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B4-204
CIGRE 2012
Requirements of DC-DC Converters to facilitate large DC Grids
C.D. BARKER, C.C. DAVIDSON, D.R. TRAINER, R.S. WHITEHOUSE,
Alstom Grid UK Ltd
United Kingdom
SUMMARY
Two DC grids may be constructed independently. As these DC grids are independently
developed, the operating DC voltage, the operating strategy or even the DC topology may be
different and hence, these two DC grids cannot be directly connected. Just as AC systems
operating at different voltages require transformers to exchange power, DC grids require DCDC converters to exchange power between networks.
A group of offshore windfarms may use a high voltage DC, multi-terminal, bulk power
transfer scheme to transfer the power to the onshore AC system. At a future stage a new
offshore windfarm may be constructed sufficiently remote from the existing scheme or the
mainland to merit a DC transmission connection; but at a lower voltage rating than the
existing scheme. An economic method of interconnection may, therefore, be the use of an
AC-DC converter at the windfarm and a DC-DC converter local to the existing offshore
converter in order to use the existing DC grid to bring the power onshore.
Many turbines actually generate an AC output from an AC-DC-AC converter. By removing
the DC-AC stage within the wind turbine a medium voltage DC collector bus could be used
as a common power interconnection between the DC outputs from multiple wind turbines. A
DC-DC converter could then be used to raise the DC voltage to a higher voltage for
transmission to an onshore DC to AC converter.
For many years the possibility has existed of utilising HVDC to supply multiple small loads
dispersed over a large geographical area. As each load may only be at a relatively low
power the cost of a 3-phase DC-AC high voltage converter may be prohibitive; but a low cost
DC-DC converter could be used to reduce the DC bus voltage at each load centre.
There are, today, many existing Line Commutated Converter (LCC) HVDC transmission
schemes in operation. By using DC to DC converters it may be possible to interconnect
these existing DC schemes with new VSC HVDC converter multi-terminal grids increasing
the utilisation of existing grids.
KEYWORDS
Voltage-Sourced, Converter, HVDC, DC-DC.
carl.barker@alstom.com
1.
INTRODUCTION
The applications for DC-DC converters within HVDC Grids are beginning to emerge as the concept of
DC-DC grids gets closer to being a reality with the increased availability of Voltage Source
Converters (VSC). The VSC can exchange power between different nodes within a network by
reversal of DC current instead of DC voltage, making the power flow between terminals very much
more flexible than can be achieved with Line Commutated Converters (LCC).
VSC technology also opens up the opportunities for novel circuit topologies and some of these can be
applied to converters designed to connect DC systems together.
There are many reasons for interconnecting two or more DC circuits and these different reasons result
in different requirements. In order to classify the general parameters of a DC-DC converter the
following nomenclature has been adopted in this paper:
High Power
Medium Power
Low Power
50 MWdc
0 MWdc
≤
≤
Vdc HIGH VOLTAGE SIDE
Low Ratio
Medium Ratio
Transferred DC Power ≥
Transferred DC Power ≤
Transferred DC Power ≤
Vdc LOWER VOLTAGE SIDE
1.5
High Ratio
≤
Vdc HIGH VOLTAGE SIDE
Vdc LOWER VOLTAGE SIDE
Vdc HIGH VOLTAGE SIDE
Vdc LOWER VOLTAGE SIDE
500 MWdc
500 MWdc
50 MWdc
≤
1.5
≤
5
≥
5
Each of the above categories has their own specific drivers in terms of capital cost, losses, volume etc.
Therefore, there is not, necessarily one DC-DC converter topology that is suitable for all applications.
Instead, it is envisaged in this paper that there will be many alternative topologies based on common
building blocks.
2.
LOSSES AND RATINGS
If losses are considered, then it can be assumed that for both ‘high power’ and ‘medium power’ loss
capitalisation will be a significant factor in the design of the DC-DC conversion process. For ‘low
power’ applications however, losses may be less significant since the whole reason for using DC may
be that it is the only practical solution to an electrical energy transfer problem and, whilst losses may
be higher than with other converter applications, as a percentage, the loss may not be as significant.
