Power compensation design

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

Figure 4-21 The gain and phase Bode plots for design example 4.7.1 (compensation design) (a) the gain plot for the power supply (h) the phase plot for the power supply.

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). 

In contrast, DSC, designed in 1960 by Watson184 and O Neill,185 is a newer, more quantitative technique that does measure Ts and TR, but also measures very precisely the electrical energy used by separate heaters under either pan to make Ts = TR (this is power-compensated DSC, useable below 650° C). The power input into S minus the power input into R is plotted against Tr. High-temperature DSC (useful for TR > 1000°C) measures the heat fluxes by Tian-Calvet thermopiles rather than the electrical power, as a function of Tr. In a heat-flux DSC, both pans sit on a small slab of material with a calibrated heat resistance. The temperature of the calorimeter is raised linearly with time. A schematic DSC curve is shown in Fig. 11.80. 

Two principal DSC designs are commercially available—power compensated DSC and heat flux DSC. The two instruments provide the same information but are fundamentally different. Power-compensated DSCs heat the sample and reference material in separate furnaces while their temperatures are kept equal to one another (Fig. IB). The difference in power required to compensate for equal temperature readings in both sample and reference pans are recorded as a function of sample temperature. Heat flux DSCs measure the difference in heat flow into the sample and reference, as the temperature is changed. The differential heat flow to the sample and reference is monitored by chromel/ constantan area thermocouples (Fig. IC). ... 

Figure 10.4 Differential scanning calorimetry (DSC) instrumentation design (a) heat flux DSC and (b) power compensation DSC. A, furnace B, separate heaters and C, sample and reference holders. (Reproduced with permission from E.L. Charsley and S.B. Warrington, Thermal Analysis Techniques and Applications, Royal Society of Chemistry, Cambridge, UK. 1992 Royal Society of Chemistry.)...

Whereas most fixed-cell instruments are power-compensation instruments (because it is possible to place heaters on the base of cells that are not removable), batch-cell instruments are available as either power-compensation or heat-flux designs. One design of a heat-flux, batch-cell instrument is the micro-DSC in (Setaram). The instrument consists of a calorimetric block into which two channels are machined. One channel holds the sample cell, the other holds the reference cell. At the bottom of each channel, between the cell and the block, is a plane-surfaced transducer. The transducers provide a thermal pathway between the cells and the block and are used to maintain the cells at a temperature identical to that of the block. The electrical signal produced by the transducer on the sample side is proportional to the heat evolved or absorbed by the sample. The temperature of the calorimetric block is maintained by a precisely thermostated circulating liquid. The liquid is raised in temperature by a separate heater and is cooled by a supply of circulating water. The precise control of the temperature of the circulating liquid allows scan rates of just 0.001°C min-1 to be attained and ensures that the calorimetric block is insulated from the surrounding environment. 

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

A totally different DSC design is used in power-compensation DSC system. Here, the sample and the reference holders are insulated from each other and have their own individual heaters and sensors as shown in Fig. 7.19. During heating, the same heating power is supphed to both microfumaces via a control circuit and ideally the temperature of both microfurnaces is identical. When a reaction takes place (e.g. melting process, endotherm) the sample temperature becomes less than that of the reference, which is recorded by the temperature sensor and is immediately compensated by the sample heater. Thus, a power-compensated DSC measures the electrical power that is required to keep both... 

Power-compensation This term is applied to the design which in its original form was introduced in 1964. Figure 2(a) shows the main features of the DSC cell - the provision of separate temperature sensors and heaters for the sample and reference. In the event of a temperature difference arising between the sample and reference, differential thermal power is supplied to the heaters to eliminate the difference and to maintain the temperature at the program value. The differential thermal power is the source of the instrument signal. 

In power compensated DSC the small size of the individual sample and reference holders makes for rapid response. The temperature sensors are platinum (Pt) resistive elements. The individual furnaces are made of Pt/Rh alloy. It is important that the thermal characteristics of the sample and reference assemblies be matched precisely. The maximum operating temperature is limited to about 750 °C. High temperature DSC measurements (750-1600°C) are made by heat flux instruments using thermocouples of Pt and Pt/Rh alloys. The thermocouples often incorporate a plate to support the crucible. The use of precious metal thermocouples is at the expense of a small signal strength. Both chromel/alumel and chromel/constantan are used in heat flux DSC equipment for measurements at temperatures to about 750 °C. Multiple thermocouple assemblies offer the possibility of an increased sensitivity - recently a 20-junction Au/Au-Pd thermocouple assembly has been developed. Thermocouples of W and W/Re are used in DTA equipment for measurements above 1600°C. The operating temperature is the predominant feature which determines the design and the materials used in the con-... 

The DTA/DSC-EGD coupled simultaneous technique and relevant equipment have been investigated since 1979 [62, 74, 75]. The Model CDR-1 power compensation DSC analyzer (ambient temperature ca 720 C) was developed by the Tian Ping Instrumental Factory (Shanghai, China). The EGD detector is a thermal conductivity detector (TCD) in the GC analyzer. The CDR-1 DSC analyzer coupled with GC was constructed using a specially designed gas conduit. A schematic diagram of the on-line coupled simultaneous DTA/DSC-EGD apparatus is shown in Figure 2.33. 

Heat flux DSC and power-compensation DSC are used to analyze most polymers. The difference between them, besides the temperature-sensing device, is the speed and accuracy at which they can heat and cool a sample specimen. The power-compensation DSC has the ability to heat and cool at very fast rates (up to 500°C/min), while the heat flux DSC effectively heats or cools at slower rates (up to 30°C/min). Their differences in heating/cool-ing are due to the design of the calorimeters and the heat transfer characteristics of each. For a heat flux DSC diagram, see Fig. 15. 

Figure 7.28 Basic design of a power-compensated differential scanning calorimeter.

For these reasons, most designers preferred to make use of the power compensation DSC for their construction (see Hohne and Kaletunf, 2009, and references given there). 

PerkinElmer Life and Analytical Sciences (PKI, Perkin-Elmer, Perkin-Ehner) offers two types of DSC modules. The first is constructed on a heat flux design, while the second design is based on the power compensation principle. Table 2.10 compares the characteristics of the two DSCs ... 

Most of the 500-/400-kV cables shown in Table 3.4 are installed in highly populated areas, hence the route lengths are limited to 10-20 km. These cables are equipped with shunt reactors for the compensation of the charging capacity, but their unit size and the total capacity are not as large due to the shorter route lengths. For these cables, only studies such as the reactive power compensation, the design of the cable itself, and the laying method are discussed in the literature. Transient studies on these cables are not available. 

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