This chapter gives an overview of overvoltage protection to industrial appliances. To commence with, the overvoltage definition and its causes have to be known in order to understand what sort of overvoltage detection should be assessed further in this project. Moreover, it is important to examine the significance of the overvoltage detection circuit in overvoltage protection devices that are currently available in market. Further analysis in overvoltage detection circuit is helpful to develop this project, meeting the circuitry requirements, as to make this project worthwhile.
Industry, public institutions and home appliances mostly depend on electrical and electronic data engineering. As the data recording devices at the production facilities are connected to office terminals and computers, voltage supplied to them should be have reliability and stability. High supply reliability, which is the fundamental significance, may cause major nuisance to the customer if there is any interruption. Moreover, it can lead to life threatening situations, for industrial customer or create severe technical and production problems. Therefore, the reliability of the supply is important to avoid any damages or losses due to devices malfunctioned.
Electrical power supply of good quality is provided by the regulated and defined voltage levels with low fluctuations, regulated value of frequency with low fluctuations and low harmonic current. To ensure a high quality of electrical supply, two basic methods can be used which are the proper use of automatic voltage and frequency control methods; by employing large interconnected power systems which by their very nature, are less susceptible to load variations and other disturbances.
An electronic insurance company in Germany reported that the costs of compensation for over voltage damage due to electromagnetic disturbances on electronic system and equipment, such as computers, measuring devices and communication systems have quadrupled within a period of ten years. The damages mostly caused by surge and voltage supply instability .
Surge damage analysis has shown that lightning discharges are the main disturbances, followed by due to switching operations in technical system and by electrostatic discharge. In spite of the lightning is the main disturbance, indirect damage caused by electromagnetic lightning disturbances is far higher than those due to direct lightning. In this case, the hardware damage is only small portion of the total impact; consequential damage is the breakdown of computer systems and malfunctioning of measuring and control systems cause the utmost proportion of loss .
In general, overvoltage occurs when the voltage in a circuit or part of it exceed the upper predetermined voltage. Overvoltage is one of the voltage unbalance condition, in which the three phase voltages differ in amplitude or displaced from their normal 120 degree phase relationship or both. The nature of unbalance includes unequal voltage magnitudes at the fundamental system frequency, either undervoltage or overvoltage and fundamental phase angle deviation . Overvoltage may cause by transient overvoltage, which is a voltage peak with a maximum duration of less than one millisecond.
There are two possible causes of overvoltages on electrical networks which are natural causes and other cause due to equipment or switching devices.
Natural Caused Over voltage
Natural overvoltages on low voltage networks are caused by lightning strikes. The high voltage potential contained in direct lightning strike on a lightning conductor leads to considerable damage of the installation. Surges of this atmospheric cause can be divided by two, which are direct strike and remote strike.
Lightning current in a lightning path causes a voltage drop at the impulse earth resistance of the earthing system, which then induces surge voltages and currents in loops inside the structure's installation circuit. The lightning strike causes the induced voltages, therefore surge currents in installation loops especially due to its magnetic interfering radiation. Consequently, if lightning strikes a feeding overhead line, there will be increased voltages on the incoming power lines. However, it can be assume that 50% of the lightning current flows into earthing system and the other 50% is distributed equally to the outgoing remote- earthed supply systems, which are the four wires of power cable or data line .
In the case of remote strikes, travelling surges either propagate along the lines or lightning strikes in the vicinity of the protected systems, thereby generating electromagnetic fields which affect the system. Damage due to surges of natural cause has shown that electronic installations, up to a distance of about 2km from the lightning point of strike, are susceptible to induced surges. When this partial of the lightning currents flow in cables, it generates transverse voltages. Thus, the transverse voltage generated between the wire and the metal cable creates stress on the insulation of the connected device between its input terminals and earth. Within a radius up to several kilometres, the electromagnetic field caused by lightning in clouds also create sudden increase in voltage .
Switching Over Voltage
Switching over voltages linked to a network's equipment create over voltages linked to a network's equipment. Even though the overvoltages much lower than natural causes, it occur much more frequent, thus causes premature ageing of the equipment. The switching overvoltages can be caused by disconnection of an open- circuit power line. It occurs when the switch opens, thus the instantaneous value of the supply voltage on the line results in a high potential difference between the system and the disconnected line. The potential difference can cause a flash back between the switch contacts that are yet to close. The line voltage then balances at a level equal to the instantaneous value of the supply voltage and the arc between the switch contacts is quenched.
Switching overvoltage too can be caused by the disconnection of an open circuit transformer, where its self capacitance is loaded by energy of the magnetic field. Therefore, the inductive capacitive circuit now oscillates until all of the energy in the ohmic resistance of the circuit is converted into heat. These overvoltages can reach amplitudes of several times the value of the nominal supply voltage.
Equipment containing electronic components is also likely to generate electrical disturbances comparable to overvoltages. The consequences of which on sensitive equipment, although not visible, are no less harmful: premature ageing and unpredictable breakdowns.
SIGNIFICANCE OF OVERVOLTAGE PROTECTION
Recently, there has been an increasing number of integrated circuit on chip that support embedded functions such as CPU, logic gates, memory and etc. In such an integrated circuit device, different power supply voltages are needed for each internal circuit. Thus this invention relates to integrated circuit devices, in particular, an integrated circuit device with a power- supply voltage detection circuit to detect that a power supply voltage goes out of preset regulation.