For high power and medium power converters the component ratings will be significantly affected by
the characteristics of the through-put power. Where the power is transferred between DC circuits in a
discontinuous manner, for example as pulses of electrical energy transferred between passive
components, then the converters must be rated for the maximum instantaneous value of the throughput voltage and current. This can lead to very high component ratings, particularly in terms of valve
voltage rating which strongly impacts on the cost of the converter. It is better, therefore, to operate the
converter in a manner which provides continuous power flow between the two DC circuits.
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Figure 1:
A Simple Buck/Boost Converter
As an example consider the simple Buck/Boost
converter shown in Figure 1 [1]. Assume that
Vd1 is larger than Vd2. When switch S1 is
closed diode D1 is reverse biased and hence
energy is transferred from DC System 1 to the
inductor L1 where it is stored as magnetic
energy. When switch S1 then opens the diode
D1 will be forward biased as energy will transfer
from inductor L1 to Capacitor C1 where it will
be stored as electrostatic energy. Energy can
then be transferred from the capacitor C1 to DC
System 2. This operation results in a pulse of
energy being drawn from DC System 1 which
would require significant DC harmonic filtering
to smooth. The load current as seen by the
components S1, D1, L1 and C1 will have a
greater value than the actual transferred DC
current requiring these components to be rated
for a higher current than the load current.
The voltage rating of the circuit is the difference between +Vd1 and –Vd2 hence the components are
rated for a voltage greater than the DC voltage of the system to which they are coupled. In addition
the operation of this circuit relies on the electromagnetic and electrostatic storage of energy
necessitating large component values. The size of the energy storage component values can, to some
extent, be mitigated by increasing the switching frequency of switch S1. However, doing this will
increase the converter switching losses. An optimum value for component ratings and losses can be
found but the ratings of the individual components within the circuit must always exceed the steadystate load. By using a circuit topology which can transfer energy in a continuous fashion from DC
System 1 to DC System 2 and vice versa, the need for rating components for a voltage and current
greater than the steady-state load value can be mitigated and hence the rating and losses of the
converter can be reduced.
The concern with regards to component ratings may not be as significant when considering low power
conversion. In this case, as in the case of losses, the cost of the conversion may not be as significant
as the requirement to actually deliver the power. As losses can be considered as less of a design factor
alternative converter topologies may be more appropriate as they can be provided for a lower capital
cost, despite the increase in component ratings.
3.
GALVANIC SEPARATION AND FAULT BLOCKING
From [2] ‘Galvanic Separation’ is defined as “prevention of electric conduction between two electric
circuits intended to exchange power and/or signals”. In the context of DC-DC converters this
definition is taken to mean that there is no direct current path between side A and side B of the DC-DC
converter. A simple example of such a connection with Galvanic Separation would be the use of a
coupling transformer between the two DC circuits, Figure 2(a).
Consider the simple DC-DC converter shown in Figure 2(b). A pole-to-ground fault in one
symmetrical monopole will result in the voltage on the healthy pole ‘jumping’ up in voltage to a
prospective value of 2 pu. With a direct current path connection, this DC voltage shift can be
transferred across to the other DC system, that is, a pole-to-ground fault on one DC system can
propagate to another DC system via a DC-DC converter.
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With very fast control response it may
be possible to inhibit the DC pole-toground fault transfer between DC
systems so long as the power
electronic valves associated with the
(a)
part of the converter connected to the
unfaulted system are rated for the
maximum voltage difference. This
voltage difference may be the
determining factor in the rating of the
valve as it may be higher than the
normal working voltage stress across
(b)
the converter. Where the converter
Figure 2:
A Simple DC-DC Converter
ratio is ‘low’ the voltage transfer may
(a) with Galvanic Separation
not be excessive and hence this
(b) without Galvanic Separation
method may provide an optimum
solution. However, with ‘medium’ or
‘high’ voltage ratios the use of Galvanic separation provides the facility of transferring the voltage
stress to another component, typically an inter-connection transformer.
