FIELD OF THE INVENTION

[001] The present invention is related to voltage source converters. More specifically, the present invention is related to a converter arrangement having a filter with a capacitor, and method of isolating the converter arrangement to protect from harmonic distortions.

BACKGROUND OF THE INVENTION

[002] Voltage Source Converter (VSC) technology is used in various power system applications to enhance power quality and energy efficiency. VSCs can be inverters, utilized for providing single or multiphase AC voltages from a DC voltage source. VSCs can also be rectifiers for having a rectified DC output from AC voltages. For example, the VSC is a IGBT (Insulated Gate Bipolar Transistor) based three-level inverter or converter

[003] In such converters, the output of the VSC can be connected to a power system. For example, a converter may be connected with an AC power grid, loads etc. A filter is generally utilized while connecting the converter with the power system. The filter can be used lower harmonic distortion in the line / grid currents. LCL (inductor- capacitor-inductor) filter is a frequently used filter in“T” topology amongst different filters used in the field of power electronic applications. This is due to the good attenuation achieved with a relatively smaller value of the components of the LCL filter.

[004] Generally, an LCL filter is designed with a tuning frequency of one third of its switching frequency. If there are any harmonic voltage components around the tuning frequency present in the line currents flowing through the filter, then these harmonics may resonate with the filter. The harmonic voltage components in the currents may get amplified at the filter capacitor, which can cause a heavy flow of current into the filter capacitor. These currents with higher magnitude can create hotspots in the capacitors film. This phenomenon damages the filter capacitors and/or the value of the filter capacitors may be deteriorated. As a result the required performance cannot be achieved from the VSC. For example, higher levels of switching ripples may be present in the output current of the inverter and can lead to failure of filter capacitor due to resonance with the filter. Taking another example, the load harmonics may not be within prescribed limits and thereby not suitable for operation due to improper Electromagnetic compatibility (EMC). Similarly, there could be overheating of line reactors and in components of protection devices such as relays or fuses due to higher harmonic content or resonance effect, and can cause damage in capacitors used in electronic/electrical devices by way of explosion of the capacitor due to higher harmonic content or resonance effect, thereby causing interruption in plant operation..

[005] Thus certain harmonic content can cause the filter elements to resonate, thereby having detrimental effect on the circuit components. There is accordingly a need to monitor the built up of harmonic content, to detect presence of harmonics at an early stage to protect the converter and components thereof.

SUMMARY OF THE INVENTION

[006] An aspect of the present invention is related to a converter arrangement for power supply between an alternating current, AC, power system and a direct current, DC, power system. For example, the converter arrangement can be used to couple an AC power grid with a battery or supercapacitor system. The converter arrangement can be used for various power system applications such as, but not limited to, active harmonic filtering, load balancing, reactive power compensation and active power generation. [007] The converter arrangement comprises a converter, a filter and a control unit. The converter is a converter with switching devices such as IGBTs, for power supply or power quality control applications. The converter is capable as operating as an inverter and as a rectifier based on mode of operation. In other words the converter can be seen as a bi-directional inverter, which can be used to monitor and control flow of power between the AC and DC systems. Thus, the converter can be used not only as an inverter to provide a single or multiphase AC voltage from the DC source, but also as a rectifier to perform conversion of AC power to DC power, as needed.

[008] The filter couples the converter output with the AC power system. The filter can be an LCL filter, with a line side reactor, a converter side reactor and a capacitor between the line side reactor and the converter side reactor. The filter can be used to filter one or more of, but not limited to, high frequency components, line side harmonics, and switching frequency components.

[009] The control unit is used for controlling the operation of the converter. Here, the control unit can control the mode of operation of the converter to act as an inverter or rectifier. Taking an example, the control unit can control power in one or more phases with the converter. The control unit can have control logic implemented with a processor (e.g. a Digital Signal Processor) for controlling the operation of the converter / converter arrangement or components thereof.

[0010] The control unit measures voltage across the line side reactor of the filter to detect harmonic content in the voltage across the line side reactor. For example, the voltage across the line side reactor can be provided to a differential circuit (e.g. an operational amplifier based circuit), the output of which can be provided to the control unit. Here, the output can be smoothened with another circuit (e.g. a rectifier circuit) before it is provided to the control unit. The control unit can optionally perform certain estimations to be get the measured voltage values. For example, a Root Mean Square (RMS) value can be calculated for the measured voltage. [0011] The voltage across the line side reactor can be measured in one or more phases. For example, the converter may be connected with three phases of the AC power system. Accordingly, the control unit can measure the voltage across the line side reactor in each of the three phases. In such a case, the control unit selects a maximum value from the one or more voltage values as the voltage value across the line side reactor.

