Page 830 - Industrial Power Engineering and Applications Handbook

System voltage regulation 241785
Saturation of power transformers as a result of lead to saturation of power transformers operating in
periodic overloading and load rejections. the system. The transformers produce high currents,
Increased ripples in the rectifier circuits. causing rich in harmonics, and cause ferro-resonance or sub-
harmonics. synchronous resonance. An UV will result in higher
Malfunctioning of some equipment, possibly because system loading than necessary and cause underutiliza-
of a fault. tion of the system capacity.
Oscillating torque in the rotating machines as a
result of load variations and harmonics present in In the following we consider the case of a transmission
the system. line, 132 kV and above, being more typical and complex
Feeding non-linear loads such as: for the purpose of reactive control. Based on this, it would
~ Induction furnaces be easier to apply appropriate reactive control to a
- Arc furnaces and arc welders distribution network and large inductive loads such as an
- Steel rolling mills arc or induction furnace.
~ Large motors with periodic loading
- Thyristor drives 24.6.2 Analysis of an uncompensated
- Railway traction which is mostly through d.c. transmission line
drives
~ And many loads which may have to be frequently
switched Current profile
All such loads generate harmonics and cause variations A transmission line can be represented, as shown in Figure
in the fundamental power frequency of the supply 24.7(b). In Tables 24.1 (a) and (b), we show typical line
qystem which leads to distortion in the sinusoidal parameters for different system voltages and line
waveform of the voltage. This distortion may affect configurations. Because of line charging capacitances,
the quality of the supply system (voltage) beyond C,’s, between conductors and conductors and ground, as
desirable limits. A non-sinusoidal and distorted supply shown in Figure 24.7(a) (higher significance in HV and
system may adversely affect the different loads EHV lines of 132 kV and above), and series inductance
connected on the system, besides leading to outage of Lo, there is a charging current, Io, even at no load and
the system itself. even when the far end of the line is open-circuited (Figure
Maintaining a near-unity p.f. 24.7(c)). Figure 24.8 describes a normal profile for such
Maintaining the frequency to near constant by suppre- charging currents. This current rises with the rise in line
sing the system harmonics. length and is highest at the generator end. As this
Maintaining the receiving-end voltage at almost the phenomenon a function of system voltage it is negligible
rated voltage.
or nil in HT lines up to 66 kV. This current is totally
capacitive, ignoring the effect of line resistance. Ro,. A
(B) Transmission of power transmission line, being a high power transfer system.
has a very low content of Ro (Table 24.1 (a)).
1 Enhancing the steady-state power transfer capability The magnitude of the charging current, Io, will depend
of the lines over long distances. or making short lines upon the content of C,, which is a measure of line voltage,
capable of transferring larger powers. size of the conductor, spacing between the conductors
2 Improving the stability of the system by supporting and between the conductors and the ground etc. Table
the voltage at key points. Without compensation, the 24.2 provides the approximate values of charging current,
stability of the system becomes a limiting factor even Io, and charging power for a few system voltages with
for shorter line lengths and the system is rendered different line configurations. The generated charging
prone to frequent outages on small disturbances. reactive power, by the line charging capacitances (C<>’s).
3 Reducing system oscillations and flickering caused flows back to the generating source and has to be absorbed
by voltage fluctuations and system harmonics as a by it, even on no load, or a part of it during light loads.
result of frequent and rapid changes in reactive power It is a strain on the field windings of the generator. as the
demand, loss of load, loss of generation or a system machine under no load is underexcited. and
fault. Excessive voltage swings may cause tripping of underexcitation is not a healthy situation for a thermal
industrial drives and even system outages. High-speed turbogenerator because,
SVCs (Static VAr compensators, Section 24.10(2))
can overcome such situations by providing appropriate A capacitive circuit magnifies the harmonic effects
reactive support during system disturbances and when present in the system, as discussed in Section
maintaining a near-flat voltage profile through the 23.5.2, and gives rise to spurious voltages and currents,
length of the transmission line. raising the normal VI and I, to V, and I,,, respectively.
3 Providing voltagc support when switching large loads. The stator windings are subject to overcapacitive
5 Improving voltage regulation: both overvoltages (OVs) voltages as a result of this, and the end turns particularly
and undervoltages (UVs) are undesirable. An OV may are endangered.
cause ageing ol‘ the equipment’s insulation and can Reduced field current reduces the voltage generated,
lead to a flashover or eventual breakdown of the which may affect the system’s stability.
terminal equipment and line insulators. It may also The generator manufacturer can define the lowest