This present invention generally relates to meters and monitors of the type that measure power consumption and/or power quality. More specifically, the present invention relates to an automatic gain switching of such meters and monitors.
In a typical electrical distribution system, electrical energy is generated by an electrical supplier or utility company and distributed to consumers via a power distribution network. The power distribution network is the network of electrical distribution wires which link the electrical supplier to its consumers. Typically, electricity from a utility is fed from a primary substation over a distribution cable to several local substations. At the local substations, the supply is transformed by distribution transformers from a relatively high voltage on the distributor cable to a lower voltage which is supplied to the end consumer. From the substations, the power is provided to industrial users over a distributed power network that supplies power to various loads. Such loads may include, for example, various power machines used by the end consumer.
At the consumer""s facility, there will typically be a meter connected between the consumer and the power distribution network to measure the consumer""s power consumption and/or power quality. The revenue meter is an electrical energy measurement device which accurately measures the amount of electrical energy flowing to the consumer from the supplier. The amount of electrical energy measured by the meter is then used to determine the amount for which the energy supplier should be compensated.
Typically, the electrical energy is delivered to the consumers as an alternating current (AC) voltage that approximates a sine wave over a time period. The term alternating waveform generally describes any symmetrical waveform, including square, sawtooth, triangular, and sinusoidal waves, whose polarity varies regularly with time. The term AC (i.e., alternating current), however, almost always means that the current is produced from the application of a sinusoidal voltage, i.e., AC voltage. The expected frequency of the AC voltage, e.g., 50 Hertz (Hz), 60 Hz, or 400 Hz, is usually referred to as the fundamental frequency. Integer multiples of this fundamental frequency are often referred to as harmonic frequencies.
While the fundamental frequency is the frequency that the electrical energy is expected to arrive with, various distribution system and environmental factors can distort the fundamental frequency, i.e., harmonic distortion, can cause spikes, surges, or sags, and can cause blackouts, brownouts, or other distribution system power quality problems. These problems can greatly affect the quality of power received by the power consumer at its facility or residence as well as make difficult an accurate determination of the actual energy delivered to the consumer.
In order to solve these problems, revenue meters have been developed to provide improved techniques for accurately measuring the amount of power used by the consumer so that the consumer is charged an appropriate amount and the utility company receives appropriate compensation for the power delivered and used by the consumer. Examples of such metering systems are well known in the art.
In addition, power monitors, and revenue meters with power monitoring capabilities, provide information about the quality of the power, i.e., frequency and duration of blackouts, brownouts, harmonic distortions, surges, sags, swells, imbalances, huntings, chronic overvoltages, spikes, transients, line noise, or the like, received by a power consumer at a particular consumer site. Blackouts, brownouts, harmonic distortions, surges, sags, swells, imbalances, huntings, chronic overvoltages, spikes, transients and line noise are all examples of power quality events. As utility companies become more and more deregulated, these companies will likely be competing more aggressively for various consumers, particularly heavy power users, and the quality of the power received by the power consumer is likely to be important. This, in turn, means that accurate and detailed reporting and quantification of power quality events and overall power quality will become more and more important as well.
For example, one competitive advantage that some utility companies may have over their competitors could be a higher quality of the power supplied to and received by the consumer during certain time periods. One company may promote that it has fewer times during a month that power surges reached the consumer causing potential damage to computer systems or the like at the consumer site. Another company may state that it has fewer times during a month when the voltage level delivered to the consumer was not within predetermined ranges which may be detrimental to electromagnetic devices such as motors or relays. Previous revenue accuracy meters which provide for measuring quality of power in general lack the necessary accuracy and power quality monitoring features to provide the consumer and the power utility with the needed information.
Problems occur since power monitors often are called upon to cover a wide range of voltage and current, such as 0 to 1000 Volts (V) Root Mean Square (RMS) and 0 to 50 Amps (A) RMS. To handle the wide range of voltage and current, one known solution is to include a mechanical or electronic switch that interchanges between different voltage and current ranges of the meter. Such switching is commonly referred to as gain switching. For example, if an input voltage exceeds the meter setting, the switch is changed to a different gain setting. The mechanical or electronic switching of the power monitor, however, may cause samples to be missed since no samples above the range are accurately recorded until the power monitor is switched.
Another known solution used to accommodate a wide range of input voltage and current is to utilize an Analog to Digital Converters (ADC) having a high bit count that handles a wide range of voltage and current. While power monitors commonly use an ADC with a bit count of 12 bits or less, ADCs with a bit count of 16 bits or higher are available. The 16 bit or higher ADCs, however, are prohibitively expensive in today""s market. In addition, the overall system design becomes more complex and more costly due to signal/noise and data processing issues. At the bottom end of the bit range, a signal/noise ratio decreases, especially in industrial applications, to produce poor quality signals. The best resolution occurs at the top end of the ADC""s bit range, e.g., 10 to 12 bits for a 12 bit ADC. High resolution and accuracy are especially important, for example, in applications such as a waveform recorder of the power monitor which allows a user to view line conditions in oscilloscope like form.
To obtain accurate readings, other known techniques include producing customized devices that accommodate a predetermined input range of voltage and current specified by the consumer. The customized power monitors contain amplifiers, for example, that provide gain to the signal to place the signal at the high bit range of the ADC. During the production process the amplifier circuit is adjusted to provide the required gain. It can be appreciated, however, that such customized devices increase manufacturing costs and complicate production procedures and logistics. In addition, the customized meters do not address problems caused by transients, such as voltage spikes and swells, that can exceed the normal operating conditions of the meter by several hundred percent. Thus, a voltage spike of, e.g., 1000 V RMS can saturate the ADC to its maximum bit count which indicates, e.g., only 120 V RMS. Such saturation of the ADC creates a clipped sample of the signal.
Accordingly, there is a need for a power monitor that is capable of monitoring, reporting and quantifying the quality of power with high level of detail and accuracy. Further, there is a need for a power monitor that guarantees no missing or clipped samples within a wide operating range of input voltages and currents. In addition, there is a need for a power monitor that eliminates production difficulties and costs associated with customized power monitors.
Such needs are met or exceeded by the present method and device for automatic control of gain switching. In general, device and method for gain switching improves power monitor and/or revenue meter operation within a wide range of input voltages and currents. Further, firmware controlled gain switching allows the power monitor to achieve an improved accuracy and waveform recording quality, and guarantees no missing or clipped samples in the waveform recordings.
More specifically, the preferred embodiment of the present invention includes an electronic circuit that splits an input channel into at least two gain channels. Thereafter, a processor controls analog to digital conversions to simultaneously sample all gain channels at a required sampling rate, (e.g., 128 samples per cycle) and reads the conversion results into buffers, preferably located in the processor""s memory. Thereafter, the processor accesses a present buffer of a first gain channel. The present buffer contains a predetermined amount of samples which represent the input signal. Next, the processor determines whether at least one of the samples contained in the present buffer is saturated. If at least one of the samples is saturated, the processor selects an alternate gain channel. Otherwise, if none of the samples is saturated, the processor determines whether a previous buffer of the first gain channel is saturated. If none of the samples from the previous buffer are saturated, the processor selects the first gain channel. Otherwise, the processor selects the alternate gain channel. Thereafter, the samples are processed from more than one of the plurality of gain channels to calculate power parameters.