Principle of elementary saturated reactor Elements of saturated Flux, current, and voltage waveforms Variation of fundamental voltage with

A variety of saturating-iron devices have been used for voltage rent (assumed At least four basic principles have been applied, but only one h

been developed for transmission-system applications, the remainder

ing been employed only in smaller sizes and at lower voltages. This i is the saturable magnetic core, whose idealized magnetization so-called polyphase, harmonic-compensated, self-saturating reactor. ristic is shown in terms of flux and current (Figure closely related to, and was developed from, the phase-multiplying type me that a sinusoidal current flows in the winding. Then the flux and frequency multiplier, of which many different versions were develop ge waveforms are as shown in Figure 36b. The flux

both in the United States and in Europe since about 1912. form is approximately square, the flux alternating between the Other classes of saturated-reactor voltage stabilizer include the levels The voltage is almost a series of impulses, but roresonant transformer, the transductor, and the ta se the flux waveform is nearly independent of the current the

compensator. The ferroresonant con tal component of the voltage is constant. In practice the

voltage transformer is manufactured in the United States only in characteristic is not perfectly flat, but is nearly linear above with sizes. The transductor has dc control windings and works as an adjust e proportional to the permeability of free space. The result is a susceptance. Its speed of response compares unfavorably with other typ characteristic of the form shown in Figure of compensator. The compensator a small positive slope. The constant fundamental voltage property essentially single-phase, and has a series as well as a shunt element. It ows directly from the saturation transitions of the core, each of which therefore suitable only for load compensation small arc furna uces a fixed voltage impulse (volt-seconds) in the winding, no matter and cannot be connected to a power-system for voltage stab rapidly the transitions take place. The fundamental voltage lags

nd the current and reactive power is absorbed.

he plain saturated reactor just described is unsuitable for use in 4.4.1. Principles of Operation _{nsmission systems because the voltage, or the current, or both, are too} orted. In transmission-system compensators the harmonics are re- The principle of the saturated reactor is shown in Figure 36. (It _{ed to an extremely low level by internal compensation. The} interesting to compare this with Figure for the The controll'

216 Principles of Static Compensators 4.4. Saturated-Reactor niques are in principle the same as used in phase-multiplying transfo Phase

ers or magnetic frequency multipliers, and a simple explanation is gi in terms of Figure 37, which shows a three-phase saturated reactor ha an additional winding in the form of a closed delta. In the absence of secondary winding, the primary currents are highly nonsinusoidal. H

ever, if the secondary winding is closed, by three-phase symmetry it be found that most of the necessary triplen harmonic components of magnetomotive forces in each limb can be provided by the current. The primary current is free of these harmonics. The secon currents are predominantly and the circuit is thus an mentary frequency-tripler, which shows the generic relationship this type of voltage stabilizer and the magnetic frequency multiplier. frequency multipliers the high-frequency power is transferred to a loa ther by direct series insertion in the secondary circuit, or by

coupling.)

The primary currents, under balanced conditions, contain no lower than 5th and 7th. It was found in the 1930s that by in reactor of suitable value in the secondary circuit, even these harmoni could be reduced. This phenomenon of partial harmonic compensati

has been exploited particularly Friedlander. reactor. Arrangement

The harmonic performance and the characteristics (part of the winding is omitted for the plain frequency tripler are not good enough for application in pow

systems, and much better characteristics are obtained with frequency m _{ppropriate design of an inductive load in the 9th harmonic mesh} of up to nine times, such as for example the treble-tripler react _{these can be reduced to around by partial harmonic}

This shares with the plain tripler the open-mesh secondary wind' _{Plain shunt capacitors can normally be used to absorb these residual} which carries predominantly 9th harmonic, but in order to generate _onics.

from a balanced three-phase system a phase-multiplying arrangeme necessary, as shown in Figure 38. There are nine limbs in the mag

core, and in common with other magnetic frequency multipliers only 4.4.2. Characteristics is unsaturated at a time. Each limb saturates alternately in both

tions, giving a total of 18 unsaturations per cycle. In the terminol e voltagelcurrent characteristic of the saturated reactor by itself is the TCR, this corresponds to 18-pulse operation and helps to wn in Figure 39. There is a slope of between 5 and (based on high speed of response of the reactor by itself. The lowest-order cha reactor rating), which depends on the reactor design and in particular teristic harmonics of the treble-tripler reactor are the 17th and the after-saturation inductance of the windings. A lower slope

FIGURE 37. Elementary frequency ler having approximately constant volt characteristic.

FIGURE 39.

Principles of Static

tance requires a more expensive and larger reactor design. The ergization is by direct closure of the compensator circuit breaker, teristic is very linear above about 10% of rated current. ugh the shunt capacitors need not be energized simultaneously. A lower slope reactance is sometimes obtained by connecting a cap sses are comparable to those of similarly rated transformers, and their tor in series with the saturated reactor. This slope-correcting capac iation with current is not unlike that of the

can be sized to make the slope zero or even negative, but typical slo te high near the reactor because of high-frequency magnetostrictive are the same as would be specified for a TCR. The effect of the slo ces, and a brick enclosure is sometimes used. Reliability is comparable correcting capacitor is shown in Figure 40. h that of similarly rated transformers.

