Optimal Hybrid Multiplexed AC-DC-AC Power Converters

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OPTIMAL HYBRID MULTIPLEXED

AC-DC-AC POWER CONVERTERS

Matthew Deakin 1*

1Department of Engineering, Newcastle University, Newcastle-upon-Tyne, UK

*email: matthew.deakin@newcastle.ac.uk

Keywords: HYBRID POWER ELECTRONICS, AC-DC-AC CONVERTER, MULTIPORT CONVERTER,

MULTI-TERMINAL SOFT OPEN POINT

Abstract

The flexibility of multi-terminal AC-DC-AC converters connected in distribution networks can be increased by changing the

sizes of the individual AC-DC converter stages and connecting the AC side of those converters to electromechanical switches

(multiplexers) to allow reconfiguration within the network. The combinations of real powers that can be transferred by such a

design can be described using a capability chart. In this work, it is proposed that the area of these capability charts is a meaningful

metric for describing the flexibility of such a device. These capability chart areas are calculated in closed form for a three-

terminal AC-DC-AC device consisting of three AC-DC converters of arbitrary sizes, allowing the optimal AC-DC converter

sizing to be determined to maximise this area. It is shown that this optimal design yields a capability chart area that is 64%

larger than the equivalent area from a conventional equally-sized AC-DC-AC converter. Converters which are optimal in other

senses are discussed, such as a design with 10% increased per-feeder real power transfer, albeit with an 8% area reduction. It is

concluded that the capability chart area is an intuitive and informative approach for describing the increased flexibility of

multiplexed AC-DC-AC converters.

1 Introduction

Electrical distribution systems require new, flexible capacity

to enable consumers to connect low carbon technologies such

as electric vehicles, heat pumps and solar photovoltaics (PV).

One way of increasing this network capacity is through AC-

DC-AC converters (called, amongst others, Soft Open Points,

DC Links, Solid State Transformers, Smart Transformers, etc)

[1]. These solutions can be installed in substations [2], in place

of normally open points [1], or in parallel with switchgear [3].

It is well-known that these solutions are typically more

expensive than conventional approaches of providing network

capacity [2]. As a result, there has been interest in proposing

Hybrid AC-DC-AC solutions that make use of low-cost

electromechanical switches to increase the flexibility of these

designs. For example, the Hybrid Open Point [1] installs AC-

DC-AC power converters in parallel with switchgear to

increase operational and planning flexibility. In the past, cost-

effective hybrid approaches have allowed increased market

share [4].

This work considers the novel Hybrid AC-DC-AC

configuration shown in Figure 1, first introduced in [5] as the

Hybrid Multi-Terminal Soft Open Point (Hybrid MTSOP).

This approach uses multiplexers (‘Feeder Selector Switches’)

to allow any converter to connect to any of the distribution

feeders at a node. It is shown that this enlarges the capability

chart of the device, increasing the power that can be transferred

by 50% and increasing the loss reduction capabilities by 13%.

The approach shows similarities with the ‘MVAC switchyard’

described in [6], which also uses multiplexers to reconfigure

power electronics to improve network capacity. It also shows

some parallels with the hybrid EV charger described in [7],

which also uses a multiplexer (‘relay matrix’) to increase the

flexibility of the outputs of a vehicle-to-grid charger, or the

phase changing soft open point [13]. On the power electronics

side, there is also a large literature on the design of multiport

converters that make efficient use of components (e.g.,

reduced numbers of solid-state switches through interleaving

[8] or developing systematic strategies for designing

topologies with low component count [9]). However, to the

best of the author’s knowledge, there is no explicit

quantification of the area of the increased capability charts that

such a Hybrid Multiplexed AC-DC-AC system can provide.

As capability charts are intuitive and well-known methods of

presenting information about device flexibility, this is a

significant gap.

In this paper, we address this gap by quantifying the area of

the capability chart of a three-terminal Hybrid AC-DC-AC

converter for any given set of three converter sizes. It is shown

that it is feasible to evaluate the capability chart analytically,

allowing the optimal converter design (in terms of maximum

capability chart area) to be determined. A number of further

cases of interest are also described to highlight the properties

of the capability chart areas and more generally properties of

Hybrid Multiplexed AC-DC-AC converters.

