Indian Institute of Technology Bombay, Mumbai-400076 .... converted to High voltage dc (HVDC) using a ... the transformer, converts HVDC to HFAC that is.
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POWER ELECTRONIC TRANSFORMERS IN SMART GRIDS Jakka Venkat, Anshuman Shukla, S. V. Kulkarni Indian Institute of Technology Bombay, Mumbai-400076 ABSTRACT Smart technologies are needed to handle the issues like renewable energy integrations into the existing power systems, reducing weight as well as size of traditional power equipments and making bi-directional power flow possible between the grid and end users of electricity. In response to these needs and the evolution of power electronic converters, power electronic transformers (PETs) became a viable option to replace the conventional transformers. This paper presents a brief review of PET focusing on its basics, topologies and applications. The role of PET in the future smart grid is also elaborated. A three-stage PET is the most popular topology among the discussed PET topologies. Simulation results for the above PET model are presented using PSCAD/EMTDC. Keywords: power electronic transformer, multi level converters, micro grid, smart grid, dual active bridge, power electronic distribution transformer, power electronic traction transformer.
I.
INTRODUCTION
Transformers are fundamental components in power distribution systems and they provide key features like isolation and voltage transformation. Conventional transformers are well researched, and they are economical, highly reliable and quite efficient. On the contrary, they pose challenges such as sensitivity to harmonics, vulnerability to system disruptions and overloads, environmental concerns regarding mineral oil, and poor performance under dc-offset as well as load unbalances [1]. In addition to the above pitfalls, conventional transformers are usually the heaviest parts of the distribution system due to their bulky iron-core and windings. The size and weight of a transformer depend on the operating flux density of the core material and maximum allowable core and winding temperature rise [2]. Higher utilization of the magnetic core and reduced size can be achieved by increasing the operating frequency.
The size and weight of transformers can be reduced by using power electronic converters which make the operation at high frequencies possible. PET also provides the following features: 1. Improved power quality 2. DC and alternate frequency AC service options 3. Integration with system monitoring and control devices 4. Elimination of hazardous liquid dielectrics. Because of these advantages, PETs are used in distribution systems, traction applications, renewable energy systems and micro grids. Brief history, classification and applications of PETs are presented in the following sections. II.
POWER ELECTRONIC TRANSFORMER
A. Background PETs were traditionally considered for stepping up or stepping down ac voltages. In 1970, W. Mc Murray form G.E. introduced a power converter circuit having a high frequency transformer [3]. A naval researcher J. L. Brooks proposed a PET consisting of an ac-ac buck converter to step down a given ac voltage in 1980 [4]. This topology has the following demerits: no magnetic isolation, inability to correct the load power factor and interference of load harmonics from the primary-voltage system. Another topology for direct ac/ac conversion has been proposed by M. Kang and P. Enjeti from Texas A&M University [5]. Reduced size and weight are achieved but it does not result in unity power factor at the input side. A three-stage PET has been proposed by E. R. Ronan and S. D. Sudhoff in 1999 [1]. This topology is one among the most popular topologies with excellent features like selfprotection, excellent power quality at the input as well as the output terminals, power-factor correction, and elimination of oil as a dielectric and coolant. However, bidirectional power flow is not possible with this topology. A cascaded Hbridge active rectifier based PET topology is explained in [6]. It has all the properties of a typical three-stage PET with bidirectional power
9th International Conference on Transformers flow capability. Many other topologies for PETs are available but they are mostly similar with the discussed ones. The classification of PET topologies is elaborated in the next subsection. B. Topologies Various kinds of PETs are discussed in the previous subsection. These configurations can be classified into three categories: a) direct ac to ac conversion without a high frequency link, b) PET without a dc-link and c) PET with a dc–link [7]-[9]. Figure 1-(a) shows the block diagram for the direct ac-ac conversion topology. In this case, the basic power electronic ac-ac converters (buck, buck-boost, boost or any other derived converters) are used to convert the ac voltage from one voltage level to another voltage level. These converters do not provide isolation between the source and load terminals. The block diagram for PET without dc-link is shown in figure 1-(b). In this configuration a high voltage ac (HVAC) input at the grid frequency is transformed to a high frequency ac (HFAC) output which is applied to the high voltage (HV) terminals of a high frequency transformer. The low voltage (LV) terminals of the transformer are connected to another converter, where the HFAC is converted to low voltage ac (LVAC). Magnetic isolation and reduced size can be achieved with the topology but power factor improvement still remains as a challenge. HVAC
LVAC
LV
LFAC LVAC
HVAC HV HFAC LFAC (b) PET without dc-link HVDC
HFAC
LV
LVDC
LVAC
HVAC HVAC
LVAC HFAC HV (c) PET with dc-link
HVDC
III.
