GB2576715A

GB2576715A - Capacitive unit for local power factor correction and system comprising multiple capacitive units

Info

Publication number: GB2576715A
Application number: GB1813872.7A
Authority: GB
Inventors: Bourn Steven; Bourn Matthew
Original assignee: Energy Facilities Uk Ltd
Current assignee: Energy Facilities Uk Ltd
Priority date: 2018-08-24
Filing date: 2018-08-24
Publication date: 2020-03-04
Anticipated expiration: 2038-08-24
Also published as: WO2020039207A1; GB201813872D0; GB2576715B; GB2576809A; EP3841648A1; GB201905557D0

Abstract

A capacitive unit 100 for local power factor correction of a multi-phase inductive load 200. The capacitive unit 100 comprises at least one capacitive module 105. Each capacitive module comprises a contactor 120 and a multi-phase capacitive load 110 connectable in parallel with the multi-phase inductive load 200 by the contactor 120. The capacitive unit 100 further comprises a central control unit 130 configured to control each of the contactors 120 of the at least one capacitive module 105 so as to disconnect the respective multi‑ phase capacitive load 110 from the multi-phase inductive load 200 when the current in a phase cable 201 providing electric power to the multi-phase inductive load 200 falls below a pre-determined threshold. The central control unit 130 is configured to connect to one or more other central control units of one or more other capacitive units and to receive operating data of one or more other multi-phase inductive loads connected to the one or more other capacitive units from the one or more other central control units.

Description

CAPACITIVE UNIT FOR LOCAL POWER FACTOR CORRECTION AND SYSTEM

COMPRISING MULTIPLE CAPACITIVE UNITS [0001] The present invention relates to a capacitive unit for local power factor correction and a system comprising multiple capacitive units.

[0002] Inductive loads are used widely in residential, commercial and industrial settings, for example in the form of electric motors and transformers used in compressor and refrigeration systems, ventilation and air conditioning, motor drives of industrial machines and others. The cost and environmental impact of operating such inductive loads is directly correlated to their power consumption. It is thus desirable to improve the efficiency of power transmission and consumption of inductive loads.

[0003] The power factor of an AC electric power system is generally considered a measure of efficiency of the system. The power factor is defined as the ratio of real or active power used by the load and apparent power transmitted to the load. An AC electric power system comprising an overall inductive load typically has apparent power greater than real power. This is due to energy that is stored in the inductive load, in the form of a magnetic field required to operate the inductive load, and returned to the source. The power factor of such a system is less than 1 or unity. The “wasted” or useless power lost to energising the magnetic field of the inductive load is also referred to as reactive power. The AC current waveform in such an inductive load is out of phase, and lags, the AC voltage waveform. The reactive power does not contribute to the useful power output of the inductive load, but adds to the power transmitted to the inductive load and thus increases the environmental impact and cost of operating the inductive load.

[0004] Power factor correction units may be used to improve the power factor and reduce the reactive power of an AC electric power system. Such power factor correction units compensate for the lagging current induced by the inductive load by creating a leading current, for example by using capacitive loads. A capacitive load gives rise to a negative reactive power, effectively cancelling the positive reactive power of an inductive load. However, conventional power factor correction units and systems do not provide the required flexibility and customizability to achieve optimized power factor correction over a range of inductive load conditions, and are not easy and convenient to operate. In practice, inductive loads may switch off, fail or break down, change in inductivity or be replaced by other loads. Furthermore, the configuration of inductive loads varies widely among existing electric power systems, such that an off-the-shelf unit may not optimally correct the power factor. Maintenance and monitoring of multiple capacitive units requires an operator to separately access each capacitive unit, making such tasks complicated and timeconsuming.

[0005] The inventors have identified the need for a power factor correction unit that is more flexible, versatile and customizable to a range of existing electric power systems comprising inductive loads, and a system with multiple power factor correction units that is easy and convenient to monitor and maintain. As such, the invention aims to at least partly address the issues of conventional power factor correction units, as described above.

[0006] According to the present invention there is provided a capacitive unit for local power factor correction of a multi-phase inductive load. The capacitive unit comprises at least one capacitive module. Each capacitive module comprises a contactor and a multi-phase capacitive load connectable in parallel with the multi-phase inductive load by the contactor. The capacitive unit further comprises a central control unit that controls each of the contactors of the at least one capacitive module so as to disconnect the respective multi-phase capacitive load from the multiphase inductive load when the current in a phase cable providing electric power to the multi-phase inductive load falls below a pre-determined threshold. The central control unit connects to one or more other central control units of one or more other capacitive units and receives operating data, such as real-time and historic operating data, of one or more other multi-phase inductive loads connected to the one or more other capacitive units from the one or more other central control units.

[0007] The modular structure of the capacitive unit allows the capacitive unit to be customized to a wide range of inductive loads based on a set of standard modules, improving the versatility of the capacitive unit. The multi-phase capacitive load is connectable in parallel with the multiphase inductive load by the contactor. The central control unit controls the contactor so as to disconnect the multi-phase capacitive load from the multi-phase inductive load when the current in a phase cable providing electric power to the multi-phase inductive load falls below a predetermined threshold. The central control unit thus ensures that the capacitive load of the capacitive modules is reliably disconnected from an electric power system as soon as it is detected that the current falls below a pre-determined threshold, for example due to the inductive load being switched off or breaking down. This prevents the capacitive load from acting as a generator and providing reactive power to an AC electric power system after an inductive load is disconnected. This improves the power factor following disconnection of the inductive load and decreases the overall power consumption of the AC electric power system. The central control unit is connected to other central control units of other capacitive units, allowing the central control unit to operate in a system of interconnected capacitive units. The central control unit receives operating data from the other central control units, such that an operator of multiple capacitive units may monitor the multiple capacitive units from a single location, improving the convenience of operating multiple capacitive units at different locations.

[0008] The capacitive unit may further comprise a plurality of probes connected to the central control unit. The plurality of probes may measure the voltage and current of each phase of the multi-phase inductive load. The central control unit may calculate operating data, such as realtime operating data and historic operating data, of the multi-phase inductive load based on the voltage and current measurements. The central control unit may further comprise a display, and display the operating data calculated by the central control unit on the display. This allows an operator to monitor both the real-time and historic behaviour of the electric power system, allowing identification of temporary or constant inefficiencies and drawbacks that should be fixed. Monitoring the power factor data may allow the operator to adjust the respective capacitive loads of the capacitive units to bring the power factor closer to unity, if required. Monitoring the harmonic frequency data may allow the operator to adjust the setup of the electrical power system (including the inductive load and the capacitive load) to suppress any identified harmonic frequencies.

