Series compensation goes the distance

1 June 2001

In places like South America series compensation is proving a very cost-effective way of increasing capacity and maintaining stability in long distance transmission corridors - avoiding the expense of new lines. Rolf Grünbaum, ABB Power Systems, Västerås, Sweden and José Luis Piñeiro, Transener, Neuquén, Argentina

Interconnecting of power systems is becoming increasingly widespread as part of power exchange between countries as well as between regions within countries in many parts of the world. UCTE (Union for the Co-ordination of Transmission of Electricity), which represents the interests of transmission system operators in 16 European countries, is a good example. Similarly, there is continuously growing co-operation between UCTE and NORDEL (the organisation for Nordic power co-operation), on AC as well as DC. Also, there are numerous examples of interconnection of remotely separated regions within one country. Examples include projects in Argentina, Brazil, Canada, the Nordic countries, and Russia.

Political detente, increasing economic co-operation between regions of the world, and, last but not least, growing deregulation and privatisation of the electricity supply industry, will all contribute to continued furthering of interconnection of power systems. For transmission of large amounts of electric power, AC is the established as well as the most cost effective option for the overwhelming majority of instances. Where transmission is over long distances, special care has to be taken to safeguard synchronism as well as to maintain stable system voltages over the interconnection, particularly when there are faults.

With series compensation, the distances over which AC power transmission is viable are becoming very large. Series compensated AC power corridors transmitting bulk power over distances of more than 1000 km are already a reality. For example, in Argentina, the latest phase of a bulk power interconnection project has been completed recently, providing economic transmission of low cost, environmentally friendly hydro power from the interior of the country to the power hungry Buenos Aires area. With the distances and power amounts involved, series compensation has been a prerequisite from the start.

Experience with series capacitors in operation has demonstrated the validity of the concept. It has been shown that in comparison with alternatives such as building of additional lines, series compensation has proved both a quicker and more cost-effective way of achieving an increase of power transmission capacity or an increase of dynamic stability in power transmission corridors.

For example, consider the choice between a series compensated 500 kV twin circuit transmission system and an uncompensated 500 kV three circuit system for the same power transmission capacity. The investment cost of the compensated alternative will typically amount to only 2/3 of the cost of the uncompensated case. What is more, the cost of the series capacitors themselves will only represent some 5-10 per cent of the total investment.

Increasing transmission capability

Series compensation reduces transmission reactances at power frequency, which brings a number of benefits for the user of the grid, all contributing to an increase in the power transmission capability of new as well as existing transmission lines. The impact of series compensation on power transmission capability is shown in Figure 1. Here, the quantity k is the degree of compensation of the series compensated system, equal to the ratio between the capacitive reactance of the series capacitor (XC) and the inductive reactance of the transmission line (XL). c is the angular difference between the end voltages of the line. In power transmission applications, the degree of compensation is usually chosen somewhere in the range 0.3 <= k <= 0.7.

For a fixed angular difference, the active power transmission capability of the line increases as the degree of compensation increases. Vice versa, for a fixed amount of power transmission over the line, the angular difference decreases as k increases, which is a measure of increased dynamic stability of the transmission system.

Series compensation: some basic mechanisms

Series compensation has been used for many years, with excellent results in AC power transmission in a number of countries. The usefulness of the concept can be demonstrated by means of well-known expressions relating to active power transfer and voltage (using the notation of Figure 1):

P = U1U2sinc/X (1)

U = f(P,Q) (2)

Here, U1 and U2 denote the voltages at either end of the interconnection, whereas c denotes the angular difference of the said voltages. X is the reactance of the transmission circuit, while P and Q denote the active and reactive power flow.

From Equation 1 it is evident that the flow of active power can be increased by decreasing the effective series reactance of the line. In other words, if a reactance of opposite sign is introduced in the denominator, a corresponding increase in power transmission is achievable without having to increase the angular separation of the end voltages, ie with the angular stability of the link unimpeded. Similarly, it is demonstrated that by introducing a capacitive reactance in the denominator of Equation 1, it is possible to achieve a decrease of the angular separation with power transmission capability unaffected, ie an increase in the angular stability of the link.

The influencing of transmission reactance by means of series compensation also opens up the possibility of optimising load sharing between parallel circuits, again allowing an increase in overall power transmission capacity (Figure 2). This is so because the introduction of series compensation enables a shifting of power from the most heavily loaded branch to the less heavily loaded one, thus making room for more power transmission overall. Another valuable feature is that active losses associated with power transmission can be decreased.

Voltage improvement

From Equation 2 it can be seen that the voltage of a transmission circuit depends on the flow of active power as well as reactive power. The reactive power contribution from a capacitive element in series with the line acts to improve the reactive power balance of the circuit, and thereby to bring about a stabilisation of the transmission voltage. This reactive power is instantaneous and self-regulatory in nature, ie it increases when the line load increases, and vice versa. It consequently contributes to voltage stability in a truly dynamic fashion. This makes series compensation a highly effective means for maintaining or even increasing voltage stability in a heavily loaded transmission circuit. And likewise, it allows additional power transmission over the circuit without upsetting voltage stability.

