Of particular interest are the parameters, which determine the a-c characteristics:
Capacitivity, power factors, resistivity and power loss. All these parameters may be determined by the measurement of any three of the quantities: volts, amperes, watts and power factor. There are several methods of measurement, some of the more common of which will be described.
1.10.1 Wattmeter-Voltmeter -Ammeter Method
This is a common and well-understood method, and was used by the Doble Company in there Type I test sets. The usual connections are shown in Figure 1-17. In addition to the power taken by the dielectric, the wattmeter measures the power taken by its own current coil, If not negligible, the I2 Rc loss in the current coil may be subtracted from the wattmeter reading. To avoid measuring losses in the ammeter, it is short-circuited while the wattmeter is being read.
Figure 1-17
With high voltages, a potential transformer to reduce the voltage to the wattmeter and voltmeter will be necessary. From the instrument readings,
EI F P
P. .= (26)
ω E
C ≈ I farads (27)
12
R= P ohms (series) or P R E
= 2 (parallel) (28)
Where P is the power in watts, E the voltage, I the current: ω = 2πf where f is the fre-quency in cps; and R the resistance.
A glance at Figure 1-9 (b) will show that in terms of the equivalent parallel circuit it is the leakage, or in-phase, component of the current on which the wattmeter reading de-pends. The total current, however, is acting to produce heating (I2R losses) in the current coil of the wattmeter. At very low power factors, therefore, it is impossible to build a wattmeter with sufficient torque without excessive heating.
1.10.2 Type M Circuit
To provide a deflecting instrument for measuring losses at low power factor, the Doble Company developed the "M" circuit. In this circuit the wattmeter is replaced by a multi-range milliammeter with sufficient sensitivity to measure the in-phase component of current. Additional circuitry provides an adjustable source of wattless, or quadrature, current which can be applied to the meter in such a direction as to oppose the quadrature current of the specimen. With proper adjustment, recognized by a minimum reading on the meter, the quadrature current is entirely eliminated from the reading on the meter, and the meter then reads the inphase component. When a factor representing a predetermined test voltage is applied to the meter scale, the meter will read directly in watts.
A practical application of this circuit is shown in Figure 1-18 in a simplified form,
quadrature voltage Vb produced across Rb by the current from the loss-free standard capacitor. Exact balance can be obtained by adjusting Rb to the value, which results in a minimum reading of the meter. The only voltage then acting on the meter is produced by the in-phase component of the specimen current Ig flowing through the resistor Ra; that is, Vb = Ig Ra. The loss in the specimen is W = EIg = EVb / Ra, and when E is fixed, W = KVb. Using the scale factor K, the meter is calibrated to read in watts.
1.10.3 Capacitance Bridge
The precision, which is obtainable with indicating instruments such as voltmeters, amme-ters and wattmeamme-ters, is limited. With careful calibration the accuracy obtainable is probably not better than one-half of one per cent. Moreover, with very small capacitances, ammeters and wattmeters rated for the resulting small currents are unobtainable. With bridge circuits, the precision obtainable depends on the accuracy with which resistors, inductors and capacitors can be adjusted, which is very high. The only instrument involved is a null-type detector, which can be very sensitive. Thus much greater precision can be obtained with bridge methods than with indicating instruments.
Bridge methods are generally used in the laboratory while deflection methods are used for field measurements where great precision is not required and where the speed and simplicity of deflection methods are an important consideration.
Figure 1-19
The simplest type of Capacitance Bridge is the Wheatstone - bridge type shown in Figure 1-19. A & B are two resistor arms, B usually being adjustable. Cx is the equivalent series capacitance of the dielectric and Rx its equivalent series resistance. Cs is a standard loss-free capacitor, which may be adjustable, and Rs a variable resistor in series with Cs. ''Gen" is an alternating-current source and D a detector which may be a telephone receiver if an audio frequency is used.
At balance, the capacitance of the dielectric is
A C B
Cx = s (29)
(Note that the ratio B/A is the reciprocal of that used when resistances are being measured by a Wheatstone bridge.)
The dissipation factor of the dielectric is
s
The equivalent series resistance of the dielectric is
B R A
Rx = s (31) 1.10.4 Schering Bridge
The Schering bridge is widely used for dielectric measurements, particularly at high voltages. A diagram of the bridge is shown in Figure 1-20. Cx is the equivalent series capacitance of the dielectric and Rx its equivalent series resistance. Cs is a standard loss-free capacitor, which at high voltage is usually-of the air or gas-filled type; A is an ad-justable resistor, B a fixed resistor paralled by variable capacitor Cp. Balance is usually obtained by the adjustment of the ratio arm A and the capacitor C p.
Figure 1-20
At balance, the dissipation factor of the dielectric is B
C R C
Tanδ =ω x x =ω P (32) The capacitance of the dielectric is
A C B
CX = S (33)
The foregoing measurements give the equivalent series capacitance and resistance of the dielectric.
It is also convenient to know the corresponding equivalent parallel capacitance and conductance (see Figure 1-21 also Figure 1-8 (a) and Figure 1-9 (a).)
Figure 1-21
The relations among these parameters are as follows (see Figure 1-21),
2 2
1 S2 S S S
P C R
C C
ω
= + (34)
2 2 2
2 2
1 S S S
S S
P C R
C G R
ω ω
= + (35)
Note that ω2CS2RS2 =tan2δ (36)
Doble Engineering Company
2.1 Introduction
The treatise "Dielectric Theory and Practice" (1) presented at the 1962 Client Conference was devoted primarily to the basic phenomena, which underlie the behavior of dielectrics. Dielectrics and their properties were defined in accordance with the standard definitions of ASA. A dielectric was defined as a medium in which it is possible to produce and maintain an electric field with little or no supply of energy from outside sources, and that a vacuum is the only perfect dielectric. An insulating material is one in which a small, but not negligible, current flows, when a potential difference is applied between any two points. It follows that an insulating material is also a dielectric and that a dielectric is also an insulating material, each having both dielectric and insulating properties. Hence, both terms are often used indiscriminately when reference is made to a given material. In the foregoing treatise (1) there were also presented the characteristics of the dielectric circuit including resistance, conductance, capacitance and power factor. Then there followed a description of the basic elements which constitute the body of dielectrics such as atoms, positive and negative ions, polar and non-polar molecules; also their behavior in direct-current and alternating-current fields. It was shown that the lateral movement of positive and negative ions and the rotation of the dipoles in the electric field accounted for many of the well-known characteristics of dielectrics such as absorption, conduction, power loss, and capacitivity, and that these were functions of field strength, frequency and temperature.
In as much as electrical apparatus, with its insulation, is used for many and diverse applications, it is exposed to wide ranges of environment, such as temperature, moisture, exposure to weather, chemicals, etc. Hence, among the insulating materials there must be a wide range and diversity of electrical and physical properties to meet these many different requirements.
It is the purpose of this treatise to describe a limited number of the well-known types of insulation, their electrical properties and characteristics under operating conditions, as functions of voltage gradient, temperature and frequency. So far as possible these characteristics are related to the fundamental behavior of the basic elements of the dielectrics, which were described in the prior treatise. The applications of these different insulations to the types of apparatus to which they are adapted are then described.
One major factor affecting the life of an insulating material is thermal degradation, although moisture, chemical contamination, corona, voltage stress, and mechanical stress may also contribute to its deterioration, especially after the material has been weakened by thermal degradation. Data obtained from accepted test procedures and from field tests have permitted the evaluation of the thermal life of several of the insulating materials. Based on these data the
(2)
(l) Numbers refer to Bibliography at the end of this chapter