PETS IN SMART GRID APPLICTIONS
A smart grid is the modernization of electricity delivery system that monitors, protects and automatically optimizes the operation of its interconnected elements. The above mentioned elements are distributed generators, energy storage installations, distribution networks and industrial as well as commercial end-users. The smart grid can be considered as the integration of energy management, information technology (IT) and telecommunication technology. Using these technologies, electricity can be delivered from suppliers to consumers in two-way communication to save energy, reduce cost and increase reliability/transparency. There are many demos for smart grids and micro grids established in Europe, U.S. and Asia; among them, future renewable electric energy delivery and management (FREEDM) [10] systems centre is one of the most attractive models for future grid architecture. In this model, PET is used as the central hub or router. Applications of PETs in a typical smart grid are shown in figure 2. From the above block diagram, a PET is capable to provide the following advantages
(a) Direct ac-ac converter HFAC
terminals of the high frequency transformer. The LV terminals of the transformer are connected to another converter which converts HFAC into low voltage dc (LVDC). LVDC is converted into LVAC using an inverter circuit. The entire DC to DC conversion part of this topology is also known as dual active bridge (DAB). Medium voltage operation can be possible with the discussed topology by using modular multi-level converters and advanced semiconductor switching devices. Applications of multilevel converter based PETs are presented in the subsequent sections.
LVDC
Figure 1 PET topologies Figure 1-(c) shows the block diagram for a PET with a dc-link (a three-stage structure). HVAC is converted to High voltage dc (HVDC) using a rectification circuit. HVDC is converted to HFAC using an inverter circuit, which is fed to the HV
1. A plug and play facility for any distributed renewable energy resource (DRER) or distributed energy storage device (DSED) 2. DRERs, DESDs, loads and the grid can be managed using any distributed grid intelligent software 3. Communication technologies can be integrated for proper utilisation of electricity 4. It can be completely isolated from the grid and operated in an islanded mode 5. It can be connected through fault isolation devices to protect from onerous conditions
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PET IFM Intelligent Fault Management
DAB AC DC
Load
DC
AC
DC AC
AC
DC
Load
IFM
Load Energy Storage Device (ESD)
Renewable Energy Resource (RER)
Figure 2 Application of PET in smart grids
Figure 2 shows the layout of a PET used in a smart grid application. In future, low voltage smart loads (DC), residential class DRERs and DESDs can be connected to the low voltage dc bus of the PET. All the loads, DRERs, DESDs and the grid can communicate with a standard communication protocol. PETs in the smart grid enable bidirectional power flow and provide features such as power factor correction, voltage sag ride through, fault current limitation and elimination of harmonics. A fault isolation device is used to isolate the PET from the grid and to provide reconfiguration capability. With the deployment of PETs into the smart grid, the following features such as smart distribution of power, effective integration of the distributed generating units to the grid and proper utilization of power in transportation systems (traction systems and hybrid vehicle charging stations) can be achieved. Some of the preferred applications of PETs are elaborated in the next section. IV.
bridge converters which are connected to both terminals of the high frequency transformer. The H-bridge connected to the primary terminals of the transformer, converts HVDC to HFAC that is fed to the transformer. The second H-bridge coverts the LV side HFAC to LVDC. The output stage is basically an inverter that converts LVDC to LVAC. DAB
The basic block diagram of a modular multilevel converter based PEDT is shown in figure 3 and its control block is shown in figure 4 [1] [6] [11][13]. This topology includes three stages. Stage1 acts as a multi-level active rectifier whose primary objective is to convert the input ac voltage with primary power frequency to a desired dc voltage. In addition to the ac-dc conversion, it also provides unity power factor at the source terminals. The second stage of PEDT or dual active bridge (DAB) consists of two H-
LV
HV
LV
LVAC
HVAC LVDC HV HVDC
HFAC
LV HFAC
Figure 3 Power electronic distribution transformer LVDC
HVDC
SELECTED APPLICATIONS
A. Power Electronic Distribution Transformer (PEDT)
HV
Input
Stage 1 (Rectifier)
Stage 2 (DAB)
Stage 3 (Inverter)
Control
Control
Control
Output
Figure 4 Control block for PEDT
B. Power Electronic Traction Transformer (PETT) In earlier days of electric railways, dc was the most common power supply for traction locomotives. Power was transmitted from the substation to the train in a low voltage range (750 V- 3000 V) since the step down of dc voltage was
9th International Conference on Transformers not feasible. But the transmission at a low voltage causes more loses. Later, single phase ac transmission with a higher voltage level (15 kV / 16.7 Hz and 25 kV / 50 Hz) was introduced to reduce the losses. A bulky transformer was generally required to be used for stepping down the ac voltage. A heavy transformer is not necessarily a disadvantage for the traditional trains as the maximum force that a locomotive can apply to pull the train is dependent on the weight of the locomotive. But, in modern passenger trains the traction equipment is distributed along the length of the train. As a result, weight and volume of the traction transformer became the main constraint parameters for the design engineers. DAB
