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Energize The hybrid transformer: a solution for industrial network power...
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The hybrid transformer: a solution for industrial network power quality

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Industrial power quality (PQ) solutions generally consist of discrete devices to deal with individual problems. If more than one problem arises, more than one device is required.

Voltage fluctuations and other disturbances are becoming more severe on the grid with the increasing amount of new load such as electric vehicle charging, automated machinery and embedded renewable energy systems. Power quality problems experienced at the grid connection point of industrial systems include current harmonics, low power factor, resonance and high-frequency interference. Problems experienced at the load side include low power factor, voltage harmonics, voltage sag and spikes, and conducted interference.

PQ problems have an effect on the performance of machinery and other items such as control systems and scada/PLC devices. With the growth in artificial intelligence (AI) and the increased use of robotics in industry, PQ will become more and more important, and the need for integrated solutions that handle all possible problems will become greater.

Fig. 1: Example of common HT configuration.

To control the voltage level, conventional distribution transformers (DTs) are complemented by various automatic voltage regulator (AVR) devices, starting from electromechanical servomechanisms, through transformer tap changers (TCs), to solid-state transformers (SSTs) [1-4]. However this AVR technology does not keep up with the present grid voltage regulation requirements, especially in terms of dynamics and accuracy. This is the fundamental reason behind the growth in the application of power electronics-based devices [2].

Most existing solutions such as statcom, DVR and SVC systems tend to deal with one problem only, such as harmonics, and treat grid and load problems separately. Static shunt compensators suffer from source voltage oscillations while static series compensators cannot operate in nonlinear load condition correctly [1].

One of the possible solutions that have been extensively investigated is the solid-state transformer (SST) or electronic transformer, where the output voltage can be completely controlled by electronics to deal with all PQ issues. Although this device satisfies all the requirements, it has proven to be far too expensive to implement at present and the concept of a hybrid transformer has gained popularity.

The hybrid transformer

A competitive alternative to AVR solutions is the hybrid  transformer (HT), combining the features of a conventional transformer and the regulatory capabilities of power converters. In this case, the converter, enabling smooth and instantaneous voltage adjustment, is designed to convert only a fraction of the transferred power. What is more, in the event of converter failure, the energy can still be transferred to the load so that reliability of the power supply is ensured. The use of HT systems, combining the high efficiency and low cost of distribution transformers with the functionality of power converters, fully meets the requirements of modern AVR, such as:

  • Fast voltage correction (within 20 ms).
  • Precise output voltage regulation as a percentage of the rated supply voltage.
  • High efficiency, reliability and safety during faults.
  • Low impedance to mitigate the effect of load changes.
  • Continuous power supply during switch operations.

The hybrid transformer incorporates the power electronics into a secondary winding of the transformer. Incoming and outgoing quality issues are handled at the point of connection (POC) of the network to the grid.

Fig. 2: Possible configurations of HTs [3].
The HT is realised by adding a “fractionally rated” power electronic converter to a regular transformer, which provides the transformer with additional control capabilities at a much lower cost than the SST. The power electronic converter enables the control of different parameters and is rated at only a fraction of the transformed power. If only±10% voltage regulation is considered, the power electronics converter can be rated at only a fraction (typically around 10 to 20%) of the transformer’s rating. This reduces the cost and complexity of the converter significantly, and it maintains high efficiency. For example, for a 13,8 kV system, if a full rating converter has to be employed (2-level inverter/rectifier) as in an SST, the peak voltage that would be seen by the IGBTs would be around 19 kV, but if the converter is rated only 10% peak voltage, the blocking voltage is reduced to less than 2 kV, making the converter design simpler, less expensive and more compact [2].

The HT concept can provide dynamic AC voltage regulation, reactive power compensation, as well as in future designs form an interface with energy storage devices. Other potential functionalities that can be realised from the HT include voltage phase angle control, harmonic compensation and sag compensation [2].

Principles and configuration

The hybrid transformer consists of a standard low-frequency transformer and a “fractionally rated” power electronic converter.

Converters

The power converter may be placed in either the primary or secondary circuit of the distribution transformer, but most configurations place the converter in the secondary, as shown in Fig. 1.

The fractionally rated power electronic converter allows the HT to achieve various control objectives depending on the configuration and the operational mode. Several configurations are possible in addition to that shown in Fig. 1. Fig. 2  shows the high-level schematics of some of the possible configurations using voltage source converters.

