Friday, August 12, 2011

Design and selection requirements for equipment and cables

1. Generators

If the rated speed of the prime mover is less than 3000 or 3600 rpm (50 or 60 Hz systems respectively), no gearbox shall be installed. The generator rated power factor shall be 0.8 lagging, unless otherwise specified. For generator ratings requiring more than three cables per phase, consideration should be given to the use of phase segregated busbar connections between the generator and its transformer or switchgear.

1.1 Operational conditions (OC classification)

The classification of operational conditions for a plant network or external network is as follows.

1. OC1 (Steady state)                         Voltage ±5%, Frequency ±2%
2. OC2 (Sustained)                             Voltage 90-95% or 105-110%, Frequency 95-98% or 102-103%
3. OC3 (Deviated)                               Voltage 70-90%, Frequency 92-95% or 103-110%
4. OC4 (Degenerated)                        Voltage <70%

1.2 Classification of generating units

1. P1    Fully independent of process fuel supply, as it has own fuel supply or dual fuel capabilities.
2. P2    Partially process dependent on fuel supply from plant processes
3. P3    Completely process dependent on non-electric process such as heat generation
4. P4    Emergency power supply, completely independent

1.3 Fault current classification of generators

1. G1   Synch. Generator with independent excitation system. Able to withstand certain short circuit current during a fault.
2. G2   Synch. Generator with dependent excitation system. Not able to withstand short circuit current.
3. G3   Asynch. Generator (Not able to control voltage or reactive power or to withstand short circuit current).

2. Switchgear

2.1 HV switchgear

Only SF6 and vacuum breakers are to be considered. Other media such as oil and air are to be considered only in those cases where extensions of panels are required in which these types of breakers are installed. Outdoor switchgear should only be considered for voltages exceeding 36 kV. Indoor gas insulated switchgear (GIS) shall only be considered for voltages of 30 kV and above when available space is limited or environmental conditions would result in unreliable operation of open terminal switchgear.

3. Power transformers

Transformers for outdoor use and rated:
-          up to and including 1600 kVA shall be of the oil filled hermetically sealed type
-          above 1600 kVA and up to and including 3150 kVA should preferably be of the oil filled hermetically sealed type, but may be of the oil filled conservator type, if so determined by the client
-          above 3150 kVA shall be of the oil filled conservator type

Oil-filled power transformers shall be installed outdoors in a fenced-in area of the substation. The fences shall have at least two lockable gates. Each transformer shall have at least 1 m clear space all around. For intake substations with large transformers, of 100 MVA and above, a blast wall between the transformer bays shall be provided. Non-sealed type transformers shall be surrounded by a gravel-filled or gravel-covered oil catchment pit which is sized to contain the total oil content of the transformer.

4. UPS

Load circuit fuse ratings should not exceed 10% of the rated output current of UPS. Fuse ratings exceeding 10% should only be permitted if the mains supply frequency is sufficiently stable to enable the UPS static bypass circuit to be relied on to operate the fuse in the event of a load circuit fault. Batteries for UPS can be any of the following three types, based on economics.

1. Vented lead-acid batteries (Plante type)
2. Vented nickel-cadmium batteries (pocket plate type)
3. Valve-regulated gas recombination type lead-acid (VRLA) batteries

Vented lead-acid or nickel-cadmium batteries should be used where battery capacities in excess of 400 Ah are required so as to avoid the need to connect batteries in parallel or where wide temperature fluctuations are expected, e.g. outside the range of 10 – 30 deg C.

5. Capacitors

All capacitors should have individually fused elements. HV capacitor banks shall be installed outdoors. Air cored reactors shall be installed in HV capacitor banks to limit the inrush currents. An interlock system shall be provided for all automatically controlled capacitor banks to prevent re-energisation when the residual voltage is above 10% Un.

6. Motors

The minimum and maximum power ratings for motors for various switchboard voltage ratings are as follows.

1. Switchboard nominal voltage LV and 3.0/3.3 kV – Max. LV motor  110 kW, Min. HV motor 132 kW
2. Switchboard nominal voltage LV and 6.0/6.6 kV – Max. LV motor  185 kW, Min. HV motor 200 kW

The installation of LV motors of higher capacity than these is acceptable, if the installation of an HV system is thereby avoidable. Cage induction motors are preferred on account of their robust construction and low capital cost. Synchronous motors, being more efficient, may be cost effective for capacities more than 10,000 kW. If low-speed and powerfactor compensation is required, synchronous motors may be used for lower ratings. They should not be considered for ratings below 2,000 kW.

