TECHNO TRENDS – 07

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       The HVDC transmission systems have led the way in providing assistance from
a source other than a generator. This transmission made a modest beginning in 1954.
But, since 1972 when the Nelson river transmission entered service in Canada. The
D.C. System controls have been used to modulate the D.C. power to help stabilize the
power frequency at either end of the link and dampen the power oscillations between
weakly connected areas of the A.C. System.

       In India D.C. connections between separate electrical areas have been built to
encourage energy trading. The systems they interconnect are relatively weak. This
paper explains the effective use of HVDC over A.C., frequency and system damping
assistance and stability control through HVDC links.


    HVDC

    Power Modulation

    Power System stability

       The meshing level between transmission networks has become in the recent
years more and more tight, due to the general trend of the utilities and system operators
to exploit the networks as close as possible to their maximum transmission capacity and
to the liberalization of the electrical industry, which is a well assessed or rapidly on
going process in many countries. In particular the application of High Voltage D.C.
links and compensated A.C. lines is becoming a frequent transmission design choice,
especially in case of very long interconnections, often combined with the adoption of
flexible A.C. transmission systems.

       In this growing interconnection scenario and in presence of high levels of power
exchanges, some network operation problems may occur: increased active and reactive
power losses, higher risks of voltage instability or collapse, stronger requirements for
the load demands following and higher criticism associated with possible grid
contingencies which typically results the most crucial and important.

       At this concern the simultaneous presence of A.C. and D.C. interconnections
may create un expected and additional contingencies, with respect to the A.C. line
faults or trips, due for instance to the D.C. line faults or communication failures at the
inverter side, with following HVDC link temporary stops.

       In all the above depicted scenarios, power modulation controls through HVDC
links is a well accepted technique to enhance A.C. systems performance. For a long
time, such a technique has been used essentially to increase the power transfer
capability of an hybrid A.C./D.C. power system and in particular to provide stability
benefits to the A.C. system. Today the practical demands of power systems operation
require the HVDC controls to be more able to regulate power flows, under adversity of
operating conditions. At the same time HVDC technology (conventional or light)
allows these systems to actively participate in the active/reactive power rescheduling.

       The D.C. current transmission offers asynchronous transfer of electrical poser.
The transfer can be between independent systems or points with in a system. Though
the treatment of these two cases will be different the essential contributions that comes
with a D.C. link are:
 Power     flow    can     be   controlled
   precisely and very rapidly. In either
   direction, not only a wheel energy
   but also to help the control of either
   or both A.C. systems. By controlling
   its power transfer, the D.C. can help
   the system operator to dictate the
   power flows in the adjacent A.C.
 By rapidly changing its power transfer, it can improve A.C. system stability. It can
   modulate its through put power to damp out past disturbance swings.
 Unlike asynchronous connection, D.C. transmits no fault current and buffers one
   network from faults in the other. As long as there is an A.C. Voltage, the D.C.
   terminal will operate.
 It can act as a voltage regulator by switching its reactive power banks or by
   adjusting control angles to absorb more or less reactive power. In this way it can
   exercise Mvar exchange with in specified limits.


       HVDC links are typically operated according to their basic regulation modes,
like individual phase rather than equidistant firing control, current control rather than
voltage control mode, rectifier injection rather than inverter extinction angle control,
power flow control etc. Some HVDC links are also provided with special dynamic
controls, capable of responding to any deviation from the normal operating condition in
the A.C. or D.C. systems, in such a way to not remain passive to special needs of the
interconnected A.C. system.

       Some times these special dynamic controls are exclusively adopted for facing
un- wanted dynamic interactions between the A.C. and D.C. systems, which may
manifest themselves in a variety of voltage, harmonic and power instabilities,
essentially related to the strengths of the A.C. and D.C. systems expressed by the short
circuit capacity. Alternatively such A.C./D.C. interactions can be usefully exploited by
using of the fast converters adoptability to achieve an over all more stable operation of
the power system under control.
         More generally a considerable flexibility degree exists in respect to the levels of
power that can be inter changed between A.C. systems interconnected through HVDC
links. This property, already fully exploited in practical applications, is used to provide
various types of D.C. power control, according to the operational needs briefly describe

Network Frequency Control:

         The frequency of a small network interconnected to a larger one by an HVDC
transmission link can be quickly controlled by means of an additional frequency feed
back loop, typically acting on the D.C. link controls, such that the small network draws
the required power change from the larger one.