There are two ways of influencing voltage and current of a distribution transformer (DT)  with a converter. On the one hand, a current can be injected on the high-voltage (HV) or low-voltage (LV) side by using a shunt connected converter as shown in Fig. 2 (configuration A). On the other hand, a voltage can be added to the HV or LV side voltage with a series connected converter (configuration B). By combining both shunt and series connected converter, basic configuration C is obtained.

Depending on the configuration, control of voltage, active and reactive power as well as active filtering and 3-phase load balancing are possible. Compared to FACTS and custom power devices, cost and volume of the HT are reduced due to the integration into one single smart asset. In practice, the principle is realised in a 3-phase configuration which allows additional functionality such as correcting voltage imbalance between phases.

Fig. 3: Basic configuration of the matrix converter [6].
One of the advantages of the configurations shown is that the energy required for applying the controls is obtained from the additional secondary winding and not from a stored energy source, as would be the case with statcom, SVC and DVR systems.

Since no active power can be applied by the converters with the concepts A and B, a change in voltage and current results in a change of the power factor. This drawback is eliminated in basic configuration C where almost arbitrary voltage and current vectors can be generated. In this version the shunt and the series converter are connected back-to-back to the same DC-link. Since the voltage added in series is only a fraction of that of the LV grid, an adaption of the converter input and output voltages is necessary to obtain a reasonable usage of the semiconductor blocking voltage. This can be done with the help of an additional transformer, a tertiary winding, or both.

Converter technology

The converter used must provide both buck (decrease) and boost (increase) functions to the output voltage, although the mitigation of voltage sags and dips is the main function.

Two technologies exist for use with the converter: AC/DC/AC voltage source converter with a DC-link capacitor, and the emerging matrix converter or matrix reactance chopper.

AC/DC/AC converter with voltage source inverter

This consists of three sections: rectifier, DC link storage and inverter. The rectifier converts the AC from the tertiary winding into DC which is fed to the DC link. The inverter converts the DC from the DC link into AC of the required voltage and phase. The DC link capacitor provides energy storage and regulates the output voltage, allowing the unit to respond fairly rapidly.

Matrix converter (MC) or Matrix reactance chopper (MRC)

This is a direct AC to AC converter with the ability to control voltage and phase angle. The MC is a power device without a DC energy storage element. The DC energy storage element used in classical converters with a voltage source inverter (VSI) is the main factor increasing size, weight and cost of the converter. This energy storage in the form of electrolytic capacitors is the most frequently damaged element in operational service. Elimination of the DC energy storage increases the reliability of the converter. The basic configuration is shown in Fig. 3.

Fig. 4: Example of a HT with an MC or matrix chopper, installed at connection terminals of an industrial plant, a building, or selected industrial loads [6].
Every input phase is connected to every output phase by a solid-state switching device, and the output voltage, frequency and phase are determined by the sequence in which the switches are closed and opened. Switches are operated by control circuits at high frequencies while input and output filters help shape the waveform. An example of an HT using matrix converters is shown in Fig. 4.

Functions in an industrial application

The HT in an industrial application has the advantage that the control is applied directly at the customer premises and not the remote transformer. One of the basic functionalities of the hybrid transformer is its voltage regulation capability. The IGBT-based power electronic converter of the hybrid transformer enables it to achieve sub-cycle response and, as a result, it can act both as a continuous and transient voltage regulator. The hybrid transformer can therefore be used to replace both a tap changing transformer as well as a dynamic voltage restorer.

Dynamic voltage regulation is one of the most important capabilities of the hybrid transformer. By controlling the output voltage magnitude, hybrid transformers can be used to perform conservation voltage regulation (CVR). Multiple independent studies have shown that CVR can result in significant energy savings.

References

[1] M Rycroft: “Addressing industrial power quality problems”, Energize, July 2018.
[2] S Bala, et al: “Hybrid distribution transformer: concept development and field demonstration”, IEEE, 2012.
[3] J Burkard and J Biela: “Evaluation of topologies and optimal design of a hybrid distribution transformer”, EPE 2015.
[4] R Strzelecki: “Hybrid stepless distribution transformer with four-quadrant AC/DC/AC converter at low voltage side – simulation tests”, Przeglad Elektrotechniczny, 94, 2018.
[5] P Szcneśniak: “A modelling of AC voltage stabiliser based on a hybrid transformer with matrix converter”, Archives of electrical engineering, Vol. 66, 2017.
[6] P Szcneśniak: “Challenges and design requirements for industrial applications of AC/AC power converters without DC-link”, Energies, April 2019.

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