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Avoidance of Overloads and Loop-Flows
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 essential. For 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.

 

Fig. 3: 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

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 enhancement.
 

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.


Parallel Compensation

In 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.

Fig. 5a: 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.
 

Fig. 5b: Voltage Collapse – Benefits of Shunt Compensation in Emergency Situations

 

Power Flow Control

Fig. 6 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.

 

Fig. 6: Avoidance of Loop-Flows by means of Power-Flow Control

 

An 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.

 

Fig. 7: UCTE: 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 Stability” requirements.

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

Recent 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?

Quebec’s 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”.
 

Fig. 8: Blackout in Italy - very close to the US Event Fig. 9: The area of the US Blackout
            (modified satellite photo)


In 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.
 

Conclusions

HVDC and 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 given.