WO2018146446A1

WO2018146446A1 - Capacitive unit for local power factor correction

Info

Publication number: WO2018146446A1
Application number: PCT/GB2018/050124
Authority: WO
Inventors: Steven A BOURN; Matthew D BOURN
Original assignee: Energy Facilities Uk Limited
Priority date: 2017-02-13
Filing date: 2018-01-17
Publication date: 2018-08-16
Also published as: GB201702317D0

Abstract

A capacitive unit (100) for local power factor correction of a multi- phase inductive load. The capacitive unit comprises at least one capacitive module (105). Each capacitive module comprises a contactor (120), a multiphase capacitive load (110) and a control circuit (130). The multi-phase capacitive load is connectable in parallel with the multi-phase inductive load by the contactor. The control circuit 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 pre-determined threshold.

Description

CAPACITIVE UNIT FOR LOCAL POWER FACTOR CORRECTION

[0001] The present invention relates to a capacitive unit for local power factor correction.

[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 do not provide the required flexibility and customizability to achieve optimized power factor correction over a range of inductive load conditions. 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.

[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. 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, a multi-phase capacitive load and a control circuit. 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 multi-phase inductive load by the contactor. The control circuit 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 control circuit of each capacitive module thus ensures that the capacitive load of that capacitive module 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.

[0007] The control circuit of the at least one capacitive module 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.

[0008] The capacitive unit may comprise a plurality of 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.

[0009] The capacitive unit may comprise at least one capacitive module per phase of the multiphase load. The current in each phase cable providing electric power to the multi-phase inductive load may be used by at least one of the control circuits of the plurality of capacitive modules for controlling a respective contactor. It is possible that the current in one phase cable deviates from the current in other phase cables, for example if the inductive load is unbalanced. Using the current in each phase cable ensures that the capacitive unit remains responsive to changes in the current in each individual phase cable. This allows for more accurate power factor correction.

[0010] Each of the control circuits of the plurality of capacitive modules may control a respective contactor 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.

[0011] 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.

[0012] 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 three- phase 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.

[0013] 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.

[0014] 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. [0015] 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.

[0016] The control circuit may comprise a current transformer that measures the current through a phase cable providing electric power to the multi-phase inductive load. This provides an accurate measure of the reactive power requirements of the multi-phase inductive load.

[0017] 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.

[0018] The capacitive unit may further comprise an isolating transformer. The isolating transformer may provide electric power to the control circuit. The isolating transformer comprises an input side and an output side. The input side of the isolating transformer may be connected to two phase cables providing electric power to the multi-phase inductive load. The output side of the isolating transformer may be connected to each control circuit of the at least one capacitive module. This allows all of the control circuits of the different capacitive modules of the capacitive unit to be powered by the electric power system that the capacitive unit is connected to for power factor correction. No separate power supply is necessary. This makes the construction of the capacitive unit simple and the operation of the capacitive unit reliable.

[0019] The capacitive unit may further comprise a first over-current protection element connected in series with the input side of the isolating transformer. Alternatively or in addition, the capacitive unit may further comprise a second over-current protection element connected in series with the output side of the isolating transformer. This reduces the risk of damage to the control circuit due to excessive currents.

[0020] Each of the at least one capacitive module of the capacitive unit may further comprise a multi-phase varistor bank. The multi-phase varistor bank is connected in parallel with the multiphase capacitive load of the at least one capacitive module. This protects the capacitive load from surge currents, thereby reducing the risk of damage to the capacitive load during operation. [0021] Each of the at least one capacitive module may further comprise a charge indicator. The charge indicator indicates whether residual charges are present on the multi-phase capacitive load of the at least one capacitive module. 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.

[0022] 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.

[0023] According to a preferred embodiment of the present invention, there is provided a capacitive unit for local power factor correction of a three-phase inductive load. The capacitive unit comprises three capacitive modules. Each capacitive module comprises a contactor, a three- phase capacitive load and a control circuit. The three-phase capacitive load is connectable in parallel with the three-phase inductive load by the contactor. The control circuit controls the contactor so as to disconnect the three-phase capacitive load from the three-phase inductive load when the current through a phase cable providing electric power to the three-phase inductive load falls below a pre-determined threshold.