[0009] The central control unit may further comprise an input device. A user may enter desired values for the pre-determined threshold of the capacitive unit into the input device. The central control unit may set the value of the pre-determined threshold of the capacitive unit based on the user input to the input device. This allows an operator or user to control operation of the capacitive unit using the capacitive unit’s input device.

[0010] The central control unit of the capacitive unit may include a time delay element. The time delay element may delay, by a time delay, controlling the contactor to connect the multi-phase capacitive load in parallel with the multi-phase inductive load after disconnecting the multi-phase capacitive load from the multi-phase inductive load. This allows any latent energy, or residual charges, in the capacitive unit to be dissipated after disconnecting the multi-phase capacitive load and before re-connecting the capacitive load. The inventors have found that re-connecting the capacitive load before residual charges have been dissipated may lead to damage to the capacitive unit. This can be avoided by including the time delay element. A user may adjust the time delay of the capacitive unit by inputting a desired time delay value into the input device. The central control unit may set the value of the time delay based on the user input into the input device. This allows an operator or user to control operation of the capacitive unit using the capacitive unit’s input device.

[0011] The central control unit may comprise a modem that communicates with an external device. The external device may be located remotely from the capacitive unit and the one or more other capacitive units. The external device may be a computer or a mobile device, such as a smart phone or a laptop, for example. The central control unit may send the operating data calculated by the central control unit and/or the operating data received from the one or more other central control units to the external device via the modem. The user of the external device may control the capacitive unit and the one or more other capacitive units using the external device, for example by inputting desired values for the respective time delays and pre-determined thresholds into the external device. The central control unit may receive instructions from the external device via the modem to switch the capacitive unit and/or the one or more other capacitive units on and off, and/or to set the value of the predetermined threshold and/or the value of the time delay of the capacitive unit and/or of the one or more other capacitive units. This allows an operator to monitor and control the multiple capacitive units from a location remote from the capacitive units, for example from an office or from home. This makes monitoring and controlling the capacitive units easier and more convenient.

[0012] The capacitive unit may further comprise a power supply unit. The power supply unit may provide a DC electric power to the contactors. The power supply unit comprises an input side and an output side. The input side of the power supply unit may be connected to two phase cables providing electric power to the multi-phase inductive load. The output side of the power supply unit may be connected to each of the contactors of the at least one capacitive module via the central control unit. The central control unit may individually connect and disconnect the contactors from the power supply unit. This may be achieved via relay modules of the central control units, which may form separate electrical connections between the power supply unit and each of the contactors and may be controlled by the central control unit. All of the contactors of the different capacitive modules of the capacitive unit may thus be powered by the electric power system that the capacitive unit is connected to for power factor correction. No separate power supply is necessary. A single power supply unit may be used to power all of the contactors. This makes the construction of the capacitive unit simple and the operation of the capacitive unit reliable.

[0013] The capacitive unit may comprise a plurality of capacitive modules, for example three capacitive modules. Each of the plurality of capacitive modules may be customized and controlled individually. The modular structure of the capacitive unit improves the versatility of the capacitive unit.

[0014] The central control unit may control each of the contactors using a pre-determined threshold of a different value. This allows each capacitive module to be disconnected or connected at a different pre-determined threshold. The capacitive modules may thus be connected or disconnected in a step-like manner, so as to more accurately match changes in the reactive power requirements of the multi-phase inductive load. This further improves the power factor and decreases the overall power consumption of the AC electric power system.

[0015] The multi-phase inductive load may be a three-phase inductive load. The multi-phase capacitive load may be a three-phase capacitive load. This allows use of the capacitive unit in three-phase electric power systems, which are the most common type of electric power system used by electric grids worldwide to generate, transmit and distribute electric power.

[0016] The three-phase capacitive load may be connected in a delta configuration. Alternatively, the three-phase capacitive load may be connected in a Y configuration. Preferably, the threephase capacitive load is connected in the same configuration as the three-phase inductive load. This allows use of the capacitive unit with commonly used connection configurations of inductive loads.

[0017] The multi-phase capacitive load may be arranged in a plurality of interconnected branches. Each branch of the multi-phase capacitive load may comprise a capacitor and a resistor connected in parallel. The capacitors give rise to a negative reactive power, counteracting the reactive power of the inductive load and thereby improving the power factor and reducing power consumption. The resistors allow residual charges or latent energy on the capacitors to be dissipated. Dissipation of the residual charges, before re-connecting the capacitive unit, reduces the risk of damage to the capacitive unit.

[0018] Each branch of the multi-phase capacitive load may comprise a plurality of capacitors that are connected in parallel. For example, each branch of the multi-phase capacitive load may comprise three capacitors connected in parallel. This further improves the customizability and versatility of the capacitive unit, as each branch can be designed to match an inductive load using a plurality of standard components.

[0019] Each branch of the multi-phase capacitive load may further comprise a plurality of resistors. Each resistor of the plurality of resistors may be connected in parallel with a respective one of the plurality of capacitors. Each resistor may thus be matched to a respective capacitor, ensuring that residual charges can be quickly and efficiently dissipated from each individual capacitor. This reduces the risk of damage to the capacitive unit.

[0020] The control circuit of the at least one capacitive module may control the respective contactor so as to connect the multi-phase capacitive load in parallel with the multi-phase inductive load when the current in the phase cable providing electric power to the multi-phase inductive load exceeds the pre-determined threshold. This allows capacitive modules to be automatically reconnected when required, thereby more accurately matching the changing reactive power requirements of the inductive load. This further improves the power factor and reduces power consumption.

[0021] The capacitive unit may further comprise a single multi-phase varistor bank. The multiphase varistor bank is connected in parallel with the multi-phase capacitive loads of the at least one capacitive module. This protects the capacitive loads from surge currents, thereby reducing the risk of damage to the capacitive loads during operation.

