HVDC Interconnections for Large Power Systems, the Path to the More Robust and Efficient Transmission Network

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HVDC Interconnections for Large Power Systems, the Path to the More
Robust and Efficient Transmission Network

L. BIZUMIC, R. Cherkaoui, O. A. Mousavi,
Ecole Polytechnique Federale De Lausanne
Switzerland
A. ARESTOVA
Power System Emergency Control Laboratory
Russia

SUMMARY

This paper discusses the plausibility of interconnecting multiple large asynchronously
operated power systems by means of HVDC technology. These DC interconnections could be made
not only between previously disconnected systems, but also, large power systems could be split into
smaller asynchronous areas connected by HVDC lines. The reliability and the security of modern
power systems are currently in decline. This claim is supported by the numerous large-scale blackouts
that have occurred in recent years. The frequency and the severity of these cascading outages are
unprecedented. This is due to many factors, but the main reasons are the growing size and complexity
of synchronously operated power systems, rising energy demands, aging equipment, electricity market
deregulation and more complex market operation, etc. The standard approach towards mitigating these
problems is usually to invest in additional resources, transition lines and interconnections.
Unfortunately, these measures also further increase the power system’s size and complexity, so the
overall effect may even be reduced reliability and security. This paper proposes that systems should be
connected exclusively by HVDC links, instead of further increasing the size of synchronous areas. In
this way, the interconnected power systems will reap all the benefits of system interconnections, while
at the same time HVDC links will act as a firewall for disturbance propagation between systems. This
would enable power systems to be operated more efficiently and reliably. To demonstrate the
effectiveness of this approach, a comparison is performed between the response to disturbances of
power systems connected by AC lines, and those connected solely by DC lines. In addition, the
influence of the sizes of the interconnected synchronous systems is analyzed and discussed. In order to
ensure accurate dynamic simulations, realistic network models of the European and Russian power
networks, developed within the scope of the ICOEUR project (Intelligent Coordination of
Operationand Emergency Control of EU and Russian Power Grids), were used in conjunction with
extensively validated software designed for simulating the dynamics of electric power systems.

KEYWORDS

HVDC, POWER SYSTEM DYNAMIC STABILITY, ENSTO-E IPS/UPS, CONTROL
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1. Introduction

Rising energy demand, the aging of the equipment, the growing usage of renewable energy sources,
the deregulation and the increased reliability security requirements of modern power systems call for
smarter operation and more advanced control strategies. This need is confirmed by the numerous
large-scale disturbances which occurred worldwide in recent years. One of the mostly used approaches
towards the mitigation of these problems is to invest additional resources in new devices and
interconnection lines. In order to increase the reliability and efficiency of the Pan-European and
Russian electrical system, it has long been proposed to interconnect the European (ENTSO-E) and
Russian power systems (IPS/UPS) [1]. The European, ENTSO-E system covers 34 European countries
and is supplying electricity to about 450 million people with an annual consumption of approximately
2500 TWh. This system consists of 5 asynchronously interconnected (by HVDC) regional groups,
these areas are: Continental Europe, Nordic area, UK, Ireland and Baltic system. The Russian
IPS/UPS system is the world’s most geographically extended power system, spanning over 8 time
zones. At the moment, with 335 GW of installed capacity IPS/UPS annually supplies about 1200 TWh
to more than 280 million consumers. Since both systems are among the largest synchronously
operated systems in the world, the coupling of these electrical networks would result in a power
system unprecedented in size and complexity, with severe effects on operation and stability. Problems
that could emerge from such coupling are summarized in Fig. 1. [5].

Fig. 1, Problems of large synchronous interconnected systems
As can be seen from the Fig. 1, for every interconnection there is an optimum size of the systems for
which the ratio between benefits and problems caused by such interconnection is optimal. Further
growth of such interconnection results in growing efforts to maintain it, and at some point those efforts
could even surpass all the benefits of coupled systems. The concern is that precisely this could happen
in the case of the synchronous coupling of the European and Russian systems. In order to mitigate
these limitations and problems, the idea of interconnecting these systems by means of HVDC
technology is presented in this paper and compared with the traditional HVAC interconnection.

