Mitigation of Voltage Sags and Swells by The Faraday Exchanger

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1 Mitigation of Voltage Sags and Swells by The Faraday Exchanger Jagadeesh Gunda, Rod Buchanan, Matthew Williams, Andrew Scobie Faraday Grid Ltd, Edinburgh, UK {Jagadeesh.Gunda, Rod.Buchanan, Matthew.Williams, Abstract The widespread implementation of electronic equipment made voltage sags and swells as the most critical power-quality issues faced by utilities and consumers. Although several power conditioning technologies exist to mitigate voltage-sags and swells, their effectiveness is often limited to certain equipment types or power-quality disturbance profiles and their wide adoption is uneconomical. Moreover, their installation often adds further control complexity to network operation. Faraday Grid Ltd (FGL) has developed a completely new technology, the Faraday Exchanger (FE) a single device featuring power flow control, voltage regulation, power factor correction and power conditioning capabilities. The objective of this paper is to demonstrate the power conditioning capability of Faraday Exchanger in mitigating voltage sags and swells through laboratory experiments on the device. The experimental results confirmed that the Faraday Exchanger competently limits the impact of sags and swells ensuring that they are inconsequential, thereby improving sag- and swellride-through performance of downstream equipment or process. 1 Introduction Power-quality refers to the ability of the grid to supply a pure sinusoidal voltage waveform with no (recurring or nonrecurring) change in its shape, magnitude and frequency. Issues caused by power quality disturbances range from disrupting the operation of consumer electronic devices to shutting down a production line of a manufacturing plant, leading to millions of dollars of industrial revenue loss. For example, the estimated annual revenue loss due to power quality problems is up to $188 billion in the USA and up to 150 billion in Europe [1]-[3]. Hence, powerquality has become a major concern to electric utilities and consumers. Any event that causes a change in the shape, magnitude, or frequency of the supply voltage is considered to be a power-quality disturbance, which can be classified into several types [4]-[7]. Voltage sags and swells are two common types of such power quality problems. They became critical due to the increased usage of voltage-sensitive electronic devices and penetration of renewable generation. Voltage sags and swells can cause incorrect operation, production downtime and economic loss, etc. The economic losses due to a single voltage sag event could range from $5500 (stainless steel manufacturing plant) to $1500K (semiconductor industry), according to a study conducted by the Electrical Power Research Institute (EPRI) [2]. The economic losses due to voltage sags could range from 2120 to per event, according to the European Power Quality survey report [3]. Voltage sags and swells are characterized by their magnitude and time duration. A sag is defined as a decrease in rms voltage below 0.9 pu lasting between a half cycle to 1 min, and a swell is defined as an increase in rms voltage above 1.1 pu lasting between a half cycle to 1 min [4]-[5]. Voltage sags generally originate from a short-term increase in current due to short circuits, lightning faults, or the startup of large loads, whilst voltage swells can originate from a short-term decrease in currents due to faults, switchoff of large loads or switching on large capacitor banks [4]- [8]. Several mitigating technologies can be applied on both the utility and the customer side to reduce the number and severity of sags and swells. Utility solutions mainly involve implementing preventive and corrective actions (e.g. change in design plans) to limit the occurrence of sags and swells, but it is not possible to eliminate them completely. Customer solutions involve installation of additional equipment, called power conditioners at the equipment interface to limit the impact of sags and swells and hence improve the ride-through capability of associated voltage sensitive equipment [9]. Literature review: There are several power conditioning technologies in the market [8]-[10]: UPS, SVC, Dynamic voltage restorer, DSTATCOM, CVT, SMPS, etc. A dynamic voltage restorer with phase jump to compensate for voltage sags is proposed in [11]. A generalized voltage compensation strategy, with continuous phase adjustment of injected voltage, to mitigate both voltage sags and swells is proposed in [12]. A novel unidirectional power flow control algorithm with DVR maximum compensation limit consideration is proposed in [13]. This methodology is aimed to suppress problems of continuous rising in dc-link