[0012] The control unit also compares the measured voltage across the line reactor with one or preset threshold values. The comparison is performed to detect a condition where the harmonic content is higher than at least one of the one or more preset threshold values. The one or more preset threshold values can be associated with one of power quality at a line side and deterioration of the capacitor of the filter. For example, the line currents can have certain harmonics (or harmonic content). These may be due to fluctuations in the line currents supplied from the AC side. Alternately, there may be a degradation (or deterioration) in the performance of the capacitor. These preset threshold values can be available with the control unit, for example in a data storage associated with the control unit.

[0013] Thus, the control unit can detect if there is any harmonic content in the line currents that can resonate with the filter components, or due to deterioration of filter components such as due to capacitor degradation. Accordingly, the control unit can take preventive actions. In accordance with the aspect, the control unit generates an open signal for a relay to isolate the converter arrangement in response to determining that the measured voltage across the line reactor is higher than the at least one preset threshold value.

[0014] The relay may be part of the converter arrangement, or external to the converter arrangement. In an embodiment, the relay controls a contactor (e.g. a circuit breaker) that is provided between the AC and DC systems. Here, the relay generates a trip signal for opening (disconnecting) the converter arrangement from the AC power system, based on the open signal.

[0015] The control unit can also generate a communication to be provided on an operator interface of the converter arrangement when the measured voltage is higher than a reference value associated with life of the capacitor. The operator interface may be part of the converter arrangement (e.g. a Human Machine Interface (HMI)) of the converter arrangement, or a remote interface connected via a communication channel with the converter arrangement. Thus, the control unit can generate an alert for maintenance or service activity when capacitor deterioration is detected. Thus, the line currents flowing through the components of the converter arrangement can be monitored for performing one or more monitoring and protection functions associated with the converter arrangement.

[0016] Another aspect of the invention accordingly provides a method for isolating a converter arrangement such as described in the above aspect, or a different converter arrangement which has at least the converter and the filter as described above. The method can be performed with a control unit as described above, or a control unit that is external to the converter arrangement. In case the control unit is external, it is required that it is operationally connected with the converter arrangement and components thereof for performing the different method steps.

[0017] The method comprises obtaining measurement of voltage across the line side reactor detect harmonic content in the voltage across the line side reactor. The voltage across the line reactor may be measured in one or more phases. In an embodiment, the voltage is measured in three phases. Here, a maximum value of the voltages in the three phases is selected as the measured voltage across the line side reactor. Further, one or more estimations may be performed to have the measured voltage. In an embodiment, a RMS value is calculated for the voltage across the line side reactor. [0018] The method further comprises comparing the measured voltage across the line side reactor value with one or more preset threshold values. The comparison is performed to detect a condition where the harmonic content is higher than at least one of the one or more preset threshold values.

[0019] In addition, the method comprises generating an open signal for a relay to isolate the converter arrangement. The open signal is generated in response to determining that the measured voltage is higher than the at least one preset threshold value. The method can also include providing a trip signal to operate a contactor connected between the AC power system and the line side reactor based on the open signal. Additionally, the method can include generating a communication for an operator interface when the measured voltage is higher than a reference value associated with life of the capacitor.

BRIEF DESCRIPTION OF DRAWINGS

[0020] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in attached drawings in which:

[0021] Fig. 1 is a simplified representation of a power system with a converter arrangement for power supply between an AC power system and a DC power system, in accordance with an embodiment of the invention;

[0022] Fig. 2 shows a scheme of operation of a control unit, in accordance with an embodiment of the invention;

[0023] Figs. 3 and 4 show a voltage measurement unit for measuring voltage across a line side reactor, in accordance with an embodiment of the invention; [0024] Fig. 5 is a flowchart of a method for isolating the converter arrangement, in accordance with an embodiment of the invention; and

[0025] Fig. 6 shows simulation results for resonance detection due to grid side harmonics and capacitor deterioration.

DETAILED DESCRIPTION

[0026] A Voltage Source Converter (VSC) is used to couple an alternating current, AC, power system with a DC power system. As an example, a VSC may be used for providing real power to loads, a compensation for unbalance, reactive power and harmonics caused by loads, wherein the loads may be unbalanced, non-linear, inductive and/or capacitive.

[0027] In such application, a frequent failure that is observed is of a filter capacitor of the VSC. This can be due to certain harmonics (harmonic content), causing the capacitor (or filter) to resonate. This may be due to harmonics in the line / grid side currents. This may also be a result of capacitor degradation (or deterioration), as it results in performance degradation of the capacitor / filter. Though the capacitance value degradation is slow varying phenomenon, it can be a potential problem if not addressed at the right time.