         An example of frequency control is represented by the SACOI link where at the
time of the original design, both the Sardinian and Corsican networks had the power
rating and droop of the D.C. line comparable with or greater than the rating of the
running generators in the A.C. system to which the line was connected: then the line
terminal could share in the frequent regulation or even perform it unaided. Conversely,
the GRITA link inter connecting the Southern Italian and Western Greek networks uses
a combination of control modes, such that the control signal applied to the current
controller is normally power flow, and it does so as long as the frequency remains with
in predetermined limits. Out side these limits, frequency control takes over to assist the
islanded network in emergency; in these operating conditions, if the minimum
transmission power is approached, the frequency controller automatically starts a
coordinated fast inversion maneuver.

Small Signal Stabilization:

         In tightly connected systems dynamic instability is rare and electro mechanical
swings are usually well damped for their characteristic frequencies; however, in case of
long and relatively weak interconnections between large systems, low frequency swing
modes may result. An Italian example of these modes has appeared in the frame work
of the planning studies related to the new A.C. line interconnection between Sardinia
and Corsica, in parallel to the existing D.C. link; another example in Italy could be
represented by the A.C. cable interconnection between Sicily and Italy. In a worst case
scenario, the response of the power system controls to the synchronizing swings
associated with these low-frequency modes can produce sufficient negative damping to
cancel the usual positive damping of the power system, so causing the occurrence of
oscillations with increasing amplitudes. For damping these oscillations a control system
to modulate the HVDC link power flow could be developed: a typical solution may be
based on the small signal modulation of the D.C. power, in proportion to the frequency
difference across the A.C. systems or to the power flow between themselves.

Large Signal Stabilization:

       While the small signal modulation above discussed is suitable to maintain the
state of equilibrium, it results often inadequate for the damping of very large
disturbances: in this case large signal stability control is thus needed to regain the state
following large disturbances, forcing for instance predefined higher steps in current
order or angle set point.

       On this concern, with the increased power ratings of recent HVDC transmission
schemes and with the introduction of VSC technology it is becoming more frequent to
assess the effect of alternative converter control strategies on the transient stability
levels of the interconnected A.C. systems. The thyristor valves used in HVDC
transmission are in fact rate to with stand considerable over loads, without adverse
effects to avoid unnecessary protective action, and this capability provides the basis for
the first peak transient stability improvement. Each particular emergency strategy, i.e.
current increase or decrease, temporary power reversal, etc., will use special purpose
controllers, responding to power frequency and even absolute phase changes and giving
the possibility to provide D.C. power burst to reduce first swing stability peaks.

       Further more in case of A.C. system interconnection through a tie line, a
disturbance occurring on one system normally causes the trip of the line to prevent the
disturbance itself affecting the other system, but at the same time the system in
difficulty losses an essential in-feed. An HVDC link instead, even if equipped only
with its basic controls, is capable to shield one system from disturbances on the other.
In addition, although the specified power flow can continue, it is possible to vary the
D.C. power setting to help the system in difficulty to the extent which the healthy
system can allow, with out putting itself in difficulty, and subject to the rating of the
link. An appropriate control is therefore capable to propagate a disturbance originated
in one system with a pre-determined attenuation to the other system, without requiring
any particular overload capability as to get through the first swing, exclusively by
timely D.C. power set point reductions.

Damping of Sub-Synchronous Resonance:

       The interaction between torsional oscillation modes of turbine generators and
system oscillation modes of the electric power transmission system, by a synchronizing
component with a relatively small negative damping, due to the resistance of the A.C.
transmission line. In addition the damping windings and the mechanical effects
resulting from stream flow, friction, etc. contribute to provide a relatively large positive
damping to the torsional modes of vibration of the turbine generator shafts, which are
therefore normally stable when connected to an A.C. transmission system. In presence
of an HVDC system, its equivalent apparent loads changes slightly depending on the
absence of control or more commonly in practice on the presence of the current control
loop. The potential destabilization of torsional oscillations due to HVDC systems is
similar to that caused by series compensated A.C. transmission lines and is commonly
referred to as sub synchronous Resonance. The dynamic interaction with D.C. system
can be solved relatively simply by providing D.C. power modulation control to cancel
the negative damping impact of the basic constant power control loop.

A.     Indian example:

       In India there are five separate regions, with a disparity of resource and demand
and a wide variation of operating frequency and voltage on a day-to-day basis. To
synchronize any two was difficult for two reasons: the need to maintain stability and
the organization necessary to schedule power exchange. The needs of resource
allocation have however demanded an ability to transfer power between them.
       System studies were carried out to evaluate the viability of interconnecting two
Indian regions using A.C. and D.C. Each is geographically large and with a substantial
demand of approximately 19,000 Mw at peak times. Each of the regions is a relatively
weak network even at 400 kv or 220 kv and consequently inter-regional
synchronization is not readily achieved.

       The study primarily investigated the post fault transient stability of two
alternative types of interconnection (A.C. and HVDC) between the regions, namely:

 A double circuit, 400 kv A.C. transmission line of nominal capacity 500 Mw,
   assuming that matching of the two system frequencies and voltage levels is
 A double circuit 400 KV AC transmission line of nominal capacity 500 Mw, with a
   500 Mw HVDC back-to-back link connected at one end.

