Rectifier Circuits—Multiphase
A widely used rectifier circuit, especially for low-voltage, high-current rectifiers, is shown in Fig. 9.4 at the left. In the days before availability of efficient silicon rectifiers and controlled rectifiers, this was the customary circuit. It is called a delta six-phase double-wye, USAS Circuit 45. An interphase transformer is used to allow 120° conduction in the rectifiers. Only a single rectifier is in the series circuit between each transformer winding and the return from the load; hence, the argument goes, it is more efficient than the bridge circuit at the right, a delta six-phase wye, double-wye, USAS Circuit 23, that has two rectifiers in series. But things are really not quite so simple. The rectifier losses are indeed cut in half. However, the circuit at left has transformer secondary windings that conduct half-wave currents and will probably have higher eddy current losses than the transformer at right. The interphase transformer introduces still more losses. Also, the capital cost is increased by the lower kVA efficiency of the secondary windings and the need for an interphase transformer.
Magnetics costs generally outweigh semiconductor costs by a considerable degree, so the circuit selection based on the lower lifetime costs must consider the long-term cost of capital as well as the operational cost of losses. However, this may be a hard sell to many users who have been buying rectifiers for years, because the myth of superiority of the interphase circuit is well established in the industry.
As a compounding factor, the transformer vendors seldom will warrant the efficiency of their transformers with anything but sinusoidal currents, so the additional losses from half-wave conduction and the added eddy currents seldom appear in quotations from them.
Commutation
Three-phase rectifiers transfer the load current from phase to phase in sequence. The current cannot be transferred instantaneously, however, because of inductance in the supply. The process of driving the current out of one phase and into another is termed commutation, and it always results in a loss of output voltage to the load. Figure 9.5 illustrates the process for a three-phase bridge rectifier. VA, VB, and VC are the line-to-neutral voltages. At time 1, the current is flowing from phase A through the positive bus to the load and returning through the negative bus to phase C. The positive bus voltage is the phase-A voltage. At time 2, the positive phase-B voltage is becoming greater than the positive phase-A voltage, and the current begins to transfer from phase A to phase B. The voltage driving the current transfer is the line-to-line voltage VBA. During this period, the positive bus voltage is the average of phase A and phase B voltages. At time 3, the current transfer into phase B is completed, and the positive bus voltage is now the phase-B voltage.
Bridge rectifier silicon controlled rectifier rectifier series half-wave rectifier circuit
Bridge rectifier rectifier circuit full-wave rectifier circuit
Rectifier monitoring solid state rectifier
Halogen converter international converters Line/ Level converter 8-channel AD/DA converter
The same process of current transfer takes place on the negative bus as well, and the output voltage of the bridge is always the difference between the positive bus voltage and the negative bus voltage. A commutation is taking place every 60°.
Commutations are driven by the voltage difference between the outgoing and incoming phases. Initially, the difference voltage is zero in a rectifier, and the di/dt is also zero. The commutating current can be visualized as a current that circulates from the incoming to the outgoing phase. It is equal to the time integral of the voltage difference divided by the sum of the source inductances in each of the two phases. The circulating current is the current in the incoming phase, and it subtracts from the current in the outgoing phase. Commutation is completed when this current is equal to the load current.
UNIT 4. POWER SUPPLYING
ELECTRICAL CABLES
ELECTRIC CABLES
1) All electric cables and wiring external to equipment shall be at least of a flame-retardant type and shall be so installed as not to impair their original flame-retarding properties; the use of special types of cables such as radio frequency cables, which do not comply to the above specifications may be permitted.
2) Cables and wiring serving essential or emergency power, lighting, internal communications or signals shall so far as practicable be routed clear of galleys, laundries, machinery spaces of category A and their casings and other high fire risk areas.
3) Cables connecting fire pumps to the emergency switchboard shall be of a fire - resistant type where they pass through high fire risk areas. Where practicable all such cables should be run in such a manner as to preclude their being rendered unserviceable by heating of the bulkheads that may be caused by a fire in an adjacent space.
4) Cables and wiring shall be installed and supported in such a manner as to avoid chafing or other damage.
5) Terminations and joints in all conductors shall be so made as to retain the original electrical, mechanical, flame-retarding and, where necessary, fire-resisting properties of the cable.
Extract from ABS Rules for Building and Classing Steel Vessels - 2005
Part 4 Vessel Systems and Machinery - Chapter 8 Electrical Systems
Section 4 Shipboard Installation and Tests
Intrinsically Safe Systems
Installation of Cables and Wiring (2005)
(a) General. Installations with intrinsically safe circuits are to be erected in such a way that their intrinsic safety is not adversely affected by external electric or magnetic fields under normal operating condition and any fault conditions, such as a single-phase short circuit or earth fault in non-intrinsically safe circuits, etc.
(b) Separation and Mechanical protection. The installation of the cables is to be arranged as follows:
i) Cables in both hazardous and non-hazardous areas are to meet one of the following requirements:
• Intrinsically safe circuit cables are to be installed a minimum of 50 mm (2 in.1 from all non-intrinsically safe circuit cables, or
• Intrinsically safe circuit cables are to be so placed as to protect against the risk of mechanical damage by use of mechanical barrier, or
• Intrinsically safe or non-intrinsically safe circuit cables are to be armored, meat sheathed or screened.
ii) Conductors of intrinsically safe circuits and non-intrinsically safe circuits are net to be carried in the same cable.
iii) Cables of intrinsically safe circuits and non-intrinsically safe circuits are not to be j in the same bundle, duct or conduit pipe.
iv) Each unused core in a multi-core cable is to be adequately insulated from earth and from each other at both ends by the use of suitable terminations.
Cables in Hazardous Areas
Cables in hazardous areas are to be armored or mineral-insulated metal-sheathed. Where these cables pass through boundaries of such locations, they are to be run through gastight fittings. No splices are allowed in hazardous areas, except in intrinsically safe circuits.
Unquote
Extract from ABS Rules for Building and Classing Steel Vessels - 2005
Part 4 Vessel Systems and Machinery - Chapter 8 Electrical Systems
Section 5 Special Systems
Cable Installation
Runs of Cables (2003)
In accommodation spaces, high voltage cables are to be run in enclosed cable transit systems.
High voltage cables of different voltage ratings are not to be installed in the same cable bunch, duct, pipe or box. Where high voltage cables of different voltage ratings are installed on the same cable tray, the air clearance between cables is not to be less than the minimum air clearance for the higher voltage side in 4-8-5/3.7.1(a). However, high voltage cables are not to be installed on the same cable tray for the cables operating at the nominal system voltage of 1 kV or less.
Installation Arrangements (2003)
High voltage cables are to be installed on cable trays or equivalent when they are provided with a continuous metallic sheath or armor which is effectively bonded to earth; otherwise, they are to be installed for their entire length in metallic casings effectively bonded to earth.
Termination and Splices (2003)
Terminations in all conductors of high voltage cables are to be, as far as practicable, effectively covered with suitable insulating material. In terminal boxes, if conductors are not insulated, phases are to be separated from earth and from each other by substantial barriers of suitable insulating materials. High voltage cables of the radial field type, i.e., having a conductive layer to control the electric field within the insulation, are to have terminations which provide electric stress control. Terminations are to be of a type compatible with the insulation and jacket material of the cable and are to be provided with means to ground all metallic shielding components (i.e., tapes, wires etc).
Marking
High voltage cables are to be readily identifiable by suitable marking.