[0028] The present invention is directed towards detecting such detrimental harmonics in the line currents for protecting the capacitor and other circuit elements. As an example, the entire converter system can be isolated from the grid during a resonance condition, by giving a trip command to protective relays and IGBTs of the converter system.

[0029] Referring to Fig. 1 , which is a simplified representation of a power system with a converter arrangement (100) for power supply between an AC power system (102) and a DC power system (104), in accordance with an embodiment of the invention. The AC power system can be an AC power grid or a three phase load, while the DC power system can be a battery system, a supercapacitor system etc. The converter arrangement can be used for various applications in such a power system such as for, but not limited to, active harmonic filtering, load balancing, providing supplemental power to the AC system, and reactive power compensation.

[0030] As shown in Fig. 1, the converter arrangement comprises a converter (106), a filter (108) and a control unit (110). The converter is a converter with switching devices such as IGBTs, for power supply or power quality control applications. The converter is capable as operating as an inverter and as a rectifier based on mode of operation. In other words the converter can be seen as a bi-directional inverter, which can be used to monitor and control flow of power between the AC and DC systems. Thus, the converter can be used not only as an inverter to provide single or multiphase AC voltages from the DC source, but also as a rectifier to perform conversion of AC power to DC power, as needed.

[0031] As shown, the filter couples the converter output with the AC power system to control power quality between the two systems. For example, a Pulse Width Modulation (PWM) output of the inverter (converter) is connected to a grid through the filter to lower harmonic distortion in the line/grid currents. The filter is provided with the converter (at the converter output, either as a part of the converter or converter arrangement) and comprises a reactor (or inductor) and / or a capacitor. The filter can be provided in any well-known configurations, for example as phi filter, multiple filter units cascaded or provided with single component of L or C (where the resonance effect may occur together with parasitic components). In the exemplary illustration, the filter is considered as an LCL filter, and as shown in Fig. 1 , the filter has a line side reactor (112), a converter side reactor (114) and a capacitor (116) between the line side reactor and the converter side reactor. The capacitor may be referred to as filter capacitor or capacitor in different parts of the description. The advantages from the invention can also apply to any significant capacitor used in the converter arrangement, which can get degraded.

[0032] The control unit is used for controlling the operation of the converter. For example, the control unit can control the converter to have required power quality / energy optimization. Here, the control unit can control the mode of operation of the converter to act as an inverter or rectifier according to the application. The control unit can have control logic implemented with a processor (e.g. a Digital Signal Processor) for controlling the operation of the converter arrangement or components thereof. The control unit is operationally connected with different components of the converter arrangement and / or the power system for the various applications.

[0033] The voltage across the line reactor (line side reactor) provides a good idea to get information about the line conditions. It is observed that the voltage across the reactor offers a very low impedance to the fundamental frequency component, and a very high impedance to the high frequency switching components of the converter. Hence, the voltage drop across the line reactor directly gives the high frequency components, which can be used for detecting the resonance condition and capacitor value degradation. The resonance detection due to harmonics in line / grid currents and capacitor value degradation using voltage measurements across the line side reactor is explained below.

[0034] Detection of resonance in the filter due to harmonics:

[0035] There could be harmonics (or harmonic content) in the currents at the line / grid side at the filter. These harmonics can cause the filter, or filter components, to resonate as the frequency components may be similar to tuning frequency of the filter / components. During the resonance condition, a huge amount of high frequency current components can flow to the filter capacitor. This causes larger amount of voltage drop across the line reactor. The voltage drop across the line reactor can be directly processed by a voltage measurement unit (202) as shown by 204 in Fig. 2.

[0036] The voltage measurement unit can have a voltage measurement stage (302) and a rectifier stage (304) as shown in Fig. 3. These can be differential circuits to have the voltage drop measured and smoothened before it is fed to the control unit. Fig. 4 shows an example hardware for detection of resonance and filter capacitor deterioration. In the embodiment of Fig. 4, the voltage measurement stage has an operational amplifier (OPAMP) based circuit, while the rectifier stage has a full wave rectifier with a DC capacitor. The rectifier stage is used to smoothen the output voltage from the voltage measurement stage. In the exemplary circuit shown in Fig. 4, the voltage measurement stage has one OPAMP, while the full wave rectifier has two OPAMPs. Hence three OPAMPs are needed for each phase. Other equivalent circuits with OPAMP or other electronic components can also be used for the purpose of voltage measurement.

[0037] The voltage across the line reactor as processed by the voltage measurement unit is provided to the control unit as shown by 206 in Fig. 2. As an example, a RMS (root mean square) value of the high frequency voltage component can be directly processed by the OPAMP based full wave rectifier with the RMS voltage detector circuit as shown in Fig. 4. It is to be noted that the voltage measurement unit may be provided as a part of the converter arrangement.