The impact of series compensation on the voltage stability of a transmission circuit is illustrated in Figure 3.

To summarise, series compensation of power transmission systems brings several important benefits:

• an increase of active power transmission capability over the circuit without violating the angular or voltage stability;

• an increase of angular and voltage stability without derating the power transmission capacity;

• a decrease of transmission losses in many cases.

Safeguarding availability

As already noted, for a certain power transmission level, there will be a certain angular separation, c. This separation, for the sake of system stability, should never be allowed to grow too much. At the same time, as demonstrated previously, series compensation has a beneficial influence on the angular separation at any given power transmission level. In a dynamic sense, this means that series compensation increases the system stability in the way that it keeps the system in synchronism for more or less severe disturbances, where, if uncompensated, the system would have been lost. We call this an increase of angular, or transient, stability of the system. With improved angular stability, availability of the power system is increased in the sense that it gets less disposed towards falling out of synchronism as a consequence of faults caused for instance by strokes of lightning.

Transient stability is intimately linked to the inertia of the power system contained in its swing masses located in turbines and generators in power plants. Once disturbed, these masses will move in relation to each other in an oscillatory way, thereby, in the worst case, jeopardising the synchronism of the power system. Here, series compensation will act to improve things, thereby safeguarding system stability and availability.

In Figure 4, the power transfer, P, is shown as a function of angular displacement, c, for two cases: with no compensation, and with series compensation. At a certain moment, a fault occurs in the system, during which the transmitted electrical power P becomes zero, while the mechanical input power to the generator remains constant, Pm. As a consequence, the sending-end generator accelerates until the fault has been cleared and the line reclosed. During this process, the angular difference of the system will have increased from its steady-state value c1 to a higher value, c2. After reclosure of the system, the transmitted power exceeds the mechanical input power and the sending-end machine decelerates. Since there is a difference in angular velocity between the machine rotor and the system, the angular difference continues to grow until the excess energy built up in the rotor has been discharged into the system.

The critical condition for post-fault system stability is that the angular displacement does not exceed ccrit, because if it does, the system cannot get back to equilibrium, and synchronism is lost. We can find the location of the maximum angular displacement, c3, in both cases shown in Figure 4 by recognising that the areas A1, As1 represent the accelerating energy and areas A2, As2 the decelerating energy during and after the fault, and letting these areas be equal. The “stability margin” in each case will then be given by the area blocked in between Pm and P(c) in the interval to the right of c3, cs3. From the graph, it is obvious that the series capacitor improves the “stability margin”. By how much will depend of the degree of compensation in each case.

Design issues

Of course, a series capacitor is not just a capacitor in series with the line. For proper functioning, series compensation requires control, protection and supervision to enable it to perform as an integrated part of a power system. Also, since the series capacitor is working at the same voltage level as the rest of the system, it needs to be fully insulated to ground.

The main circuit diagram of a state of the art series capacitor is shown in Figure 5. The main protective device is a varistor, usually of the ZnO type, limiting the voltage across the capacitor to safe values when faults give rise to large short circuit currents through the line.

A spark gap is used in many cases, to enable bypass of the series capacitor in situations where the varistor is not sufficient to absorb the excess current during a fault sequence.

Finally, a circuit breaker is incorporated in the scheme to enable the switching in and out of the series capacitor as needed. It is also needed for extinguishing of the spark gap, or, in the absence of a spark gap, for bypassing of the varistor in conjunction with faults close to the series capacitor.

In designing of the series capacitor, one needs to distinguish between the required behaviour of the series capacitor in the event of internal faults as well as external faults (Figure 6). Internal faults are malfunctions of the power system in the same section of the transmission line as the one containing the series capacitor. External faults are those located outside the segment of the line which contains the series capacitor.

For external faults, in practically all cases it is required that the series capacitor is not bypassed in conjunction with the fault, ie the series capacitor has to stay in the circuit during and after the fault sequence in the system. This means that during the fault, the series capacitor has to endure that part of the fault current which flows through it, without any damage to the series capacitor. To enable this, the ZnO varistor protecting the series capacitor must have sufficient thermal capacity to withstand the heating caused by the fault current for as long as it flows through the circuit.

After the fault has been cleared, the voltage across the varistor drops, and the varistor stops conducting. This means that the series capacitor is instantaneously and automatically going back into operation. The inherent speed of this procedure is crucial to system behaviour after the fault. As mentioned already, fast reinsertion of the series capacitor is often a prerequisite for post-fault stability of the system, and, as a matter of fact, may be one of the key reasons for having series compensation in the scheme in the first place.