[0022] Each of the at least one capacitive module may further comprise a charge indicator. The charge indicator may indicate whether residual charges are present on the multi-phase capacitive load of the at least one capacitive module, and whether the contactor of the at least one capacitive module is open or closed. This allows an operator to take account of residual charges and detect failure of the residual charge dissipation mechanism, or to ensure that the capacitive module is energized when connected to the inductive load. This makes use of the capacitive unit safer. [0023] Each of the at least one capacitive module may further comprise a module over-current protection element. The module over-current protection element is connected in series with the multi-phase capacitive load of the at least one capacitive module. This protects the capacitive load from excessive currents, reducing the risk of damage to the capacitive unit.

[0024] According to an embodiment of the present invention, there is also provided a system comprising at least two capacitive units for local power factor correction of a respective multiphase inductive load. Each capacitive unit comprises at least one capacitive module. Each capacitive module comprises a contactor and a multi-phase capacitive load connectable in parallel with the multi-phase inductive load by the contactor. Each capacitive unit further comprises a central control unit that controls each of the contactors of the respective capacitive module so as to disconnect the multi-phase capacitive loads of the respective capacitive module from the respective multi-phase inductive load when the current in a phase cable providing electric power to the respective multi-phase inductive load falls below a pre-determined threshold. The central control unit of a first capacitive unit is a master control unit and the central control unit of a second capacitive unit is a slave control unit. The master control unit and the slave control unit are in data communication with one another. The master control unit receives operating data of the multi-phase inductive load connected to the second capacitive unit from the slave control unit. The master control unit sends instructions to the slave control unit so as to set the pre-determined threshold of the second capacitive unit. This allows an operator to monitor and control each of the at least two capacitive units from a central location, making the system easy and convenient to use.

[0025] The master control unit (but not the slave control unit) may comprise a modem that connects to an external device. The master control unit may send operating data of the multiphase inductive load connected to the first capacitive unit and operating data of the multiphase inductive load connected to the second capacitive unit to the external device via the modem. The master control unit may also receive instructions from the external device via the modem. The master control unit may set the pre-determined thresholds of the first capacitive unit based on the received instructions, and may send the instructions to the slave control unit to set the predetermined threshold of the second capacitive unit based on the received instructions. This allows an operator to monitor the at least two capacitive units from a location remote from the capacitive units, making the system even more convenient and easy to use.

[0026] The invention is described below, by way of example only, with reference to the drawings, in which:

[0027] Figure 1 is a schematic diagram showing an example of how the capacitive unit of the present invention may be connected to an AC electric power system;

[0028] Figure 2 schematically shows the capacitive unit with three capacitive modules; [0029] Figure 3 shows a detailed schematic diagram of the capacitive unit of Figure 2;

[0030] Figures 4a and 4b respectively show schematic diagrams of a three-phase capacitive load in a delta configuration and in a Y configuration; and [0031] Figure 4c is an exemplary circuit diagram of the three-phase capacitive load of Figure 4a. [0032] The following description is merely exemplary in nature and is not intended to limit the scope of the present invention, which is defined in the claims. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0033] The present invention may be used for power factor correction of an inductive load in an AC electric power system, in which electric power is supplied to the inductive load by an AC electric power source. Preferably, the present invention is used in a multi-phase AC electric power system, comprising a multi-phase inductive load and a multi-phase AC electric power supply. Further preferably, the present invention is used in a three-phase AC electric power system, comprising a three-phase inductive load and a three-phase AC electric power supply. The capacitive unit will in the following be described in terms of its preferable application in a threephase AC electric power system. However, the capacitive unit described in the following could be readily adapted for application in other multi-phase AC electric power systems, such as twophase, four-phase or higher-phase AC electric power systems, or in a single-phase AC electric power system.

[0034] Figure 1 shows an AC electric power system comprising a power supply 300 and an inductive load 200. The inductive load 200 is connected to the power supply 300 via phase cables 201a, b, c. Specifically, Figure 1 shows a three-phase AC electric power system, comprising a three-phase AC electric power supply 300 and a three-phase inductive load 200 connected thereto via three phase cables 201a, b, c. A capacitive unit 100, specifically shown as a three-phase capacitive unit 100 in Figure 1, is connectable in parallel to the inductive load 200. This ensures that the capacitive unit 100 can be easily installed in existing electric power systems without the need to disconnect the inductive load 200 from the power supply 300. The capacitive unit 100 is connected in parallel to the inductive load 200 via connection lines 101a, b, c. The capacitive unit 100 may be connected to the inductive load 200 either locally or at the distribution side of an inductive load 200 that, for example, comprises several smaller inductive loads. This allows the capacitive unit 100 to be installed by the consumer of electric power, or the user of the inductive load 200, to allow for local power factor correction and reduce the cost and environmental impact of the inductive load 200.

[0035] The capacitive unit 100 may be located in a housing, such as a glass reinforced plastic (GRP) housing. All components of the capacitive unit 100 may be enclosed by the housing. The capacitive unit 100 may be a stand-alone unit. The capacitive unit 100 thus can be delivered and installed as a single unit.

[0036] Figure 2 shows an embodiment of the capacitive unit 100 comprising three capacitive modules 105, 105’, 105”. The three capacitive modules 105, 105’, 105” are connected in parallel. Each of the capacitive modules 105, 105’, 105” comprises a capacitive load 110, 110’, 110” and a contactor 120, 120’, 120”. The capacitive unit 100 further comprises a central control unit 130. The central control unit 130 controls the contactors 120, 120’, 120”, so as to open and close the contactors 120, 120’, 120”. Although three capacitive modules 105, 105’, 105” are shown in Figure 2, the capacitive unit 100 may include fewer or more capacitive modules. For example, the capacitive unit 100 may include only one capacitive module 105, or the capacitive unit 100 may include only capacitive modules 105 and 105’. Alternatively, one or more additional capacitive modules may be connected in parallel to the three capacitive modules 105, 105’, 105” in the capacitive unit 100.