LVAC
HVAC
HV HVDC
HFAC
LVDC
LV HFAC
Figure 5 Power electronics traction transformer
Literature on power electronic transformer based traction systems is available in [14]-[17]. Figure 5 shows the block diagram for a power electronics traction transformer (PETT). Its control is similar to PEDT. A similar topology of PET can also be used for wind power stations and photovoltaic plants to feed the generated electricity into power grids [18]-[20]. C. PETs in Wind and Solar Applications In traditional power systems, power generating plants are located at remote geographical locations and power is transferred to consumption centers over long distance transmission lines. The scene is changing gradually with many distributed generating units, including both the renewable and non-renewable, are getting deployed. But the initial cost for implementing these technologies is expensive and availability of the resources are unpredictable as they are mostly weather based. Therefore, it is difficult to operate the power system containing them with the conventional setup. By using PET, some of the above
drawbacks can be resolved. Applications of PET in the wind and PV plants are narrated in this subsection. Application of PETs to the wind farms provides the following features: frequency and voltage control, regulation of active and reactive power, and quick response to power system transients at various speeds. As the wind farm equipped with PETs can perform both the active and reactive power flow control under a variable wind speed condition, maximum energy can be captured and the mechanical stress on the wind turbines can be reduced [18]. HVDC transmission can be implemented for the long distance offshore wind-farms by using a modified structure of PETs. In the HVDC transmission, LVAC or MVAC at the wind farm is converted to HVDC by using the rectifier and dual active bridge portion of the PET. The converted dc power is transmitted to the on-shore system where the dc voltage is converted back into an ac voltage using an inverter circuit. Block diagram for such a system is shown in figure 6. DC AC
DC
DC AC
DC
DC AC
AC
DC AC
AC
DC AC
AC DC
DC
AC
Grid
HVDC DC
AC
Figure 6 PET based off-shore wind farm
In modern days, solar energy is considered to be a major contributor of power generation among the available renewable resources. Hence, the research on PV systems with the focus on the medium voltage grid integration, maximum power point tracking, multi-level inverter applications and better utilization of the resources is vigorously pursued. The traditional PV systems consist of PV modules which are connected in series-parallel combinations to form arrays and they are further combined to feed inverters. The power conversion and the grid integration for large scale PV plants become more complex as the size of installation increases [18]. In conventional PV plants, the generated DC voltage is regulated using a DC-DC converter and is fed to an inverter. The ac voltage at the inverter output terminals is stepped up to match the grid voltage. Many such systems are connected in parallel depending on the powerrating of the plant [19] [20]. In such systems, large DC link electrolytic capacitors are needed
IEEMA TRAFOTECH 2014 due to the presence of double frequency power pulsation component on the DC side. Also, the use of line-frequency transformers increases the size of the system and a high switching frequency at the inverter output increases the complexity associated with the output filters. A modified topology based PET can be integrated to the PV system to mitigate the above disadvantages. The block diagram of a PET based PV plant is shown in figure 7.
AC load Grid DC
AC
AC
Battery DC
DC
DC
DSED DC
DC load
AC
PV DC
DC
DRER
Figure 8 AC micro grid
PV DC DC PV
AC DC
DC
DC DC load
AC
AC Grid
DC DC PV
AC DC
DC AC
DC Battery
Grid
DC DC AC
DSED DC
DC DC
AC DC
AC load
PV
DC
DC
AC
DRER
Figure 9 DC micro grid
Figure 7 PET based PV plant
The system offers the following advantages: 1. 2. 3.
4.
5.
Application of the high frequency transformer reduces the size and weight of the system Semiconductor devices can be efficiently used Due to the modular structure, it is possible to replace or shutdown one zone at a time without affecting the entire system Due to the reduction in current harmonics on the DC-link, thin film capacitors with smaller size and longer life can be used [19] Optimum control-techniques can be used to control the series connected multi level inverters to control both active and reactive power flows between the grid and the plant
D. PETs in Micro Grids Small scale renewable energy based micro-grids can reduce the dependency on fossil fuels for electric power generation as they can change many consumers of electricity into producers. The key components of micro-grid systems are utility grid, distributed storage energy devices, distributed renewable energy resources, transformers and power electronic converters. Figure 8 and figure 9 show the traditional ac and dc based micro grids respectively [21] [22]. Both need an additional transformer to step down the voltage. The dc side and ac side loads of the ac and dc micro-grids are connected through different converters.
