Switchboard, main

In a fixed construction, all the feeders in the switchboard, feeding the various load points, are securely mounted in the assembly and rigidly connected to the main bus. In the event of a fault in one feeder on the bus side, a shutdown of the entire switchboard may be required. A process industry or critical loads can ill afford such an arrangement. However, since this is the most cost-effective switchboard, it is also the most common type and is used extensively. It also suits all applications, except a process industry or critical loads, which may not be able to afford a total shutdown or prolonged downtime in the event of a fault. In such cases a draw-out type switchboard will prove to be a better choice as discussed below. A fixed-type construction may further be classified as follows. 

Figure 3.20 shows the transient voltage response at the main electrical switchboard for a commercial building due to capacitor bank switching by the utility. A moderate... 

FIGURE 5.2 Main service switchboard indicating elements of a ground system. 

Figure 6.103 (a) Low voltage switchboard for coal mines. A flameproof main compartment (with two doors) is fitted with a terminal compartment at right side (incoming power lines) and at left side (outgoing lines). 

The incoming and bnsbar section switching devices are usually air-break circuit breakers, which can be fixed or withdrawable from the main frame or enclosure. They can be 3 or 4-pole depending upon whether a 3 or 4-wire snpply is required. Some low power switchboards may use load-break switches for these functions. 

A good rule-of-thumb guide is to assume that between 15% and 25% extra capacity will be required. Hence the chosen rating will be 115% to 125% of the best-known estimate at the early design stage. This requirement also applies to power transformers and their main cables or overhead power lines, and to outgoing feeder cables to auxiliary switchboards and motor control centres. It does not usually apply to individual motor consumers, see Chapter 1. 

A modem plant requires more information, events and alarms to be made available at the main control room than was generally the case in the past. This has been made much easier to achieve by the use of computer networking and fibre optical technology. Most of the information that is available at the switchboard can be transferred to the main control room so that, for example, a one-line diagram presentation can be made on a computer desk-top monitor (man-machine interface, MMI). 

HV motor control centre LV main switchboard (no motors) LV main motor control centre LV auxiliary motor control centre HV motor terminals at full-load LV motor terminals at full-load HV motor terminals at starting LV motor terminals at starting... 

A preliminary estimate of the load is Si, which may be assumed to consist of a certain amount of motor load and some static load. The motor load can be assumed to be connected to the main high voltage switchboard and at lower voltage switchboards. Let the high voltage motor load be Sh and the lower voltage motor load be Sim Assume the static load Sis to be connected to the lower voltage switchboards. Typical power factors for these loads are 0.87, 0.85 and 0.97 respectively. The total active and reactive power estimates are,... 

For a new plant it may be assumed that all the generators that are connected to the main switchboard have the same rating and parameters, i.e. identical machines. Similarly their transformers may be assumed to be identical. The total capacity of the generators 5gen must be greater than the load 5ioad-... 

Suppose a main switchboard feeds load through transformers of different ratings and impedances. For the purpose of estimating fault current at an early stage in a project it is reasonable to combine all the distribution transformers into one equivalent transformer. The equivalent rating of all the... 

Three transformers feed a load from a main switchboard. Their ratings and impedances are,... 

The contribution of fault current from the main switchboard motors is... 

The total of these currents is the peak asymmetrical sub-transient fault current which is 61,375 amps. This is a conservative summation because it assumes that the three peaks occur at the same time. The fault making duty of the main switchboard must be greater than this value of current,... 

Whichever method is used it is not usually necessary to include the contribution of fault current from induction motors, because such current will have decayed to almost zero at the fault clearance time. If there are large motors connected to the main switchboard then their contribution will be similar to a generator and should be included, see sub-section 7.2.7 and Reference 3 therein, and sub-section 11.8.5. 

All these considerations apply to HV motors, particularly if they are fed directly from the main generator switchboard. LV motors can often be grouped together and considered as one large equivalent motor. It is sometimes possible to ignore the contributions from LV motors because their circuits often have a low X-to-R ratio, which causes the motor contribution to decay very fast. Also, the connected cables, busbars and transformers in the circuit will tend to attenuate the motor fault contribution. 

LV motors can occasionally be ignored when HV switchboard faults are being calculated but this will depend npon circumstances, e.g. the number of intermediate voltages exist in the system, whether there are many small motors or a few large motors, the average route length of motor and transformer feeder cables. On offshore platforms it is advisable to seriously consider the LV network. LV motor control centres will be inflnenced by their motor loads, and the effect of motor contribution will mainly be determined by the fnse, contractor and circuit breaker configurations. 

Consider the systems shown in Figures 11.11 to 11.13. Assume that all the generators are rated at 20 MVA, with sub-transient reactances of 0.15 pu and the main switchboard operates at 11 kV. The symmetrical making current duty of the switchboard is 30,000 amps. Ignore motor contribution in this example. Calculate the per-unit reactance Xr required for each reactor in the three different systems. 

The largest motors that are started direct-on-line at the main generator switchboard are large compared with the smallest generator e.g. 20% or larger. 

The standing load at a switchboard is high (80% or more) compared with its feeder capacity. This is especially a problem at the main generator switchboard when b) applies, see sub-section 12.2.2.4. 

Very and extremely inverse relays are used in systems where the fault level downstream is low when compared with that available at the point of main supply, e.g. a main generator switchboard. They are also nsed where coordination with upstream or downstream fuses is necessary. 

The system receives its main power from a circuit breaker (6) or contactor in the upstream switchboard or motor control centre. This switchgear will contain the main power protective relays... 

Each switchboard in the main distribution network can have its own one-line diagram displayed, and for the more complicated switchboards each busbar section can be displayed. The active and reactive power flows and current flows can be shown for each item where suitable analogue transducers have been fitted. This is particularly beneficial for the main power generation switchboard. AU names and tag numbers can be shown for clarity. A diagram similar to Figure 16.1 can be displayed. 

If there is a large number of motors to restart (or also called reaccelerate) then there will be a large surge of main feeder current if they all start at the same time. This will cause a severe volt-drop at the common busbars, which will cause the run-up times to be extended and the possibility of the overcurrent protection relay at the main feeder circuit breaker to trip. Similarly if there is a group of motor control centres all in the same state of restarting their motors, then their common busbar e.g. main generator switchboard, will experience a significant volt-drop which may not be acceptable. 

Change of status of main circuit breakers, motor starters, transformer feeders, busbar bustie circuit breakers, switchboard interconnectors, earthing switches, withdrawn devices. 

A switchboard consists of 20 circuit breakers. Each circuit breaker has, two indicator lamps each taking 1 amp continuously, a tripping solenoid taking 5 amps for one second, and a spring charging motor for reclosing which takes 3 amps for 30 seconds. The battery needs to supply current for 4 hours when a mains failure occurs. The ampere-hour (AH) duty is -... 

The relays would be fitted into the main switchboard, not at intermediate locations along the overhead line route. 

Single-line diagram of each main switchboard. 

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