2 voltage. A generalized DVR using direct converters is proposed in [14] to mitigate voltage sags, swells, unbalance, harmonics, and flickers as well. In [15], an enhanced sag compensation strategy is proposed, which mitigates the phase jump in the load voltage while improving the overall sag compensation time. A dynamic voltage restorer (DVR) without dc link using direct ac-to-ac-convertor-based topology is proposed in [16]. A series voltage regulator for a distribution transformer to compensate voltage sags and swells is proposed in [17]. 2 Aim of the Investigation Several power-conditioning technologies have been developed to limit the impact of voltage sags and swells. Some technologies sit on the utility side and others on the consumer side but in either case, these technologies often come as auxiliary equipment and sit outside the main energy-conversion-and-distribution process. Hence, their effectiveness is limited to mitigating specific power quality disturbances or specific instances of the same power quality problem. For example, different equipment, and even the same equipment of different brands will have different swell/sag sensitivities [6]. A thorough solution would require several of these technologies, thus their wide adoption would not be economically viable. Moreover, they can pose further operational problems by each adding a level of complexity to the network. Both utilities and consumers are eagerly awaiting a single fundamental technology which, unlike current technologies, sits inside the main energy-conversion-anddistribution process and adapts itself to resolve most of the electromagnetic disturbances. Faraday Grid Ltd has developed a completely new technology, the Faraday Exchanger which can be used as an umbrella solution to mitigate many power quality problems. The objective of this paper is to present Faraday Exchanger as a comprehensive solution for re-solving voltage sags and swells; validate it's sag/swell-resolution capability using laboratory experiments. 3 The Faraday Exchanger Technology The Faraday Exchanger is a proprietary technology of Faraday Grid Ltd. It is an autonomous power flow control device made up of an electromagnetic core, a primary winding, a secondary, and a control winding (Figure 1). The magnetic flux in the core is continuously modulated by controlling the current flowing through the control winding. The control winding current creates a virtual air gap and hence dynamically controls the electromagnetic coupling between the primary and secondary winding. An onboard control system continuously monitors the input and output of the FE device to command the control winding. The control system is completely self-managed and requires no network connection, managing the immediate geographic location to maintain a stable and efficient power flow. The Faraday Exchanger stores energy in the magnetic core in a synchronous manner, allowing it to provide inertia to the system. The Faraday Exchanger is able to provide volt/var regulation, power flow control, power factor correction and power conditioning capabilities independently and simultaneously. The Faraday Exchanger has been designed to have the ability to be a drop in replacement for a transformer within an electricity network, however because of it s operational performance it also makes redundant capacitor banks, tap-changers, STATCOMs, harmonic filters, DVR, and others. If the control system fails, then the Faraday Exchanger will default to passive control of the voltage (as a transformer would) having no impact on the network. The Faraday Exchanger is completely scalable in its core design principals, meaning it can be utilised in any application at any power level within any electricity system, from consumer devices, through to HV grid and generation. Fig. 1 Simplified Faraday Exchanger Schematic While the exact designs and modelling details of the Faraday Exchanger cannot be disclosed due to intellectual property rights, being trade secrets of Faraday, the general designs and modelling information is available in [18]. Unlike current technologies that come as parallel devices, sitting outside the main energy generation and supply chain, the Faraday technology will sit inside the main energy conversion process and adapt its response to control the electromagnetic disturbances. This enables the Faraday Exchanger to limit the impact of numerous voltage disturbances and pass a pure sinusoidal voltage downstream. 4 Experimental Validation 4.1 Sag/Swell-Ride-Through Criteria In general, sag/swell-ride-through refers to the ability of an electric equipment to satisfactorily perform its intended operation even during the periods of voltage sags and swells without degrading equipment lifetime. Sag/swellride-through capability of a one-port equipment (e.g. generator, motor, refrigerator, etc) significantly differs from a two-port equipment (e.g. transformer, faraday exchanger, etc). While one-port equipment with ride-

