81
6
Emergency Generators
Emergency generators (Figure 6.1) are used in order to provide consumers in the
system entitled to standby service in the event of breakdown or shutdown of the
normal power supply. They consist of the generator, the flywheel and the motor.
Figure 6.1 Schematic of circuit for standby generators
For the dimensioning of the generator power to supply a large number of consu-
mers, the connected load with the coincidence factor must be taken as the basis.
This connected load should be about 60 % of the rated load for the standby genera-
tor. The prime mover must be designed for the effective power and the generator for
the apparent power. The feed-in of the network to the automatic standby power con-
trol takes place through the secondary distribution, to which other consumers are
also connected. The functional and operational reliability of standby generators is
ensured only when the planning takes account of all requirements for channeling
air intake and exhaust air, room ventilation, sound attenuation and fuel supply. The
main components are:
Analysis and Design of Low-Voltage Power Systems. Ismail Kasikci
Copyright 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-30483-5
82 6 Emergency Generators
. Generator
. Battery system
. Emergency standby control
. Fuel system
. Exhaust system with sound attenuation
. Fans for room ventilation
. Air intake and exhaust ducts
The starting power (Figure 6.2) and the correction factor (Figure 6.3) for consu-
mers with a high starting power, taking into account the still permissible voltage
dip, are calculated as follows and an additional 20 % reserve is planned to cover
future energy requirements.
Figure 6.2 Starting power/rated standby generator power [42]
Figure 6.3 Correction factor k [42]
6.1 Generator-Specific Limiting Operational Values 83
The starting power:
IA
Sst 1:3 P k (6.1)
In
Generator power:
PrM grG
SrG (6.2)
cosj
Motor drive power:
SrG cosj
PrM (6.3)
grG
The meanings of the symbols are:
IA Starting current in A
Sst Consumer starting power in kVA
In Rated current in A
k Correction factor for operating conditions
P Effective power in kW
PrM Motor drive power in kW
SrG Generator power in kVA
cosj Power factor
6.1
Generator-Specific Limiting Operational Values
. Range of application 1, Communal facilities After a switching time of maxi-
mum 15 seconds, 100% of the consumer power for the required safety equip-
ment must be supplied (Table 6.1).
Table 6.1: Electrical installations in hospitals and communal facilities
Characteristic parameter Communal facilities in hospitals
Static frequency deviation 5% 4%
Dynamic frequency deviation ± 10 %} ± 10 %
Frequency adjustment time 5s 5s
Static voltage deviation ± 2.5 % ± 1%
Dynamic voltage deviation +20 %; ±20 % ± 10 %
Voltage adjustment time 4s 4s
Steady state short circuit current 3 In (3 s) 3 In (3 s)
Total harmonic distortion of voltage 3 ~ < 5% 3 ~ and 1 ~ < 5 %
. Range of application 1, Electrical installations in hospitals After a switching
time of maximum 15 seconds, 80 % of the consumer power (consumers of
the required safety equipment and operationally important consumers) in a
84 6 Emergency Generators
maximum of two steps and after a further 5 seconds 100 % of the total con-
sumer power must be supplied (Table 6.1). For inductive consumers the
large starting current must also be considered (Table 6.2).
Table 6.2: Standard values for starting currents
Startup procedure IA/IrM
Direct startup 4 ... 6
Squirrel cage rotor via star delta 2 ... 3
Squirrel cage rotor via frequency converter 1.5 ... 3
Squirrel cage rotor via phase control 2 ... 4
. Type classes 2 and 3 Connecting and disconnecting loads takes place in accor-
dance with the regulations.
Table 6.3: Type class 2 and 3
Characteristic parameter Class 2 Class 3
Static frequency deviation 5% 3%
Dynamic frequency deviation ± 10 % ± 7%
Frequency adjustment time 5s 3s
Static voltage deviation ± 1% 1%
Dynamic voltage deviation + 22 %; ±18 % + 20 %; ±15 %
Voltage adjustment time 6s 4s
Steady state short circuit current 3 In (3 s) 3 In (3 s)
Total harmonic distortion of voltage 3 ~ < 5% 3 ~ < 5%
6.2
Planning a Standby Generator
The following features must be considered during the planning of standby genera-
tors:
1. Characteristic parameters of the network, such as type and number of active
conductors of the feed-in, type of ground connections (mostly TN-S systems)
2. Determination of the power requirements with the coincidence factor
3. Expected three-pole and single-pole short circuit currents at the feed-in point
4. Distribution of phase loads for the operational equipment of the electrical
system
5. Switching between network and standby power with mutual mechanical or
electrical locking
6. Connection of the standby generator to the existing network takes place up to
125 A through a 5-pin CEE plug-and-socket device with H07RN-F line.
7. System ground can be in the form of a concrete footing electrode, ring con-
ductor or buried grounding electrode.
6.3 Example: Calculation of Standby Generator Power 85
6.3
Example: Calculation of Standby Generator Power
The following data are known for a three-phase motor:
Speed 1480 rpm
Rated power 22 kW
Rated current 42 A
Power factor cos j = 0.88
Starting current direct: factor 5.5
Starting current star-delta: factor 2.3
Voltage dip 15 % in accordance with communal facility regulations
For the correction factor k, from Figure 6.3 we find a value of 0.97. Starting power
with direct startup:
IA
Sst 1:3 P k 1:3 22 kW 5:5 0:97 152:6 kVA
IrG
Starting power with star delta connection:
IA
Sst 1:3 P k 1:3 22 kW 2:3 0:97 63:8 kVA
IrG
For 15 % voltage dip, from Figure 6.2 we find a factor of 1.25. The required
standby power for the delta connection is:
Sst 152:6 kVA
SrG 122 kVA
x 1:25
and for the Y/D connection:
Sst 63:8 kVA
SrG 51 kVA
x 1:25
