The Influence of Distributed Generation on Relay Protection Technology

In the face of increasingly serious environmental and energy problems, the development and utilization of renewable energy has become the focus of current research work. Distributed generation can use renewable energy to generate electricity, which is an important measure to solve the current environmental and energy problems. In the last century, the term distributed generation first appeared in developed industrial countries in the West. With the development of smart grids and energy development technologies, distributed generation technologies will surely lead the revolution in the power industry.

The distributed power supply is often connected to the distribution network in situ. The relay protection in the traditional low-voltage distribution network system is designed on the basis of single-side power supply and radiation type structures. Generally, only simple protection can be used to meet the requirements. Grid relay protection requirements. With the large number of distributed power supply DGs in the power grid, the power flow will change, and the short circuit current of each line or bus will also change when a short-circuit fault occurs in the system. There are many new problems in relay protection. Traditional simple protection has been Can not meet the requirements.

1 Distributed Generation Definition

Distributed generation was first proposed by the U.S. in the Public Utilities Management Policy Act of 1978. Its definition is: the traditional form of power generation, which is different from centralized power generation, long-distance transmission, and large Internet, is directly deployed near the distribution network or load. Small, modular (power generation between kW and 50mW) power generation facilities that can operate economically, efficiently, and reliably. Distributed generation is to meet the needs of specific users and to support the operation of the existing distribution network, distributed in the vicinity of the user.

Distributed power generation technologies mainly include: combined heat and power generation and thermoelectric cooling, fuel cell technology, solar power generation technology, wind power generation technology, biomass energy power generation technology, small hydropower technology, ocean energy power generation technology, and geothermal power generation technology.

Energy-storage devices are also included in distributed generation. Their significance is to improve energy efficiency and improve the economics of the system, including: battery storage, superconducting energy storage, supercapacitor energy storage, flywheel energy storage, compressed air, and pumping Energy storage and so on.

Distributed generation technologies and energy storage technologies are collectively referred to as distributed generation. Different technologies are used for distributed generation. There are high-frequency alternating current, industrial frequency alternating current and direct current. The corresponding grid-connected methods include direct grid-connected and inverter-connected grids. Among them, the wind power generation can use the two kinds of inverter grid-connected and direct grid-connected, and the rest can only adopt one kind of grid-connected mode.

The distributed power supply can be operated independently, in standby operation, or in parallel with the system. Its operation mode is: distributed power supply operates independently; distributed power supply is standby operation; distributed power supply and system are operated in parallel.

Distributed power sources can be used as peaking power sources to reduce grid losses or provide power to remote areas, while also saving investment in power transmission and transformation, economic benefits, and improving the reliability of power supply.

2 Influence of Distributed Generation on Relay Protection Technology

Most of the traditional distribution network is a radiating structure of a single-side power supply. Its protection configuration is relatively simple. There are two main configuration methods for domestic power grid relay protection:

2.1 using traditional three-phase current protection

Instantaneous current quick-break protection, definite time current quick-break protection and over-current protection. Instantaneous current quick-break protection can instantaneously cut off the fault, but it cannot protect the full length of the line. When the setting value is short-circuited according to the end of the protected line, the maximum three-phase short-circuit current flowing through the protection should not be set by the protection action. The definite time current quick-break protection can protect the whole length of the line, and the setting value is determined according to the line terminal fault sensitivity and the instantaneous current protection of the adjacent line. The overcurrent protection setting is generally set according to the maximum load current of the line and the overcurrent protection of the adjacent line, and the full length of the adjacent line can be protected.

2.2 Using Inverse Time Overcurrent Protection
Inverse time overcurrent protection is generally intended to remove faults near the power supply as quickly as possible. This is a protection-time limit related to the size of the short-circuit current in the protected circuit. The greater the short-circuit current, the shorter the protection time limit; the protection time limit is related to the fault's distance, and the farther the action time is, the longer it will be.

Since most of the faults in the distribution network are transient faults, a three-phase automatic reclosing device should be configured in the non-full-cable lines to ensure that the power supply can be quickly restored after a fault.