[0037] Each of the capacitive modules 105, 105’, 105” may be identical. Alternatively, different capacitive modules 105, 105’, 105”, for example including capacitive loads 110, 110’, 110” with different capacitances, may be used. Each of the capacitive modules 105, 105’, 105” may be chosen from a set of standard capacitive modules. It may also be possible to customize each of the set of standard capacitive modules. This allows the capacitive unit 100 to be highly customizable to match the inductive load 200. Changes to the inductive load 200 may be matched easily by adding further capacitive modules to the capacitive unit 100, exchanging one of the capacitive modules 105, 105’, 105”, or changing the capacitive load 110, 110’, 110” of one or more of the capacitive modules 105, 105’, 105”. This makes the capacitive unit 100 highly flexible to hardware changes in the inductive load 200. Using three capacitive modules 105, 105’, 105” ensures a high flexibility of the capacitive unit 100, while maintaining a simple construction. Using more than three capacitive modules 105, 105’, 105” may further improve the customizability and flexibility of the capacitive unit 100. Using less than three capacitive modules 105, 105’, 105” may make the construction of the capacitive unit 100 simpler. The inventors have found using three capacitive modules 105, 105’ 105” to be an effective trade-off between simplicity and flexibility of the capacitive unit 100.

[0038] A detailed schematic diagram of the capacitive unit 100 of Figure 2 is shown in Figure 3. The capacitive unit 100 comprises three capacitive modules 105, 105’, 105”. Each of the three capacitive modules 105, 105’, 105” comprises the capacitive load 110, 110’, 110”, the contactor 120, 120’, 120”, a charge indicator 160, 160’, 160”, and a module over-current protection element 170, 170’, 170”. The capacitive unit 100 further comprises a central control unit 130, a power supply unit 140, a varistor bank 150, and a thermal conditioning system comprising a thermostat 180 and a fan 182. The central control unit 130 comprises a display 132, relay modules 134, 134’, a time delay element 135, a modem 136, an input device 137, and a connection module 138 for connecting to one or more other capacitive units 100’. The one or more other capacitive units 100’ may be identical to the capacitive unit 100, except for the modules of the central control unit 130. The capacitive unit 100 may be a master capacitive unit 100, including a master control unit 130, and the one or more capacitive units 100’ may be one or more slave capacitive units 100’, including slave control units. The slave control units may not include a modem 136. A plurality of probes comprising current measurement devices 133, 133’, 133” may be connected to the central control unit and measure the voltage and current of each phase of the multi-phase inductive load 200. The central control unit 130 receives current measurements of the current flowing through phase cables 201a, b, c from current measurement devices 133, 133’, 133” and further receives voltage measurements of the voltage at the phase cables 201a, b, c.

[0039] The capacitive module 105 will be described in the following. Each of the three capacitive modules 105, 105’, 105” may correspond to the capacitive module 105 described below, and comprise identical components to the capacitive module 105 described below, unless stated otherwise.

[0040] The contactor 120 is for switching capacitive loads, such as capacitive load 110. The contactor 120 may comprise two switching terminals. A holding voltage, or control voltage, may be applied to the switching terminals. The contactor 120 is closed, so as to connect the capacitive load 110 to the inductive load 200, when the holding voltage applied to the switching terminals of the contactor 120 exceeds a pre-set minimum voltage. The contactor 120 is open, so as to disconnect the capacitive load 110 from the inductive load 200, when the holding voltage applied to the switching terminals is below the pre-set minimum voltage.

[0041] The contactor 120 may be of a type that achieves minimal arcing on the coil contacts. For example, the contactor 120 may be an AF30-30-30 by ABB ®. The holding voltage that is applied to the switching terminals of the contactor 120 may be in the range from 20 to 500V, preferably in the range from 20 to 30V, for example about 24V. The contactor 120 may support an AC electric voltage with a frequency in the range from 25 to 400 Hz, preferably in the range from 40 to 70 Hz. The contactor 120 may support a peak voltage of up to 690V, preferable in the range from 100V to 300V. The contactor 120 may be switchable, between an open and closed state, at a frequency in the range from 0.01 to 1 Hz, preferably from 0.04 to 0.4 Hz.

[0042] The holding voltage across the switching terminals of the contactor 120 is provided by the power supply unit 140 and controlled by the central control unit 130. The central control unit 130 is connected to current measurement devices 133, 133’, 133” and includes a relay module 134 comprising one or more switches. The current measurement devices 133, 133’, 133” may be placed around respective phase cables 201a, b, c. The phase cables 201a, b, c provide electric power to the inductive load 200. The current measurement devices 133, 133’, 133” may measure the current (such as a reactive current) in the phase cables 201a, b, c. Each of the current measurement devices 133, 133’, 133” may be a current transformer. The central control unit 130 may receive current measurements from the current measurement devices 133, 133’, 133”. [0043] The relay modules 134, 134’ comprise one or more switches, for example two switches. Each relay module 134, 134’ may be used to control a fixed maximum number of capacitive modules 105, 105’, 105”, for example up to two capacitive modules as in the embodiment of Figure 3. The one or more switches of each relay module 134, 134’ are individually controllable or switchable by the central control unit 130. Each switch of the relay modules 134, 134”, when closed, may create a closed electric circuit including an output side of the power supply unit 140 and the switching terminals of a respective contactor 120, 120’, 120”. Each switch of the relay modules 134, 134’, when closed, applies the holding voltage to the contactor 120, thereby closing the contactor 120. When the current measurements received by the central control module 130 from the current measurement devices 133, 133’, 133” fall below the pre-determined threshold, the central control module 130 may open the switches of the relay modules 134, 134’. This interrupts the electric circuit including the output side of the power supply unit 140 and the switching terminals of the contactors 120, 120’, 120”, breaking the holding voltage applied at the switching terminals of the contactors 120, 120’, 120”. The contactors 120, 120’, 120” may thus disconnect the capacitive loads 110, 110’, 110” of the respective capacitive modules 105, 105’, 105” from the inductive load 200.

[0044] The pre-determined threshold is stored or pre-set in the central control unit 130. The predetermined threshold may be input into the central control unit 130 by a user using the input device 137. The pre-determined threshold may be freely adjustable by a user. The predetermined threshold may be determined, for example, based on the reactive current output of the capacitive load 110 of the capacitive module 105. The pre-determined threshold may correspond to a current in the range from 0.2 times to 1.5 times, preferably from 0.9 times to 1.1 times, the reactive current output of the capacitive load 110. For example, if the reactive current output of the capacitive load 110 is 90 A, the value of the pre-determined threshold may be 90 A (provided the inductive load 200 consumes only a reactive current component), such that the capacitive load 110 is disconnected from the inductive load 200 when the current in a phase cable 201 a, b, c to the inductive load 200 falls below 90 A.