The location of the push-button station should not be further away from the motor than 1.5 m and the operator should be able to see the ammeter when operating the discharge valve.

7. Metering, protection and control equipment

Voltage transformers’ rated output shall, as a minimum, be equal to the connected burden of the protection, control and/or monitoring devices, including the lead burden, plus 25% spare, rounded up to the next standard rating.

8. Cables, wires and accessories

Unless the soil will definitely remain free from contamination, lead sheathed underground cable shall be used in oil and chemical plants. HV multicore cables shall have a minimum cross sectional area of 25 mm2 and a maximum cross sectional area of 240 mm2. For LV cables, maximum cross sectional area shall be 185 mm2 for motor cables and 240 mm2 for distribution cables. For power, lighting and control cables, the minimum cross section shall be 2.5 mm2, except for signalling and indication purposes, where a minimum cross section of 1.5 mm2 may be used. Wires in conduit shall have a minimum cross section of 2.5 mm2, except for the phase connection between a switch and a luminaire, where 1.5 mm2 can be applied. Wiring colours shall be

-          blue – neutral,
-          brown – phase,
-          black – switched phase,
-          green/yellow – earth

Local rules requiring other colours shall prevail

Minimum bending radii for cables shall be as following:

Cable type
Voltage class
Number of cores
Bending radius
Lead-sheathed cables
<= 12 kV
>=12 kV
Non-lead sheathed cables

Instrument, telecom and computer/data cables shall be laid in trenches or on trays separated from those used for HV and/or LV cables. While laying cables along side pipelines, a clear distance of at least 300 mm between cable and pipe (including insulation) shall be maintained. When crossing such pipelines, cables should cross underneath buried pipelines except where the depth measured at the top of the pipeline is more than 1 metre. HV cables may be laid in the same trench with LV cables. If no barrier exists between these two, a clear space of at least 600 mm shall be maintained. In order to protect against lightning-induced currents, in all cable trenches at least one separate earthing wire, also referred to as Parallel Earthing Conductor (PEC), shall be installed. Individual cables emerging from floors or soil shall be protected against mechanical damage by means of galvanised steel pipes or rigid PVC pipes. These pipes shall extend at least 100 mm above ground or floor level.

On trays or racks, HV cables shall be segregated from LV cables. Cable numbers shall be marked on the cables along their routes and at both termination points. For underground cabling, the spacing between cable numbers should not exceed 10 m, and for above ground cabling, 25 m.

Flexible cables on plants shall be limited as far as practicable to portable equipments, hand lamps, soot blowers etc. An earth continuity conductor, equal in cross-sectional area to the largest phase conductor shall be provided. This requirement applies even when the cable is armoured.

When calculating the rating of cables, the overall rating factors should not be lower than

Direct buried
In air

9. Lighting and small power equipment

Industrial flurorescent lighting in ‘white’ colour shall be used for illumination. High pressure discharge lamps should be used to light tall buildings and large areas. Low sodium discharge lamps shall not be used, as they constitute a fire hazard in the event of breakage. Hand-held lamps shall be rated for a maximum 50 V a.c. supply.

Plant lighting circuits shall be protected with maximum 16 A fuses or MCBs, but not loaded higher than 12 A. Plant lighting distribution boards shall include 10% spare outgoing circuits. Adjacent luminaires shall not be supplied from the same circuit.

The number of emergency luminaires in relation to the total number of fittings shall be determined as follows:
-          utility area: 20%
-          process area: 10%
-          administrative area: 5%
-          control room and auxiliary rooms: 50% (including 10% connected to inverter system)
-           substations, field auxiliary rooms, compressor and generator buildings: 30%

The types of portable equipment to be used in both industrial and non-industrial areas shall be one or more of the following:

-          double or reinforced insulation equipment, Class 2 of IEC 60536, connected to the mains via 30 mA RCCB, protecting both the supply cord and equipment.
-          42 V equipment, Class 3 of IEC 60536, connected to a safety-extra-low-voltage circuit by using double-wound safety isolating transformers, complying with IEC 60742 (SELV system)

In restrictive conductive locations which can be defined as locations where it is likely that a person could be in contact with conductive parts at two or more points simultaneously, no electrical hand-held equipment connected to an external supply shall be used. Instead, air-driven equipment or equipment with built-in batteries or an air-driven generator shall be used.

