Avoidance of Overloads and
How HVDC and FACTS can improve the System Performance
The development of the electric power industry follows the increase
of the demand on electrical energy closely. Global studies show,
that within the next 20 years the power consumption in developing
and emerging countries is expected to increase to more than 200 %
and in industrialized countries about 40 %. This means that the size
and complexity of electrical networks will increase further.
Therefore, enhancement of the transmission systems will become
simplified understanding of transmission systems,
they can basically be classified according to the structures in Fig.
1. It can be seen, that there are many similarities to
transportation systems. In meshed power systems, problems can occur
with loop-flows and local overloads. Bulk power transmission
requires loss minimization and reserve capacity in case of outages
of such highly loaded power corridors. Due to stability reasons weak
systems must be supported.
Fig. 1: Kinds of Transmission Systems
To enhance such transmission systems, impedance control (series
compensation), parallel compensation or load-flow control can be
introduced. The principle of power transmission is shown in Fig. 2,
which shows the basic equation and the influencing parameters.
Fig. 2: Power Transmission - The basic Equation
Elimination of Bottlenecks
in Transmission – Prevention of Overloads and Loop–Flows
Fig. 3 can be regarded as an “Application Guide” for system
enhancement. Local overloads (shown in red) can be decreased by load
displacement through impedance control in case of parallel line
configuration, ref. to the figure. If an overloaded line is
connected in series to other less loaded lines, a power-flow
controller will be needed to force power management.
Elimination of Bottlenecks in Transmission Systems – Prevention of
Overloads and Loop-Flows
With the installation of new power plants short-circuit currents can
exceed the maximum allowable levels, therefore Short-Circuit Current
Limitation (SCCL, refer to the HVDC / FACTS newsletter of May 2003)
will be essential.
Series compensation is used to improve system stability and to
increase the transmission capacity in radial or bulk power long
distance AC systems. Referring to the equation in Fig. 2 a series
capacitor reduces the line impedance X, hence the power
transmission P will increase.
This principle can also be applied in meshed systems for balancing
the loads on parallel lines. The simplest form of series
compensation is the Fixed Series Compensator (FSC, see Fig. 4) for
reducing the transmission angle, thus providing stability
Fig. 4: Series
Compensation – Thyristor Controlled (TCSC) and Fixed (FSC)
Thyristor Controlled Series Compensation (TCSC) uses a variable
impedance to control the load flow. It can also be applied very
effectively for damping of power oscillations.
weak systems, voltage control by means of parallel compensation is
applied to increase the power quality. Fig. 5a gives examples for
improvement of the voltage profile for different system and load
conditions when using a Static Var Compensator (SVC) for fast
control of shunt connected capacitors and reactors.
Shunt Compensation – SVC: Improvement of Voltage Profile
Shunt compensation can also be employed as a “local” remedy against
voltage collapse which can occur when large induction machines are
connected to the system, see Fig. 5b. After system faults the
machines load the power system heavily with high reactive power
consumption. The remedy for such faults is strong capacitive power
injection, for example by using either an SVC, a STATCOM (Static
Synchronous Compensator) or just mechanically switched capacitors.
However, if the voltage collapse extends over an area which is too
large, only an HVDC can act as a barrier against the spread of it.
Voltage Collapse – Benefits of Shunt Compensation in Emergency
Power Flow Control
gives an example of power-flow control in a meshed system. An HVDC-B2B
(Back to Back) can easily be used as Grid Power Flow Controller. It directs the power flow in the desired direction and
defines the amount of power via its control. Using these means, loop-flows
can be avoided very efficiently. In the example in Fig. 6, the
installation of a new power station (System B, in red), causes the
loop-flow, however through implementation of a power-flow controller, the
initial power flow of 200 MW from System A to B can be restored easily.
Avoidance of Loop-Flows by means of Power-Flow Control
example of a study on the West European UCTE system is shown in Fig. 7. A
total of 500 MW should be transported from Hungary to Slovenia.
It can be seen, that in a system with uncontrolled power flow, the load is
spread widely through the neighboring systems. Only a limited amount of
power is flowing directly to the target location. Using a FACTS
controller, however load flow can be significantly improved, thus
providing a basis for power purchase contracts between the two countries.
Load-flow improvement with FACTS / HVDC
Phase Shifting Transformer versus HVDC and FACTS
Phase-shifting transformers have been developed for transmission system
enhancement for steady state system conditions. The operating principle is
voltage source injection into the line with a series connected
transformer, which is fed by a tapped shunt transformer. It is very
similar to the UPFC (Unified Power Flow Controller), which uses VSC
(Voltage-Sourced Converter)-Power Electronics for coupling of both shunt
and series transformers. Thus, overloading of lines and loop-flows in
Meshed Systems and in parallel line configurations can be eliminated.
However, the speed of phase-shifting transformers when changing the phase
angle of the injected voltage via the taps is very slow. Switching times between 5 and 10 s per tap are common, which may sum up to 1 minute or
more, depending on the number of taps.
A rule of thumb for successful voltage or power flow restoration under
transient system conditions is a response time of approx. 100 ms, this
being with respect to voltage collapse phenomena and “First Swing
Such fast reaction times can easily be achieved when using FACTS or HVDC
controllers. Their response times are fully suitable for fast support
during system recovery. Hence, dynamic load-flow restoration requires the
application of power electronics such as HVDC and FACTS.
In conclusion, phase-shifting transformers and similar devices using
mechanical tap-changers can only be used for very limited tasks with slow
reaction times under steady state system conditions.
blackouts in parts of Europe and the U.S. (Italy, see Fig. 8 and USA, Fig.
9) have demonstrated the necessity for system improvement by means of new
communication technologies, enhanced system control & protection and by
using advanced transmission solutions to avoid overloads, loop-flows,
voltage collapse and system outages.
In various documents published on the WEB (ref. to HVDC / FACTS newsletter
in September 2003), it is reported that the major characteristics of the
blackout in the USA was a voltage collapse which spread through the
synchronous systems in the North-East US and Ontario, Canada.
Quebec Canada was not affected – What are the reasons?
major interconnections to the blacked-out areas are DC-Links. These
DC-Links act like barriers against cascading events such as voltage
collapse and frequency decline. They disconnected the systems at the right
point and at the right time. Hence, a spread of the collapse to Quebec has
been avoided. However, as the system was being restored, the DC links
assisted the process by providing “Power Injection”.
Blackout in Italy - very close to the US Event
of the US Blackout
Italy, the blackout was as large as the one in America. Reasons were trips
on the interconnection lines from Switzerland and France (Fig. 8) in a
high power import demand situation in Italy. “Normal” power import for
Italy is about 5 GW. On September 28, the demand was increased to 6.7 GW
due to power station outages in combination with country-wide wide
celebration activities for the “White Night”. 20 min after the first
inner-Swiss line trip, a second line tripped in Switzerland, and
immediately, a cascade of sequential line-trips occurred on all
interconnections to Italy.
Hence, the “White Night” turned into a “Black Night”. Luckily, this
happened during the weekend. Remedies like increase of generation in Italy
and strengthening the interconnections are under discussion.
FACTS controllers have been developed to improve the performance of
transmission systems. Excellent on-site operating experience is being
reported, and the technology became mature and reliable.
Examples of HVDC and FACTS applications are described and solutions for
load-flow and voltage quality improvement are presented. A brief overview
of the recent, severe Blackout events in United States and in Europe is