In Sect. 7.1.7 the reactive power of line-commutated converters was discussed.
With phase control and a control angle (X with natural commutation there is a
b
c
Fig. 8.22a - c. Sector control. a Delay phase control; b advance phase control; c delay and advance phase control
phase displacement <PI of the fundamental oscillation of the supply system current with respect to the voltage. With fully-controlled connections and smoothed dc current without vacancies <PI is equal to the control angle a. (see Eq. (7.80) ). For the single-phase or three-phase ac supply system such a load with inductive reactive power is usually undesirable.
Besides the reactive power-saving half-controllable connections described in Sect. 7.1.8 a new technique called sector control can be employed to avoid this (Fig. 8.22). The dc voltage Ud of a converter in single-phase bridge connection and the fundamental frequency component il of the ac current on the line side are plotted assuming resistive load on the dc side (no smoothing inductance Ld ) •
With normal phase control and a control angle a. the fundamental component of the line current is retarded by the phase angle <PI (Fig. 8.22a). If, however, the phase control is carried out from the end of the voltage half-cycle beginning with the termination angle ~, there is a phase advancement of the fundamental component of current (Fig. 8.22b). In the supply system capacitive instead of inductive reactive power flows. This technique, however, cannot be used with natural commutation with line-commutated converters (see Fig. 7.18). Instead forced commutation must be employed. Finally, formation of the dc voltage can be carried out by simultaneously employing a. and ~ control. When a. and ~ are the same size the fundamental component il of the line current remains in phase with the line voltage. Here again phase control can only be by means of forced commutation at the end of the half-cycle [8.15, 8.23, 8.24, 8.28, 8.38].
Quenchable Asymmetric Bridge Connection. Figure 8.23 shows realization of the sector control technique with the asymmetrically half-controllable bridge connec- tion employed in converter locomotives and motor coaches of the Federal German Railways [8.20]. By incorporating a quenching capacitor Ck and two auxiliary thyristors Tl' and T2', the main thyristors T1 and T2 can be turned off
Self-commutated Inverters
u t
I
Fig. 8.23. Quenchable asymmetric bridge connection
175
•
i O~~~~~~~--~~~~--
Fig. 8.24. Voltage and current waveforms for the quenchable asymmetric bridge
without having to wait for the commutating voltage from the ac supply system.
The connection is called the quenchable asymmetric bridge connection. It considerably reduces the reactive power consumption of the converter.
The fundamental power factor cos </>1 can be made capacitive. The power factor A is thereby brought nearer to the desired value, namely 1.
Figure 8.24 shows the voltage and current waveforms of quenchable asym- metric bridge connection. An operating condition is depicted with the termination angle ~ beginning at the end of the half-cycle. The current in the controllable valves T1 and T2 is thereby shortened. It is lengthened in the uncontrollable diodes Dl and D2 (see Fig. 7.24). During the quenching processes the current temporarily flows via the alternate of the quenching thyristors T1' and T2'. The ac current i on the line side is advanced by the phase angle </>1 with respect to the ac voltage u. The ac voltage source is loaded with a capacitive component.
With sector control it should be remembered that the single-phase or three- phase ac system generally contains non-negligeable inductances Lk• The quench- ing capacitor Ck must therefore interrupt the current in the line inductance Lk and the magnetic energy stored there, LkP /2, must be absorbed. In order that no excessive transient voltages are thereby generated and, moreover, no unnecessary power oscillations occur the circuit shown in Fig. 8.23 has been modified to include a capacitive intermediate energy store coupled via diodes (see Fig. 13.21 ).
Quenchable Half-Controllable Bridge Connections. Figure 8.25 shows several variations of half-controllable bridge connections in which the control branches are made quenchable by means of quenching capacitors Ck and auxiliary connections.
