A computer program product readable by a computer and comprising computer program instructions that, when executed by the computer cause execution of a computer process for estimating short-circuit current at a given bus of an electric power grid, the grid comprising at least one transmission system and at least one active distribution grid, the process comprising: estimating (400) initial values and proportion for currents due to demand and production based on available, statistical information; refining estimates for the proportion of current due to demand and production using real-time measurement (404) of voltage and current of the given bus; and upon detection (406) of a voltage change event, estimating the short circuit current, short circuit capacity or short circuit ratio of the given bus and / or the local network based on the measured change in voltage and current.

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