Motor differential protection device based on fault component principle

A microcomputer protection device is designed based on the principle of fault component. The protection device based on this technology is introduced to make up for the shortcomings of the existing motor comprehensive protection device. Combined with the motor comprehensive protection device, it can meet the protection requirements of large high-voltage motors. And by comparing with the traditional differential protection, the superiority of the differential protection using the fault component as the braking amount is explained. The hardware design of the protection device and the protection principle are introduced, and the algorithm for signal processing is given.
Keywords : microcomputer protection; fault component; ratio differential; Fourier algorithm

A Motor Differential Protection Device Based on Fault Component

BI Rui , WEN Yang-dong , XU Hua-li

(School of Electric Engineering and Automation ,Hefei University of technology ,Hefei 230009,China )

Abstract : A microcomputer-based motor protection device based on fault component is designed in this paper. The device compensates the insufficiency of the microcomputer-based motor integrated protection device designed previously. Using the two devices in practice can meet the demands of high-voltage motor. Compared with traditional differential protection based on all components, differential protection based on fault component show its advantage. The design of device's hardware, the theory of protection and the algorithm of signals processing are introduced in the paper.
Keywords : microcomputer-based protection ;fault component;percentage differential protection;Fourier algorithm

0 Introduction
Large high-voltage motors are widely used in power plants, chemical plants and other large enterprises as expensive electrical main equipment. If a serious fault occurs and the motor burns out, it will seriously affect the normal production and cause huge economic losses. Therefore, it is necessary to provide it with perfect protection. The existing motor comprehensive protection device is mainly aimed at small and medium-sized motors, providing them with current quick-break, thermal overload inverse time overcurrent, two-stage time-limited negative sequence, zero-sequence current, rotor stagnation, long start time, frequent start and other protection functions. However, for large-capacity motors above 2000KW, it is impossible to meet the requirements of protection sensitivity and speed in the event of internal faults. Therefore, this device is developed and combined with the comprehensive protection device to provide more reliable and sensitive protection measures for high-voltage motors. This device is designed as a three-phase longitudinal differential, because the 3KV, 6KV, and 10KV power grids where the motors with large capacity above 2000KW are located may be power grids with the neutral point of the transformer grounded through high resistance. The three-phase longitudinal differential protection can not only serve as the main protection for the short circuit between the stator winding and the lead wire of the motor, but also as the main protection for the single-phase grounding fault, acting on instantaneous tripping. 1 Overall hardware structure of the device The hardware structure diagram of the device is shown in Figure 1.

The device uses Intel's 80C196KB as a processor. In order to improve the system's anti-interference ability and prevent the program from running away, a MAX705 chip is used as a hardware watchdog. All digital input and output are introduced and led out by the pins of the EPLD (erasable programmable logic circuit) chip, and the switch quantity (not marked in the figure) enters the EPLD after the optical coupler. In addition, the EPLD is also responsible for the address decoding of the peripheral chip select in the system and the realization of the digital logic gate circuit. The A/D uses the AD7874 of the American Analog Devices Company. The AD7874 is a four-input 12-bit data acquisition system with high sampling accuracy. Moreover, since it can sample 4 signals at the same time, it can reduce the error caused by non-simultaneous sampling of signals. This device exchanges data with the host computer in serial communication mode. In order to meet the requirements of different communication methods that different users may adopt, this device is designed with two communication interfaces, RS-232 and RS-485, and users can freely choose according to their needs. 2 Protection principle and algorithm description 2.1 Protection principle and advantages The difference between various types of differential protection is mainly reflected in the composition of the braking amount. The braking amount in the traditional longitudinal differential protection is composed of the fault component superimposed on the through current, while the braking amount of the fault component longitudinal differential protection is only composed of the fault component. The specific analysis is as follows. 2.1.1 The composition of traditional differential and fault component-based longitudinal differential Take the following simplest two-side power supply as an example. (As shown in Figure 2)