For internal faults, the requirements on the series capacitor are somewhat different. For internal faults, when the faulty line section is taken out by opening of the line breakers, the series capacitor is taken out with it. Consequently, it is not required to carry fault current once the line section has been disconnected. Before that can happen, however, it must still carry the fault current. The tripping of the faulted line will take some 50-100 ms to execute. For that period, the series capacitor must endure the full fault current without suffering any damage. This is again where the ZnO varistor comes in. Only this time, the spark gap placed in parallel with the varistor will usually be allowed to operate and thus take over the fault current from the varistor after a certain time, normally less than 1 ms. By this means, the varistor rating can be lowered, very significantly.

In cases of three-phase faults, a relatively rare event in transmission systems, this may be the specified sequence of events for the series capacitor. For single-phase faults, which are much more common, it may in fact be specified that the spark gap does not operate. Then the rating of the ZnO varistor has to take this into consideration.

If the spark gap has indeed operated, the next thing which happens is that the bypass breaker of the series capacitor is closed, whereupon the series capacitor is bypassed altogether. After clearing of the fault, the bypass breaker is reopened, and the series capacitor is ready for service again once the line is reinserted into operation.

Gapless schemes (Figure 5, right hand digram) are also frequently chosen. Such schemes rely exclusively on ZnO varistors for overvoltage protection, plus the bypass breaker for bypassing of the series capacitor. This kind of scheme may be the optimum solution in cases where the fault level of the power transmission system is limited at the location of the series capacitor, and consequently, no excessive amounts of energy need to be dissipated by the varistor for any foreseeable fault situation.

The Argentinian case

Striking examples of power system growth can currently be found in Latin America. As economies develop all over the continent, electric power demand is naturally growing as well. Large amounts of electricity need to be transported over vast distances, between regions of countries as well as between countries. Here, series compensation is the natural option and the technology is being used ever more extensively.

At the end of 1999, four series capacitors rated together at 430 Mvar were commissioned together with the 4th line of Transener´s 500 kV AC power transmission corridor, completing the latest phase of a power transmission project enabling vast amounts of hydro power to be transmitted from the Comahue region in southwestern Argentina to the large consumer areas around Buenos Aires, a distance of more than 1000 km (Figure 7). The corridor has four parallel 500 kV AC lines, all series compensated. With the 4th, compact type (Figure 8), series compensated line in operation, the overall power transmission capacity of the system has reached 4600 MW.

All in all, there are ten series capacitors in operation in the power corridor, rated together at close to 2400 Mvar at 500 kV (Figure 9). The purpose of the series capacitors is to enable an increase of the power transmission capability of the corridor.

Without series capacitors, several additional 500 kV lines would have had to be added to enable stable transmission of the same amount of power. It is not difficult to imagine how much more that would have cost, in time, money, and environmental impact. In fact, it would not have been a feasible proposition.

The benefits of series compensation as an alternative to building new lines were established from the very start, where an evaluation was made between two series compensated 500 kV lines and three uncompensated lines, for transmission of the initial 1650 MW of power (Figure 10). The series compensated alternative came out as considerably less costly, and with this, the series capacitor option was firmly established for the continued development of the power corridor.

The evolution of the power transmission corridor over the years can be summarised as follows in the fact file below...

The ratings of individual series capacitors range from less than 100 Mvar to close to 700 Mvar.

Future additional extension of series compensation in the power corridor has been prepared for, to achieve a power transmission capacity for the four lines totalling 5200 MW.

The ten existing series capacitors are all protected against faults by ZnO varistors.

The series capacitor control and protection system is microprocessor based and uses optical current transducers for current measurement with optical fibres for signal transmission (Figure 11). This system offers several benefits:

• no relay protection equipment is located on the EHV platforms;

• no auxiliary power is required on the platforms;

• the optical current transducers are powered solely by means of light generated at ground level.

Furthermore, maintenance costs are reduced by continuous self monitoring using a microprocessor based system, allowing increased maintenance intervals.

The control and protection system supervises all functions of the series capacitors and provides protective action in the event of faults such as capacitor overload or unbalance, flashover to platform, or varistor overload.

To accomplish this, several quantities are measured by means of digital optical current transformers (DOCT):

• line current;

• capacitor unbalance current;

• spark gap current;

• varistor current;

• platform flashover current.

The DOCT consists of a current transducer in the high voltage current busbar and an optical interface module in the control room. In the transducer, the current is sampled and converted to a digital value. This value is transmitted in the fibre system to the interface. The converter circuit in the transducer is optically powered by light sent from the interface to the transducer in the same fibre which transmits the measured value. The transducer is operated at a sampling rate of 2000 Hz, using low powering levels for the converter circuit. This allows for low power laser diodes to be used as the supply power source.

... Evolution of power transmission corridor


Technical data for series capacitors in the Transener power corridor

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