Introducing LOC technology for input measuring circuits
Transformers have influenced the design of measuring equipment for over 100 years. During this time the restrictions imposed on us by transformers have become an accepted limitation on the performance of input measuring circuits. Another technique now exists, enabling transformer-less circuits to be used which deliver high performance in practice and allow a more flexible approach.
RESTRICTIONS IMPOSED BY TRANSFORMERS
Traditionally transformers have been used where high levels of galvanic separation are required. However, the very principle of winding insulated wires around an iron core causes disadvantages.
- Both the weight and size are considerable, because of the high content of iron and copper.
- Stable mounting of transformers causes restrictions, particularly in the field of portable equipment.
- Greater effort is required to overcome the isolation problems for higher separation voltages.
- To a lesser extent problems exist with disturbance caused by magnetically induced noise.
- Frequency restriction. Linearity is normally restricted to a bandwidth of a few hundred Hz, and 45 to 65 Hz is common. Wider frequency ranges are possible with switched tappings, which raise problems of control and isolation. Harmonic restrictions are normally up to the fifth, maximum seventh, harmonic.
- DC signals cannot be transformed. Only DC components on an AC signal can be transformed, making it difficult to provide an accurate reproduction of the DC content.
- High internal losses.
- High complex inductive load.
- Normally restricted to voltage or current signals.
- Difficulties exist in the linear transformation of process signals.
LOC (LINEAR OPTO-COUPLER) TECHNOLOGY
Opto-couplers have been used for many years to galvanically separate outputs. This is most commonly seen in the form of the communication port for data transmission on a meter. Here a simple on/off digital signal exists. Transformers, however, are linear. If a linear transformer is to be replaced by an opto-coupler, the opto-coupler must also be made linear.
With correction techniques it is now possible to produce a linear response via a circuit using an opto-coupler. This revolutionises measuring elements, as the restrictions on frequency response/harmonics, signal processing, weight and size etc are removed, providing a revolutionary hardware platform for input measuring circuits.
The analogue input is supplied to a servo amplifier via a pre-scaler circuit which is used to set up the input measuring range. Galvanic separation is achieved by means of an opto-coupler which uses one send diode and two receive diodes. With the use of feedback circuits, a regulated transresistance amplifier and a passive output filter, an effective linear response is achieved. (Figure 1).
- A very small size and low weight.
- A small movable mass. This is important for portable devices where the jolting of larger masses causes stress on the PCB.
- An easily achieved high isolation voltage.
- A wide linear frequency response – from DC to 100kHz (120kHz-3dB) depending on the filter used. This allows process signals such as ripple control and PLC signals to be processed.
- It is possible to adapt these circuits for sensing almost any sort of DC signal, including DC voltage, DC current, DC process signals – for example 0-20mA, DC excitation currents, pressure, temperature, path sensors.
- Very low internal power consumption (typically <0.1W per channel).
- High input impedance. This is important for high impedance sensor outputs.
- No predetermined measurement range. This allows a customer-defined measurement range to be provided.
- The same input circuit technology can be used for 0-20mA or ± 450V RMS. Larger currents (>100mA) are possible with the use of an external shunt.
APPLICATIONS OF LOC TECHNOLOGY
- Freely configurable input connections are possible.
- Ideal for modular applications.
- Together with modern micro-processor techniques it is possible to build a complete measuring device with an extremely high sample rate, allowing high degrees of accuracy and resolution.
Typical specifications are:
- Accuracy 0.1% (higher accuracies for RMS values).
- User-defined current range possible, e.g. 0 to 5A or 0 to 100A, dependent only on the shunt which is used.
- User-definable voltage range per channel, e.g. 0-450 Vac (phase neutral).
- Frequency range, e.g. DC to 50 kHz.
A TYPICAL LOC CIRCUIT BOARD
A typical LOC circuit board consists of four analogue channels and six binary channels. Each analogue channel can be used for either voltage or current, although the addition of an external shunt for high currents is required. It is therefore necessary to rethink the principles for describing the voltage and current circuits. Traditional descriptions such as current circuit and voltage circuit become outdated. With this design there are simply four analogue channels in the circuit board which can be allocated to either voltage or current inputs. By using a second or more circuit boards, multiples of four analogue/six binary channels can be made.
ENERGY TARIFFS AND EN50160
Energy tariffs are traditionally associated with a meter. The number of tariffs, maximum demand, active and reactive measurements are all carried out by or linked to the meter.
The concept of added value for higher quality energy supply has its roots in North America and South Africa and in the precision manufacture of items such as microprocessors. This concept is now entering Europe and providing new challenges in the field of energy billing and measurement.
An example is the AMD project in Dresden, where a higher quality energy is being provided at a higher tariff. Confirmation that the higher quality energy rate is applicable is determined by sophisticated equipment that combines the functions of power quality measurement and disturbance recording. A new billing device is thus being introduced to the market which is very different from a meter. The power monitoring system used is based on LOC technology. The exacting specifications include the normal features of event capture, as well as transients, wave form triggers (curve shape), power and power swings, and THD triggers for
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LOC technology and parallel processing allow both Star and Delta measurements to be carried out with the same channels, reducing the number of channels required. Additional features such as measurement of imbalance are easily provided with LOC technology.
Software frontiers have in many cases been pushed back further than those of existing hardware platforms, because transformer input technology has been frozen in time. LOC technology now provides an advanced, high performance alternative which allows higher performance hardware platforms to be created. Together with advanced software and the latest processors, a significant advance in performance is now at hand.