The structure of this paper is as follows. In Section 2, we define

the AC-DC-AC capability chart area, to show the sense in

which this metric describes operational flexibility achieved by

designs using the multiplexed approach. In Section 3, we

proceed to calculate these areas analytically for three

combinations of systems, enabling a utility to understand the

potential benefits of the approach quantitively. Finally, in

Section 4 we draw salient conclusions.

(a) Conventional three-

terminal AC-DC-AC

converter

(b) Multiplexed three-terminal

AC-DC-AC converter

Figure 1: as compared to a conventional AC-DC-AC

converter, which typically splits the power ratings equally

between feeders (a), the proposed multiplexed design has

asymmetrically sized converters  connected to feeders

through a bank of multiplexers (b) to increase device

flexibility.

2. AC-DC-AC Capability Chart Areas

The goal of this work is to evaluate the capability charts of

Hybrid Multiplexed AC-DC-AC converters for a three-

terminal case. In this section, the principles of the Hybrid

Multiplexed AC-DC-AC converter are first outlined to give

the reader a clear understanding of how multiplexing AC-DC

power converters increases performance. The device

capability charts are then described, and the Capability Chart

Area defined mathematically to give an unambiguous

description of the proposed metric that will be evaluated.

Finally, necessary preliminaries are presented to give the

reader a fuller understanding the results presented in Section 3.

2.1 Principle of Operation

The proposed Hybrid Multiplexed AC-DC-AC device was

first described in [5], and so only a brief introduction to the

operating principles of the device is given here. A

conventional AC-DC-AC converter design would consist of

three equally sized legs that are hard-wired to the feeder on

which they are connected [10]. This approach has the

advantage of simplicity, with the amount of capacity that can

be drawn from a given feeder being fixed at 1/3 pu.

In contrast, the Hybrid Multiplexed AC-DC-AC converter has

three converters, each with a different converter size, and with

the AC side of the converters connected to feeders through a

multiplexer, as shown in Figure 2. As in the conventional

design, the total per-unit capacity of the AC-DC converters is

1 pu. However, the power that can be transferred by the device

can be increased by 50% to 1/2 pu. Assuming the cost of

devices is proportional to the total power capacity of the AC-

DC converters, and the maximum power transferred is the

limiting factor, the cost can be reduced by 33% [5].

This increase in maximum power transfer can be shown by

considering a system with three AC-DC power converters,

with sizes = (1/2, 2/5, 1/10) pu, as shown in Figure 3. As

the converter with 1/2 pu can be connected to any one of the

three feeders, it can be seen that the power transfer for any

feeder is increased compared to a non-configurable design

with equal sizing. As well as this increase in maximum power

transfer, there are also many combinations of feasible power

transfers – for example, the two smaller converters can be

connected in parallel to one feeder, or to different feeders

(Figure 3(b)).

Figure 2: The three-terminal Multiplexed AC-DC-AC

converter consists of three multiplexers connected to the AC

side output of AC-DC converters to allow reconfiguration.

(a) First configuration

(b) Second configuration

Figure 3: Reconfiguration of power converters enables the

capacity connected to each feeder to be changed depending

on the needs of the network – the first configuration (a)

allows 1/2 pu to be transferred only between Feeder 1 and 2,

whilst the second configuration (b) allows 1/2 pu to be

transferred between Feeder 3 and Feeders 1 and 2.

2.2 Capability Charts and Capability Chart Areas

For the purposes of this work, the combinations of power

transfer that are achievable by the Multiplexed AC-DC-AC

converter are considered a measure of the flexibility of the

device. These combinations can be described via a Capability

Chart, which can be described mathematically as follows. Let

∈{0,1}3 be a vector representing the state of each

multiplexer (as in Figure 2), such that

∑= 1 ,

and let ∈{0,1}3×3 be a matrix concatenating these vectors,

i.e., = [,,]. The capacity connected to each feeder

 ∈3 is therefore

 =

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摘要：

1OPTIMALHYBRIDMULTIPLEXEDAC-DC-ACPOWERCONVERTERSMatthewDeakin1*1DepartmentofEngineering,NewcastleUniversity,Newcastle-upon-Tyne,UK*email:matthew.deakin@newcastle.ac.ukKeywords:HYBRIDPOWERELECTRONICS,AC-DC-ACCONVERTER,MULTIPORTCONVERTER,MULTI-TERMINALSOFTOPENPOINTAbstractTheflexibilityofmulti-termina...

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