Smart Transformer For Distribution System

Smart transformers work independently to constantly regulate voltage and maintain contact with the smart grid in order to allow remote administration (if needed), and to provide information and feedback about the power supply and the transformer... - Narendiran S, Dr. Sarat Kumar Sahoo

A smart transformer (ST) provides the exact amount of power that is needed, and responds instantly to fluctuations within the power grid, acting as a voltage regulator to ensure that the optimized voltage is undisturbed. STs are programmed to, as a default, provide an voltage optimized power supply that directly addresses their facility’s energy needs. ST can help large commercial facilities use power more efficiently to save money, power and go greener.

A solid state transformer (SST) with managerial role in the electric distribution grid is generally called Smart Transformer (ST). Smart transformers work independently to constantly regulate voltage and maintain contact with the smart grid in order to allow remote administration (if needed) and to provide information and feedback about the power supply and the transformer. Moreover this type of transformers are used in Point of Common Coupling (PCC) in a microgrid for voltage control – and it acts as a protecting device for electrical equipments during power fluctuations.

Fig. 1: Smart transformer concept…

The ST has to have some intelligence to meet the requirement of future power systems. A possible idea of ST, based on power electronics and communication technology is depicted in Fig.1. The main aim of this article is to show the role of ST in distribution grid system with its hardware control and communication topology with its role in distribution system. Some important characteristics of ST are:

  • They give exact amount of power that’s needed and respond instantly to fluctuations within the grid
  • ST act as a voltage regulator and it ensures that the optimised voltage is undisturbed, because they directly reduce energy consumption & green house gas emissions
  • ST immediately reduce power consumption by providing a stable, optimal power supply that supplies electrical equipment with its ideal voltage
  • They also protect electrical equipments with its ideal voltage
  • ST as a default, provide a voltage optimised power supply that directly addresses their energy needs.

Power converter cells for ST

Once chosen the ST, the next step is choosing, which should be the basic block of the ST, the power converter cell.
The following goals are achieved using power converter cells with ST:

  • Isolation for the different voltage level
  • Reduced core size
  • Minimal loss.

ST hardware control system

The set of hardware control blocks include ST, instrumentation, control gate drives, electronic On Load Tap Changer (OLTC), transformer builds the voltage regulation block, which is also responsible for the secondary voltage regulation.

The interface module blocks are data converters, which convert variables and events of the control system – and send them to the master module. In the master module the data of all the interface modules are packed and sent to the Programmable Logic Control (PLC) interface which converts the data to be sent through the medium voltage line. The data coming from a set of transformers are received by the gateway block, which concentrates and sends them to the wireless interface. From this block, the data are sent to the power utility where it is received and processed as in Fig. 2.

Fig. 2: Smart transformer hardware control …

Communication topology for ST in distribution system

This system is designed for applications in rural distribution networks. This system consist of a distribution transformer equipped with an electronic OLTC for automatic voltage regulation and a communication system.

The PLC channel is used to establish a path between the distant locations, where the transformer is installed, and the gateway that concentrates the data from other smart transformer installed in that region. The gateway also converts the data into a wireless communication channel.

The amount of data transferred and received by the smart transformer makes the PLC suitable for this application, since it does not need a high data transmission rate. on the other hand, the wireless channel needs a higher data transmission rate, since it concentrates the data from a set of smart transformers, which prompts cell phone and internet topologies a good choice. A communication topology for employment in smart transformer in distribution system in shown in Fig. 3.

Fig. 3: Communication topology for smart transformer…

Area of applicability

These days, controlling the power transfer through the PCC has become a major issue.

  • Smart transformers when used at the PCC, it controls the active power exchange between a microgrid and the utility grid dependent on the state of both networks and other information communicated to the ST
  • To control the active power, the ST uses its taps that change the microgrid side voltage at the PCC. This voltage based control of the ST is compatible with the voltage based droop control of the units in the microgrid
  • ST is a more powerful component providing faster and superior voltage regulation, as it can regulate the voltage, the frequency and the harmonic behaviour of each feeder
  • A smart transformer enables to control the power exchange between a microgrid and the utility network by controlling the voltage at the microgrid side within certain limits.
  • The distributed generation units in the microgrid are equipped with a voltage based droop control strategy. This controller reacts on the voltage change making the smart transformer an element that controls power exchange without the need for communication to other elements in the microgrid.

Role of ST in the distribution system

In grid connectivity:

  • ST are controlled dynamically allowing facilities to monitor and manage the transformers directly during the period of power fluctuations – and helping them ensure that their power supply remains voltage optimised even when new demands are being placed upon it
  • The power transfer between a microgrid and the utility grid is actively controlled without the need for communication to all microgrid elements
  • Protect the power system from load disturbances by isolating source and load harmonics, transients and
  • voltage sags
  • Enhance the power quality by summarizing the loads to the mains with identical phase current even for unbalanced loads
  • Providing unity power factor with sinusoidal currents under non-linear loads
  • Coordinate fault re-closing over sub-grids zones when connecting to other STs
  • Accept direct connection to future medium voltage DC power transmission, low voltage DC grid, storage systems and renewable energy systems.

Issues solved through ST in distribution system

The ST can also help in overcoming several issues associated with distribution network, which are difficult to deal with conventional off-load and on-load tap changer transformers, some of the attributes of smart transformers are listed below,

  • Reduction in grid losses
  • Improved power quality & supply reliability.

Merits of smart transformer

Smart transformer receives attention by researchers due to the following advantages:

  • ST controls the bidirectional power transfer between utility grid and microgrid
  • ST is able to aggregate information to determine its set point of power exchange
  • ST enables to exploit the microgrid as a controllable entity, because the utility network only needs to communicate to the ST instead of all microgrid elements
  • Sustainable, Reliable, Reduced overall weight & volume.

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