ELECTRICAL DESIGN CRITERIA

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.).