Device for Measuring Average Values of Electrical Loads

DEVICE FOR MEASURING AVERAGE VALUES OF ELECTRICAL LOADS

V. V. Litvak, V. V. Prokopchik, V. M. Makarov

(Presented by the scientific seminar of the Department of Electrical Systems and Networks)

In connection with the introduction in 1967 of a new electricity tariff, the basic charge of the two-rate tariff for electricity for industrial consumers with an annual load maximum of not less than 500 kW is calculated according to the value of the power declared by the consumer that participates in the maximum load of the power system.

However, the introduction of this economically effective system of payment for electricity, aimed at leveling the load curve of the power system, does not produce the expected effect, since power-supply organizations have practically no instruments for monitoring the declared power.

It should be noted that, beginning in 1969, special active-energy electricity meters equipped with indicators of maximum power averaged over 15 or 30 min. [1] began to be used for periodic monitoring of the declared power of enterprises.

However, a number of shortcomings of these meters casts doubt on the necessity of their widespread introduction at enterprises. In particular, the maximum indicator of these meters has a return time to the initial state of about 9 sec. Return is required when passing from one measurement interval to another. This leads to significant measurement errors, especially when the electricity meter is connected through current and voltage transformers with large transformation ratios. In addition, the contact clocks used to control the electricity meter have low accuracy of operation and low accuracy of actuation on the setting scale. Also of considerable importance is the circumstance that replacing ordinary meters with meters that record the maximum, purchased in the GDR and not produced by domestic industry, entails significant costs.

For periodic measurements of the value of the declared power, it is advisable to use the billing electricity meters already available to consumers. For this purpose a device has been developed, the average value of the load being determined with its aid as the ratio of the number of revolutions of the electricity-meter disk measured over the desired time interval to the duration of this interval. For direct-connected meters this is written as follows:

\[ P_{\text{av}}=\frac{N}{K\cdot t}, \quad \text{kW}, \]

where \(N\) is the number of revolutions of the disk over the time interval \(t\);

\(K\) is the meter constant, rev/kWh.

The number of revolutions of the meter disk is determined by means of a photorelay, which does not require disconnecting the meter from the network or entering its housing. The photorelay (Fig. 1) is made as an attachment to the billing meter and responds to the movement of the red mark on the disk.

Fig. 1. Schematic circuit diagram of the pulse sensor. \(T_1\) and \(T_2\) are MP26-type transistors; \(R_1\) is a photoresistor of type FSK-Sh or FS-3A; \(R_2\)–\(R_7\) are resistors, respectively, of \(100;\ 1.2;\ 0.27;\ 1;\ 0.056;\ 1.8\ \text{k}\Omega\); \(P_1\) is a RES-10 relay, passport RS4.524.302.

The device consists of several functional blocks (Fig. 2). Pulse sensor (photorelay) 1 is electrically connected to memory blocks 3 and 5 of the first and second channels. The memory block of each channel is built on three step switches of type ShI-11, which makes it possible to store 999 pulses without resetting the readings. Each hundred pulses in the memory blocks corresponds to 5 volts, and 10 pulses to 0.5 volts.

Fig. 2. Block diagram of the device: 1 — pulse sensor; 2 — circuit for comparing two variable quantities; 3 and 5 — memory blocks of the first and second channels, respectively; 4 and 6 — recording blocks of the first and second channels, respectively; 7 — program block.

Registration units 4 and 6 of channels I and II, respectively, are multistage electric-pulse counters; the number of stages corresponds to the number of stages in memory units 3 and 5. The comparison circuit 2 for two variable quantities has direct and feedback connections with memory units 3 and 5 and registration units 4 and 6, and is built on voltage balancing using two diode rectifier bridges [2].

The program unit is made with step finders, using a synchronous motor with an interrupter as the time-pulse generator. The device operates as follows.

After being installed on the window of the electric-energy meter by means of clamp locks and being switched on, the photo relay (pulse sensor) responds to the motion of the meter disk, producing a sequence of voltage pulses sent to one of the memory units. The step finders of the memory unit of one of the channels, receiving control pulses, accumulate information with their moving contacts (in the form of an alternating-current voltage) for the comparison circuit.

At the end of the specified time interval, the program unit gives a pulse to switch on the comparison relay, which feeds information on the number of pulses in each of the memory units into the comparison circuit. Depending on the program, the comparison circuit retains in the device memory the larger or smaller value of the number of pulses from the two recorded in the memory units of channels I and II, while erasing the information of the other channel, thereby freeing this channel to receive information in the next time interval.

By setting the program unit to 15 or 30 min (or another value), the entire survey period is automatically divided into the specified intervals. In this case, the device itself selects from all the obtained values the maximum or minimum average value of the load power, depending on the program assigned to the comparison circuit, i.e., it gives the average-weighted maximum or minimum load over the investigated time period.

In addition, the program unit can be assigned a program for switching the device into operation during a 24-hour period. Then the device will switch on only in the specified time intervals, for example during the hours when the maximum load of the power system occurs.

Main technical data:

1. Supply voltage — 220 volts alternating current, 50 Hz.
2. Power consumption — not more than 40 VA.
3. Operating time of the comparison circuit — not more than 0.6 sec.
4. The operating error of the program unit under the daily program depends on the frequency of the supply network and does not exceed ±2 min.
5. Weight — not more than 17 kg.

Conclusions

1. A device has been developed that makes it possible to measure the average value of an electrical load (active or reactive), using for this purpose the billing electricity meters or technical metering meters available to consumers without any change in their design. The measurement error depends only on the accuracy class of the installed electricity meter.

1. If several sets of such measuring devices are available, the power-supply organization can carry out systematic monitoring of the magnitude of the contracted demand at a large number of enterprises.

Trial operation of the device has shown the high accuracy of its operation, its ease of maintenance, and the possibility of its industrial use.

REFERENCES

1. A. V. Mikhalkov. Metering of active energy by meters with indication of the load maximum. Promyshlennaya energetika, 1970, No. 7.

2. V. L. Fabrikant. Fundamentals of the Theory of the Design of Measuring Instruments for Relay Protection and Automation. Vysshaya shkola, 1968.