Abstract : A special protection device for low-voltage lines was designed, which can be used in conjunction with circuit breakers to protect against overload, grounding, overvoltage, and undervoltage faults in the lines, thereby improving the electrical safety and reliability of low-voltage power distribution systems, simplifying the design of distribution cabinets, and increasing the degree of automation.
Keywords : Low-voltage line protection; Inverse time curve; Overcurrent protection; Hardware circuit
0 Introduction
Currently, low-voltage (AC not exceeding 1000V or DC not exceeding 1500V) power distribution protection mostly uses molded case circuit breakers, fuses, or residual current devices (RCDs) to achieve instantaneous tripping and long-delay protection. However, many molded case circuit breakers lack sufficient operating accuracy and are difficult to achieve selective coordination between circuits, which may cause cascading tripping between circuits and escalate the accident. In addition, molded case circuit breakers do not have functions such as real-time signal monitoring and display, event logging, and communication networking.
Therefore, this paper designs a low-voltage line protection device, which, when used in conjunction with a circuit breaker, can protect the line from overload, grounding, overvoltage, undervoltage and other faults.
1. Design of Low-Voltage Line Protection Devices
This low-voltage line protection device is designed for AC 400V (or 690V) voltage levels and is installed in low-voltage feeder cabinets, using either embedded or DIN rail mounting. Normal operating conditions include: operating temperature -10℃ to +55℃, altitude not exceeding 2000 meters, no significant corrosive gases in the environment, humidity ≤95%, and no condensation. To meet the protection requirements for line overload and ground faults in power distribution standards, it is designed with two-stage definite-time protection and inverse-time protection (standard inverse-time, extreme inverse-time, and 8 other curves), as well as undervoltage and overvoltage protection. The device consists of a hardware platform and a software platform. The hardware block diagram is shown in Figure 1.
Figure 1 Hardware Composition Block Diagram
1.1 Design of Main Hardware Circuits
In low-voltage systems, the starting of a high-power motor can cause a momentary drop in the mains voltage. To prevent malfunctions caused by this voltage drop, the device's power input range is designed to be AC 85V to AC 265V. In some applications, the low-voltage control circuit may use DC power (DC 110V or DC 220V), therefore the power supply needs to support both AC and DC. Commonly used linear power supplies cannot adequately meet these requirements; therefore, a switching power supply solution is used to design the device's power supply. This device uses switching power supply chips from PI for its power supply design, with an overall power output of approximately 8VA. The power supply must meet a 2kV power frequency withstand voltage (power frequency withstand voltage levels can be found in GB 14048-2012 "Low-voltage Switchgear and Controlgear") and pass a Level 4 surge test. In the switching power supply, the transformer design is simplified by using the transformer design software included with PI Expert. The switching transformer design is briefly described as follows: The topology is flyback, the feedback type is a secondary TL431, and the input voltage is a general-purpose (85-265)V. The output voltage and power are designed according to actual conditions. If output voltage isolation is required, the output can be set to discrete in the overlay options. When designing the transformer, parameters such as efficiency, magnetic flux density, core, and frame need to be comprehensively considered. Sometimes, adjusting the efficiency will cause changes in magnetic flux, which will degrade the transformer's performance. After the transformer design is completed, PCB design should be performed. Refer to the layout and routing recommended by PI Expert to reduce loops and prevent unpredictable interference signals.
The voltage and current signals in the low-voltage feeder are high-voltage, high-current signals relative to the internal acquisition circuit of this device, and need to be converted into low-voltage, low-current signals. When selecting voltage and current transformers, the product characteristics must be considered. For example, when selecting current transformers for ordinary electrical measuring instruments, only the overload capacity of 120% of the rated value is considered, but protection devices require protection-grade current transformers with overload capacities of 5P10, 10P20, or even higher. The design of the sampling circuit needs to comprehensively consider parameters such as resistor power, voltage, temperature drift coefficient, and accuracy. For example, when designing a current sampling circuit using a 10P20 current transformer, a sampling resistor capable of withstanding 20 times the overload (secondary side of the transformer) must be considered. After selecting the sampling resistor, the subsequent signal processing circuit is designed. The signal processing circuit includes filtering and amplification circuits. Low-pass filtering is generally used in the filter circuit design to remove unwanted interference information, and the filter cutoff frequency must match the software sampling frequency. The signal amplification circuit design needs to consider parameters such as signal range and linearity, and segmentation processing may be necessary. The device directly uses AC amplification and, in conjunction with software, performs true RMS calculations, vector calculations, and other tasks.