Fault blocking capability within a DC-DC converter stops a fault on one DC system propagating to the
other DC system, that is, it acts as a firewall. Consider Figure 2, a DC-DC short circuit will not
cascade through the DC-DC converter if the AC link voltage collapses.
4.
HIGH POWER, LOW RATIO CONNECTIONS
In AC systems a common means of achieving a voltage change is to use an auto-transformer. The
auto-transformer does not offer galvanic separation between the two AC voltage levels and is best
suited to low ratio applications; “In most cases an auto-transformer is not a particularly favourable
proposition for voltage transformation ratios above 2” [3]. An equivalent in a DC grid would be the
interconnection of two DC systems operating at a similar voltage and using the same topology (for
example, symmetrical monopole or bipole).
+Vd1
+Vd2
-Vd1
-Vd2
Figure 3: A High power, low Ratio DC-DC Converter
One possible implementation of the basic converter for this application, based on the “Alternate Arm
Converter” [4], [5] is shown in Figure 3 and converts DC-AC-DC. Equally, the now well established
Modular Multi-Level Converter topology, such as the ALSTOM Grid MaxSine generation 1, could be
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used. The two DC-AC converters are 3-phase in order to provide a power through-put capability
equivalent to any AC system connection converter that forms part of the DC grid. If higher levels of
power transfer were required a multi-phase AC connection between the two systems could be utilised.
The form of the AC connection shown in Figure 3 need not be at a frequency such as 50 Hz or 60 Hz.
Higher frequencies, such as 500 Hz, are proposed in order to minimise the component ratings of the
individual cells within the converter for example increasing the AC frequency allows the capacitor
value (mF) to be reduced whilst maintaining the same power electronics module. In addition the AC
connection need not be sinusoidal. Alternative waveshapes such as a square-wave or trapezoidal can
be used, along with the converter to optimise both component ratings and converter losses.
In Figure 3 each valve in each of the two three phase converter bridges shown consists of a ‘Director
Switch’ in series with a series connection of ‘Full-Bridge’ sub-module. The Full-Bridge modules are
rated for the DC voltage of the associated DC system whilst the director switch is rated for the DC
voltage of the associated DC system plus an additional offset voltage which could result from a poleto-ground fault in the DC system. Hence, in this configuration fault propagation between DC
networks can be blocked without having Galvanic separation.
Where the two DC schemes are of different topologies, for example one DC system is a ±320 kV
symmetrical monopole whilst the other is a +500 kV monopole (possibly part of a bipole scheme) the
shift in ground reference would give rise to a circulating DC component of current between the two
DC systems via the ‘AC’ connection, making this arrangement unsuitable for such an application.
The ‘AC’ system in Figure 3 is common to both DC/AC converters and hence the AC line-to-line
voltage and phase current is common. Where both DC System 1 and DC System 2 are of a similar
voltage magnitude the AC quantities can be optimised to minimise the converter losses. The losses of
this arrangement would be approximately 75% and the cost approximately 80% when compared to a
fundamental frequency back-to-back. However, as the DC voltage difference increases the selection
of AC quantities will be less optimised for one, or both, of the DC/AC converters. Considering this,
plus the need for additional valve rating to avoid transferring earth shifts this circuit is considered best
suited to ‘Low’ DC voltage ratio applications.
5.
MEDIUM POWER, MEDIUM RATIO CONNECTIONS
Power transfer requirements between two DC systems may not always require a multi-phase AC
coupling in which case a single phase interconnection can be used. This can be particularly useful
where the DC voltage ratio across the converter is ‘medium’ or ‘high’ and an transformer coupling
between the AC connections of the DC/AC converters is to be used, see Figure 4.