       Operation of the system was simulated under peak load flow conditions with
transfer through the inter-regional tie. With the regions interconnected, three phase
faults of different intensities were applied at representative locations around the
networks. The variations with time of generator rotor angles and terminal voltages were
examined together with the power flows in the interconnection.
B.      Canadian Example:

        To give an example, over 60% of the load of the province of Manitoba in
Canada is supplied by the Nelson river HVDC project which brings hydro-generated
power 900 km to the load center at Winnipeg. The project consists of two bipoles. It
supplies four A.C. systems which, though synchronous, tend to oscillate at a natural
frequency of 0.5 to 1 Hz. If the tie line to one of the four systems opens, a power
imbalance will result. The HVDC controls respond to other the DC power flow by the
same amount as the tie line was previously carrying, so that the disturbance is cancelled

               A further control of tie-line power oscillations is necessary. It responds
to changes in the AC bus bar voltage angle, a convenient measure of the change in tie-
line flows within the critical frequency of oscillation, and alters DC power flow,
bringing substantial damping improvement to the operation of the network.
Alternatively the modulation of power is possible in response to the frequency of one or
               A major benefit of HVDC transmission compared to AC is the inherent
robustness of the inter connection in the face of difficult A.C. system conditions and its
ability to isolate an A.C. system from the worst effects of a transient disturbance in an
adjoining network. An HVDC link will continue to regulate power even under the
conditions of varying A.C. voltage, frequency and phase angle. The ability of the
HVDC to cope with the varied AC conditions can even permit the interconnection to be
made with DC when it would be impossible with A.C.

              Fig. 5, 6 and 7 show results of a three phase fault applied to one circuit
of an AC transmission line in region B. If there is no AC or DC interconnection, the
system on its own can recover from this fault. In contrast, Fig.5 shows that, when the
regions are connected with an AC tie, the tie will be lost with continuously increasing
oscillation of both terminal voltage and line power flow in Region B. The AC line
power is measured in one of the circuits that supplies the link.

                However, the results for the same study with an HVDC back-to-back
link in series with the AC interconnection can be seen, in Fig. 6, to be stable with the
voltages and power flow returning to new stable values with the voltages and power
flows returning to new stable valves with decreasing oscillations.          With the AC
interconnection the effect of faults was also transferred to region A resulting in similar
unstable oscillation in region A as shown in Fig.5. However, when the interconnection
include back-to-back DC link there is little disturbance in region A as shown in Fig.7.
The DC link has buffered the disturbance in Region B from region A and ensured
stability of the whole system.

                These studies results match the performance of the real systems in which
it has been found that even when conditions have permitted the interconnection using
an AC time, the system will become unstable, but with the HVDC tie, operation is

Additional Control benefits of HVDC:

        Power flows can be controlled through an HVDC link in response to control
inputs and these can be programmed to act for the overall benefit of the AC system or
systems. In addition to power oscillation damping or power modulation already
described, it is possible to apply rapid changes in the level of power transfer in response
to changed system conditions. Such applications are specific to the system conditions
and must be evaluated for each case. One strategy that has been successfully applied is
to reduce the power through the DC link in response to dropping AC voltage in the
supplying system. The principle is to relieve a system under stress by reducing some of
its load, a strategy less disruptive than load shedding, since no customer is interrupted.
There will however be a reduction of power supplied to the receiving system, which
must then draw on its reserves during the disturbance.
       Finally the power exchange can be adjusted over a slightly longer time scale to
provide frequency control, provided that the change in power flow that will assist one
system can readily be supplied by the other.

       Dynamic over voltages can arise on one side of a DC link when power flow is
interrupted by a voltage collapse on the other side. An HVDC link can be utilized to
control voltage through its inherently variable power factor. The valve firing process is
maintained in effect developing a DC by-pass on the faulted side, the current through
the by-pass being controlled to regulate the voltage on the healthy side.

       As a regular feature, steady state assistance can be provided to either system
because there is reactive power available at the converter. By advancing the firing angle
the converter will absorb more reactive, after which filter switching may be

       Power electronics have proved a powerful force in contributing to power system
stability. Damping signals can modulate the power transferred by a HCDC link to
stabilize oscillations where methods based on generators control reach.

       By avoiding the synchronization of two systems, a DC link transfers power but
offers a barrier to the transfer of faults. The paper has demonstrated that instability
occurring when two regional systems are synchronized can be solved by the insertion
of a back-to-back DC link. A substantial array of control options becomes available
when such a link is introduced. Both the power passed through can be adjusted to
relieve a system under stress and the reactive power can be made sensitive to the
system’s transient requirements of voltage control.

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