Abstract: Traditional distribution networks typically exhibit a single-source radial power flow from the power source to the system. When distributed wind farms are integrated, short-circuit faults alter the original distribution network structure, potentially causing power flow reversal. The short-circuit current provided by the distributed wind farm also affects the coordination between relay protection systems. This paper analyzes the principle of traditional current protection and its impact on relay protection after the integration of distributed wind farms. A relay protection method combining current limiting and current protection is proposed, and a system simulation model is established using PSCAD/EMTDC. Simulation data is used to verify the correctness of this protection scheme.
1 Introduction
Traditional distribution networks typically operate in a single-source radial or dual-source open-loop configuration, with power flow from the system to the loads. Therefore, current protection in traditional distribution networks fully meets relay protection requirements. However, with the depletion of fossil fuels, the severity of environmental pollution, the occurrence of accidents in traditional large power grids, and the development of the power industry worldwide, there is a growing demand for efficient, green, and economical energy sources. Wind energy is a renewable and pollution-free green energy source. Wind power development technology is maturing and offers significant economic and social benefits, making it one of the most promising new energy sources today. However, the decentralized integration of wind power and the increasing capacity of wind farms make the protection of distribution networks with wind farms a challenge. While decentralized wind power integration into distribution networks has gained widespread attention in China, the increasing number and capacity of wind farms have altered the power flow structure of traditional distribution networks, severely impacting relay protection. Therefore, a systematic analysis of the impact of distributed wind farm integration into distribution networks on line relay protection is of great significance.
In recent years, the large-scale integration of wind farms has brought about significant concerns regarding distribution network relay protection. Previous research on wind power has largely focused on internal fault analysis within wind farms and voltage stability control of distribution networks incorporating wind farms. However, research on the magnitude of short-circuit currents provided by wind farms and the protection of the entire system has been limited, with even less research on the feasibility of protection schemes for distributed wind farms connected to the grid. But as wind farm capacity increases, so does the short-circuit current, impacting the sensitivity and selectivity of distribution network relay protection.
This paper analyzes the characteristics of non-three-phase short-circuit faults after the integration of distributed generation (DG) and proposes a fast protection scheme. Based on the location of the DG connection point, the protection feeder is divided into regions, with directional longitudinal protection configured in the upstream region of the DG, while overcurrent protection is retained along the entire feeder. A current-limiting resistor is proposed to be connected at the DG's output. When a short circuit occurs, the current-limiting resistor limits the short-circuit current to a very small range, thus ensuring coordination between protection systems. However, problems arise: the current-limiting resistor causes a voltage drop at the DG's output, and secondly, the resistor consumes energy in the system. The paper comprehensively analyzes the shortcomings of traditional relay protection schemes and proposes a relay protection scheme that coordinates current-limiting reactance with current protection. Simulation data verifies the feasibility of the protection scheme.
2. Traditional Three-Stage Current Protection and Its Characteristics
Distribution networks are widely distributed throughout the power system, and their safe operation is directly related to the safety of people's lives and property. Therefore, the protection of distribution networks must be simple, safe, and reliable. Three-stage overcurrent protection precisely possesses these characteristics, and thus is widely used in distribution network protection and has been recognized in practical applications. Three-stage overcurrent protection mainly includes instantaneous overcurrent protection, time-delayed overcurrent protection, and definite-time overcurrent protection.
2.1 Instantaneous overcurrent protection
Instantaneous overcurrent protection, also known as time-limited instantaneous overcurrent protection or instantaneous stage I current protection, is a current protection that reacts to the increase in the amplitude of the short-circuit current and operates instantaneously. Its setting principle is to ensure that the protection will not maloperate when a short-circuit fault occurs at the outlet of the next level line. The setting is based on the operating current being set to avoid the maximum short-circuit current when the protected line is short-circuited externally. The setting formula is shown in equation (1).
(1)
In the formula: K1rel is the instantaneous overcurrent protection coefficient after considering various influences, which is generally taken as 1.2~1.3; Kd is the short circuit type coefficient; Eph is the system equivalent potential; Zsmin is the equivalent impedance of the protection back-side system under the maximum operating mode; ZL is the positive sequence impedance of the protected line; and I3k.max is the effective value of the periodic component of the primary current when a three-phase short circuit occurs at the end of the protected line under the maximum operating mode of the system.
To determine the instantaneous overcurrent protection range, assuming the fault occurs at line λZL, the short-circuit current is:
In the formula, Zs is the equivalent system impedance of the protected back-side system.
The schematic diagram and operating characteristics of the instantaneous overcurrent protection are shown in Figure 1.
It is evident that traditional instantaneous overcurrent protection cannot protect the entire length of the line, and the protection range is greatly affected by the system operating mode and the type of short circuit.
