Page 532 - Industrial Power Engineering and Applications Handbook

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1 Excessive heating of magnetic cores, as a result of 16.7 Starting of a DG set
harmonic frequencies due to hysteresis and eddy
current losses (equations (1.12) and (1.1 3)). 16.7.1 Through an auto-mains failure (AMF)
2 Overloading of neutral conductors due to third
harmonic currents flowing through the neutral. The scheme
size of the neutral must be increased by 150-200% This is a common scheme to bring a standby DG set on
of the normal size in such cases, depending upon line automatically on the failure of the main source of
the severity of the harmonics. supply with the help of a battery backed-up ignition
3 Higher voltage stresses, V, (equation (23.1)) may scheme. On the failure of the main supply, generally three
lead to dielectric breakdown. ignition pulses are given to the engine to auto-start the
4 Ageing of insulation of the generator and other machine. If the engine fails to start at three attempts, a
equipment connected to such a system. further pulse is blocked. To start, the engine is made
5. Noise and resonance problems (Section 23.5.2(C)) automatic. The generator excitation or the field current
in the electrical distribution and communication is also pre-set, which adjusts the generator voltage to the
networks. required level automatically through the AVR. Provision
6 Problems during a parallel operation due to distorted is also made to start it manually in case the AMF scheme
quantities of currents and frequencies. Non- fails and to also facilitate routine testing and give it more
sinusoidal current will cause a non-linear impedance flexibility. The scheme may be briefly described as follows
(mostly reactive) voltage drop on load in the (see the control and scheme drawing in (Figure 16.10).
generator windings. The terminal voltage will also
contain harmonic quantities. It is possible that even Control supply
a fast-responding AVR may not be able to maintain
the required sinusoidal voltage waveform in the A battery backed-up d.c. source of control supply is
whole cycle and cause circulating currents. Generator provided for the AMF panel and engine ignition. The
manufacturers recommend that not more than 40% control scheme, as illustrated, generally consists of a
of such loads be connected on the system at a time, 220 or 240 V a.c. source of supply, with a transformer
when the remaining loads are linear. To suppress rectifier unit, to provide a 24 or 48 V d.c. control voltage,
the third harmonics, use of YlA or A/Y transformers to charge the battery as required and a battery back-up of
is common to feed such loads and filter out the suitable capacity.
third harmonics. For more information refer to
Section 23.6.1. Selection scheme

For all such loads, the machines will require a derating. Auto or manual selection (switch SW2)
For appropriate selection it is therefore essential first to A switch SW2 is provided for the selection of the engine
determine the content and magnitude of harmonics and to be started in auto or manual modes:
then to consult the manufacturer for the selection of the
machine. To determine the magnitude of harmonic Auto mode In auto mode the engine starts
contents. refer to Section 23.7. automatically on failure of the a.c. bus voltage, V,,
through an undervoltage or bus voltage relay (Relay
code 27) (Figure 16.10). The relay is provided on the
16.6.3 Special loads generator control panel with time delay contacts. Time
delay is provided to allow a pause to the generator if
Loads of welding sets, which have intermittent duty cycles, the normal supply is quickly restored.
are mostly single phasc, cause low p.f. and stress the
generator windings intermittently. Unbalanced loads, such Manualmode
as single phase loads, distributed unevenly cause current
unbalance and low p.f. Local control
All loads that may overstress the gcnerator windings In manual mode, the engine can be started and stopped
fall in this category and call for special consideration. locally, through a pair of start and stop push buttons
The manufacturer must be consulted for the right choice Nos 17 and 18, provided on the AMF panel (Figure
of machine. 16.1 1).
There are no ready-made formulae to deal with such a
situation, except experience. It will largely depend upon Local remote control (switch SW3)
the skill of the engineer to make the right choice of This is provided to facilitate the manual start and stop of
machine to meet the requirements in consultation with the engine from a remote point such as through a remote
the manufacturer and also observing certain disciplines station.
as noted above, such as maintaining the p.f. between 0.8
lagging and unity, suppressing harmonics as far as is Three-attempts start
practical, and maintaining a balance of loads. It is also
desirable that neutrals are not interconnected when more A three-attempts start facility is provided through a
than one machines is operating in parallel to eliminate sequential timer (ST) in auto mode. A starting relay
circulation of third harmonic currents. (SR) gives three ignition impulses to the engine. If the