Safety isolating low voltage transformers may be rated for 250, 630 or 1000 VA.

10. Battery installations

When the total capacity of vented batteries exceeds 20,000 VAh, a separate battery room shall be provided in the substation and have an access door from within the substation. Valve regulated batteries of any capacity do not require a separate battery room. A water tap, eye-wash basin, sink and drain shall be installed in the room. Exhaust fan motors shall have type of protection ‘e’ or ‘d’, gas group C, exhausting to the outside of the battery room. The luminaires and convenience outlets shall be suitable for Zone 1, gas group IIC.

Design Practice - Earthing

1. HV systems

HV system neutrals shall be earthed at each source of supply. For grid infeed system voltages above 36 kV, the neutral point of transformers should be solidly earthed, unless otherwise required by the public utility.Transformer feeders to HV switch boards with a system voltage not exceeding 36 kV shall be resistance earthed. The rating of each resistor shall be such as to limit the earth fault current to a magnitude approximately equal to the rated full load current of the supply equipment (generator or transformer). When multiple generators are operated in parallel, only one earthing resistor may be put in line to avoid circulating current due to zero sequence triplen harmonics. If a generator with an earthing resistor trips in such a system, an alarm should be generated for alerting the operators to switch on the earthing resistor of another generator manually. The earthing resistor shall be rated to withstand the earth fault current for a duration of not less than 10 s.

2. LV systems

LV electrical system neutrals at each source of supply shall be solidly earthed. The system of earthing shall be designated TN-S in accordance with IEC 60364-3. The earth loop impedance shall be such as to effect circuit disconnection in a time not exceeding 1 s under solid earth fault conditions.

3. UPS systems

AC UPS systems shall have their neutrals solidly earthed. This applies equally to single and three phase systems. The inverter neutral shall be connected to the neutral of the bypass mains neutral, which is solidly earthed. If isolating transformers are required in the bypass supply, then inverter neutrals and transformer secondary neutrals shall be solidly earthed to the instrument ‘clean earth’ system. DC systems supplying instrumentation loads and switchgear control and protection loads shall be earthed through a high resistance earth fault monitoring circuit with a sensitivity of 5 mA. DC supplies for telephone systems shall be solidly earthed at the positive pole in line with normal telecommunication practice.

Design Practice - Protection and Control

1. Switchgear

The operating time of short-circuit protection shall be as short as possible and shall in any event not exceed the through-fault withstand duration of the switch board. For HV and LV switch boards, the rated short circuit withstand duration corresponding to rated withstand current shall normally by 1s. The maximum permissible operating time of circuit protection shall not therefore exceed 1s.

2. Transformers

Distribution transformers shall normally be connected Dyn. The protection may consist of phase short-circuit and earth-fault by means of two-stage overcurrent relays. Stage 1 shall be IDMT and set to detect secondary side faults. Stage 2 shall be instantaneous in operation and set to detect primary side faults only. For primary voltages up to and including 6.6 kV, fused contactors may be used.

3. Capacitors

HV capacitor banks shall comprise individually fused capacitor units. For large capacitor banks exceeding 1000 kVar, the capacitors shall be connected in double-star with unbalance protection monitoring the star-point voltages. Capacitor failure shall trip and bank and produce an alarm. Individual capacitors shall be controlled by contactors, circuit breakers, or, for LV applications, fused switch units. The switching devices shall be rated for at least 1.5 x In and must be able to withstand transient inrush up to 100 x In.

4. Motors

Overload protection of electric motors that are required to remain operational in the event of fire shall be set at a minimum of 300% of the motor full load current in accordance with NFPA 20. Such motors shall be clearly identified.

5. Power and convenience outlets

Each LV power and convenience outlet circuit shall be protected by a phase short circuit protective device and by an ELCB/RCCB. The ELCB operating current shall be 30 mA for circuits less than 125 A and 300 mA for circuits equal to or greater than 125 A.