[0024] The control circuit includes a time delay element. The time delay element delays, by a time delay, controlling the contactor to connect the three-phase capacitive load in parallel with the three-phase inductive load after disconnecting the three-phase capacitive load from the three- phase inductive load. Each of the three control circuits of the three capacitive modules controls the contactor based on the current in a different one of three phase cables providing electric power to the three-phase inductive load. The capacitive unit further comprises an isolating transformer for providing electric power to each of the three control circuits of the three of capacitive modules. The input side of the isolating transformer is connectable to two phase cables providing electric power to the three-phase inductive load. The output side of the isolating transformer is connected to each of the three control circuits of the three capacitive modules. Using three capacitive modules makes the capacitive unit highly customizable and versatile. The capacitive unit is usable in the three-phase AC electric power systems that are most common. Generation of unnecessary reactive power by the capacitive unit is avoided, improving the power factor and reducing power consumption and the environmental impact. Power supply to the control circuits of each of the capacitive modules is simple and reliable. There is thus provided a power factor correction unit that is highly flexible, versatile and customizable to a wide range of existing electric power systems comprising inductive loads.

[0025] According to an embodiment of the present invention, there is also provided a capacitive unit for local power factor correction of an inductive load. The capacitive unit comprises at least one capacitive module. Each capacitive module comprises a contactor, a capacitive load and a control circuit. The capacitive load is connectable in parallel with the inductive load by the contactor. The control circuit controls the contactor so as to disconnect the capacitive load from the inductive load when the current through the inductive load falls below a pre-determined threshold. This allows the capacitive unit to be used for power factor correction of a single-phase inductive load.

[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 2a schematically shows the capacitive unit with one capacitive module;

[0029] Figure 2b schematically shows the capacitive unit with three capacitive modules;

[0030] Figure 3 a shows a detailed schematic diagram of the capacitive unit of Figure 2a;

[0031] Figure 3b shows a detailed schematic diagram of the capacitive unit of Figure 2b;

[0032] Figures 4a and 4b respectively show schematic diagrams of a three-phase capacitive load in a delta configuration and in a Y configuration; and

[0033] Figure 4c is an exemplary circuit diagram of the three-phase capacitive load of Figure 4a.

[0034] 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.

[0035] 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 three- phase 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 two- phase, four-phase or higher-phase AC electric power systems, or in a single-phase AC electric power system.

[0036] 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.

[0037] 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.

[0038] Figure 2a shows an embodiment of the capacitive unit 100 comprising one capacitive module 105. The capacitive module 105 comprises a capacitive load 110, a contactor 120 and a control circuit 130. The contactor 120 may be closed, so as to connect the capacitive load 110 in parallel to the inductive load 200 via connection lines 101a, b, c. The contactor 120 may be opened, so as to disconnect the capacitive load 110 from the inductive load 200. The control circuit 130 controls the contactor 120, thereby opening and/or closing the contactor 120.

[0039] Figure 2b shows an embodiment 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", a contactor 120, 120', 120" and a control circuit 130, 130', 130" . Each of the capacitive modules 105, 105', 105" of the capacitive unit 100 of Figure 2b may correspond to the capacitive module 105 of the capacitive unit 100 of Figure 2a. Although three capacitive modules 105, 105', 105" are shown in Figure 2a, the capacitive unit 100 may include fewer or more capacitive modules. For example, the capacitive unit 100 may include only capacitive modules 105 and 105'.

Alternatively, one of more additional capacitive modules may be connected in parallel to the three capacitive modules 105, 105', 105" in the capacitive unit 100.

[0040] 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.