[0045] The central control unit 130 may individually open the switches of the relay module so as to break the holding voltage applied to the switching terminals of the respective contactor 120, and open the contactor 120. The central control unit 130 may open an electric circuit formed between the isolating transformer 140 and the switching terminals of the contactor 120. This disconnects the capacitive load 110 from the inductive load 200. It can be avoided that the capacitive load 110 acts as a reactive power generator when the reactive power consumed by the inductive load 200 falls below the reactive power generated by the capacitive load 110. This improves the power factor of the AC electric power system, thereby reducing the power consumption and the environmental impact.

[0046] The time delay element 135 may delay, by a time delay, controlling the contactor 120 to connect the multi-phase capacitive load 110 in parallel with the multi-phase inductive load 200 after disconnecting the multi-phase capacitive load 110 from the multi-phase inductive load 200. Put another way, the time delay element 135 implements a delay period after disconnecting the capacitive load 110 in which the central control unit 130 is prevented from closing the contactor 120. The time delay may be pre-determined and pre-set in the time delay element 135. The time delay may be input into the central control unit 130 by a user using the input device 137. The time delay may be freely adjustable by a user. The time delay ensures that residual charges, or latent energy, may be dissipated from the capacitive load 110 before re-connecting the capacitive load 110 to the AC electric power system. The inventors have found that connecting the capacitive load 110 before residual charges are dissipated may damage the capacitive unit 100. Implementing the time delay reduces the risk of damage to the capacitive unit 100.

[0047] The time delay element 135 may, for example, be a implemented in software running on the central control unit 130. The time delay may be set such that residual charges are allowed to dissipate from the capacitive load 110 after opening the contactor 120 and disconnecting the capacitive load 110. For example, the time delay may be set such that at least more than 90%, or preferably more than 99%, of residual charges are dissipated. The time delay may be calculated based on the RC time constant of the capacitive load 110, or each capacitor resistor pair of the capacitive load 110. For example, the time delay may be larger than twice, and preferably larger than three times, the RC time constant of the capacitive load 110 or each capacitor resistor pair of the capacitive load 110. For example, if the RC time constant of the capacitive load 110 is in the range from Is to 60s, the time delay may be in the range from 2s to 120s, preferably from 3s to 180s.

[0048] The power supply unit 140 may provide electric power to the contactors 120, 120’, 120” via the relay elements 134, 134’. The power supply unit 140 may provide the holding voltage that is applied to the switching terminals of the contactor 120. The power supply unit 140 comprises an input side that is connected to two connection cables 101a, b or two phase cables 201a, b of the AC electric power system. The input side may be connected to any two of the connection cables 12

101a, b, c or phase cables 201a, b, c. Electric power is provided to the power supply unit 140 from the AC electric power system. The power supply unit 140 comprises an output side that is connected to the switching terminals of the contactor 120 via the relay module 134’ of the central control unit 130, so as to provide the holding voltage to the switching terminals of the contactor 120. The holding voltage may be a DC voltage. The power supply unit 140 may output a DC voltage suitable to switch the contactor 120, for example a DC voltage in the range from 20 to 30 V, such as about 24 V. The central control unit 130, in particular using the switch of the relay module 134, may break the holding voltage applied to the contactor 120 via the power supply unit 140.

[0049] The power supply unit 140 provides the holding voltage to the switching terminals of each of the contactors 120, 120’, 120”. Each of switches of the relay modules 134, 134’ and the switching terminals of each of the contactors 120, 120’, 120” may be connected to the output side of the power supply unit 140. Thus, a single power supply unit 140 may be used to provide the electric power required to control all capacitive modules 105, 105’, 105”.

[0050] In addition, the power supply unit 140 may be used to power the modem 136 and the thermal conditioning system of the capacitive unit. The thermal conditioning system comprises a thermostat 180 and a fan 182. The thermal conditioning system may be connected between a positive terminal and a negative terminal of the output side of the power supply unit 140.

[0051] The varistor bank 150, in particular a multi-phase (three-phase) varistor bank 150, is connected in parallel to all of the capacitive loads 110, 110’, 110”. The capacitive unit 100 may comprise only a single varistor bank 150. The varistor bank 150 may be connected in a delta or Y configuration, so as to match the connection configuration of the capacitive loads 110, 110’, 110”. The varistor bank 150 provides surge protection to the capacitive unit 100. The varistor bank 150 may include varistors with a discharge current in the range from 1 to 50 kA (8/20ps), for example about 12 kA (8/20ps), such as the Z500 varistor (MOV) assemblies by PD Devices Ltd. [0052] The charge indicator 160 may indicate whether residual charges are present on the capacitive load 110. The charge indicator 160 may also indicate whether a respective contactor 120 is open or closed. The charge indicator 160 may comprise a single indicator lamp, such as a single LED. The charge indicator 160 may indicate to a user whether charges are present on the capacitive load 110 when the capacitive unit 110 is switched off. The user can thus determine whether it is safe to carry out maintenance of the capacitive load 110 or the inductive load 200, and whether it is safe to switch the capacitive load 110 on. The charge indicator 160 provides an additional layer of protection in case of failure of other charge dissipation mechanisms. This makes use of the capacitive unit 100 safer for a user. The indicator lamp of the charge indicator 160 may, for example, be an LED chosen from the 501 SM series or the 504 05 series by CamdenBoss ®.

[0053] The module over-current protection element 170 is connected in series with the capacitive load 110 to prevent excessive currents at the capacitive load 110, and optionally across the contactor 120, so as to prevent damage to the capacitive module 105. The module over-current protection element 170 may, for example, be a miniature circuit breaker, such as the Pl MB 3P D63 by Lovato electric.

[0054] Each of the switches of the relay modules 134, 134” may break the holding voltage applied to the switching terminals of a respective contactor 120, 120’, 120”. The central control unit 130 may control all of the switches of the relay modules 134, 134” so as to control all contactors 120, 102’, 120” based on a comparison of the same current measurement (for example the current through a single phase cable 201a, b, c or an average current in the phase cables 201a, b, c) with the same pre-determined threshold. This makes configuration of the central control unit 130 simple.