2. HVDC Technology

HVDC Technology can be efficiently used to solve a large number of problems and technical
challenges. By using the HVDC, it is plausible to avoid some of the limitations of AC systems and to
benefit from various advantages. Some of the most important advantages of HVDC technology are the
ability to transfer bulk power over long distances, to interconnect systems operating at different
frequencies and fast control of power flows. Also, a very important characteristic of the HVDC is that
it acts as the firewall against perturbations, not allowing disturbances to propagate from one system to
another. This way, the number of fault events spread through the system is not anymore in direct
correlation with the size of interconnection.
Today, there are two HVDC technologies. The first one is very mature and has been proven in large
number of projects, HVDC Classic. The newer one, HVDC Voltage Source Converter (HVDS VSC)
represents state of the art transmission technology, offering a variety of advantages over the HVDC
Classic. Unfortunately, although HVDC VSC certainly has important advantages, it is still quite
limited with the power capacities, its power ratings are around 400MW. Since 400MW per line is not
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enough for interconnection proposed here, this paper only takes HVDC Classic links into
consideration. Nevertheless, all results and solutions presented here also hold for VSC technology, and
once the capacity limitations are surpassed, VSC technology will also be attractive technology for EU-
RU interconnection. Even today, in combination with HVDC Classic, VSC could further improve the
performance of the suggested interface.

3. Network modelling

For the purpose of the simulations in this paper, a simplified equivalent system model of the
ENTSO-E and the IPS\UPS power systems was used (Fig. 2). This model of the ENTSO-e –IPS/UPS
super grid, has been defined and tested in the scope of ICOEUR project and it is widely available for
public use [2]. This test case retains the dynamical characteristics of the ENTSO-e and IPS/UPS test
case and at the same time its small scale allows the researchers to focus on emergency problems. The
simulations presented here were performed using the renowned dynamic power system analysis tool,
EUROSTAG together with a newly developed API feature which can be used to create and implement
complex control strategies, which allows the simulation of a wide area monitoring and control system
coupled with the classical electromechanical model of the Power System.

Fig. 2 ENTSO-E – IPS/UPS test power network model
A logical first step towards full systems interconnection would be to use existing interface lines
between these two systems. Table 1 (following mainly [3]) gives an overview of the existing interface
lines between ENTSO-E and IPS/UPS.
Table 1. Existing ENTSO-E IPS/UPS interface lines
ENTSO-E RG CE IPS/UPS
Voltage, kV Length, km Capacity, MVA
Substation Country Substation Country
Rzeszow Poland Khmelnitska NPP Ukraine 750 395 2600
Vel’ke Kapusany Slovakia Mukachevo Ukraine 400 51 900
Sajoszeged Hungary Mukachevo Ukraine 400 142 692
Albertirsa Hungary Zakhidnoukrainska Ukraine 750 479 2600
Kisvarda Hungary Mukachevo Ukraine 220 96 312
Tiszalok Hungary Mukachevo Ukraine 220 54 310
Rosiori Romania Mukachevo Ukraine 400 115 831
Isaccea Romania Pivdennoukrainska Ukraine 750 409 2600
Isaccea Romania Vulkaneshty Moldova 400 59.7 955