Id 1'1 1'1
~l 1'1 1'1
+u tUI tU tUI
a b
Id fd
C Ud 1'1 1'1
'u tUI
e
Fig. 8.25a-e. Quenchable half-controllable bridge connections. a Non-uniform double-way connection with one quench capacitor; b non-uniform double-way connection with separate quench arms; c non-uniform double-way connection with separate quench arms and slow quenching; d non-uniform double-way connection with separate quench arms and a unipolar storage capacitor; e non-uniform double-way connection (one ac pole controlled) with one quench capacitor and separate unipolar storage capacitors
Circuit a employs a common quenching capacitor Ck and two auxiliary thyristors. In circuit b each of the controllable main branches contains a quenching capacitor Ck and an auxiliary thyristor in addition to diodes and a resistor RL to charge the quenching capacitor. In circuit c the ac voltage source u is in series with each of the control arms. The rate of the current rise in the control arm is therefore slowed due to the inductive component of the mains and transformer. In all of these circuits the quenching capacitors Ck must absorb the magnetic energy stored in the mains and transformer inductances which is released when the mains current is interrupted. This leads to relatively large values of capacitance. Both circuits d and e employ storage capacitors C. These capacitors ( which need only withstand voltage in one direction) store the magnetic energy built up in the mains and transformer inductances. The values of the quenching capacitors Ck can therefore be reduced. However the number of auxiliary diode branches must be increased. In addition, blocking diodes must be added to the quenchable main branches.
Figure 8.26 illustrates the voltage and current curves of circuit d. The current iv begins to flow when one of the main thyristors is triggered at a phase angle r:t = o. It commutates with an initial overlap Uo from the non-controlled main branch to the triggered main branch. The quenching process is initiated by the triggering of the auxiliary thyristor at a phase angle of 1t -~. When the voltage Uc exceeds the momentary value of the mains voltage the mains current begins to decrease becoming zero at 1t-y. Before the end of the half-cycle the storage capacitor C is discharged through the two primary thyristors onto the dc load. The dc voltage ud
Self-commutated Inverters 177
2lC wt
b
Fig. 8.26a,b. Quenchable half-controllable bridge connections with separate quench arms and a unipolar storage capacitor (see Fig. 8.25a). a Voltage UL on line side, voltage Ue on capacitor and ac voltage Ud; b current iv on valve side of transformer; parameters: Id = IdN, Ud =0.5 Udb ukl = 5%, UkL = 1 %, and idealized quenching process are assumed
uJ u u.i u
a b
wt c
Fig. 8.27a-c. Waveform of the current in single phase mains with half-controllable bridge connections. a Delay phase control; b advance phase control; c sector control
is composed of the shaded areas in the voltage diagram. The line side current leads the line phase voltage, e.g., its fundamental wave has a capacitive component.
Phase and Sector Control. Figure 8.27 illustrates the line current for the various control methods. Using phase control with delay angle r:t. (a) the line current i lags the line voltage u (inductive). Using phase control with a lead angle ~ (b) the line current has a positive phase shift (capacitive). This mode of operation assumes quenchable main branches. With sector control (c) a combination of delay phase
Sector control with prevailing delay
Delay phase control (full controlled)
Delay phase control (half controlled)
Sector control (symmetrical)
O~~~~~~~~~~---~
Sector control with prevailing advance
Capacitive
UdlUdi
Advance phase control (half controlled)
Fig. 8.28. Reactive power of the fundamental wave of current for different control methods (angle of overlap u = 0 and smoothing inductance Ld -> 00 )
control (with a delay angle r:t) and advance phase control (with a lead angle B) is used. The fundamental oscillation of the current i can be held in phase with the line voltage u. This method requires quenchable main branches as well.
Reactive Power and Power Factor. Figure 8.28 shows the range of reactive power (related to the fundamental wave of the current) possible for each of the different control methods. Using a delay phase control with the half-controllable circuit (ignoring any overlap and assuming sufficient smoothing) the reactive power (for fundamental wave of current) describes a semi-circle in the region of inductive reactive power. For the advance phase control this semi-circle lies in the region of capacitive reactive power. Using sector control the operating point may be set anywhere within the shaded area of the diagram. The reactive power factor cos <p of the fundamental wave of current can be set to be inductive, unity or capacitive. Due to the waveshape (non-sinusoid) of the current distortion power loss is present. The (total) power factor A. is less than unity.
Applications. Circuits employing advance phase control and sector control offer many advantages especially in the area of traction. Compared to half-controllable bridge circuits they achieve a considerable improvement in the power factor for single-phase railway systems.