[0055] Alternatively, the central control unit 130 may control each of the switches of the relay modules 134, 134”, so as to control each of contactors 120, 120’, 120”, based on the current (for example reactive current) in a different phase cable 201a, b, c. The current in each of the phase cables 201a, b, c may be used to open or close a respective contactor 120, 120’, 120”. For example, the current measurement device 133 may measure the current in phase cable 201a, the current measurement device 133’ may measure the current in phase cable 201b, and the current measurement device 133” may measure the current in phase cable 201c. The currents in all phase cables 201a, b, c may be taken into account when opening or closing the contactors 120, 120’, 120” so as to switch the capacitive modules 105, 105’, 105”. This allows the capacitive unit 100 to react to changes in the current in one particular phase cable (and not the other phase cables), which could arise if the inductive load 200 is or becomes unbalanced. Alternatively, it is possible that the central control unit 130 uses the current in the same phase cable (one of phase cables 201a, b, c), or an average current in all phase cables 201a, b, c, to control switching of the contactors 120, 120’, 120”.

[0056] The central control unit 130 may control each of the switches of the relay modules 134, 134’ so as to open a respective contactor 120, 120’, 120” if the current in a respective phase cable 201a, b, c falls below a pre-determined threshold of a different value. So, each of the capacitive modules 105, 105’, 105” may switch based on a different pre-determined threshold. The capacitive modules 105, 105’, 105” may thus be switched in succession, and not all at once. For example, the first capacitive module 105 may switch based on a first threshold value, the second capacitive module 105’ may switch based on a second threshold value (smaller than the first threshold value), and the third capacitive module 105” may switch based on a third threshold value (smaller than the first and second threshold values).

[0057] When the current in phase cable 201a falls below the first threshold value (for example because the reactive power consumption of the inductive load 200 decreases), the contactor 120 of the first capacitive module 105 opens, so as to disconnect the respective capacitive load 110 and the first capacitive module 105. Disconnecting the first capacitive module 105 will reduce the reactive power generation of the capacitive unit 100, thus matching the reactive power generation of the capacitive unit 100 to the reduced reactive power consumption of the inductive load 200 and improving the power factor. If the current in phase cable 201b continues to decrease (for example because the reactive power consumed by the inductive load 200 continues to decrease), so as to fall below the second threshold value, the second capacitive module 105’ may be disconnected. This again improves the power factor. If the current in phase cable 201c continues to decrease, so as to fall below the third threshold value, the third capacitive module 105” may be disconnected. So, it is possible to switch the capacitive modules 105, 105’, 105” separately at different times to maintain the power factor closer to unity compared to a case in which the capacitive modules 105, 105’, 105” were switched at the same time. The reactive power provided by the capacitive unit 100 may be reduced more gradually, in a step-like manner, so as to more accurately match a decrease in reactive power consumption of the inductive load 200.

[0058] Additional capacitive modules may be connected to the output side of the power supply unit 140 via additional switches in, if appropriate, additional relay modules of the central control unit 130, in parallel to the capacitive modules 105, 105’, 105”. Using additional capacitive modules allows for a more gradual reduction in the reactive power provided by the capacitive unit 100, which may further decrease the power consumption and environmental impact.

[0059] The central control unit 130 may receive, for example in real-time, current and voltage measurements of each of the phase cables 201a, b, c connected to the multi-phase inductive load 200. The central control unit 130 may calculate operating data, such as real-time operating data and historic operating data, of the multi-phase inductive load 200 based on the voltage and current measurements. Such operating data may include some or all of real power data, reactive power data, apparent power data, power factor data, harmonic frequency data, voltage data and current data.

[0060] The central control unit 130 may display the calculated operating data of the multi-phase inductive load 200 on the display 132. This allows a user to easily identify real-time operating data of the multi-phase inductive load, quickly recognize an inefficient set-up of the capacitive unit 100, or trouble shoot if components of the capacitive unit 100 or the multi-phase inductive load 200 are damaged. The central control unit 130 may also compile historic operating data and display historic operating data on the display 132. This allows a user to observe operation of the capacitive unit 100 over an extended period of time.

[0061] The central control unit 130 may be powered by a connection to two connection cables 101a, b or phase cables 201a, b. The connection may include suitable fuses, for example 1 Ampere fuses, to protect the central control unit from surge currents.

[0062] The connection module 138 of the central control module 130 may be used to connect the capacitive unit 100 to one or more other capacitive units 100’, in particular to connect the central control unit 130 to one or more other central control units (slave control units) of one or more other capacitive units 100’ (slave capacitive units 100’). The connection module 138 may form, for example, an RS485 interface with the slave control units. Up to twenty slave control units may, for example, be connected to the central control unit 130. This allows the capacitive unit 100 to be used in a system together with other capacitive units 100’. Each of the capacitive unit 100 and the other capacitive units 100’ may be a self-contained unit in a single housing. Each of the capacitive unit 100 and the slave capacitive units 100’ may be installed in a different location, and be connected to a different multi-phase inductive load 200.

[0063] The central control unit 130 may act as a master control unit 130, whereas all other central control units may act as slave control units. The master control unit 130 may have more functionality than a slave control unit. For example, the slave control unit may, compared to the master control unit 130, not comprise a modem 136. The slave control units may thus be simpler and cheaper than the master control unit 130.

[0064] The central control unit 130, acting as the master control unit 130, may receive operating data of one or more other multi-phase inductive loads connected to the one or more other capacitive units 100’ from the one or more slave control units. The central control unit 130 may send this operating data of the one or more other multi-phase inductive loads to an external device. The slave control units may calculate the operating data of a respective multi-phase inductive load based on voltage and current measurements of the multi-phase inductive load. [0065] The central control unit 130 may send instructions, such as instructions received from the external device, to the one or more slave control units to set the value of a respective pre16 determined threshold. The central control unit 130 may also send instructions to the one or more slave control units to set the value of a respective time delay. The central control unit 130 may also send instructions to the one or more slave control units to turn the one or more other capacitive units 100’, or one or more capacitive modules of the one or more other capacitive units 100’, on or off. Each of the other capacitive units 100’ may thus be controlled via the central control unit 130. This improves the convenience of a system comprising multiple capacitive units 100, 100’, and allows a user or operator of the system to quickly interact with all capacitive units 100, 100’ of the system without needing to travel to the location of each of the capacitive units 100, 100’.