[0041] A detailed schematic diagram of the capacitive unit 100 of Figure 2a is shown in Figure 3a. The capacitive unit 100 comprises one capacitive module 105. The capacitive module 105 comprises the capacitive load 110, the contactor 120, the control circuit 130, a varistor bank 150, a charge indicator 160, and a module over-current protection element 170. The capacitive unit 100 further comprises an isolating transformer 140, a first over-current protection element 144, a second over-current protection element 142, and a bleed resistor 180.

[0042] 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. [0043] 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.

[0044] The holding voltage across the switching terminals of the contactor 120 is provided by the isolating transformer 140 and controlled by the control circuit 130. The control circuit 130 includes a current measurement device 132 and a time delay element 134. The current measurement device 132 may be placed around one or more of phase cables 201a, b, c. The phase cables 201a, b, c provide electric power to the inductive load 200. The current measurement device 132 may measure the current (such as a reactive current) in one or more of phase cables 201a, b, c.

[0045] The current measurement device 132 may be a normally open switch. When the current measurement device 132 measures no current, it may be open. When the current measured by the current measurement device 132 exceeds a pre-determined threshold, the current measurement device 132 may be closed. The current measurement device 132, when closed, may create a closed electric circuit including an output side of the isolating transformer 140 and the switching terminals of the contactor 120. The current measurement device 132, when closed, applies the holding voltage to the contactor 120, thereby closing the contactor 120. When the current measured by the current measurement device 132 falls below the pre-determined threshold, the current measurement device 132 opens. This interrupts the electric circuit including the output side of the isolating transformer 140 and the switching terminals of the contactor 120, breaking the holding voltage applied at the switching terminals of the contactor 120. The contactor 120 may thus disconnect the capacitive load 110 of the capacitive module 105 from the inductive load 200.

[0046] The pre-determined threshold is stored or pre-set in the current measurement device 132. The pre-determined threshold may be adjustable. The pre-determined threshold may be determined 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.

[0047] The current measurement device 132 may break the holding voltage applied to the switching terminals of the contactor 120, so as to open the contactor 120. The current

measurement device 132 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.

[0048] The current measurement device 132 may be a current transformer or current switch. For example, the current measurement device 132 may be a current switch of the CSW series by E. C. Products Limited, such as the CSW-NO-ASP. The pre-determined threshold may be in the range from 1.5 to 200 A. The current measurement device 132 may measure or monitor a current in the phase cables 201a, b, c in the range from 1.5 to 200 A.

[0049] The time delay element 134 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 132 implements a delay period after disconnecting the capacitive load 110 in which the control circuit 130 is prevented from closing the contactor 120. The time delay may be pre-determined and pre-set in the time delay element 134. This 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.

[0050] The time delay element 134 may be, for example, an ON delay single timer relay, such as the LEDO 12-230V AC/DC by Broyce Control. 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 3 s to 180s.

[0051] The isolating transformer 140 may provide electric power to the control circuit 130. The isolating transformer 140 may provide the holding voltage that is applied to the switching terminals of the contactor 120. The isolating transformer 140 comprises an input side that is connected to two phase cables 101a, c of the AC electric power system. The input side may be connected to any two of the phase cables 101a, b, c. Electric power is provided to the isolating transformer 140 from the AC electric power system. The isolating transformer 140 comprises an output side that is connected to the switching terminals of the contactor 120 via the control circuit 130, so as to provide the holding voltage to the switching terminals of the contactor 120. The holding voltage may be an AC voltage. The isolating transformer 140 may output a voltage suitable to switch the contactor 120, for example an AC voltage in the range from 20 to 30V, such as about 24V. The isolating transformer 140 may, for example, be a transformer of the PRI230 series by legrand®. The control circuit 130, in particular the current measurement device 132, may break the holding voltage applied to the contactor 120 via the isolating transformer 140. A center tap of the output side of the isolating transformer 140 may be connected to electric ground or earth.