3 through capability is supposed to perform its intended operation even during sags and swells, two-port equipment with ride-through capability not only perform its intended operation but also help downstream equipment to ridethrough sags and swells. To avoid this confusion, we use the term sag/swellresolution to represent the sag/swell-ride-through capability of power-flow control equipment. Based on this, we can say that a general two-port device has an excellent sag/swell-resolution capability if it can clamp the secondary or downstream voltage (rms value) sufficiently within an acceptable-voltage-margin, even during a sag/swell applied on the primary or upstream side. The acceptable-voltage-margin is network specific and varies based on the operating characteristics of other equipment (i.e. protection and switchgear equipment). In this paper, we consider ±15% of the nominal voltage as an acceptable-voltage-margin as this margin is more than enough to avoid malfunctioning, degradation of equipment-reliability and of equipment-lifetime that occur due to voltage sags and swells. The sag/swell-resolution capability of the Faraday Exchanger is analysed against this acceptable-voltage-margin. 4.2 Experimental Setup and Procedure The schematic diagram of the experimental setup is shown in Figure 2. A programmable power supply is used to generate the supply voltage of desired magnitude and shape. The supply (primary) and load (secondary) voltage waveforms are captured with voltage sensors of resolution 50kHz. The waveforms captured during the experiment are monitored on a programmable oscilloscope as well as processed in Matlab. The tested Faraday Exchanger was a single-phase two-port four terminal device with 220/110V voltage rating and 40 kva power rating. An inductive load with aa power-factor of 0.95 is connected on secondaryside. As the device was single-phase, only single-phase sags and swells are tested. Due to limitations of our internal laboratory testing equipment and power supply circuits, this testing procedure is carried out at a voltage level of 50V rms on the primary and 23 V rms on the secondary. Fig. 2 Test circuit diagram Numerous test cases (i.e. voltage versus time) for voltage sags and swells were prepared (as per IEEE Std [4]) and applied on primary side of both the Faraday Exchanger and a transformer of the same rating. The secondary (or load) voltage was monitored and checked against the acceptable-voltage-margin during the sag or swell period. 4.3 Results of Voltage Sags In the first test, instantaneous, momentary, and temporary sags of specific magnitude and duration were applied on the primary side of both a transformer and the Faraday Exchanger. The lowest-load-rms-voltage during each sag period is shown in Table 1., Table 2., and Table 3. The experimental results showed that the transformer was unable to clamp the load voltage within the acceptablevoltage-margin; whereas the Faraday Exchanger was capable to effectively hold load-voltage within the acceptable margin. As an example, the response of both devices in terms of instantaneous and rms voltage waveforms during a specific sag (70% sag for 0.6s) is shown in Figure 3. Table 1. Lowest load-rms-voltage (%) during instantaneous sag Time duration of @0.3s Table 2. Lowest load-rms-voltage (%) during momentary sag Time duration of @2s Table 3. Lowest load-rms-voltage (%) during temporary sag Time duration of @30s

4 Fig. 3 Voltage waveforms with 70% sag for 0.6s Figure 3. shows an instantaneous sag of 70% applied at 5.0s and lasts for 0.6s. This decrease in primary voltage results in a reduced secondary voltage (68.8%) in case of the transformer. However, under the same conditions, the Faraday Exchanger s secondary voltage is still at 95.33% which falls within the acceptable-voltage-margin during the swell period (i.e. 5s to 5.6s). Similar operation can be observed for all other sag test cases. 4.4 Results of Voltage Swells In the second experiment, instantaneous, momentary, and temporary swells of specific magnitude and duration were applied on the primary side of both the transformer and the Faraday Exchanger. The highest-load-rms-voltage during each sag period is shown in Table 4., Table 5., and Table 6. Again, the experimental results showed that the transformer was unable to clamp the lowest-load-rmsvoltage within the acceptable-voltage-margin; whereas the Faraday Exchanger was capable effectively to hold loadvoltage within the acceptable margin. As an example, the response of both devices in terms of instantaneous and rms voltage waveforms during a specific sag (140% swell for 0.6s) is shown in Figure 4. Table 4. Highest load-rms-voltage during instantaneous swell Time duration of @0.3s Table 5. Highest load-rms-voltage during momentary swell Time duration of @2s Table 6. Highest load-rms-voltage during temporary swell Time duration of @30s In Figure 4, an instantaneous swell of magnitude 140% is applied at 5.0s and lasts for 0.6s. The transformer produces a proportional increase (38%) in the secondary voltage and hence the downstream equipment may not ride-through these swells. But the Faraday Exchanger limits the secondary voltage to 105.1% which is still within the acceptable-voltage-margin. Similar operarion can be observed for all other swell test cases. To sumup, while the transformer sees a proportional voltage sag/swell corresponding to a voltage sag/swell on the primary side, the Faraday Exchanger maintains the secondary voltage within ±15% of the nominal voltage irrespective of the primary voltage sag/swell. This voltage margin is sufficient to overcome the impacts of voltage sags/swells (relay/contactor dropout and pickup, malfunctioning of PLC, etc.). Hence, the Faraday Exchanger is not only self-resilient to sags and swells but also but also makes the downstream equipment to successfully ride-through the sags and swells.

