Power compensation

This device controls the generator and maintains a steady-stale armature voltage automatically within the predefined limits. It also serves to control the reactive kVAr loading during a parallel operation or when the machine is being used as a synchronous condenser for reactive power compensation through a quadrature droop control (QDC) as noted below. 

Consider Figure 23.29(a), where, the p.f. of a power circuit is to be improved from cos 0, to cos 02- If kVAri is the reactive component of power at p.f. cos 0, which is to be improved to kVAr2, at p.f. co.s 0i, through the reactive power coinpensation, then the reactive component of power compensated or kVAr rating of the required capacitor banks... 

Central Board of Irrigation and Power. India. Workshop on Reactive Power Compensation Planningand Design. Dec. (1993). 

Experiments were performed in tlie SIMULAR calorimeter using the power compensation method of calorimetry (note that it can also be used in the heat flow mode). In this case, the jacket temperature was held at conditions, which always maintain a temperature difference ( 20°C) below the reactor solution. A calibration heater was used to... 

The power compensating DSC (Figure 156) uses a sample and a reference. During a controlled temperature program the sample and the reference are held at equal temperatures by adjusting the heat supplied to them by separate... 

Figure 156. Power compensating DSC (picture Universitat de Lleida)...

Two types of DSC measurement are possible, which are usually identified as power-compensation DSC and heat-flux DSC, and the details of each configuration have been fully described [1,14]. In power-compensated DSC, the sample and reference materials are kept at the same temperature by the use of individualized heating elements, and the observable parameter recorded is the difference in power inputs to the two heaters. In heat-flux DSC, one simply monitors the heat differential between the sample and reference materials, with the methodology not being terribly different from that used for DTA. Schematic diagrams of the two modes of DSC measurement are illustrated in Fig. 9. 

Fig. 9 Schematic diagrams illustrating the sample cell configurations for (a) power-compensation and (b) heat-flux modes of DSC detection. Each cell system is contained in the furnace assembly, and the differential heat flow between sample and reference is monitored as the experimental observable and ultimately is plotted as a function of the system temperature.

The equipment for power-compensated DSC involves two parallel temperature measurement systems. Sample (ca. 50 mg) and reference in... 

Power-compensated differential scanning calorimetry (DSC) apparatus (S = sample R = reference). 

Adiabatic calorimeters are complex home-made instruments, and the measurements are time-consuming. Less accurate but easy to use commercial differential scanning calorimeters (DSCs) [18, 19] are a frequently used alternative. The method involves measurement of the temperature of both a sample and a reference sample and the differential emphasizes the difference between the sample and the reference. The two main types of DSC are heat flux and power-compensated instruments. In a heat flux DSC, as in the older differential thermal analyzers (DTA), the... 

Whereas the heat flux DSC measures the temperature difference between the sample and the reference sample, power-compensated DSCs are based on compensation of the heat to be measured by electrical energy. Here the sample and the reference are contained in separate micro-furnaces, as shown in Figure 10.6(b). The time integral over the compensating heating power is proportional to the enthalpy absorbed by or released from the sample. 

Figure 10.6 Schematic representation of (a) heat flux DSC and (b) power-compensated DSC.

FIGURE 2.12. Schematic Representation of Heat-flux DTA and Power Compensation DSC... 

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

The CPA [188], marketed by ThermoMetric AB (Sweden), is frequently used in Europe. It operates on the principle of power compensation, which is based on the supply or withdrawal of heat to and from the reactor, respectively, in order to keep the temperature at the set-point and, thus, to compensate for energy differences (either shortage or surplus). The heat is supplied or withdrawn by means of special (Peltier) elements, which produce a cold or a hot surface area if subjected to an electrical current. An accurate measurement of the heat supply/withdrawal is possible as the heat flow is directly proportional to the current supplied to the Peltier elements. 

Fig. 4.6. Schematic diagrams of the power-compensation and heat-flux modes of DSC measurement.

In the DTA measurement, an exothermic reaction is plotted as a positive thermal event, while an endothermic reaction is usually displayed as a negative event. Unfortunately, the use of power-compensation DSC results in endothermic reactions being displayed as positive events, a situation which is counter to IUPAC recommendations [38]. When the heat-flux method is used to detect the thermal phenomena, the signs of the DSC events concur with those obtained using DTA, and also agree with the IUPAC recommendations. 

The two basic types of reaction calorimeters commonly used for safety assessments are isothermal (including both heat flow and power compensation calorimeters) and adiabatic. 

In power compensation calorimeters, the jacket temperature is set slightly below the desired reaction temperature. A heater in the reaction mass maintains the set temperature. A change in electrical power to the heater compensates for any change in reaction temperature. This provides a direct measure of the heat produced by the chemical reaction. 

As mentioned above, titration methods have also been adapted to calorimeters whose working principle relies on the detection of a heat flow to or from the calorimetric vessel, as a result of the phenomenon under study [195-196,206], Heat flow calorimetry was discussed in chapter 9, where two general modes of operation were presented. In some instruments, the heat flow rate between the calorimetric vessel and a heat sink is measured by use of thermopiles. Others, such as the calorimeter in figure 11.1, are based on a power compensation mechanism that enables operation under isothermal conditions. 

In a power compensation DSC (figure 12.3), the sample and the reference crucible holders consist of two small furnaces, As and Ar, each one equipped with a temperature sensor, Bs or Br, and a heat source, Cs or Cr. The furnaces... 

Figure 12.3 Schemeofa power compensation differential scanning calorimeter. A sample furnace Ar reference furnace B temperature sensor of the sample furnace Br temperature sensor of the reference furnace C resistance heater of the sample furnace Cr resistance heater of the reference furnace D cell S sample R reference.

Androsch, R., Moon, I., Kreitmeier, S., and Wunderlich, B. (2000). Determination of heat capacity with a sawtooth-t rpe, power compensated temperature-modulated DSC. Thermochimica acta. 357-358,267-278. 

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