[0052] The first over-current protection element 144 is connected in series with the input side of the isolating transformer 140. The second over-current protection element 142 is connected is series with the output side of the isolating transformer 140. The over-current protection elements 142, 144 prevent excessive currents from being applied to the control circuit 130 and the switching terminals of the contactor 120. The first and second over-current protection elements 142, 144 may, for example, be miniature circuit breakers, such as the PI MB 2P C02 by Lovato electric and S200M miniature circuit breaker by ABB ®.

[0053] The varistor bank 150, in particular a multi-phase (three-phase) varistor bank 150, is connected in parallel to the capacitive load 1 10. The varistor bank 150 may be connected in a delta or Y configuration, so as to match the connection configuration of the capacitive load 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 (8/20μ8), for example about 12 kA (8/20μ8), such as the Z500 varistor (MOV) assemblies by PD Devices Ltd.

[0054] The charge indicator 160 may indicate whether residual charges are present on the capacitive load 110. The charge indicator 160 may comprise an indicator lamp per phase of the capacitive load 110. Each of the indicator lamps may be connected to a different phase of the capacitive load 110. The charge indicator 160 may indicate to a user whether charges are present on the capacitive load 1 10 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 lamps of the charge indicator 160 may, for example, be LEDs chosen from the 501 SM series or the 504 05 series by CamdenBoss ®.

[0055] 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 PI MB 3P D63 by Lovato electric. The bleed resistor 180 provides isolation of the capacitive unit 100 from the inductive load 200. The bleed resistor 180 may, for example, have a resistance of 470 kQ and a power rating of 3W.

[0056] A detailed schematic diagram of the capacitive unit 100 of Figure 2b is shown in Figure 3b. Compared to the capacitive unit 100 shown in Figure 3 a, the capacitive unit 100 of Figure 3b comprises three capacitive modules 105, 105', 105" . Each of the three capacitive modules 105, 105', 105" may correspond to the capacitive module 105 described in relation to Figure 2a. Each of the three capacitive modules 105, 105', 105" comprises the capacitive load 110, 110', 110", the contactor 120, 120', 120", the control circuit 130, 130', 130", the varistor bank 150, 150', 150", the charge indicator 160, 160', 160" and the module over-current protection element 170, 170', 170" . Each of these components may correspond to the respective component described in relation to Figure 3a. The capacitive unit 100 further comprises the isolating transformer 140, the over-current protection elements 142, 144 and the bleed resistor 180.

[0057] The isolating transformer 140 provides the holding voltage to the switching terminals of each of the contactors 120, 120', 120". Each of the control circuits 130, 130', 130" and the switching terminals of each of the contactors 120, 120', 120" may be connected to the output side of the isolating transformer 140. Thus, a single isolating transformer 140 may be used to provide the electric power required to control all capacitive modules 105, 105', 105".

[0058] Each of the control circuits 130, 130', 130" may break the holding voltage applied to the switching terminals of a respective contactor 120, 120', 120". Each of the control circuits 130, 130', 130" may control a respective contactor 120, 102', 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, as shown in Figure 3b, the current measurement device 132 of the first capacitive module 105 may measure the current in phase cable 201a, the current measurement device 132' of the second capacitive module 105' may measure the current in phase cable 201b, and the current

measurement device 132" of the third capacitive module 105" 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 each of the capacitive modules 105, 105', 105" uses the current in the same phase cable (one of phase cables 201a, b, c) to control switching of the respective contactor 120, 120', 120".

[0059] Each of the control circuits 130, 130', 130" may open a respective contactor 120, 120', 120" if the current in the 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, with reference to Figure 3b, 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).

[0060] 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.

[0061] Additional capacitive modules may be connected to the output side of the isolating transformer 140, in parallel to the capacitive modules 105, 105', 105" . Such additional capacitive modules may use or measure the current in any of the phase cables 201a, b, c. 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.

[0062] 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" .

[0063] 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 μ¥ (with a voltage rating of 540V), of 60 μ¥ (with a voltage rating of 440V), or of 70 μ¥ (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.

[0064] 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 3b 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 μ¥ capacitors connected as in Figure 4c.