5 5 Conclusions This paper proposed Faraday Exchanger as a comprehensive solution for resolving voltage sags as well as voltage swells. The experimental results confirmed that the Faraday Exchanger can limit the impact of voltage sags and swells as it ensures that the downstream voltage is always maintained within the acceptable margin, irrespective of any upstream voltage sags or swells. Hence, Faraday Exchanger is not only self-resilient to voltage sags and swells but also makes the downstream equipment or process resilient to voltage sags and swells. This not only helps utilities improve their supply quality, reliability, and overall customer satisfaction but also helps industrial consumers to save millions of dollars revenue loss due to power quality disturbances. The revenue loss due to all power quality problems could reach more than a hundred billion dollars (e.g. 150 billion per annum for Europe alone [3]), revenue loss due to voltage swells alone can take up 29% of the overall loss and voltage sags could take upto 60% [19]. As evidenced by our results, the network-wide deployment of Faraday Exchangers will directly reduce the revenue loss originating from poor power quality and power factor. Moreover, it further increases power quality-related savings by making investment on relevant mitigating technologies redundant. For example, the cost of a typical 2 MVA dynamicvoltage-restorer (DVR) to mitigate voltage swells and sags is about $ [2], which may not be needed in a network employing Faraday Exchangers. In conclusion, netowork-wide adoption of the FG technology will result in improved power quality and reliability, increased equipment lifetime, and reducedcapital investment in mitigation technologies. These factors are directly related to improved profitability of network operators and industrial consumers; therefore, our technology ultimately contributes to economic productivity and welfare. Fig. 4 Voltage waveforms with 140% swell for 0.6s 6 References [1] Why Poor Power Quality Costs Billions Annually and What Can Be Done About It, Schneider Electric, Whitepaper, [2] Technical and Economic Considerations for Power Quality Improvements, Final Report, EPRI, Palo Alto, CA, [3] J. Manson, R. Targosz, European power quality survey report, Leonardo Energy Initiative, Nov [4] IEEE Standard : Recommended Practice for Monitoring Electric Power Quality., c1-81, [5] IEC Electromagnetic Compatibility Standard: IEC 61000, [6] Bollen, M. H.: Understanding power quality problems: voltage sags and interruptions. IEEE press, [7] Dugan, R.C., Mark, F.M., Surya, S., Beaty, H.W.: electr-ical power systems quality. McGraw-Hill, NY, [8] T. Kang, S. Choi, A. S. Morsy and P. N. Enjeti, "Series Voltage Regulator for a Distribution Transformer to Compensate Voltage Sag/Swell," in IEEE Transactions on Industrial Electronics, vol. 64, no. 6, pp , June [9] M. F. McGranaghan, D. R. Mueller and M. J. Samotyj, "Voltage sags in industrial systems," in IEEE Transactions on Industry Applications, vol. 29, no. 2, pp , Mar/Apr [10] A. M. Rauf and V. Khadkikar, "An Enhanced Voltage Sag Compensation Scheme for Dynamic Voltage Restorer," in IEEE Transactions on Industrial Electronics, vol. 62, no. 5, pp , May [11] J. G. Nielsen, F. Blaabjerg and N. Mohan, "Control strategies for dynamic voltage restorer compensating voltage sags with phase jump," APEC Sixteenth Annual IEEE Applied Power Electronics

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