Page 76 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
74 Chapter Two

is to prevent the standby generator back-feeding an earth fault on the
supply authority’s system. For some installations neutral voltage dis-
placement protection may be expensive or impracticable, and in such
cases it is likely that agreement could be reached on the use of some
other parameter such as rate of change of frequency, rapid phase angle
change, or unbalanced voltages.
Full tests on the protective equipment must be undertaken and recorded
by the installer to the satisfaction of the network operator. Where the
standby supply is connected to a high-voltage system the network operator
has a duty to witness the tests; where it is connected to a low-voltage
supply the operator may, at its discretion, wish to witness the tests.
The loaded generator is unlikely to produce a pure sine wave of volt-
age and when it is connected to the normal mains it will cause har-
monic currents to flow. As with any distorting load connected to the
U.K. system, the harmonic currents must be within the limits set by
Engineering Recommendation G.5/4 (ER G.5/4) published by the
Electricity Association. More is written about ER G.5/4 and harmonics
in Chap. 4 of this book.

The Low-Voltage Neutral Connections
Where a low-voltage standby supply is intended to run in parallel with
the normal supply, the two neutrals may be solidly connected. For TN-
C-S (PME) systems this is the simplest arrangement as the standby
supply neutral may be solidly earthed and connected to the normal sup-
ply neutral; the changeover from one supply to the other is then effected
by means of triple-pole contactors or circuit breakers. However this will
probably not be practicable, it is likely that the resulting triplen har-
monic current flow in the neutral will exceed the limits set by ER G.5/4
and some other procedure must be adopted.
One procedure is to provide a single-pole neutral contactor in the con-
nection to the generator star point, and to arrange for this to be open
when running in parallel. With this arrangement the standby supply
star point is not used and the system relies upon the normal supply for
single-phase loads, for earthing, and for triplen harmonic currents; all
the zero-sequence currents flow from the normal supply phase conduc-
tors and return additively in the neutral conductor. Harmonic current
flow in the phase conductors of the normal supply must not exceed the
limits set by ER G.5/4. The generator manufacturer must provide suffi-
cient information for the network operator to be able to calculate the har-
monic current flow before permission for paralleling is given.
There must be electrical interlocks to ensure that:
■ It is not possible to connect the two supplies in parallel unless the
neutral contactor is open.

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