Rectifier/transformer control

As in the case of power stations, where there is known to be considerable variation in operating conditions due to tidal changes, or in estuary waters variations in salinity, automatic control systems may be desirable. For such systems the current output of the transformer-rectifier is controlled by thyristor or transductors. Sensing electrodes are permanently installed on selected piles and transmit the electrode potential of the steel back to the controlling device. This type of system enables the most economic amount of current to be provided under all operating conditions. 

Much of the inefficiency in the cell results in heat generation a cell will heat itself almost to boiling when fed brine at about 40°C. The self-heating tends to lower the cell voltage because the electrical resistance of the cell decreases with increasing temperature. As modern rectification equipment (transformers, controllers, and rectifiers) is very efficient, the conversion of AC electricity into electrochemical products is essentially dependent on the cell efficiency. Typically, AC-kWhr/lb product = 1.03 DC-k Whr/lb product. 

Electrostatic separator with motor, drive, switches but excluding rectifiers, transformers, air valve controls. FOB cost = 35 000 at a (solids nominal capacity, kg/s)... 

When installing multiple transformers in parallel, one must ensure that their impedances are equal. If this is not so, sharing of the load is unequal and there will be circulating currents between adjacent transformers. These currents directly result in a loss of efficiency, which can lead to early failure, faulty power metering, and a loss in reliability. In the special case of rectifier transformers, the current in each is dictated by the rectifier control systems, but if they have been given different phase shifts to minimize harmonic distortion (Section 8.3.1.3), it is again necessary to ensure that their impedances are equal. 

Iligh-voltage controllers which regulate prirnaiv input voltage to the rectifier and wiper transformer and house primary current-limiting protection, meters, and instrumentation are designed for local or remote operation. 

A battery backed-up d.c. source of control supply is provided for the AMF panel and engine ignition. The control scheme, as illustrated, generally eonsists of a 220 or 240 V a.c. source of supply, with a transformer rectifier unit, to provide a 24 or 48 V d.c. control voltage, to charge the battery as required and a battery back-up of suitable capacity. 

Fig. 8-5 Circuit diagram illustrating the principle of a potential-controlled transformer-rectifier.

Current-controlling rectifiers are constructed in general on the same circuit principles as potential-controlling rectifiers only with them, the protection current is converted to a voltage via a constant shunt in the control circuit and fed in as the actual value. With devices with two-point control, the ammeter has limiting value contacts that control the motor-driven controlled transformer. 

Fig. 8-7 Principle of a transformer-rectifier control device with set-limiting potentials.

Anodes are connected to the object to be protected or to the transformer-rectifier by insulated conductors that are resistant to mineral oil (e.g., Teflon-coated cable) with a cross-section of 2.5 mm of Cu. The transformer-rectifier must meet the demands according to Ref. 6 and have the capability for monitoring and controlling its operation. The life of the anodes is in every case designed to be at least 15 years. 

Six iron anodes are required for corrosion protection of each condenser, each weighing 13 kg. Every outflow chamber contains 14 titanium rod anodes, with a platinum coating 5 /tm thick and weighing 0.73 g. The mass loss rate for the anodes is 10 kg A a for Fe (see Table 7-1) and 10 mg A a for Pt (see Table 7-3). A protection current density of 0.1 A m is assumed for the coated condenser surfaces and 1 A m for the copper alloy tubes. This corresponds to a protection current of 27 A. An automatic potential-control transformer-rectifier with a capacity of 125 A/10 V is installed for each main condenser. Potential control and monitoring are provided by fixed zinc reference electrodes. Figure 21-2 shows the anode arrangement in the inlet chamber [9]. 

Solid-State DC Drives. The controlled-thyristor rectifier and separate-field DC motor is the solid-state motor drive in greatest use. The combination provides control over at least a 10 1 speed range, plus an additional two to three times by field weakening. Depending upon the power level, the rectifier is operated directly from the AC supply lines, or via a transformer. Typical speed regulation of 2% can be accomplished with a single control system. The horsepower and speed limitations are set by the DC motor, not by the semiconductor rectifiers. The DC motor and rectifier can be combined to any required power level. 

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Automatically Controlled Modular System This method employs one large manually controlled transformer-rectifier used in conjunction with a number of modular cabinets located adjacent to each item of plant requiring protection. The main transformer-rectifier feeds d.c. to each of the module units and the modular unit provides the exact amount of current required by the item of plant in question. 

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