2.2 Time-limited instantaneous overcurrent protection
Instantaneous overcurrent protection cannot protect the entire length of the line, so faults outside its protection range must be cleared by time-delayed instantaneous overcurrent protection. This is called time-delayed instantaneous overcurrent protection, also known as stage II protection. Its setting principle is to avoid the maximum short-circuit current at the end of the instantaneous overcurrent protection range of the next-level line. The setting formula is:
2.3 Time-limit overcurrent protection
Timed overcurrent protection, also known as overcurrent protection or stage III protection, serves as a remote backup protection when the main protection of the downstream line fails to operate or the circuit breaker fails to operate, and also acts as overload protection. The setting principle is to avoid setting it above the maximum load. The setting formula is:
(5)
3. Impact of Distributed Wind Farm Access on Traditional Distribution Network Relay Protection
Traditional distribution network lines typically employ a three-stage current protection system working in tandem to meet relay protection requirements. However, with the integration of distributed wind power, the traditional single-source power supply model has been altered, leading to corresponding changes in the distribution network structure and power flow distribution.
3.1 Impact of Distributed Wind Farm Integration on the Selectivity of Current Protection in Traditional Distribution Networks
As shown in Figure 2, in a traditional distribution network, the power flow always originates from the power source and flows to the line, regardless of whether a short-circuit fault occurs. The current protection characteristics of a traditional distribution network are that the three-stage coordination of the current-level line, plus the coordination between lower-level lines, fully meet the requirements of line relay protection. However, when a distributed wind farm is connected to the distribution network, as shown in Figure 3, the original distribution network structure is altered. When a fault occurs upstream of the distributed wind farm's access-level line protection (F1 or F2, typically a busbar or an adjacent line on the same busbar), the reverse short-circuit current provided by the distributed wind farm will cause the protection to malfunction. When a fault occurs downstream of the distributed wind farm (F3), the boosted current provided by the distributed wind farm makes the sensitivity excessively high compared to the original distribution network, even extending the protection range of the upper-level line to the lower-level line, thus affecting the coordination between the protection of upper and lower-level lines.
3.2 Impact of Distributed Wind Farm Access on Reclosing and Fuse Sectionalizer Coordination
Traditional power distribution network lines are sometimes equipped with reclosing devices at the beginning. Whether it is front-acceleration or rear-acceleration, the principle is the same: after the recloser is disconnected, the reclosing is detected to see if the fault is isolated. For single-power supply networks, it can correctly disconnect or reclose.
When a distributed wind farm is connected, if a fault occurs at the point where the distributed wind farm is connected as shown in Figure 4F4, the distributed wind farm will still provide a large pipeline current to the fault location after the reclosing switch at the beginning of the line is disconnected. As a result, the arc cannot be extinguished smoothly after the reclosing switch is opened, and the reclosing will fail. In severe cases, the reclosing device will be damaged.
In addition, the excessive short-circuit current provided by the distributed wind farm can cause the sectionalizer to count incorrectly, leading to maloperation or failure of the protection system. It can also cause the original distribution network to lose its coordination with the fuses, thus affecting the sectionalizer, fuses and their coordination.
In summary, when large-capacity distributed wind farms are connected to the distribution network, the excessive short-circuit current provided during short circuits affects the coordination between traditional current protection systems. Therefore, adopting a method that combines current limiting and current protection is an economical and applicable solution to meet the relay protection requirements of the distribution network lines.
4. Distributed wind farm integration into power distribution network current limiting and protection scheme
The excessive short-circuit current provided by distributed wind farms is the main factor affecting the coordination of relay protection in traditional distribution network lines. Therefore, a protection scheme that combines current limiting protection and current segment protection is proposed for short-circuit current, as shown in Figure 5.
4.1 Protection Principle of Coordinated Current Limiting and Current Protection
The experiment sets the current-limiting reactance value. When a short-circuit fault occurs in the relevant lines of the system, if the distributed wind farm outlet current-limiting protection device senses the fault current, the current-limiting protection will be activated, thereby limiting the additional short-circuit current, and the system protection will operate correctly.
4.2 Principle of Current Limiting Protection Device
The equivalent diagram of the current limiting device is shown in Figure 6. At the initial stage of operation, all thyristors are turned on. A portion of the current flows through the primary winding of the series-coupled transformer. This current is coupled to the secondary winding and charges the DC inductor through the bridge circuit, resulting in a gradually increasing DC current in the DC inductor. Simultaneously, a portion of the AC current flows through the bypass inductor connected in parallel to the primary winding of the transformer and gradually decreases. After several cycles, the charging process ends, and the system enters a steady-state operation phase. The primary and secondary currents of the transformer are defined as Ia and Ib, respectively, with a turns ratio of N. The line current relationship is Ia:Ib=1:N. The current ILd in the DC inductor is equal to the peak value of the secondary winding current, i.e., ILd=2Ib=2NIa. During steady-state operation, the current ILd in the DC inductor is close to a constant, so dILd/dt≈0. Therefore, the voltage drop across the DC inductor is close to 0, which means the voltage drop across the secondary winding of the transformer is very small. Consequently, the voltage drop across the AC inductor connected in parallel to the primary winding is almost zero. During steady-state operation, the voltage drop of the current limiter is mainly caused by the leakage reactance, winding resistance, and thyristor voltage drop of the series transformer. In high- and medium-voltage systems, this voltage drop is negligible. When a short-circuit fault occurs, a large voltage drop is suddenly applied to the primary side of the transformer, and the AC inductor immediately experiences a steady-state short-circuit current ILa, while the DC inductor current ILD subsequently increases. Due to the presence of AC and DC reactors, the fault current is suppressed and does not rise sharply. Through a proper control strategy, the DC inductor and bridge circuit are taken out of the fault circuit operation, and the AC current-limiting inductor fully assumes the current-limiting function. The size of the bypass AC current-limiting inductor is determined by the allowable short-circuit current level of the system, i.e.
