Presented at CEPEX99, Poznan
Poland, March 1999
Multiterminal HVDC for High Power Transmission in Europe
by Michael Häusler, ABB Calor Emag Schaltanlagen AG, Mannheim, Germany
A multiterminal Direct Current transmission system (MTDC) is economically competitive and
technically feasible. Existing MTDC schemes with up to four terminals are described, and
parameters important for their energy availability are presented. Based on experience one
can expect that a MTDC also with a higher number of terminals provides access to remote
power sources with the same energy availability as a point-to-point HVDC connection.
Trans European Networks (TEN) are developed to support the economic co-operation
between East and West Europe. Electrical networks as a part of it allow for the competition
between suppliers of electric energy. Consequently the cost for electric energy should
decrease. Electric networks are also able to improve the energy availability at comparatively
small additional cost.
Extensive feasibility studies have been performed for the realisation of the planned High
Power Transmission System between East and West Europe. According to these studies a
multiterminal DC transmission system (MTDC) should be best suited to provide the facilities
for the intended power exchange. Such a MTDC would need less investment cost and less
space than a conventional AC transmission system for the same power. DC is able to adapt
easily to changing network conditions. This facilitates the operation and administration of the
power flow control of the high power transmission system.
Most existing DC schemes have been realised as point-to-point connections. They are
accepted as a means to provide access to remote power sources with the same energy
availability as from local power plants. Their main disadvantage is their limitation to power
exchange between only two partners. Contrary to it a MTDC allows to connect all interested
partners within the reach of the transmission routing.
Because of the lack of experience with multiterminal DC schemes one might be concerned
about its somewhat more complex control in comparison with point-to-point schemes. Some
people even fear that the energy availability could suffer from it. Therefore it is the objective
of this paper to provide inside into the main factors affecting the energy availability of a
MTDC and to estimate the performance of energy availability based on experience with
existing HVDC schemes.
Experience with existing multiterminal HVDC schemes
Since about 10 years several multiterminal HVDC (MTDC) were implemented. They all
started as conventional two terminal systems. Examples are the following schemes :
•The connection Sardinia-Corsica-Italy (SACOI)
•The Pacific Intertie in the US
•The connection Hydro Quebec-New England Hydro from Canada to US.
More information is given in Table 1.
Table 1 Some multiterminal HVDC schemes
Name Year of Power/MW Length/km Configuration
SACOI 1967 200 406 - Two monopolar terminals
1986 50 - Three monopolar terminals in
(200, 50, 200 MW)
Pacific Intertie 1970 1600 1360 - Two bipolar terminals 1600 MW
1984 400 - Upgrade of existing terminals to
USA 2000 MW
1989 1100 - Four bipolar terminals in parallel
Pole 1+2: 2000 MW
Pole 3+4: 1100 MW
Hydro Quebec- 1990 1200 1480 - Two bipolar terminals 1st stage
Hydro 1991 800 - Two bipolar terminals 2nd stage
Canada/USA 1992 250 - Three bipolar terminals in parallel:
Radisson 2250 MW
Nicolet 2138 MW
Sandy Pond 1800 MW
The experience gained with these schemes proves that the energy availability at the delivery
terminals remains in the same order as in the case of point-to-point DC schemes. The major
items affecting the energy availability in MTDC schemes are presented in the following.
Major influences on the energy availability
Energy availability at any receiving terminal of a MTDC is a measure of the amount of energy
that could have been transmitted over the MTDC to this terminal, limited only by forced and
scheduled outages of converter station equipment and DC transmission lines. The influence
of forced outages of these two major components of the transmission system on energy
availability is considered more detailed in the following.
The converter stations of a later MTDC are in the first stage of course identical to those
designed for point-to-point operation. The two major additions for MTDC operation are some
more DC switchgear needed to adapt to the different configurations of a MTDC and the
special control functions for MTDC operation. Such control functions are:
• Balancing of current orders between the different stations so that the sum of inverter
currents always equal the sum of rectifier currents.
• Ramping of DC power between two or more stations on demand.
• Control of the voltage profile on the DC system to maintain efficient operation within the
constraints imposed by equipment ratings.
• Managing of overload for different converters.
• Relocating of power between terminals after a forced outage.
These functions can be accomplished without reducing the energy availability of a terminal.
The Master Control of a point-to-point DC or a MTDC system is normally supported by a
redundant telecommunication system. The telecommunication system is today an integrated
part of the pole control or the Master Control. This concept has proven its high reliability in
operation of both point-to-point DC and MTDC systems.