These transmission lines were operated as an integrated part of IPS/UPS and power system “Mir” until
1995 when Poland, Hungary, Slovakia and Czech Republic were synchronously interconnected to
UCTE. For a synchronous coupling of ENTSO-E RG CE and IPS/UPS some of these lines need to be
refurbished and partly reconstructed. Based on Table 1 and for the purpose of this paper, interface
lines were first modelled by 5 AC lines (for the first set of simulations) and then those lines were
converted to HVDC lines (for the second set of simulations). Although, it is known that by converting
AC lines to HVDC their power transfer capability can be increased up to twofold, in this paper
additional benefits of capacity increase are not considered, so all HVDC lines are modelled with
precisely the same power ratings as the original AC lines.
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4. Control strategy
As it has been already mentioned, HVDC technology offers a variety of additional features
compared to traditional AC. Here, the ability to quickly control power flows has been used, and a very
simple but effective control strategy is presented. The idea behind this control is based on the fact that
a generator tripping will immediately result in frequency drop in the affected system, and the larger
the loss in generated power, the more severe the frequency fall. At the same time, the generator’s
speed governor would act and generators in the affected system would respond to loss of frequency by
increasing their power outputs, trying to compensate for the loss in generation power and to prevent
further frequency drop. Once the frequency decline is stopped, additional generation should be
activated to return the frequency to its original value. It is important to notice, that in the case of
systems interconnected by AC lines, the affected system is the whole interconnection (frequency is the
same in the whole system), while in a case of DC interconnection only one system containing the
faulted generator is affected. Unfortunately, here one can notice a certain drawback of purely HVDC
interface concept. Since the systems are not synchronously connected, the inertia of the affected
system is smaller than the inertia of one large synchronously interconnected system. This means that
the frequency drop in the affected system would be more severe, and that the system is left alone to
cope with the disturbance. This is not a problem for small disturbances, but for large disturbances (e.g.
power plant loss) it could represent a serious drawback of the HVDC interconnection concept.
The solution for this problem is quite simple. We can imagine the HVDC lines monitoring systems
frequencies at both line ends and adjusting their load flow according to frequency differences.
Basically this would mean that HVDC lines would respond to the generator loss in one system
similarly to the other generators, increasing or decreasing power flow to, or from the affected system.
The regulating power would be drained directly from the adjacent system. Of course, changing power
flows from adjacent system would result in frequency change in that system and generator responses,
very similar like in a case of AC connection. The important difference is that the HVDC lines should
not put the other system in danger and should limit the effect of the disturbance on the other system to
an acceptable level. Also, a control curve could be set such that the HVDC lines do not act at all for
small disturbances, while for more severe events their action would be in direct correlation with
frequency values. If frequency in the affected system approaches a first frequency shading value,
HVDC lines should offer all possible support, increasing their power flows to the maximum levels that
are still safe for the adjacent system.[4]
Thanks to this control, practically all the advantages of both AC and HVDC interconnections are
preserved, the systems operate interconnected and during the disturbances they can help each other,
but the risk of severe disturbances and consequences (e.g. large scale blackout) are limited to one
system only. Also, once the frequencies in both systems are restored to their original values, the power
transferred over HVDC lines would automatically return to its original set point values.
All control is localized in HVDC controllers and since there are no any complicated calculations
involved it is very fast. Thanks to this, there is no need for any new communication lines, complicated
control centers or sensitive network data sharing.
5. Simulations and results
AC lines in place, 500MW generation loss
The first case presented here represents a relatively small disturbance, which frequently occur in
practice. One typical system operating scenario was simulated, 12500MW of active power was
exported from the UPS/IPS system to Europe. At one point (t=10s) a 500MW generator loss in central
Europe was simulated. A very simplified representation of the system and simulated event is shown in
Fig. 3. The results of the simulation are presented in Fig. 4 and 5.

Fig. 3 Simplified system representation
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LINE 5 Unit: MW [sim1] S_OBS_1 + S_OBS_2 + S_OBS_3 + S_OBS_4 + S_OBS_5 + S_OBS_6
Fig 4 Interface load flows Fig 5 Overall power transfer between systems
As can be seen from the Fig 4 and 5, due to the generator outage in Europe, the power flows between
the systems have changed. This small disturbance does not represent a threat to either of the systems,
but since the power transfer between the systems has changed, dispatcher corrective measures are
necessary to return power flows to the agreed (traded) values.