PET
AC load
Grid DC
DC load
Battery DC DSED DC PV DC DRER
Figure 10 PET connected micro grid
A PET based micro-grid is shown in figure 10 which integrates the transformer, the rectifier and the inverter into a single module and provides following advantages: 1. Power factor improvement as seen from the grid 2. Fast fault isolation ability provided by the fully controlled PET 3. Power management for both ac and dc loads can be embedded into the PET module. Different operating modes for PET based microgrids are explained in [22]. V.
SIMULATION ANALYSIS
Different types of configurations and applications for PETs are explained in the previous sections. The three stage PET topology is most popular one among the discussed topologies and it can be used in many applications. It is implemented
9th International Conference on Transformers in PSCAD/EMTDC software. In the simulations, inbuilt blocks for power devices (switching devices, transformers, active and passive components etc.) and user defined control algorithms have been adopted to implement the model. Parameters considered for the study are shown in table 1. Table 1 Simulation parameters Grid voltage (line-line)
420 V, (50 Hz)
Base frequency of the transformer
5 k Hz
Voltage ratio of the transformer
600:300
HVDC
600 V
LVDC
400 V
Switching frequency for the DAB
5 kHz
Switching frequency for the inverter
1 kHz
Sampling time for simulation runs
5 µs
Figure 11 and figure 12 show the input voltages and currents respectively. Figure 13 shows the simulation results for the input phase voltage and the corresponding phase current on the grid side. Here, dq-vector based controller is used to maintain unity power factor at the input terminals. Individual DC-link voltages of the rectifier are shown in figure 14. In the simulations, HV and LV DC link voltages are maintained at 600 V and 400 V, respectively, through a proper control circuitry. Figure 15 shows the simulation results for the primary and secondary voltages of the high frequency transformer. Here DAB output voltage is maintained at 400 V. The above dc voltage is converted to an ac voltage using an inverter with sinusoidal pulse width modulation (SPWM) as is shown in figure 16. Filters can be used to obtain a smooth ac waveform. Figure 17 shows the active and reactive powers supplied by the grid to the load, and it demonstrates that the reactive power supplied from the grid is negligible.
magnetic isolation, control of power factor at the input terminals, reduced dimensions and size. Magnetic isolation is achieved through a transformer which is compact due to its high operating frequency (typically 20 kHz). A unity power factor condition at input terminals is achieved through an active rectifier. Another distinguishing feature is the availability of a dc bus at a low voltage level that is suitable for integrating renewable energy sources such as PV. In this work, a simulation model for the threestage PET configuration has been implemented using PSCAD/EMTDC. It has been demonstrated through the simulations that the configuration is capable of providing unity power factor at the input terminals irrespective of load conditions. It also maintains a constant DC-link voltage under variable grid voltage conditions. The future work includes studies on multi-level modular converters with intelligent control suitable for smart grid applications. The simulations will also be verified through a scaled hardware implementation.
Figure 11 Grid voltages
CONCLUSION
Figure 12 Grid currents
Power electronic transformers are expected to play a key role in the emerging smart grids. Amongst the various topologies that have been discussed in the literature, a three stage topology with a DC-link has emerged as the most popular one for power electronic transformers. The topology offers several advantages such as
ACKNOWLEDGEMENT The authors would like to thank the Department of Science and Technology (DST), New Delhi, India for providing the partial financial support for this work under the research project “Modular Power Electronic Converters for High Power Applications” and to Crompton Greaves Ltd for
IEEMA TRAFOTECH 2014 providing fellowship to the author (Venkat) for pursuing PhD.
Figure 13 Phase-A voltage and phase-A current
Figure 17 Active and reactive power supplied by the grid References [1] Ronan, E.R.; Sudhoff, S.D.; Glover, S.F.; Galloway, D.L., "A power electronic-based distribution transformer," Power Delivery, IEEE Transactions on , vol.17, no.2, pp.537,543, Apr 2002 . [2] Kang, M.; Enjeti, P.N.; Pitel, I.J., "Analysis and design of electronic transformers for electric power distribution system," Industry Applications Conference, 1997. ThirtySecond IAS Annual Meeting, IAS '97., Conference Record of the 1997 IEEE , vol.2, no., pp.1689,1694 vol.2, 5-9 Oct 1997.
Figure 14 HVDC and LVDC
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Figure 15 Primary and secondary voltages of the high frequency transformer
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Figure 16 SPWM modulated ac voltage
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