The design of low voltage line protection device

The introduction


The low pressure (exchange does not exceed 1000 v or dc does not exceed 1500 v) distribution protection choose molded case circuit breaker, fuse, or more, residual current protective device to realize quick break, long time delay protection, but a lot of molded case circuit breaker movement precision is not enough, difficult to achieve selective and collaboration between level, jump, expanding accident may lead to even higher and lower. In addition, molded case circuit breaker does not have signal real-time monitor display, the event log and communication network, and other functions.


Therefore, this article designed a low voltage line protection device, use with circuit breaker can overload of lines, grounding fault protection, overvoltage, undervoltage, etc.


1 the design of low voltage line protection device


Low voltage line protection device for AC 400 v or 690 v voltage grade of products, installed in the low voltage feeder ark, using embedded or guide the installation. Products in the normal working conditions: temperature is 10 ℃ ~ + 55 ℃, no higher than 2000 meters above sea level, no corrosive gas environment, humidity 95% or less, no condensation. To meet the power distribution in the standard of line overload, ground fault protection requirement, the design has two definite time protection and inverse time protection (standard inverse time, extreme inverse time of 8 kinds of curve), the other with under-voltage protection and over voltage protection and other protection function. The device is composed of hardware platform and software platform. Hardware block diagram is shown in figure 1.


Figure 1 hardware block diagram 


1.1 the main hardware circuit design


When high power motor starting in low voltage system, may cause power grid voltage instantaneous reduced. Device to prevent voltage caused by misoperation, power input range is designed for AC 85 v ~ 265 v AC; In some places, low voltage control circuit will use a DC power supply (DC 110 v or 220 v direct current (DC)), so the power needed to support both ac and DC. Commonly used linear power supply can not be very good to satisfy these requirements, so the switch power supply scheme to design the device power supply. This device USES PI do power switching power supply chip design company, the overall power around 8 va, between the input and output of power supply to meet 2 kv power frequency withstand voltage (power frequency withstand voltage level can see the GB 14048-14048 "low-voltage switchgear and control equipment"), surge test can pass level 4 ?


Low-voltage feeder of the voltage and current signals relative to this device internal acquisition circuit belongs to high voltage, high current signals, needs to be into a low voltage, low current signal. When choosing voltage transformer, current transformer, to combine product characteristics, such as general electric logging instrument chooses the current transformer, consider only overload capacity is 120% of the rating, but the protection should be considered using 5 p10, 10 p20 even higher level protection current transformer overload capacity. Sampling circuit design should be considered when the power, voltage, temperature drift coefficient of resistance, accuracy and other parameters. If use 10 p20 current sampling circuit, current transformer design 20 times also want to consider to withstand overload (transformer secondary side) sampling resistance. After sampling resistance selection, and then design level of signal ?

1.2 the software design


Low voltage power supply system harmonic source will exist at present stage, brings to the power grid harmonic pollution, and low voltage line protection device to select measurement algorithm based on non sinusoidal signal. Based on non sinusoidal signal algorithm including Fourier algorithm, after the first order difference half-wave Fourier algorithm, true RMS algorithm, etc. Fourier algorithm can break down the information, the entire harmonic in protection products is widely used. If there is a frequency offset, decaying dc component with a signal, the need to take the corresponding measures, otherwise cause calculation error.


For circuit overload, ground fault, low voltage line protection with inverse time protection function. Inverse time can be simply interpreted as: the greater the current, the faster the protection action, current ?

    Ir*t=K                             (1)

Among them, the coefficient K, r according to the different situation of the protection and take different values, generally in the end of the protected line head end and short circuit, current changes under the condition of small, the set time limit over-current protection, definite time limit can be thought of as a special kind of inverse time characteristic, namely r = 0; And short circuit, current larger changes in the end of the line first, then USES the inverse time characteristic very much, r = 1; Transmission line using a general inverse time characteristic, namely 0 < r < 1; Response overheating over-current protection, and fuse with special occasions is used inverse time characteristic, namely r = 2.


Typical inverse time characteristic curve as shown in figure 2, figure I/IOPR said current flow in a multiple [8].

 

Figure 2 typical inverse time curve


The inverse time protection of the device


(2)


Type:


T - trip time


K - factor (see table 1)


I - current measurements


Is - programmed threshold method


Alpha coefficients (see table 1)


L - ANSI/IEEE coefficient (see table 1)


Tp - time factor


The inverse time overcurrent protection curve characteristics as shown in table 1.


Table 1 the inverse time overcurrent protection curve characteristics


Type (2) the alpha = 0.02 direct calculation is difficult, can use a look-up table method, Taylor expansion, such as curve fitting method to calculate


(1) using the look-up table method, make X = (I/Is), between 1.1 ~ 20 X change, change step for delta X, a X0.02 each step length calculation, calculation result will be stored in the block to the EEPROM, the actual current fluctuate range, so the calculation step length shoulds not be too large or too small, will affect the calculation accuracy, X leads to t ultra poor; Step size is too small, block or increase the EEPROM overhead.


(2) using the look-up table method, the actual value and can be read directly, when X is equal to not equal by calculating the required data interpolation algorithm, block but EEPROM spends too much.


(3) according to the Taylor series expansion, which can be calculated in X, relative error is 0.44% when n = 5, meet the calculation precision of time want to ?


2 the actual application


A petrochemical project in need of a few low pressure important feeder circuit overload, unbalanced and grounding protection, overload with two pieces of over-current protection and inverse time protection, and can cooperate with the background of electric power monitoring and control system to read parameters, communication protocol is MODBUS RTU, can directly display on protection current, switching state and fault information, can record closing information, fault information.


Low voltage line protection devices are introduced in this paper two fixed time limit over-current protection, through the way of internal calculation of zero sequence current judgment ground fault, according to the current unbalanced three-phase current value do calculation, with Chinese LCD display, record, failure of the closing, communications, and other functions, fully meet the requirements. Low voltage line protection


3 the end


Low voltage line protection device can measure the three phase current, three-phase voltage, residual current, power, frequency and power parameters, such as measuring parameters can be displayed on the device, can also through the RS - 485 communication port to the background monitoring system that can overload of lines, grounding, over-voltage and under-voltage fault protection. Low voltage line protection device is designed for low voltage feeder, can be used to power plant electrical monitoring, factory automation, building electrical distribution and petrochemical and other places.


Article source: the modern building electric 6, 2015.


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