1.2 Software Design
Current low-voltage power supply systems often contain harmonic sources, causing harmonic pollution to the power grid. Therefore, low-voltage line protection devices need to employ measurement algorithms based on non-sinusoidal signals. Non-sinusoidal signal-based algorithms include Fourier algorithms, first-order differential half-wave Fourier algorithms, and true RMS algorithms. Fourier algorithms can decompose the information of each integer harmonic and are widely used in protection products. If frequency shifts or attenuated DC components appear in the signal, appropriate measures must be taken; otherwise, calculation errors may occur.
Low-voltage line protection devices are equipped with inverse-time protection functions to address line overload and ground faults. Inverse-time protection can be simply understood as: the larger the current, the faster the protection operates; the smaller the current, the longer the protection operates. In power system relay protection, inverse-time current protection is widely used for the protection of generators, transformers, motors, and transmission lines. Inverse-time overcurrent protection is typically based on the following time-current inverse-time characteristics:
Ir*t=K （1）
Where K is a coefficient, and r takes different values depending on the application of the protection: generally, when there is a short circuit at the beginning and end of the protected line and the current change is small, a time-delay overcurrent protection is used, which can be considered a special inverse-time characteristic, i.e., r=0; while when there is a short circuit at the beginning and end of the line and the current change is large, a very inverse-time characteristic is used, i.e., r=1; usually, transmission lines use a general inverse-time characteristic, i.e., 0.
A typical inverse-time characteristic curve is shown in Figure 2, where I/IOPR represents the current overcurrent factor [8].
Figure 2 Typical inverse time limit curve
The inverse time protection of this device complies with...
In the formula:
t - Trip time
K - Coefficient (see Table 1)
I - Current measurement value
Is - The threshold value set by the program.
α - Coefficient (see Table 1)
L - ANSI/IEEE coefficients (see Table 1)
Tp - Time Factor
The inverse time overcurrent protection curve characteristics are shown in Table 1.
Table 1. Inverse Time Overcurrent Protection Curve Characteristic Table
In equation (2), it is difficult to calculate directly when α=0.02. Methods such as table lookup, Taylor expansion, and curve fitting can be used for calculation.
(1) Using the lookup table method, let X = (I/Is), X varies between 1.1 and 20, with a step size of ΔX. Calculate X0.02 once for each step size and store the calculation result in EEPROM. Since the actual current has a fluctuating range, the calculation step size should not be set too large or too small. If it is too large, it will affect the accuracy of X calculation and cause t to exceed the tolerance; if the step size is too small, it will increase the EEPROM overhead.
(2) Using the lookup table method, the actual value can be read directly when it is equal to X, and the required value is calculated by interpolation algorithm when they are not equal, but the EEPROM overhead is too large.
(3) X can be calculated by Taylor series expansion. When n=5, the relative error is 0.44%, which meets the time accuracy requirements of the calculation, but the amount of calculation is large.
(4) The curve fitting algorithm simplifies the calculation process by replacing complex curves with easily calculated curves. The key is to select the correct fitting curve.
2. Practical Application
In a certain petrochemical project, overload, unbalance, and grounding protection is required for several important low-voltage feeder circuits. The overload protection requires two-stage overcurrent protection and inverse time protection, and can cooperate with the background power monitoring system to read parameters. The communication protocol is MODBUS-RTU, and the current, opening and closing status, and fault information can be directly displayed on the protection device. It can also record opening and closing information and fault information.
The low-voltage line protection device introduced in this article features two-stage time-limit overcurrent protection, determines ground faults by internally calculating zero-sequence current, performs current imbalance calculations based on three-phase current values, and includes a Chinese LCD display, opening and closing records, fault records, and communication functions, fully meeting the requirements. The secondary schematic diagram of the low-voltage line protection device is shown in Figure 3. In Figure 3, current transformers (1TA~3TA) are used to isolate and transform the main circuit current. The secondary signal from the current transformers is input to this device, which performs corresponding overload and ground fault protection based on the actual current conditions. To control the opening and closing of the circuit breaker, corresponding shunt windings and closing windings need to be added; automatic closing by the device is not required. Therefore, Figure 3 does not show a closing winding, only a shunt winding. A pulse signal is needed to disconnect the shunt winding, or the normally open contact of the circuit breaker needs to be connected in series.
Figure 3. Secondary schematic diagram of low-voltage line protection device application
3. End
Low-voltage line protection devices can measure parameters such as three-phase current, three-phase voltage, residual current, power, frequency, and energy. These parameters can be displayed on the device or uploaded to a backend monitoring system via RS-485 communication. They can protect against overload, grounding, overvoltage, and undervoltage faults in the line. Specifically designed for low-voltage feeders, these devices can be used in power plant electrical monitoring, factory automation, building electrical distribution, and petrochemical applications.
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About the author:
Zhao Bo (1982-), male, undergraduate degree, mainly engaged in the design and application of motor controllers.