Figure 2(a) shows the fault network. The traditional longitudinal differential protection compares the total current on both sides. Here, the "total current" (Is _and Ir ₎ includes two parts: the through current _{Ith (i.e., the load current Ip} in Figure 2(b) ) generated by the unequal electromotive force of the two power sources ( _Er ≠Es _{) and the fault current components Is △} and Ir _△ (as shown in Figure 2(c)) generated by the reverse voltage -U _△ before the fault at the fault point. The differential current composition can be expressed as follows:

Figure 2(c) shows the fault component network. Therefore, the fault component longitudinal differential protection can be expressed as follows:


Figure 4 shows the fault component longitudinal differential protection curve, whose slope passes through the origin, so it can be replaced by equation (8).

2.1.2 Analysis of sensitivity and selectivity
From equations (1) and (5), it can be seen that there is no difference in the operating current between the traditional longitudinal differential protection and the fault component longitudinal differential protection. However, in terms of operating current, from equations (2) and (6), it can be seen that the restraining current using the fault component is only related to the balanced similar network of the equipment and has no direct relationship with the power supply on both sides, and therefore has nothing to do with the through current. In the traditional differential protection, the current on both sides contains the through current generated by the unequal power supply on both sides, which forms a restraining effect and reduces the protection sensitivity. Therefore, the differential protection of the fault component has a higher sensitivity than the traditional longitudinal differential protection.
According to the balanced similar network, it can be deduced that the fault component differential protection has the following relationship in the case of internal faults:

Since the minimum value on the right side of the above equation is 2.0 (Zs _and Zr _are both inductive impedances, and the phase angle difference between the two is always less than 90°), in the case of internal faults, the longitudinal differential protection with If _△ (If ₎ and Ith _△ as the differential current and the restraining current always has:

that is, the restraining coefficient is Kres _{. △} ≤2.0.
The fault component longitudinal differential protection, in the case of internal short circuit , defines the protection action area with K _{res.△ ≤2.0, and in the case of external short circuit, defines the protection braking area with K res.△} ≥0.056. There is a large buffer zone between the braking area and the action area, which shows that the protection has extremely good action selectivity. Theoretically, the fault component differential protection effectively eliminates the influence of unbalanced current under normal load components, and can make the dead zone of the action characteristic near the origin of the coordinate very small, and the fault component differential current threshold ΔI _f.min and the fault component inflection point braking current ΔI _th.min can be obtained to be small. In order to prevent the protection from false action under external faults, the slope of the action characteristic curve is usually taken as large as possible while meeting the sensitivity requirements. It can be seen from formula (9) that |I _{f Δ} /I _{th Δ} | ≥ 2.0, and the sensitivity is very high (Z _f is allowed to be larger). It can be seen that this protection has not only high selectivity but also high sensitivity.
2.2 Digital filtering and algorithm for processing signal effective value
This device adopts a method combining subtraction filtering and Fourier algorithm. The Fourier algorithm assumes that the input voltage and current are periodic functions, and can be decomposed into sine and cosine functions using the Fourier series, so it has a strong function of filtering out high-order harmonics. However, the algorithm itself cannot filter out the attenuated non-periodic components, so a subtraction filter unit is added before Fourier filtering to filter out the attenuated non-periodic components to reduce the error.
The subtraction filter equation is as follows: 3 Conclusion This device has perfect functions. Users can put in or out protection by setting the protection throw-in and throw-out characters. At the same time, the device can select different fixed values ​​according to the state of the motor, that is, whether it is starting or running, so it has good flexibility and adaptability. Monitoring the CT state makes the protection more reliable. Compared with traditional differential protection, it has higher sensitivity and selectivity, and can meet the needs of actual production more ideally. At the same time, in the face of the current enterprises' requirements for industrial production process automation, this actual demand adds a serial communication interface to the protection, which is in line with the development trend of current protection devices.