Transformer couplings, as shown in Figure 4, can also be used to provide Galvanic Separation
between the two DC systems. In the case shown, if the two DC systems are both symmetrical
monopoles the connection transformer will experience the DC voltage stress resulting from a pole-toground fault across the windings, not transferring the DC voltage step across the electromagnetic
transformer coupling. Also, where one DC system is a symmetrical monopole whilst the other is a
bipole, as in the example discussed in 4 above, the electromagnetic transformer coupling will block
any DC current flow across the ‘AC’ connection. In both cases the connection transformer can
experience a high level of DC voltage on the windings and hence this connection transformer must
take this DC voltage stress into account in the insulation design.
As with the direct AC coupled converters discussed in 4, the ‘AC’ connection does not have to be at
50 Hz or 60 Hz nor does it have to be sinusoidal. Various optimisations can be made between the
converter and the connection transformer in order to optimise the overall design. In this case however,
the advantages of operating at higher frequencies must also consider the impact on the design of the
connection transformer. With increasing frequency the transformer core size can be reduced, thereby,
reducing cost and volume (which could be a significant factor if the equipment is to be located
5
offshore on a platform). However, without introducing special materials into the transformer core
design the reduction in volume is non-linear, having a reducing impact with increasing frequency.
The losses of an arrangement such as that shown in Figure 4 using an AC connection frequency of
500Hz would be approximately 105% and the cost approximately 75% when compared to a
fundamental frequency back-to-back.
Figure 4: A Medium power, medium Ratio DC-DC Converter
6.
CONNECTING A VSC AND LCC INTERCONNECTION
To date the majority of HVDC in operation is point-to-point interconnection using Line Commutated
Converter (LCC) technology. Such DC interconnections have provided and continue to provide the
most cost effective means for bulk power transmission. However, with large, bulk power transmission
systems there may be applications where there is an advantage in connecting a VSC with an LCC
HVDC connection.
+Vd2
VSC - DC System 2
LCC - DC System 1
+Vd1
-Vd2
Figure 5: An LCC/VSC DC-DC Converter
A major difference between an LCC and a VSC is that in a VSC connection the DC polarity always
remains the same, irrespective of power transmission direction whilst an LCC scheme reverses the
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polarity of the DC voltage to reverse power transmission direction. Any DC-DC converter which
interconnects these two systems must, therefore, be able to exchange power between the two DC
systems whilst simultaneously maintaining the DC voltage polarity on one side of the converter and
allowing the voltage on the other DC terminal to reverse. Such a converter is shown in Figure 5.
In Figure 5 the VSC side of the DC-DC converter consists of a three phase converter with each valve
in the converter comprising a Full-bridge converter rated at Vd2 voltage in series with a Director
switch rated at Vd2. As the Director switches are unidirectional this converter can only withstand one
DC voltage polarity. On the LCC connected side of the DC-DC converter each converter valve is
composed entirely of Full-bridge sub-modules. As the full bridge sub-module can connect the submodule capacitor in either polarity energy can be transferred from the LCC DC system to the AC
connection within the DC-DC converter irrespective of the polarity of the DC.
Figure 5 shows the AC connection within the DC-DC converter as a 3-phase connection but, as with
the previous cases considered, a multi-phase AC connection can be used for high power connections
whilst a single-phase converter can be used for medium power transmission levels. Also, the AC
connection may be at a higher frequency than 50/60 Hz and non-sinusoidal. Considering a 500Hz
connection the losses and cost would be comparable to a fundamental frequency back-to-back
converter connection of the same power rating but the footprint (and hence offshore platform
requirements) would be reduced, reducing the overall cost of the scheme.
7.
DC CONNECTION OF OFFSHORE WINDFARMS
There are many applications being discussed for DC interconnections associated with offshore DC
windfarms. The majority of applications presently under review consist of a point-to-point DC
connection from onshore to an offshore platform and then to an offshore AC collector grid which
connects the individual wind turbines through a distribution system. The wind turbines themselves,
based on present technology, will probably consist of an AC generator connected to an AC-DC-AC
converter within the wind turbine nacelle. Hence, the conversion process from generation to load is
AC-DC-AC-DC-AC. The offshore AC distribution grid which is the middle ‘AC’ within this
conversion chain actually consists of many medium voltage three-phase AC cables interconnecting
windfarm clusters. Because of the high capacitance within the cables, line-end reactors are normally
included on the offshore platforms in order to improve the voltage profile under light load conditions.
