Chapter A General rules of electrical installation design

B34
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B - Connection to the MV public
distribution network
MV distribution
panels for
which standby
supply is
required
Automatic
changeover
panel
Busbar
transition
panel
From standby generator
P y 20,000 kVA
To remainder
of the MV
switchboard
Fig. B33 : Section of MV switchboard including standby supply
panel
5 The consumer substation
with MV metering
5.2 Choice of panels
A substation with MV metering includes, in addition to the panels described in 4.2,
panels specifically designed for metering and, if required, for automatic or manual
changeover from one source to another.
Metering and general protection
These two functions are achieved by the association of two panels:
b One panel containing the VT
b The main MV circuit-breaker panel containing the CTs for measurement and
protection
The general protection is usually against overcurrent (overload and short-circuit) and
earth faults. Both schemes use protective relays which are sealed by the powersupply
utility.
Substation including generators
Generator in stand alone operation
If the installation needs great power supply availability, a MV standby generator set
can be used. In such a case, the installation must include an automatic changeover.
In order to avoid any posssibility of parallel operation of the generator with the power
supply network, a specific panel with automatic changeover is needed (see Fig. B33).
b Protection
Specific protective devices are intended to protect the generator itself. It must be
noted that, due to the very low short-circuit power of the generator comparing with
the power supply network, a great attention must be paid to protection discrimination.
b Control
A voltage regulator controlling an alternator is generally arranged to respond to a
reduction of voltage at its terminals by automatically increasing the excitation current
of the alternator, until the voltage is restored to normal. When it is intended that
the alternator should operate in parallel with others, the AVR (Automatic Voltage
Regulator) is switched to “parallel operation” in which the AVR control circuit is
slightly modified (compounded) to ensure satisfactory sharing of kvars with the other
parallel machines.
When a number of alternators are operating in parallel under AVR control, an
increase in the excitation current of one of them (for example, carried out manually
after switching its AVR to Manual control) will have practically no effect on the voltage
level. In fact, the alternator in question will simply operate at a lower power factor
(more kVA, and therefore more current) than before.
The power factor of all the other machines will automatically improve, such that the
load power factor requirements are satisfied, as before.
Generator operating in parallel with the utility supply network
To connect a generator set on the network, the agreement of the power supply utility
is usually required. Generally the equipement (panels, protection relays) must be
approved by the utility.
The following notes indicate some basic consideration to be taken into account for
protection and control.
b Protection
To study the connection of generator set, the power supply utility needs some data
as follows :
v Power injected on the network
v Connection mode
v Short-circuit current of the generator set
v Voltage unbalance of the generator
v etc.
Depending on the connection mode, dedicated uncoupling protection functions are
required :
v Under-voltage and over-voltage protection
v Under-frequency and over-frequency protection
v Zero sequence overvoltage protection
v Maximum time of coupling (for momentary coupling)
v Reverse real power
For safety reasons, the switchgear used for uncoupling must also be provided
with the characteristics of a disconnector (i.e total isolation of all active conductors
between the generator set and the power supply network).
Schneider Electric - Electrical installation guide 2008