(6)
In the formula: ω is the angular frequency of the power system, Un is the rated phase voltage of the system, and Isel is the allowable short-circuit current.
Considering the most severe short-circuit condition where the inductance of the DC inductor continuously increases for half a cycle, the relationship between the allowable increase in current Δim and the DC inductance value La during a short circuit is as follows:
(7)
In the formula: ULdFm is the peak value of the maximum possible voltage applied to the DC reactor during a fault.
To minimize the size of the DC reactor, the inductance value should be designed to minimize the energy stored in the inductor under fault conditions. The energy stored in the DC inductor under fault conditions is:
(8)
In the formula: Im is the current flowing through the DC inductor during steady-state operation. Substituting equation (7) into equation (8), and finding the extreme value of equation (8) with respect to the variable Ld, we can obtain:
(9)
The above equation shows that selecting the DC inductor by increasing the current in the DC inductor to twice its normal steady-state operating value within half a cycle after the short circuit occurs will optimize the DC inductor design. Therefore, the design of the DC inductor and bridge circuit parameters is not directly related to the system's short-circuit current level.
4.3 Advantages of Current Limiting and Current Protection Design
Based on the above analysis, when a distributed wind farm is connected to the distribution network, the original distribution network line relay protection uses a combination of current limiting and current protection, which has the following main advantages:
(1) Currently, all fan outlets need to be equipped with low voltage ride-through protection devices, that is, when the system fails, the fan will not disconnect from the grid within 625ms in the low voltage range. This time limit is far beyond the general system current protection time limit, so current limiting is more necessary, and the current limiter and low voltage protection also form a cooperative relationship when the fault occurs.
(2) For general current-limiting reactors or resistive current-limiting devices, when the system fails, although the current is limited, a large amount of energy is consumed or a voltage drop is generated. For this solution, a new type of solid-state short-circuit current-limiting reactor is adopted to overcome the above disadvantages.
(3) The switching time of the current limiting protection is much lower than the operating time of the protection device, ensuring the coordination between the current limiting and the current protection time.
(4) This solution is more economical and simpler than other power direction protection components or additional protection devices.
5. Numerical Examples for Verification
The simulation example uses the equivalent model shown in Figure 5. The system has three busbars, one of which is connected to a distributed wind farm. The simulation parameters are as follows: Distribution network voltage: 10.5kV; System: Minimum operating impedance: 0.54Ω, maximum operating main reactance: 0.64Ω; Distributed wind farm: Minimum operating main reactance: 0.75Ω, maximum operating main reactance: 1.75Ω; Line parameters: Line L1 main reactance: 0.88Ω, Line L2 main reactance: 0.96Ω, Line L3 main reactance: 0.85Ω, Line L4 main reactance: 2.55Ω, Line L5 main reactance: 1.75Ω, Line L6 main reactance: 1.8Ω, Line L7 main reactance: 3.5Ω. Reliability coefficients for setting calculations: Current segment I reliability coefficient: 1.2, Current segment II reliability coefficient: 1.1, Current segment III reliability coefficient: 1.2, Self-starting coefficient:
1.1, Response coefficient: 0.9.
Table 1 shows the protection settings before the system is connected to the distributed wind farm. When the distributed wind farm is connected to the R6 busbar of the protection, the short-circuit current at each protection point is shown in Table 2.
Therefore, it can be seen that when a three-phase short-circuit fault occurs at the beginning of the system bus or adjacent line after the distributed wind farm is connected, the protection R3 will trip the two-stage circuit breaker. According to traditional protection (without the distributed wind farm connected), the protection cannot operate when a fault occurs on the system bus, especially on adjacent lines. Therefore, the connection of the distributed wind farm will cause the protection to malfunction. When a short-circuit fault occurs on lines L6 and L7, the increased short-circuit current provided by the distributed wind farm makes its corresponding protection range too large, with the short-circuit current of R6 increasing to 2.65V. The test data after connecting the line protection device at the distributed wind farm outlet is shown in Table 3.
Comparing Tables 2 and 3, it can be seen that the short-circuit current is significantly reduced after the current limiting protection device is connected. Thus, the current limiting protection and the three-stage protection work together to meet the relay protection requirements of the distribution network lines after the distributed power source is connected to the distribution network.
6 Conclusions
This article analyzes the current protection principle, protection setting principle, and sensitivity analysis. It systematically discusses the impact of distributed wind farm integration on the power flow distribution and current protection of traditional single-source distribution networks. A novel solid-state current protection device is proposed for integration at the distributed wind farm outlet, enabling it to cooperate with traditional current protection to meet the relay protection requirements of the distribution network lines. A simulation model is established using PSCAD/EMTDC, and data extraction and analysis verify the correctness of the proposed solution.