The purpose of an overvoltage protection circuit is to protect sensitive electronic circuitry from damage or stress that may result from the application of a voltage that exceeds a preset acceptable range. Frequently, electronic devices such as televisions, stereos, or PCs are plugged into such overvoltage protection circuits which are connected directly to a wall socket. During operation, if the walls socket voltage rises above preset value (e.g 230V) due to an electrical spike, the detection circuit will detect the overvoltage, therefore send signal to the control circuit, which then clamp or short the excess voltage to ground, preventing the line voltage from rising above preset value. Therefore, it will protect the connected devices from damage.
2.4 OVERVOLTAGE PROTECTION REVIEW
In general, the electronic devices are configured in Figure 2.1. By referring to Figure 2.1, an electronic device comprises a power supply system an electronic circuit. The power supply system comprises an AC to DC power supply and DC to DC power supply. The power supply system receives commercial power, from a power receiving terminal. The AC to DC power supply is used to converts the AC received power to DC power while DC to DC power supply used to generate a desired level of DC voltage.
Figure 2.1: Electronic device.
Furthermore, by referring to figure 2.2, DC to DC power supply comprises a power circuit, a control circuit and overvoltage detection circuit. The power circuit receives power from input terminals and outputs a desired level, stabilized DC voltage to output terminals. The overvoltage detection circuit, which connected to the output terminals, senses any voltage anomalies. When the output voltage becomes abnormally high or low, the overvoltage detection circuit outputs the stop signal to the control circuit. The control circuit turns on or off the power circuit according to the signal received via the control terminal .
From the general block diagram of electronic devices shown, the overvoltage detection is important to check if the voltage across terminals is equal or higher than predetermined value, therefore send signals for further action to the control circuit.
Figure 2.2: DC to DC power supply
In addition, overvoltage protection also can be equipped with monitoring device, in order to monitor the statistics of the unbalance voltages. The overvoltage protection device too may includes an electro-optical relay unit, that may be optically- linked to overvoltage protection device. The electro- optical relay unit has a digital data port for setting system conditions, metering and event reporting. The output may include a trouble alarm or indicator as well as alarms at different set points of over voltage .
In general, a sensor is used to sense external quantities and often being called as transducers. A transducer is a device that converts one physical quantity into another, plus different transducers convert a wide range of physical quantities. Transducers of a third class take an analogue input quantity and generate from it a digital output. In some instances the output is a simple binary representation of the input, as for example in thermostat, which produces one of two output values depending on whether temperature is above or below a certain threshold . Therefore, this example too can be applied to overvoltage sensor. However, representing analogue quantity by means of a digital quantity is, by necessity, an approximation. Indeed, in many cases, the error caused by this approximation is small compared with the noise or other errors within the system therefore can be ignored. When describing sensors, it is important to quantify their characteristics and performances .
This defines the maximum and minimum values of the quantity that the sensor is designed to measure.
This is the smallest discernible change in the measured quantity that the system is able to detect
This is the difference between a measured value and its true value. Errors can be divided by two which are random errors and systematic errors.
It describes the maximum expected error associated with a measurement and may be expressed as an absolute value or range.
It is a measure of the lack of random errors produced by a sensor. Devices with high levels of precision will produce repeated output with less variance.
In overvoltage protection device, the overvoltage sensor plays important role in detecting the input voltage within or beyond the predetermined voltage. Therefore, the top requirement for overvoltage sensor should be able to compare the input voltage to the voltage reference set earlier.
To begin with, the overvoltage sensor should comprise two comparison circuits. The first comparison circuit is for the overvoltage detection circuit. This circuit may consist of voltage dividing resistor, comparator and voltage reference source. The voltage reference is set to upper limit of the normal voltage range; therefore if the input voltage supplied exceeds the voltage limit, the comparator will be activated.
On the other hand, the second comparison circuit too comprises voltage dividing resistors, comparator and voltage reference source. However, the voltage reference here is set to lower limit of the normal voltage range. Thus, whenever the input voltage supplied much lower than the voltage limit, the comparator will be activated .
The sensor should precisely detect either overvoltage or undervoltage precisely with very minimum false alarm. The nominal phase voltage input supply is followed under the Malaysia Grid code, which are 230 Vrms +10% -5% . Besides accuracy and stability, the sensor too should be operating relevantly with quick time response, in order to detect anomalies from damaging the electronic circuits for further development.
CASE STUDIES/ AVAILABLE PRODUCTS.
Nowadays, there is advanced AC Voltage Transducers manufactured in the market. Most of products are improvised by equipped by wide range of voltage and frequency, temperature compatibility, high accuracy and efficiency. One of them is the MiltiTek AC Voltage Transducer. Its model M100 series voltage transducers are designed to measure AC voltage in single and three phase system. They convert the AC signal input voltage to a DC output which is directly proportional to the input signal. For the M100 VA1 1 phase, it is self powered, which is no auxiliary calibrated voltage is required. This product used to measure voltage in energy management systems, switchboards, generator and telemetry controls .
Other product, which is CR4500 Series, also the true RMS voltage transducers are designed for applications where AC voltage waveforms may not purely sinusoidal. It is much precise and accurate than other devices. Therefore, its applications mostly used in quickly varying voltage supplies and for harmonic voltages. It has highest precision and the outputs are isolated from inputs. From its datasheet, it is noted that its operating temperature is 0°C to 60°C. Moreover, its response time to be 250ms .
Based on the patents and products reviewed, most of the voltage transducer or sensor is a common application that used to monitor the characteristic of the voltage respective to time. It is also important for the sensor to detect any changes or varies in voltage with less time response. The sensor may equip with indicating lights or sound alerts to indicate user if there is any voltage abnormalities. Moreover, some of the sensor too may provide the relay for further protection, which is tripping the circuit whenever major anomalies that may damage the equipment occurred.
Therefore, voltage sensor and indication design is put into priority to be studied in this project. Moreover, it is an objective to design a voltage sensor that is cheaper than in the market.