Reactive-Power Compensators
3.3 THE SATURATED REACTOR (SR) .1 Configuration
3.4.6 Operating Characteristics of a TCR
THE THYRISTOR-CONTROLLED REACTOR (TCR) 59 systems add on the system side for fundamental frequency, but they subtract and cancel for 5th and 7th—and for all higher frequencies of [6(2n+ 1)± 1, nc0, 1, 2, . . . ]—order.
Figure 3.14 shows the harmonic-current content in a 12-pulse scheme at var- ious locations. The large reduction in harmonic content achieved with the 12- pulse TCR greatly alleviates the requirements for harmonic filters. Instead of employing tuned 5th and 7th harmonic filters, as in the 6-pulse TCR, only high- pass filters may prove adequate. Once again, the harmonic reduction is asso- ciated with enhanced costs because of the increased number of thyristors and the presence of a special double-secondary transformer as well as a complex firing sequence. An additional advantage that occurs with the 12-pulse TCR is increased reliabiilty. Should one of the 6-pulse TCR units fail, the other TCR unit continues to operate, although with half the reactive-power rating. The 12- pulse TCRs have higher overload capabilities than the 6-pulse ones.
Those TCRs with pulse numbers higher than 12 are not used in practice, although their use would ensure a drastic reduction in harmonics. The reason for this lack of use is that they have become too complex and expensive; for instance, a 3-secondary-winding transformer would be needed for an 18-pulse TCR. Furthermore, the precision required in firing control to ensure the sym- metrical firing of the thyristors is not easily attainable.
60 PRINCIPLES OF CONVENTIONAL REACTIVE-POWER COMPENSATORS
Absorption Limit BSVC
= BL
Production Limit B= 0SVC
VSVC
a = 180° a = 140°
a = 90°
a = 180°
a = 100°
a = 110°
a = 120°
a = 140°
a = 120° a = 90°
ISVC
BSVC
BTCR
BL BL
(a) (b)
Operating Range
Figure 3.16 Different characteristics of an SVC: (a) the voltage–current characteristic and (b) the SVC TCR susceptance characteristic.
flowing in the TCR. Note thatBTCRis the variable susceptance in the foregoing equivalent of the TCR; it is given by Eq. (3.15).
For a general SVC, which can be considered as a black box with an unknown but purely reactive circuit inside, the overall compensator susceptanceBSVCcan be defined with the following equation:
ISVCcVjBSVC (3.23)
In the simple case of a TCR, the compensator susceptance is
BSVCcBTCR (3.24)
Usually, three kinds of characteristics are of interest while analyzing an SVC, as described in the paragraphs that follow.
Voltage–Current Characteristic or Operating Characteristic This shows the SVC current as a function of the system voltage for different firing angles, as depicted in Fig. 3.16(a). This V-I characteristic is given in a very general sense. No control system is assumed to vary the firing angle, and any operating point within the two limits is possible depending on the system voltage and the setting of the firing angle (other currents and voltages may be shown, too). This characteristic clearly illustrates the limits of the operating range, and it may include the steady-state characteristics of the various possible controls. This characteristic is the usual way in which the system engineers prefer to look at the compensator, because the characteristic shows the steady-state performance of the SVC plant.
THE THYRISTOR-CONTROLLED REACTOR (TCR) 61 SVC TCR Susceptance Characteristics These illustrate the change of the total SVC susceptance when the TCR susceptance is varied, as shown in Fig.
3.16(b). The susceptance characteristic for this case is very simple because BSVC c BTCR. Note that the TCR susceptance is negative, indicating that the TCR is an absorbing reactive component. These characteristics are of most interest to control-system analysis—the controls affect the TCR firing angle, whereas the total susceptance BSVC influences the power system.
Current Characteristics For more complex SVC arrangements, especially those including TSCs, it is not easy to see how the various branches contribute to the total SVC current. Therefore, these characteristics show the branch cur- rents as a function of the total SVC current, and they are important for deter- mining steady-state current ratings of the various components. (An example of such a characteristic is given in Fig. 3.38, shown later in this chapter.) 3.4.6.2 Operating Characteristic With Voltage Control The operating range of Fig. 3.16(a) can be reduced to a single characteristic of operating points if the effect of the voltage control is incorporated. Let us assume that the com- pensator is equipped with the voltage control shown in Fig. 3.17(a). The system voltage is measured, and the feedback system varies BTCR to maintainVref on the system. This control action is represented in the operating characteristic in Figure 3.17(b) by the horizontal branch marked ascontrol range. This charac- teristic shows the hard-voltage control of the compensator, which stabilizes the system voltage exactly to the set pointVref.
Two system characteristics—system 1 and system 2—are depicted in Fig.
3.17(b) that illustrate the decline in system node voltage when the node is
Controls Power System
Vref V ISVC
(a) (b)
Undervoltage Range Production Limit
Control Range
Overload Range Absorption System 1
System 2 System 2′ Absorption Limit
Current Limit
A1 A2 Vref
ISVC Production
− + BTCR
ISVC VSVC
Σ
Figure 3.17 The operating characteristics of a TCR with voltage control: (a) an SVC control system and (b) theV-Icharacteristic.
62 PRINCIPLES OF CONVENTIONAL REACTIVE-POWER COMPENSATORS
loaded inductively and reactive power is absorbed. The corresponding operating points for the two system conditions are A1 and A2. If the system voltage of system 2 rises, a new characteristic—system 2′—results. Operating pointAthen moves to the right and reaches the absorption limit of the compensator. Any further increase in system voltage cannot be compensated for by the control system, because the TCR reactor is already fully conducting. The operating point A2 will, therefore, move upward on the characteristic, corresponding to the fully on reactor connected to the system (ac908). The compensator then operates in theoverload range, beyond which a current limit is imposed by the firing control to prevent damage to the thyristor valve from an overcurrent. At the left-hand side, the compensator will reach theproduction limitif the system voltage drops excessively; the operating point will then lie on the characteristic of the undervoltage range.