These line-end reactors take up space on the platform as do the three-phases of equipment associated
with each connection. More importantly, each of the DC-AC converters will, to some extent,
introduce AC harmonics into the AC systems. With different converters (those associated with wind
turbines and those associated with the power transmission) the harmonic profile can be quite complex.
Taking this into account along with the large amount of capacitance and inductance in the offshore AC
grid and a corresponding lack of damping because of the absence of any significant loads offshore
leads to a situation where the offshore AC system is likely to suffer from resonance-associated
problems and the risk of overvoltage and consequential tripping. An alternative, which attempts to
overcome the difficulty associated with the offshore AC grid is to only utilise DC offshore.
Figure 6 shows an example of a possible future windfarm DC connection that does not need an
offshore AC grid and thereby eliminates the problems associated with such a system. In Figure 6 the
wind turbine generators generate DC at their terminals. These wind turbines are collected into small
‘clusters’ and these clusters are connected to the medium voltage terminals of a DC-DC converter. In
order to minimise capital cost and to minimise footprint on an offshore platform, the medium voltage
converter consists of a full-bridge phase arm and a capacitor phase arm. Where the medium voltage
power is high, exceeding the current rating of the VSC sub-modules it is possible to parallel up the
full-bridge arm. This converter is connected to a single-phase step-up transformer to a single-phase
thyristor bridge. Thyristors were selected for this stage as they have a significantly higher power
density compared to present day VSC technology and the technology is well proven. Whilst on the
medium-voltage side the converters can be connected in parallel the higher voltage thyristor
converters are series connected. In this way a group of wind farm clusters can be arranged in order to
7
give a high voltage DC connection without any individual DC-DC converter having to produce a very
large voltage ratio.
Individual series connected thyristor bridges in Figure 6 can be taken out of service by bypassing the
converter, thereby reducing the overall DC operating voltage of the interconnection. As the onshore
converter is constructed from full-bridge VSC sub-modules the converter can continue to operate with
a reduced DC voltage compared to the peak converter AC voltage. The method of bypassing and reconnected series connected thyristor bridges is a well known technique going back to the early
mercury-arc converters [6], hence, an out of service thyristor converter can readily be brought back
into service following an outage. The land based converter is able to operate with bypassed thyristor
bridges and hence lower DC voltages, as it is a full-bridge converter which is able to operate at lower
DC voltages.
A difficulty when designing DC offshore windfarm connections is that when the wind isn’t blowing it
is necessary to supply the auxiliary loads associated with the wind turbines from the onshore AC
system. Typically the load of the wind turbine, which is mostly composed of protection and auxiliary
loads, is only a small percentage of the power that the wind turbine is designed to generate. The
required power flow is therefore asymmetric. It may be possible to supply this auxiliary load from a
separate supply, possibly AC, run in parallel with the high power DC connection. Alternatively, in the
arrangement shown in Figure 6 it would be possible to supply a small windfarm load associated with
one or more windfarm clusters by reversing the power flow through the DC-DC converters, operating
the thyristor bridges as inverters. As losses will be of less significance in this operating mode and
considering that the actual power being supplied onshore to offshore is relatively small the thyristor
bridge can be operated with a relatively high extinction angle in order to mitigate the risk of
commutation failures.
AC System
The overall losses of a scheme such as that shown in Figure 6 compared to an offshore AC grid with a
single point-to-point VSC HVDC connection are expected to reduce by in excess of 10%. However,
the greater benefit may come from an improved stability of the offshore grid.
Figure 6: An Offshore Windfarm DC Grid with Integrated DC-DC Converters
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8.