Just as in the TCR compensator, the voltage-stabilized operation

be biased into the leading power-factor region by means of shunt _{4.5. SUMMARY}

tors. These may be designed as filters if the system resonances require

although this is usually not necessary for the characteristic harmonics able 3 summarizes the comparative merits of the main types of compen-

the reactor. tor. Note that some specialized types are omitted. For example, the

A step-down transformer is normally used for EHV connectio _{yristor-controlled transformer has many properties in common with the} because it is uneconomic or impractical to design the reactor for dir yristor-controlled reactor. The dc controlled transductor is probably not connection above about 132 The transformer may have a load ta ve because of its slow response. Breaker-switched capacitors are changer that can effectively alter the knee-point voltage of the compens tted. They are less flexible than the TSC, although they have tor on the high voltage side. It is possible to insert capacitors in seri re widely used and they certainly have lower losses.

with the transformer as well as the reactor, to obtain flat stabilization _{he comparisons in Table 3 are not hard and fast, because each type of} both the EHV and the compensator busbars. In other respects the ste _{can be designed with a wide range of properties. There is} down transformer would be similar to the one used with the TCR co efore no general way of deciding that one type is better than another. pensator. A general arrangement is shown in Figure 4. mparisons can be realistically made only in specific circumstances and The slope-correcting capacitors can make the saturated reactor comp ith respect to specific operating criteria. For example, it is meaningless susceptible to subharmonic instability especially on weak syste _{say that the TSC has lower losses than the TCR, unless the reactive} and it is normal practice to have a harmonic damping filter in parallel _{wer is specified at which the losses are evaluated. This reactive power} the capacitors. This filter as well as the capacitors must have a vo ould be representative of the average operating point of the compensa- rating compatible with any transients which might occur during throughout its lifetime.

or other system disturbances, and their overload capability limits _{Again, it is often pointed out that the synchronous condenser has a} otherwise substantial overload capability of the saturated-reactor comp w speed of response. But the speed of response can be increased by unless the capacitor is bypassed by a spark gap or some reasing the exciter ceiling voltage: this has actually been done on device. The slope-correcting capacitor also slows the response of t me synchronous condensers to the point where they have sufficient

compensator. eed of response to perform comparably with static compensators. Other

teria, such as cost and reliability, then determine which type of

With Shunt and should be used.

slope-Correcting _{r the ultimate flexibility of control, with present-day technology, the} compensator is the best type. The TSC, which may be combined the TCR, generally results in a loss characteristic which is lower in ng regime, and may enhance the control flexibility. The TCR ith Slope-Correcting _{can also be designed to have some useful overvoltage-}

capability which is not available in the plain TSC. or an absolute minimum of maintenance the

has advantages, but it has almost no control flexibility and it

0 require appreciable expenditure on damping circuits to avoid any

Current ssibility of subharmonic instability. It does have overload capability

40. characteristics for saturated reactor compensator with seri in limiting although the exploitation of (slope-correcting) and shunt capacitors.

TABLE 3

Comparison of Basic Types of Compensator

Quality Synchronous Condenser TCR (with shunt TSC (with TCR Polyphase capacitors where necessary) where necessary) Saturated Reactor Construction Rotating machine Thyristor equipment Thyristor equipment Transformer type with

with static reactors with static capacitors shunt capacitors and capacitors

Reactive Power Leading

Capability (indirect) (indirect) (indirect)

Control Continuous Continuous

Response Slow

Harmonics Very good

Discontinuous Continuous (continuous with

vernier

Fast, but system- Fast, but system- Fast, but system.,

dependent dependent dependent

slowed by

correction capacitors Filters may be Good, but filters Good, up to 17th necessary depending may be necessary and 19th on system conditions with TCR

--

Losses G o o d , but Good. but

with lagging current with leading current with lagging current Flexibility Good within Excellent

(=program- limitations of' response speed of control) Phase-balancing Limited Ability Overvoltage Good Limitation

Good; excellent Poor if TCR is added

Good Limited Limited

Moderate None (or very Good, within

limited with limitations of vernier TCR) slope-correction

capacitors

Rotating Inertia Yes No No No

Accuracy of Good Compensation

Direct EHV N o Connection

Starting Slow

Excellent Good; excellent Good

if TCR is added

Reactor - No No

Capacitors

-

Yes

Saturated reactor

-

no Capacitors

-

yes Fast, with minimal Fast, with some Fast, with some

222 Principles of Static

involve bypassing the series slope-correctin

Chapter.

with a spark gap, or other device.

condenser also has a overload In addition, because stability which is useful in some

In document [T.J.E.miller] Reactive Power Control in Electric Systems (Page 122-126)