[0066] The modem 136 of the central control module 130 may connect to an external device, for example via an Internet connection. The external device may, for example, be a computer desktop or a mobile device, such as a mobile phone or a laptop. The external device may be located remotely from the central control module 130, for example in a different room or building from the capacitive unit 100. The central control module 130 may send operating data, such as real-time operating data or historic operating data, of the multi-phase inductive load 200 to the external device via the modem 136, such that the operating data can be manipulated and/or displayed on the external device. The central control module 130 may also send the operating data received from the slave control units to the external device via the modem 136. As such, the operating data of all capacitive units 100, 100’ may be provided to the external device from a single location, via modem 136. The external device may store or display the operating data received from the central control unit 130. A user or operator of a system comprising capacitive units 100, 100’ may monitor the operating data of the multi-phase inductive loads 200 from a location remote from the capacitive units 100, 100’ using the external device, improving convenience for the user.

[0067] The user may control the capacitive unit 100 and the other capacitive units 100’ using the external device. For example, the user may individually or collectively set the pre-determined threshold and/or the time delay of each of the capacitive units 100, 100’, or may individually or collectively switch each of the capacitive units 100, 100’, or one or more capacitive modules 105, 105’, 105” of the capacitive units 100, 100’, off and on. In this regard, the central control unit 130 may receive instructions from the external device via the modem. The instructions may be generated in response to a user input into the external device, and include instructions relating to desired values of the pre-determined thresholds and time delays and/or a state (on, off, standby) of the capacitive units 100, 100’. The central control unit 130 may set the pre-determined threshold, 17 the time delay and/or the state of the capacitive unit 100 based on instructions from the external device relating to the capacitive unit 100. The central control unit 130 may send the instructions to the slave control units via the connection module 138 to set the pre-determined threshold, the time delay and/or the state of the other capacitive units 100’ based on instructions from the external device relating to the other capacitive units 100’. The user or operator may thus monitor and control a system comprising multiple capacitive units 100, 100’ via a master control unit 130 using a single external device.

[0068] Figures 4a and 4b show two examples of a three-phase capacitive load 110. Figure 4a shows a capacitive load 110 connected in a delta configuration. Figure 4b shows capacitive load 110 connected in a Y (or WYE) configuration. The configuration of the capacitive load 110 may be chosen so as to match the configuration of the inductive load 200 it is connectable to. The capacitive load 110 comprises a plurality of interconnected branches, for example one branch per phase of the AC electric power system. Each branch comprises a capacitor 112a, b, c and a resistor 114a, b, c. Each capacitor 112a, b, c is connected in parallel to a respective resistor 114a, b, c. The resistor 114a, b, c provides a current path for residual charges or latent energy stored in the respective capacitor 112a, b, c to be dissipated. For example, if the capacitive load 110 is disconnected from the AC electric power system, any residual charges on capacitor 112a may flow through corresponding resistor 114a so as to be dissipated. The resistors 114a, b, c may be matched to the capacitors 112a, b, c such that the time of charge dissipation is short, and shorter than the time delay implemented by time delay element 134, 134’, 134”.

[0069] Figure 4c shows a further example of a three-phase capacitive load 110 connected in a delta configuration. Each branch of the capacitive load 110 comprises three capacitors. A resistor is connected in parallel to each capacitor. A first branch comprises capacitors 112a, 112a’, 112a” and resistors 114a, 114a’, 114a”. Capacitors 112a, 112a’, 112a” are connected in parallel. Each of resistors 114a, 114a’, 114a” is connected in parallel to one of capacitors 112a, 112a’, 112a”. A second branch comprises capacitors 112b, 112b’, 112b” and resistors 114b, 114b’, 114b”. Capacitors 112b, 112b’, 112b” are connected in parallel. Each of resistors 114b, 114b’, 114b” is connected in parallel to one of capacitors 112b, 112b’, 112b”. A third branch comprises capacitors 112c, 112c’, 112c” and resistors 114c, 114c’, 114c”. Capacitors 112c, 112c’, 112c” are connected in parallel. Each of resistors 114c, 114c’, 114c” is connected in parallel to one of capacitors 112c, 112c’, 112c”. Providing multiple capacitors in each branch of the capacitive load 110 makes it easier to match the capacitive load 110 to the inductive load 200. Connecting a resistor in parallel to each capacitor may ensure that the time for charge dissipation can be kept short and determined for each capacitor individually. For example, each of the capacitors may have a capacitance of 35 pF (with a voltage rating of 540V), of 60 pF (with a voltage rating of 440V), or of 70 pF (with a voltage rating of 440V). Each of the resistors may have a resistance of 470 kQ. The RC time constant may be in the range from 16s to 32s.

[0070] The capacitive unit 100 may generate reactive power so as to accurately match the reactive power consumed by an inductive load 200. This improves the power factor and decreases the power consumption and environmental impact of the AC electric power system that the capacitive unit 100 is connected to. It has been shown that using the capacitive unit 100 shown in Figure 3 may reduce the power consumption of an inductive load over 30 kW by about 18-20%. This reduction in power consumption has been achieved using three capacitive modules 105, 105’, 105”, each with a capacitive load 110, 110’, 110” including nine 35 pF capacitors connected as in Figure 4c.

[0071] The capacitive unit 100 has been described in relation to a three-phase AC electric power system, with a three-phase power supply 300 and a three-phase inductive load 200. The capacitive loads 110, 110’, 110” were described as three-phase capacitive loads to match the three-phase inductive load 200. However, the capacitive unit 100 may also be used in singlephase, two-phase, four-phase or higher phase AC electric power systems.

[0072] The foregoing description of the preferred embodiments has been provided for the purposes of illustration and description. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but where applicable may be interchangeably used in combination with other features to define another embodiment, even if not specifically shown or described. The description is therefore not intended to limit the scope of the present invention, which is defined in the claims.

