The access of distributed power supply (DG) has changed the single power supply structure of distribution network, making the original relay protection device no longer applicable. According to the directional comparison longitudinal protection scheme, the protection slave controller is designed with LPC1768 as the main chip. Software system is designed according to protection logic. With the performance of single-chip computer becoming more and more superior, embedded relay protection device has become the main protection equipment of power system.
After DG is connected to the distribution network, the protection scope and sensitivity of the protection device are changed, and the distribution network is transformed into a multi-power structure. The original relay protection devices such as three-stage overcurrent protection and distance protection reflect only a little electrical information at the installation site, which is no longer applicable. In order to solve the problems caused by DG access, the mature longitudinal protection in transmission network is used as primary protection, while the three-stage overcurrent protection is used as backup protection. This paper uses ARM Cortex-M3 series 32-bit MCU LPC1768 with low price and superior performance to design, and puts forward its design scheme.
The basic principle of directional comparative longitudinal protection is that in a protected area, a station-level host is set up in the substation and a protection slave is set at the breaker switch; the protection slave is responsible for judging whether there is fault current and fault direction and uploading, without considering the influence of DG access on short-circuit current; the main station judges the fault point by calculating the fault information detected by the protection slave. The direction module is installed inside the protection slave to measure the fault direction at the installation point. When the slave controller’s directional module outputs high level “1”, it indicates that there is a fault, and the direction information is upstream fault; when the slave controller’s directional module outputs low level “0”, it indicates that the detected fault information is downstream or no fault. Fault information is obtained from slave computer and uploaded to main station through optical fiber. Distribution system often has a large number of branch lines, thermostatic element and the network structure is complex. The simple distribution network structure is shown in Figure 1. In fault location algorithm, correlation matrix L is used to describe the direct relationship between slave and line segment. The correlation matrix L is m*n matrix, m is the number of protection slaves and N is the number of line segments. The element Lij denotes the connection relationship between the first node and the J branch. When the first node is directly connected to the j branch and the branch J is in the forward direction of the node i, Lij = 1; when the second node is directly connected to the j branch and the branch J is in the reverse direction of the node i, lij=-1; when the node I is not directly connected to the shoe, Lij = 0. The positive direction is that the substation side points to the user side of the distribution system. The fault information uploaded from the slave controller is represented by a row vector G. The elements in vector G correspond to the protection slave one by one. From the value of the element in vector P, it can be found that the value of the sixth element is 1. Based on this, it can be judged that L6 is a fault section, which is consistent with the actual situation. Microcomputer relay protection device based on directional comparative longitudinal protection can be divided into seven basic functional modules: CPU main system, analog input/output module, directional module, power supply module, auxiliary detection module, man-machine interface and communication module. LPC1768 is a microcontroller based on ARM Cortex-M3. It is an ideal choice for real-time signal processing and control. LPC1768 extends MAX706 watchdog chip. Watchdogs are used to monitor MCU activities.
When the system is in an unknown state, such as program running or entering a dead cycle, the watchdog does not receive the trigger signal from the MCU within 1.6 seconds, then the output of WDO becomes low and resets the signal to the system.
LPC1768 expands the FM24CK64 chip to store the set parameters. FM24CL64 write operation can be completed in a delicate way, which can effectively avoid the interference of noise or power fluctuation, so it has higher reliability. LPC1768 extends Flash memory M25P64. M25P64 has 8M bytes of storage space for word libraries.
The PT and CT secondary sides of the acquisition line enter the control board through the transformer ZMCT112C, and are processed by the three-terminal filter ACF321825. CT or PT transformer converts the voltage and current signals into the voltage matching the analog input channel of the connected measuring equipment, and can completely isolate the measuring equipment from the transformer circuit, thus improving the insulation and anti-interference ability of the equipment.
The main system uses IR2113s chip to drive IGBT, which is a fully controlled power switch device. IR2113 is a dual-channel, gate-driven dedicated driver chip. The working voltage of the high-voltage terminal can reach 500V, which can provide up to 4A driving current, meet the driving requirements of IGBT and reduce the size of the driving circuit. Auxiliary detection includes switch detection and energy storage detection. Auxiliary detection signals need to pass through the optocoupler to improve the anti-interference ability of the equipment. The optocoupler is used to reduce the interference of signal to the controller and enhance the security of the system. TLP521 is chosen here. The power module is responsible for providing power supply for many chips in the controller. The power supply module receives power from Pt. 230V voltage is transformed into 12V, 5V, 15V by LB05-10D0515 module of Mornsun company. 5V is converted to 1.8V and 3.
3V by SPX1117-1.8 and SPX1117-3.3.
12V provides power supply for switch-on drive.
Power direction judgement uses directional elements to determine the direction of overcurrent. The simple principle of directional element is that voltage Uu and current Ui are collected through zero-crossing comparator first; when Uu or Ui is positive half-cycle, the corresponding zero-crossing comparator outputs high level, and when Uu or Ui is negative half-cycle, the corresponding zero-crossing comparator outputs low level; the square wave voltage of comparator outputs is sent to SCM via regulator, XOR gate. The man-machine interface module is mainly the display screen driven by ULN2003. ULN2003 is a high-voltage and high-current Darlington transistor array. It is suitable for 5V TTL and used to control controllable high-power devices. Longitudinal protection scheme relies heavily on fault information and requires reliable and rapid access to fault information, so communication requirements are stringent. Nowadays, the commonly used wireless communication technologies such as GPRS in distribution network can not meet the requirements. Optical fiber communication is chosen here. Based on uc_II operating system as the system software platform, it can ensure that many tasks of the slave protection device can be completed orderly and on time.
Its core is to deal with the priority of tasks and interruptions. In the application program of pilot protection based on optical fiber communication, the system mainly completes information acquisition, communication, switch output and basic supporting functions.
Among them, the tasks of data preprocessing, communication and switching output have the highest real-time performance, and the tasks with the highest priority are set in the design. Other main functional tasks: auxiliary detection, blocking and alarm, backup protection, etc.
, the priority of basic tasks is next. Man-machine interaction and system setting tasks: data recording, parameter modification, display, etc., priority is lower than other major functional tasks. Task scheduling and interrupt scheduling are accepted after system initialization. According to the directional comparative longitudinal protection theory, a protection slave controller for distribution network with DG is designed in this paper. In practical operation, anti-electromagnetic interference, master-slave coordination and islanding of distributed generation should also be considered.