[0038] Thus, the voltage drop across the line side reactor can be measured with the control unit. As shown by 206 in Fig. 2, the control unit receives the voltage across the line reactor. The measured voltage across the line reactor can be compared with one or more preset threshold values. The comparison can be performed with the control unit to produce an open or error signal as shown by 208, when a condition where the harmonic content is higher than a line side (or other) reference value is detected. [0039] The open signal can be provided to a relay (210) to isolate the converter arrangement during the resonance condition to protect the filter capacitor. The relay may be a protection relay that is part of the converter arrangement. The relay can provide a trip signal as shown by 212, to a contactor or circuit breaker (such as 118 shown in Fig. 1) provided to protect the converter arrangement by electrical isolation. For example, the relay can provide a trip signal to a circuit breaker based on the open signal.

[0040] Detection of filter capacitor deterioration:

[0041] The capacitor deterioration shifts a tuning frequency of the output filter (or components thereof) towards the switching frequency of the converter. If the value of the capacitance of the filter capacitor is reduced, then the tuning frequency of the converter (as inverter) increases inversely proportional to the square root of the new value of capacitance. For example, if the capacitance value is reduced to 80% of initial capacitance, then the resonant frequency increases to about 1.12 times of the initial value of the tuning frequency of the output filter. The loss of the filter capacitor increases the tuning frequency and hence the high frequency components of the converter (acting as inverter) can be injected to the AC power system (grid). This causes a higher voltage drop across the line side reactor. This also can be detected as similar to detection of grid resonance discussed above.

[0042] In this case, the control unit generates a communication signal as shown by 214 in Fig. 2, for providing a communication with an operator interface (216) of the converter arrangement. The operator interface may be part of the converter arrangement (e.g. a Human Machine Interface (HMI)) of the converter arrangement, or a remote interface connected via a communication channel with the converter arrangement. Thus, the control unit can generate an alert as shown by 218, for maintenance or service activity when capacitor deterioration is detected. [0043] Thus, the line currents flowing through the filter of the converter arrangement can be monitored for performing one or more monitoring and protection functions associated with the converter arrangement. The one or more preset threshold values associated with one of power quality at a line side and deterioration of the capacitor of the filter can be made available with the control unit (e.g. in a data storage or a memory associated with the control unit).

[0044] Thus, the control unit can detect if there is any harmonic content in the line currents that can resonate with the filter components, or due to deterioration of filter components such as due to capacitor degradation. Accordingly, the control unit can take preventive actions as mentioned above. Thus, the converter arrangement can be isolated to protect the arrangement from effects of detrimental harmonic content.

[0045] Referring now to Fig. 5, which is a flowchart of a method for isolating a converter arrangement. The converter arrangement can be similar to the one described in conjunction of Figs. 1 to 4, or a different converter arrangement which has at least the converter and the filter as described hereinabove. The method can be performed with a control unit such as 110, or a control unit that is external to the converter arrangement. In case the control unit is external, it is required that it is operationally connected with the converter arrangement and components thereof for performing the different method steps.

[0046] At 502, the method comprises obtaining measurement of voltage across the line side reactor to detect harmonic content in the voltage across the line side reactor. The voltage can be measured with the voltage measurement unit such as 202. In an embodiment, the voltage across the line reactor is measured in one or more phases. For example, the voltage is measured in three phases. Here, a maximum value of the voltages in the three phases is selected as the measured voltage across the line side reactor. Further, one or more estimations may be performed to have the measured voltage. In an embodiment, a RMS value is calculated for the voltage across the line side reactor.

[0047] The method further comprises comparing the measured voltage across the line side reactor value with one or more preset threshold values at 504. The comparison is performed to detect a condition where the harmonic content is higher than at least one of the one or more preset threshold values. For example, resonance conditions due to line side harmonics or capacitor deterioration can be detected from the comparison.

[0048] At 506, the method comprises generating an open signal for a relay to isolate the converter arrangement. The open signal is generated in response to determining that the measured voltage is higher than a preset threshold value (as explained above). Optionally, at 508, the method includes providing a trip signal to operate a contactor (such as 118) based on the open signal. The method can also include at 510 to generate a communication signal for an operator interface when the measured voltage is higher than a reference value associated with life of the capacitor.

[0049] An inverter system has been modeled, where a voltage source with frequency f _n has been connected in series with the grid in each phase to represent resonance at the grid side. The RMS values of the voltage across line reactor for the loss of capacitance and the grid side resonance are as shown in Fig. 6.

[0050] Thus resonance conditions due to line side harmonics and capacitance degradation can be closely monitored using the invention disclosed herein. The converter arrangement can be isolated during resonance conditions. Additionally, the capacitor performance can be monitored to detect service / maintenance needs.