REMOTE LOAD INTERCONNECTION VIA MULTI-TERMINAL HVDC
There are certain locations in the world where the communities are widely dispersed and each
community may only contain a relatively small number of people. From an electrical perspective this
means long distance transmission of relatively low power, from a few kW to a few MW. Such power
levels are difficult to transmit efficiently over long distances using AC because of the regulation
effects on the transmission line. Local generation can, and frequently is, used for such small
communities but the cost of the fuel can be very high when both the cost of the fuel and the cost of
fuel transport are considered. However, it may be possible to cost effectively transmit this power
using DC.
Consider a scenario where there are ten communities spread along a several hundred mile corridor.
Each communities load may vary from, say, nothing to 2 MW, hence the total power to be transmitted
varies from 0 MW to 20 MW. Because of the long distances over which the power is to be transmitted
the DC current needs to be kept as low as possible. By using a reasonably high DC voltage the DC
current can be kept low, so if we assumed a DC voltage at the sending converter of -100 kVdc the
maximum load current would be 200 Adc. The DC transmission could be done as a two wire
transmission circuit, with one wire at -100 kVdc and one at nominally 0 kVdc. Alternatively, the
connection could be made with a single wire at -100 kVdc and using the earth as a return path.
The cost inhibitor for such a long distance,
medium power HVDC scheme has, to date, been
the cost of each of the receiving converters, as
each one, in the scenario outlined above, must
be rated for -100 kVdc but only 20 Adc.
However, with imaginative use of the now
established VSC building blocks, alternative
converter arrangements can be envisaged which
can overcome the high capital cost obstacle.
The power transmitted is ‘Low’ hence, as
discussed in 2 above the capitalised cost of
losses can be neglected.
Figure 7 gives a proposal for a local community
DC-DC converter. The idea of using a DC-DC
converter is two fold. Firstly, the -100 kVdc
stress is taken, in this circuit, by a single
converter arm, reducing the DC voltage to a
lower level where a DC-AC converter can be
connected to supply the local community.
Studies have shown that typically the total
number of power transistors and capacitors
reduce to less than 30% of a full voltage 3-phase
DC-AC converter.
Figure 7: A Low Power, High Ratio DC-DC
Converter
The converter shown in Figure 7 is known as a
“Cascade Converter”. Each of the resonant
circuit blocks are tuned to the same frequency.
At each half cycle of this resonant frequency
charge is transferred down the stack, from one
resonant circuit to the next until, at the bottom
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stage of the converter where energy can be extracted. The operation is similar to that of the
Cockcroft-Walton generator [7], except that, in this instance power flow is generally from highvoltage to low-voltage.
9.
CONCLUSION
The paper has introduced a number of potential future developments and applications of power
electronics to both realise future DC Grids and to facilitate better power transmission.
BIBLIOGRAPHY
[1]
[2]
[3]
[4]
[5]
[6]
[7]
N. Mohan, T.M. Underland, W.P. Robbins, “Power Electronics: Converters, Applications and
Design”, John Wiley & Sons, 1989, p81
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=151-12-26
A.C. Franklin, D.P. Franklin, “J&P Transformer Book, 11th Edition”, Butterworths, 1983, p162.
D.R.Trainer, C.C.Davidson, C.D.M.Oates, N.M.MacLeod, R.W.Crookes, D.R.Critchley, “A
New Hybrid Voltage-Sourced Converter for HVDC”, CIGRÉ session 2010, paper B4-111.
M.M.C.Merlin., T.C.Green., P.D.Mitcheson, D.R. Trainer, D.R.Critchley, R.W.Crookes, “A
New hybrid Multi-Level Voltage-Source Converter with DC Fault Blocking Capability”, IET
9th International Conference on AC and DC Power Transmission, London, October 2010.
T.C.J. Cogle, “The Nelson River Project”, Electrical Review, 23 November 1973.
http://en.wikipedia.org/wiki/Cockcroft_Walton
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