Both point-to-point DC and MTDC schemes can be controlled and protected completely
without telecommunication (and consequently without Master Control). Loss of
telecommunication does not affect the possibility to schedule power transmission between
available terminals. Power can be ramped and modulated with the same speed as in normal
The control system at each terminal ensures that all available converters recover safely and
fast after the loss of a converter group or after a DC line fault, even with telecommunication
out of service. For this purpose e.g. in the Hydro Quebec-New England Hydro scheme a
special control function, the Decentralised Dynamic Voltage Controlled Recovery (DDVCR)
has been developed. During the recovery the DDVCR at the rectifier(s) “senses” how much
current the remaining inverters can take. The power flow will then be restored to prefault
levels up to the detected limits of the remaining inverters. The DDVCR concept will secure
that the inverter(s) of the MTDC are not overloaded.
Experience with existing HVDC links proves that the contribution of control and protection to
forced outages and energy unavailability is small. It can reasonably be expected that this
relation will persist also with more than four terminals of a HVDC system.
In a MTDC system with parallel connected converters only one inverter station can control
the DC voltage. If additional inverters are on line these will operate in current control mode,
and consequently at a higher extinction angle. In comparison to point-to-point DC systems
additional inverters therefore have a higher control margin and operate with less probability
of commutation failures.
To summarise, the contribution from the control system to the energy unavailability due to
forced outages between any two terminals of an MTDC system remains the same as for a
point-to-point DC system.
Also the DC switchgear for MTDC converter stations is principally the same as in point-to-
point DC connections. The switching sequences are co-ordinated by the controls in a similar
way. People ask about the need for HVDC breakers and its influence on the performance of
a DC scheme. So far in existing DC schemes real HVDC breakers are applied only for
switching from normal bipolar operation to monopolar operation with metallic return and
back. The metallic return breakers (MRTB) have to be designed for voltage stresses at the
neutral point of a bipolar DC scheme. The cost of this equipment in comparison to the
additional gain in flexibility of operation is more than justified. It is of course possible to
design HVDC breakers for the full rated DC voltage. Prototypes for 500 kV rated DC voltage
and 2000 A rated DC current have been successfully tested in the Pacific Intertie scheme.
The increased flexibility and speed of control with such breakers in comparison to switching
sequences based on HVDC disconnectors did, however, so far not justify the increased cost
for it. Therefore, no existing DC scheme uses HVDC breakers for rated DC pole voltage.
Transmission DC overhead lines
The energy availability is affected also by the DC overhead lines. From this point-of-view the
most critical case is a scheme with only one bipolar line which is the normal configuration of
a point-to-point DC system. Since ground return is normally not permitted beyond some
minutes of operation any persistent fault on a DC line would cause a loss of 100 % of the
MTDC systems are less affected by such a fault. Only the energy exchange between the
terminals at both sides of a faulted section will be interrupted. Energy exchange between
terminals connected by healthy lines may continue. In addition MTDC systems are open for
extension to a meshed system which provides the same advantages as known from AC
systems. This means that in a meshed system the outage of any line section will not cause a
reduction of transmission capability.
Expected energy availability of receiving terminals in a MTDC
For the sake of simplicity the reliability structure of a receiving terminal of a MTDC for 100 %
power is represented by the two terminals engaged in a specific power exchange and the DC
line between them (Fig.1). Experience with existing DC schemes is published regularly by
Group 14 of CIGRE. Typical values of non-availability UA and frequency of faults FOR of
their major components can be allocated to the described simplified MTDC system. In order
to respect the worst case of a system with only one single bipolar line the additional DC
switchgear (DC-Sw) to bypass intermediate converter stations is also respected.
From the simplified structure in Fig.1 one concludes that the energy availability for 100%
power of a MTDC terminal connected by a single bipolar line of 1800 km distance to the
sending end and three intermediate terminals reaches due to forced outages in the given
example 98,64 %. The expected rate of forced outages reaches about 13.7 per year. These
figures are similar to those of point-to-point DC schemes.
2 Stations DC-Sw DC-Line
UA ≤ 1.2 % 0.03 % per 0.04 % per
Intermediate station 1000 km
FOR ≤ 6 /a 0.15 /a, intermediate 4 /a, 1000 km
Fig. 1 Simplified structure for the energy availability of a MTDC
It is evident that a meshed DC grid for bulk power transmission offers increased energy
availability. The technique to operate meshed DC grids is available. This feature can be used
to practically eliminate the influence of the DC lines on energy availability. Thus in the given
example the energy availability of 100 % power at a receiving terminal would increase to
about 98.8 % and the rate of forced outages would decrease to about 6 per year.