Voltage
Variations
Voltage at switchgear bus shall be
maintained within ± 10% of the normal value for steady-state conditions, and ±
15% of the nominal value for transient during start of motor. Voltage at motor
terminal shall not be more than 20% of nominal value during starting. Over-voltages
shall not exceeding 110% of nominal for more than 2 seconds.
Frequency
Variations
The power system frequency shall be
maintained within ± 5% of the nominal value for steady-state and transient
conditions including starting of motor.
Load
Analysis
The overall power requirements shall be
determined in the Electrical Load Study Report, which includes the load
schedules for each switchgear. The calculations shall be confirmed during
detailed design stage.
The electrical load schedule is developed
using Mechanical Equipment List as the main reference for Mechanical loads. In
the absence of design information, non-process loads are estimated from similar
installations. All loads, efficiencies and power factors are estimated or
typical for the equipment selected. Loads are classified according to their
nature as continuous, intermittent and standby duty. Intermittent loads are
factored by a duty cycle to enable them to be summed with continuous loads into
average load.
Total loads are calculated as maximum
operating load, peak operating load. It's used to determine the nature of the
demand load and thus the electrical equipment requirements. In general the
power supply must be suitable for the peak demand load, as well as any
transient load changes imposed due to specific consumers.
Load shedding schedule shall be introduced
to maintain continuous operation of several essential process loads by single
unit operation of generator in case of sudden failure of the second unit
meanwhile the third – standby unit is being on the transition period of
parallel operation or under maintenance.
Classification
The electrical load is classified
according to its service as normal and essential loads:
Normal loads – loads typically related to
production, and those of which the loss of supply would not create an unsafe
condition or result in damage to equipment. The loads prevail under normal
operating conditions.
Essential loads – loads related to the
safety of personnel and equipment but which are suitable for short breaks in
the power supply without detriment (such as during starting of emergency
generators). Such loads are to be supplied by emergency generators.
Non-electrical loads – will be items
driven by non-electrical prime movers (eg. Pneumatic drives). These may include
in the load schedules as a reference.
Power
System Studies
Power System Studies shall be simulated
using the computer based calculation software. Prospective short circuit
current calculation shall be calculated on the basis of all generators
connected regardless of the installed sparing. Motor starting studies will be
performed to ensure acceptable system performance on starting of the largest
electric motors.
Load flow analysis shall be performed to
check voltage profiles and circuit loading conditions under steady state
conditions based on the design loads. The worst case accountable on load flow analysis
is the condition where one unit of gas turbine generator is under maintenance
or overhauled and only one unit Gas Turbine Generator is running following a
sudden trip of the other unit.
Electrical
Protection
The minimum electrical protection
requirements shall be as follows:
Gas turbine and diesel engine driven
generators shall have protection on phase and neutral over-current, earth
fault, over-speed, percentage differential (unless rated less than 750 kVA),
loss of excitation/field, voltage restrained phase over-current, negative phase
sequence and reverse power. Over/under frequency and under/over voltage shall
also be provided, however for the emergency generators this shall automatically
be disabled when in emergency operation. Transformers shall have protection on
phase over-current, short-circuit, restricted earth fault, backup earth fault,
differential (if greater than 1250 kVA), over-temperature and over-pressure.
Motors shall be protected against
overload, single phasing; stall/locked rotor and short circuit. Thermal
overloads shall be of the single-phase protection type. Motors 30 kW to 75 kW
shall be protected against earth fault using core-balance CTs. Motors 90 kW and
above shall be provided with electronic motor protection relays and protection
against over-current and negative sequence current. MV motors shall further be
provided with thermal protection by winding and bearing RTDs. Bus-Tie CBs shall
be provided with phase over-current protection and where applicable shall be
provided with synchronism check protection.
Incomers shall be protected against
over-current / short circuit, earth fault and under voltage. Outgoing feeders
shall be protected against over current / short circuit and earth fault. All
main circuit breakers shall be provided with lockout protection and trip
circuit supervision.
Protection
Coordination
Protection relay coordination studies
shall be carried out during detailed design to determine the discrimination
settings for protective devices to ensure that any possible fault or overload
is cleared and isolated with minimum disruption.
The calculation, setting and coordination
of the main or primary protection system shall be based on the following
operating conditions:
• Minimum
fault level.
• Maximum
fault level.
• Load
priority (main process loads, supporting process loads, non process loads
etc.).