[0065] 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 single- phase, two-phase, four-phase or higher phase AC electric power systems.

[0066] 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;

a multi-phase capacitive load connectable in parallel with the multi-phase inductive load by the contactor; and

a control circuit configured to control 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 pre-determined threshold.

2. The capacitive unit of claim 1, wherein the control circuit 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.

3. The capacitive unit of claim 1 or 2, comprising a plurality of capacitive modules.

4. The capacitive unit of claim 3, wherein the capacitive unit comprises at least one capacitive module per phase of the multi-phase load.

5. The capacitive unit of claim 4, wherein the current in each phase cable providing electric power to the multi-phase inductive load is usable by at least one of the control circuits of the plurality of capacitive modules for controlling a respective contactor.

6. The capacitive unit of claim 3, wherein each of the control circuits of the plurality of capacitive modules controls a respective contactor using a pre-determined threshold of a different value.

7. 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.

8. The capacitive unit of claim 7, wherein the three-phase capacitive load is connected in one of a delta configuration and a Y configuration.

9. 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.

10. The capacitive unit of claim 9, wherein each branch of the multi -phase capacitive load comprises a plurality of capacitors that are connected in parallel.

11. The capacitive unit of claim 10, wherein each branch of the multi-phase capacitive load further comprises a plurality of resistors, each resistor of the plurality of resistors being connected in parallel with a respective one of the plurality of capacitors.

12. The capacitive unit of any one of the preceding claims, wherein the control circuit comprises a current transformer configured to measure the current through a phase cable providing electric power to the multi-phase inductive load.

13. The capacitive unit of any one of the preceding claims, wherein the control circuit of the at least one capacitive module is configured to 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.

14. The capacitive unit of any one of the preceding claims, further comprising an isolating transformer for providing electric power to the control circuit, the isolating transformer comprising an input side and an output side, wherein the input side of the isolating transformer is connectable to two phase cables providing electric power to the multi-phase inductive load, and wherein the output side of the isolating transformer is connected to each control circuit of the at least one capacitive module.

15. The capacitive unit of claim 14, further comprising at least one of i) a first over- current protection element connected in series with the input side of the isolating transformer and ii) a second over-current protection element connected in series with the output side of the isolating transformer.

16. The capacitive unit of any one of the preceding claims, wherein each of the at least one capacitive module further comprises a multi-phase varistor bank connected in parallel with the multi-phase capacitive load of the at least one capacitive module.

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

18. 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.

19. A capacitive unit for local power factor correction of a three-phase inductive load, the capacitive unit comprising:

three capacitive modules, each capacitive module comprising

a contactor;

a three-phase capacitive load connectable in parallel with the three-phase inductive load by the contactor; and

a control circuit configured to control the contactor so as to disconnect the three- phase capacitive load from the three-phase inductive load when the current through a phase cable providing electric power to the three-phase inductive load falls below a predetermined threshold;

the control circuit including a time delay element configured to delay, by a time delay, controlling the contactor to connect the three-phase capacitive load in parallel with the three-phase inductive load after disconnecting the three-phase capacitive load from the three-phase inductive load; each of the three control circuits of the three capacitive modules being configured to control the contactor based on the current in a different one of three phase cables providing electric power to the three-phase inductive load;

the capacitive unit further comprising an isolating transformer for providing electric power to each of the three control circuits of the three of capacitive modules, the isolating transformer comprising an input side and an output side, wherein the input side of the isolating transformer is connectable to two phase cables providing electric power to the three-phase inductive load, and wherein the output side of the isolating transformer is connected to each of the three control circuits of the three capacitive modules.

20. A capacitive unit for local power factor correction of an inductive load, the capacitive unit comprising at least one capacitive module, wherein each capacitive module comprises:

a contactor;

a capacitive load connectable in parallel with the inductive load by the contactor; and a control circuit configured to control the contactor so as to disconnect the capacitive load from the inductive load when the current through the inductive load falls below a pre-determined threshold.