DC lines in place, 500MW generation loss

The scenario is that of the previous simulation. This time however, the interface between the systems
is purely HVDC. The results are presented in Fig. 6 and Fig. 7.
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Fig. 6 Interface load flows Fig7. Overall power transfer between systems
Based on Fig. 6 and Fig 7, it is clear that the disturbance was contained in one system and perturbation
was prevented from spreading to the other system. Since the disturbance was small enough, the HVDC
lines control did not act and the agreed power transfers remained unchanged during the whole
simulation.
AC Lines in place, 4000MW generation loss
In this scenario, starting from the same system state as in previous simulations, a 4000MW generation
loss in central Europe was simulated at t=10s. Results obtained by simulation are presented in Figs. 8-
11.
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Fig. 8 AC interface active power load flows Fig. 9, Overall power transfer between systems
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Fig. 9 Machine angles at interconnected nodes Fig. 10 System frequencies
As can be seen from the results, the 4000MW generation loss in the European system caused interface
load flows to change dramatically. Overall power transfer between the systems was increased by more
than 1500MW. This value is expected and it is completely in accordance with the results of one
previous study were it was concluded that in case of a 3000MW generation loss, almost 1500 MW of
regulating power flow will cross the interface area [1]. This regulating power flow caused the already
heavily loaded Line 1 (Pink) to overload due to an additional 600MVA of flow, and its protection to
act some 43s later at t=53. At that moment, the angles of the machines at interconnected nodes started
to increase, leading to the total breakup of the interface and a loss of synchronism in the system just a
couple of seconds later. This loss of synchronism would have a devastating effect on the European
system and would almost certainly lead to large scale blackouts.
DC Lines in place, 4000MW generation loss

The same scenario is repeated once again, this time with HVDC lines in the interface. The HVDC lines are
controlled as described in section 4 of this paper. The simulation results are presented in Figs 11-13.If the Fig 13
is observed, it can be seen that the frequency in the European part (blue line) started declining immediately after
the generation loss happened. A couple of moments later, once the frequency drop reached a certain point, the
HVDC control kicked in and started increasing power flows on HVDC lines. This increased load flow caused the
frequency in the Russian system to start falling also, but the fall is less severe than for a directly affected system.
Since the power flow through HVDC lines can be precisely controlled, the overloading of Line 1 is prevented
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and this critical situation is successfully brought under control. Once the additional generation resources are
activated, and the frequencies in both systems return to normal, HVDC control will automatically return power
flows to their original values.

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Fig 11. Active power load Fig 12 Power transfer Fig 13. Systems frequencies
flows between systems
Conclusion
Based on the results presented in this paper, it is clear that purely HVDC asynchronous
interconnection of the European and Russian power systems represents a very interesting and effective
solution. All the advantages of the HVDC technologies combined with the proper control strategies
should insure that the coupling of these two systems results in benefits greatly surpassing all the
necessary efforts and problems. Even more, since it has been shown that two large asynchronously
interconnected systems offer better performance and security than one large synchronously operated
power system, a new question emerges: Are the current central European and Russian power systems
of optimum sizes or even they should be split to a couple of smaller asynchronously operated clusters
interconnected by HVDC?
This interesting idea of grid segmentation has already been proposed for the US system [6]. The
large power system could be split to smaller asynchronously interconnected clusters. The control
presented in this paper could then be used to provide support during the disturbances among all
clusters. This way, it should be plausible to significantly improve the reliability and stability of today’s
power systems, blackouts would be limited only to the size of clusters and large scale blackouts
threatening to endanger the whole network would be prevented. This idea becomes even more
interesting and attractive with the development of HVDC technology and renewable and fast changing
energy sources (wind, solar). The important question is, how the borders of these clusters should be
determined and what are the optimum locations for HVDC lines connecting them. These criteria
should not be only technical but also economic, political, geographical and others.
BIBLIOGRAPHY
[1] UCTE/IPS-UPS Study, Synchronous Interconnection of the IPS/UPS with the UCTE, 2008
[2] A. Arestova, U. Häger, A. Grobovoy, et al., “SuperSmart grid for improving system stability at the
example of a possible interconnection of ENTSO-e and IPS/UPS”, PowerTech Trondheim, 2011
[3] WG SYSTINT:“European, CIS and Mediterranean Interconnection: State of play 2004. 2nd
SYSTINT Report”, EURELECTRIC, Brussels, 2005.
[4] Softening the Blow of Disturbances,” H.K. Clark, A.A. Edris, M. El-Gasseir, K. Epp, A. Isaacs,
D.A. Woodford, IEEE Power and Energy Magazine, January/February 2008
[5] W. Breuer, D. Povh, D. Retzmann, E. Teltsch and X. Lei. “Role of HVDC and FACTS in Future
Power Systems”. CEPSI 2004 Shanghai, 2004.
[6] George C. Loehr, “And There Will Be Blackouts...”, American Education Institute
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