Claims

1. A capacitive unit for local power factor correction of a multi-phase inductive load, the capacitive unit comprising at least one capacitive module, wherein each capacitive module comprises:

a contactor; and a multi-phase capacitive load connectable in parallel with the multi-phase inductive load by the contactor; and wherein the capacitive unit further comprises a central control unit configured to control each of the contactors of the at least one capacitive module so as to disconnect the respective multi-phase capacitive load from the multi-phase inductive load when the current in a phase cable providing electric power to the multi-phase inductive load falls below a pre-determined threshold, and wherein the central control unit is configured to connect to one or more other central control units of one or more other capacitive units and to receive operating data of one or more other multi-phase inductive loads connected to the one or more other capacitive units from the one or more other central control units.

2. The capacitive unit of claim 1, further comprising a plurality of probes connected to the central control unit and configured to measure the voltage and current of each phase of the multi-phase inductive load, wherein the central control unit is configured to calculate operating data of the multi-phase inductive load based on the measured voltage and current.

3. The capacitive unit of claim 2, wherein the central control unit comprises a display, and wherein the central control unit is configured to display the operating data calculated by the central control unit on the display.

4. The capacitive unit of any one of the preceding claims, wherein the operating data comprises real power data, reactive power data, apparent power data, power factor data, harmonic frequency data, voltage data and/or current data.

5. The capacitive unit of any one of the preceding claims, wherein the central control unit further comprises an input device, and wherein the central control unit is configured to set the value of the pre-determined threshold of the capacitive unit based on a user input to the input device.

6. The capacitive unit of any one of the preceding claims, further comprising a power supply unit for providing a DC electric power to the contactors, the power supply unit comprising an input side and an output side, wherein the input side of the power supply unit is connectable to two phase cables providing electric power to the multi-phase inductive load, and wherein the output side of the power supply unit is connected to each of the contactors of the at least one capacitive module via the central control unit, wherein the central control unit is configured to individually connect and disconnect the contactors from the power supply unit.

7. The capacitive unit of claim 6, wherein the central control unit comprises one or more relay modules configured to form separate electrical connections between the power supply unit and each of the contactors, and wherein the central control unit is configured to control the one or more relay modules so as to individually connect and disconnect the contactors from the power supply unit.

8. The capacitive unit of any one of the preceding claims, wherein the central control unit includes a time delay element configured to delay, by a time delay, controlling the contactor to connect the multi-phase capacitive load in parallel with the multi-phase inductive load after disconnecting the multi-phase capacitive load from the multi-phase inductive load.

9. The capacitive unit of claim 8, wherein the central control unit further comprises an input device, and wherein the central control unit is configured to set the value of the time delay based on a user input to the input device.

10. The capacitive unit of any one of the preceding claims, wherein the central control unit comprises a modem configured to communicate with an external device, and wherein the central control unit is configured to send the operating data calculated by the central control unit and/or the operating data received from the one or more other central control units to the external device via the modem.

11. The capacitive unit of claim 10, wherein the central control unit is configured to receive instructions from the external device via the modem to switch the capacitive unit and/or the one or more other capacitive units on and off, and/or to set the value of the predetermined threshold and/or the value of the time delay of the capacitive unit and/or of the one or more other capacitive units.

12. The capacitive unit of any one of the preceding claims, comprising a plurality of capacitive modules, preferably three capacitive modules.

13. The capacitive unit of claim 12, wherein the central control unit is configured to control each of the respective contactors using a pre-determined threshold of a different value.

14. The capacitive unit of any one of the preceding claims, wherein the multi-phase inductive load is a three-phase inductive load and the multi-phase capacitive load is a three-phase capacitive load.

15. The capacitive unit of claim 14, wherein the three-phase capacitive load is connected in one of a delta configuration and a Y configuration.

16. The capacitive unit of any one of the preceding claims, wherein the multi-phase capacitive load is arranged in a plurality of interconnected branches, each branch comprising a capacitor and a resistor connected in parallel.

17. The capacitive unit of claim 16, wherein each branch of the multi-phase capacitive load comprises a plurality of capacitors that are connected in parallel, preferably three capacitors that are connected in parallel.

18. The capacitive unit of any one of the preceding claims, wherein the central control unit is configured to control the contactors so as to connect the multi-phase capacitive loads in parallel with the multi-phase inductive load when the current in the phase cable providing electric power to the multi-phase inductive load exceeds a second pre-determined threshold.

19. The capacitive unit of any one of the preceding claims, further comprising a single multi-phase varistor bank connected in parallel with all of the at least one capacitive module.

20. The capacitive unit of any one of the preceding claims, wherein each of the at least one capacitive module further comprises a single charge indicator for indicating whether residual charges are present on the multi-phase capacitive load of the at least one capacitive module.

21. The capacitive unit of any one of the preceding claims, wherein each of the at least one capacitive module further comprises a module over-current protection element connected in series with the multi-phase capacitive load of the at least one capacitive module.

22. A system comprising at least two capacitive units for local power factor correction of a respective multi-phase inductive load, each capacitive unit comprising at least one capacitive module, wherein each capacitive module comprises:

a contactor; and a multi-phase capacitive load connectable in parallel with the multi-phase inductive load by the contactor; and wherein each capacitive unit further comprises a central control unit configured to control each of the contactors of the respective capacitive module so as to disconnect the multi-phase capacitive loads of the respective capacitive module from the respective multi-phase inductive load when the current in a phase cable providing electric power to the respective multi-phase inductive load falls below a pre-determined threshold, and wherein the central control unit of a first capacitive unit is a master control unit and the central control unit of a second capacitive unit is a slave control unit, wherein the master control unit and the slave control unit are in data communication with one another, and wherein the master control unit is configured to receive operating data of the multi-phase inductive load connected to the second capacitive unit from the slave control unit, and wherein the master control unit is configured to send instructions to the slave control unit so as to set the pre-determined threshold of the second capacitive unit.

23. The system of claim 22, wherein the master control unit comprises a modem configured to connect to an external device, and wherein the master control unit is configured to send operating data of the multiphase inductive load connected to the first capacitive unit and operating data of the multiphase inductive load connected to the second capacitive unit to the external device via the modem.

5

24. The system of claim 23, wherein the master control unit is configured to receive instructions from the external device via the modem, to set the pre-determined thresholds of the first capacitive unit based on the received instructions, and to send the instructions to the slave control unit to set the pre-determined threshold of the second capacitive unit based on the received instructions.