Direct-current Meters

Functions of a Direct Current Meter. — A direct current meter is an instrument intended for the measurement of electrical quantity in a direct current circuit. There are two main classes of direct current meters, (I) ampere-hour meters and (II) watt-hour meters. An ampere-hour meter measures the product of the current in amperes flowing in a circuit and the time in hours during which the flow is maintained. A watt-hour meter measures the product of the power in watts and the time in hours during which the flow of power is maintained.

Direct Current Ampere-hour Meters.— Ampere-hour meters are used by electrical undertakings for measuring the supply of electricity to domestic and industrial consumers. These undertakings are under a statutary obligation to maintain the voltage at consumers' terminals at a declared value within close limits; assuming that the supply voltage is maintained at the declared value, an ampere-hour meter can be calibrated to register in terms of kilowatt-hours at this voltage. This principle is accepted as satisfactory in most countries where the voltage at consumers' terminals is maintained within narrow limits of the declared voltage, and since direct current ampere-hour meters are, in general, more reliable and less costly than direct current watt-hour meters the practice has much in its favour.

In addition to the foregoing, ampere-hour meters are used for measuring the current consumption in battery charging, electro-deposition and other electrolytic or industrial processes and in some instances they exercise a controlling function over these operations. Many types of ampere-hour meter have been manufactured in the past, the most important being electrolytic meters and motor meters. Theoretically the former are capable of very accurate registration but in practice the working results are not so good as with motor meters, and the latter are preferred by most supply authorities.

Kilovolt-ampere Meters

Influence of Power-Factor on Supply Costs. — The cost of supplying electrical energy to industrial consumers depends not only on the kWh consumed but also on the power-factor of the load and the maximum demand. Where there is no power-factor tariff, the consumer who requires 100 kVAh at unity power-factor pays exactly the same price for current as the consumer who takes 200 kVAh at 0.5 power-factors. But the cost of the supply in the second case is higher than in the first. The PR losses in the transmission and distribution system and also in the generators and transformers are four times as great in supplying the consumer at 0.5 power-factors and the effective use which can be made of the supply system is less.

It has been computed that the average losses due to units unaccounted for are of the order of 12 per cent*, and a substantial part of these losses results from reactive power circulating in the supply system without doing useful work. The cost of these losses per kWh sold is comparable to the fuel cost per kWh. Any improvement which can be. effected in the power-factor of the load will result in a lowering of the cost per kWh by reducing the PR losses in the supply system, and will enable capital expenditure locked up in cables and plant to be used more effectively, thus further reducing the all-in costs.

Since consumers having a load with a low power-factor incur losses which result in an increase in the average cost per kWh supplied, it does not appear equitable to expect consumers with a high power-factor to share these additional costs. Accordingly, tariffs have been devised whereby the charge to the consumer is related to the cost of the supply. One such tariff consists of two parts, the first being a fixed charge based upon the consumers' maximum demand in kVA, and the second, a running charge based upon the consumption in kWh. Under this tariff the consumer with a high power-factor will, other things being equal, incur a lower fixed charge than the consumer with a low power-factor, and the latter may, by the installation of devices for the improvement of power-factor, reduce his fixed charge. This is an encouragement to industrial users of electrical energy to improve their power-factor and thus to reduce the cost of the energy supplied.

KVA Meters of Various Types

Many types of the kVA meters have been suggested and a few have been produced commercially. Of these, some are too complicated or too expensive to survive in a competitive world and others have not fulfilled the expectations of their designers from the standpoint of performance. A rectified current consists of a succession of half-waves, all acting in the same direction. The same applies to a rectified voltage and consequently, the effects of phase displacement between current and voltage on the alternating-current side still persist on the direct-current side, as the peaks of the pulses do not coincide. The introduction of smoothing devices can minimize this effect and conceivably a single-phase kVA meter may be devised which is capable of giving an acceptable performance at a comparatively high cost. But single-phase kVA meters are not required in practical working and a three-phase instrument consisting of the combination of three direct-current watt-hour meters, complete with six rectifiers, smoothing devices, transformers and the associated means for summating the three registrations on one register and demand indicator becomes so costly and bulky as to be out of the question so far as commercial metering is concerned.

Another form of kVA meter which recently has been produced commercially on a comparatively small scale consists of a polyphase meter of conventional pattern, combined with a form of phase-shifter and a power-factor relay. The phase-shifter is connected to the three-phase supply and has a large number of tappings brought out to contact studs arranged in a circle. A rotary shaft driven by a motor and a step-by-step mechanism carries four contact blades which are arranged to sweep over the contact studs. From the contact blades, connections are taken to the voltage coils of a polyphase meter and a power-factor relay. The latter is so arranged that, so long as the voltage and current applied to its voltage and current windings respectively, are in phase, the relay contacts remain open. If there is an alteration in the power-factor of the main circuit there will be introduced a phase displacement between the voltage and current applied to the coils of the power-factor relay. This produces a torque and results in the closure of a pair of contacts in the relay, which in turn starts up the motor and causes the rotary shaft to transfer the contact blades to another set of contact studs.

The voltage derived from the phase-shifter is constant in magnitude if the line voltage is constant, but there is a difference of approximately 10 deg. in phase between adjacent studs. When the contact blades reach a set of studs from which the derived voltage differs by less than 10 deg. in phase displacement from the phase of the. current, the power-factor relay contacts open and the motor driving the rotary shaft stops. By this means the voltage applied to the relay and also to the meter is always maintained substantially in phase with the current in the main circuit. The motor which moves the contact blades is reversible, and the direction in which it moves is determined by the power-factor relay; this latter closes one pair of contacts if the current is leading with respect to the voltage across the relay and another set if the current is lagging. Thus, the motor is always caused to move in such a direction as to reduce the phase displacement between the voltage and current applied to the relay windings. By these means the polyphase meter is always working under conditions approximating to unity power-factor in the main circuit. The error in the kVA measurement resulting from this small departure from the ideal condition does not exceed plus or minus 0.75 per cent, above the normal meter error when measuring kWh.

In addition to the meters already described, which give a measurement of kVA derived from the integration of kVAh, there is another class of kVA maximum demand indicator operating on a thermal basis. Most of these instruments measure current values only and are calibrated for use at a declared voltage. A few, however, are influenced by voltage as well as current. Again, the majority is for use on single-phase circuits, although one or two exceptions are constructed for polyphase work.

 М.А. Беляева и др. «Сборник технических текстов на англ. языке»