Power system relay protection devices should meet the four basic requirements, namely: selectivity, quickness, agility and reliability. The following sections discuss the different effects of DG on the two relay protection configurations and the automatic reclosing, as well as some effects on the quadratic protection relay.

2.2.1 The main performance of DG three-phase current protection

1 Causes non-faulty line sensitivity to increase, protection against misoperation, so that protection will lose selectivity, and will expand the scope of the accident. Before DG access, when other lines fail, the short-circuit current is only provided by the system; after accessing DG, the DG and the system will provide a short-circuit current to the fault point, which will increase the fault current of the faulty feeder and increase the sensitivity, which may cause serious Protection misbehaving. The greater the access capacity of the DG, the greater the impact on the sensitivity of the protection.

(2) The sensitivity of the protection of the line is reduced, and protection is refused when it is severe; the sensitivity of the downstream protection is improved, and the protection is misoperation. When the downstream line of the DG access point fails, the short-circuit current is only provided by the system before the DG accesses. After accessing the DG, the DG and the system simultaneously provide the short-circuit current, but the DG upstream protection can only sense the short-circuit current provided by the system. Moreover, this fault current is smaller than before the access to DG, which results in a decrease in the protection sensitivity. In severe cases, this may cause protection refusal. On the other hand, the protection current of the downstream DG is larger than that before the access to the DG, resulting in increased sensitivity, and even in case of seriousness, it may cause protection misoperation. And the greater the capacity of the accessed DG, the greater the impact on the protection sensitivity.

When the DG capacity of the access system is fixed, the impact on the relay protection is small when it is incorporated into the end of the line. At the same time, a protection directional element should be added upstream of the DG to ensure that the protection will not be mistaken when the DG upstream or system side fails. . When the position of the access DG is fixed, the capacity of the access should be ensured, and the fixed value of the downstream protection of the DG and the adjacent line protection setting are increased to meet the requirement of selectivity; the sensitivity of the upstream protection of the DG is checked; When the DG comes out of operation, the protection has sufficient sensitivity and protection range.

2.2.2 Influence of DG on Inverse Time Overcurrent Protection

After the DG is connected, the magnitude and direction of the short circuit current will change, which will bring the following effects to the inverse time overcurrent protection:

Downstream of 1DG, the short-circuit current in the upstream decreases, which increases the time limit for protection action, which is not conducive to the rapid removal of the fault.

Downstream 2DG faults, the downstream short-circuit current increases, the protection operation time is shortened, and the protection quickness is improved.

2.2.3 Influence of DG on automatic reclosing In the case of grid faults, transient faults account for the largest number, and installing automatic reclosing can improve the reliability of the supply and reduce the number of line blackouts. The incorporation of DG will cause hidden failures such as failure of reclosing due to automatic reclosing and non-synchronized closing. For example, after the line trips due to a fault, the formed islands maintain power and voltage near the rated value. The distributed power source may not jump off the line during reclosing action. The fault current at the fault point will be maintained by the distributed power supply. , resulting in failed reclosing, non-synchronous closing, etc., and even caused damage to the grid equipment.

3 other solutions

In view of the influence of DG on three-stage overcurrent protection and automatic reclosing, it is proposed in the literature that the use of a series reactor to limit the short-circuit current can eliminate the coordination problem between the distributed power supply and the protection to a certain extent and ensure the correct operation of the automatic reclosing. Or according to the concept of wide-area protection, using the current amplitude comparison to reduce the fault to a smaller area, and use the phase relationship between the current to find the fault line, this method has a very good value in the project.

Distributed generation based on new energy has brought both opportunities and challenges to the traditional power system. DG can make full use of clean energy, promote China's sustainable development, and make up for the shortcomings of traditional power grids in environmental protection and reliability. The traditional relay protection and automatic reclosing device based on the design of single-ended power supply system must keep pace with the times and make corresponding adjustments to maintain the safe and stable operation of the system. Otherwise, the incorporation of distributed power supply will inevitably lead to protection rejection. Problems such as moving or misbehavior may seriously cause a large-scale blackout and damage the interests of the national economy.

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