2011 The International Conference on Advanced Power System Automation and Protection

*Corresponding author (email: xiwen@tongji.edu.cn) APAP2011 www.apap2011.org

An adaptive relaying algorithm for the protection of distribution networks integrated with wind farms

ZHANG WenHao, JIN LiJun, YAN ShuJia

Tongji University, Shanghai, 201804, China

Abstract: With the increase of wind plants integrated to the power system, the effect of the wind power generation on the system has become more and more severe. The connection of wind power plant changes the short-circuit current in distribution network. Protections now used must be adjusted and changed to ensure correct fault clearing with right method, and maintain the safe and stable operation of the distribution system. In this paper, cases including the wind farm cut-in and cut-off, winddifferent farm capacity, different integration points, and different fault types have been studied to get changes of fault current. After detailed investigation, an adaptive algorithm is proposed to deal with the effect of wind farm integration on relays in distribution system. A model system in Matlab/Simulink is used to show the effectiveness of the proposed algorithm.

Keywords: Adaptive relaying, wind farm, distribution system

1 Introduction

Global adoption of renewable energy is increasing due to growing concern over climate change, increasing costs associated with conventional generation, and decreasing capital investment costs of renewable energy technologies [1-2]. Specifically, wind power represents the most technologically mature renewable alternative and recognized as a cost effective generation source in both large and small power systems. With the improvement of wind power technology and increase of wind power capacity, the impact of wind power on the grid has become an important re-search topic.

Among the world's popular wind power technologies, doub-ly-fed induction generator (DFIG) is now more commonly used. Although many works have been published on the distributed generation, DFIG has different fault characteristics because wind turbines do not have independent power excitation so it cannot provide a stable short-circuit current when fault occur [3-5]. When it accesses to distribution networks, it may result in protec-tive relays’ un-tripping or mis-tripping which need to be solved urgently.

With the increase of wind plants integrated to the power sys-tem, the effect of the wind power generation on the system has become more and more severe. The connection of wind power plant changes the short-circuit current in distribution network [6-8]. Many works have been done to study the behavior of relays after the integration of wind farms [9-10]. Protections now used must be adjusted and changed to ensure correct fault clearing with right method, and maintain the safe and stable operation of

the distribution system. In this paper, various complicated operating modes are investi-gated for distribution networks interconnected wind farms, such as the cut-in and cut-off of the wind farm, the number of wind farms, wind farm capacity, connecting position, and fault types. Models of the distribution system integrated with wind farms have been built and detailed simulation and analysis have been carried out to get the fault features under various operating modes. Based on the short circuit current calculation and analysis after the wind farm connected to distribution network, the adaptive instantaneous protection is put forward to solve the problem of malfunction or refusal that original instantaneous protection may appear.

2 Effect of wind farms integration into distribution system

In order to investigate the relays’ performance of distribution system after the integration of wind farms, the fault currents un-der different operating conditions have been studied to get the effects of wind farms.

(1) Fault current with wind farm cut-in and cut-off

The main protective relays in distribution system usually adopt the traditional current protection, so the integration of wind farms has severe effect on relays. A typical configuration of distribution system with wind farms is shown in Figure 1. Seen from Figure 2, the relay R2 measured fault current for a three-phase fault at F1

with wind farm cut-in is larger than that under cut-off condition,

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2011 The International Conference on Advanced Power System Automation and Protection

which means the relay R2 would have a higher tripping sensitivi-ty.

Figure 1 Distribution system connected with wind farm

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Figure 2 Fault current at R2 with wind farm cut-in and cut-off

(2) The effect of wind farm capacity on relays

After the integration of wind farm into the distribution system, if a fault occurs in the system, wind farm would contribute fault current to the fault point, and the fault current contribution de-pends on the capacity of wind farm, which would have different effect on the relay performance.

Suppose a three-phase fault occurs in line L2 in the system in Figure 1, the performance of relay R2 can be investigated to get the effect of different wind farm capacity. Seen from Figure 3, with the increase of wind farm capacity, the measured fault cur-rents by relay R2 would increase accordingly because of the its increasing contribution to the fault current, which makes the relay more sensitive and results a larger protection zone.

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Fault Current under different Wind Farm Capacity

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Fig.3 fault current at R2 under different wind farm capacity

(3) The effect of different integration point

For wind farms integrated into distribution system at different location, it will have different impact on the relay measurements. The following system shown in Figure 4 is used to study the ef-fect on relay R1 and R2 when wind farms are integrated at Bus B and C respectively.

Fig.4 wind farm connected to different location

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a) Fault current at R1

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b) Fault current at R2

Figure 5 wind farm connected to different location

Seen from Figure 5, for the fault happening at F1, relay R1

measured a larger fault current when wind farms are integrated at

2011 The International Conference on Advanced Power System Automation and Protection

Bus C. Relay R2 got a larger fault current when wind farms are integrated at Bus B, which is because wind farm at Bus B would supply large fault current. So measured fault currents at R1 and R2

face different effect for different integration point, which would bring difficulty to the relay setting and coordination.

(4) The effect of fault location

Figure 6 Distribution system connected with wind farm

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Figure 7 Fault current of R1 with Varied fault location

For the case of faults at different locations of distribution sys-tem integrated with wind farm, the fault currents measured at a relay installed place also change correspondingly.

Suppose fault happens at different places in system in Figure 6, the fault currents measured by relay R1 are displayed in Figure 7. It is clear that fault current changes because of the distance be-tween fault location and power sources. The closer between fault and sources, the larger fault current is.

(5) The effect of fault types

Under different fault types, the fault current measured by relay would be different apparently. Fault location F2 in system in Fig-ure 6 is taken to illustrate the effect of faulty types on fault current and relay performance. Precisely as deducted from the system analysis, three-phase-to ground fault has largest fault current. The setting of current relay must accommodate to the different fault conditions.

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Fig.8 Fault current at R1 for different fault types

3 Proposed adaptive relaying algorithm

The traditional overcurrent relay’s protection zone would change with the operating mode of the system and may lose se-lectivity after the integration of wind farms. In order to overcome these shortcomings, the setting of the relay must adaptively change with the change of wind farm’s operating mode and the fault types.

As we know from the power system analysis, the traditional fundament frequency fault current can be approximately calcu-lated as,

sf f

s l

EI K

Z Z�

� (1)

whereKf is the coefficient indicating fault type, for three-phase fault it

equals 1 while for phase-to-phase fault it equals 3 2 ;Es is the equivalent voltage of the system; Zs is the source impedance; Zl is the line impedance from relay installed place to fault

point. Referring to (1), the adaptive relay setting can be expressed as,

sset rel f

s l

EI K K

Z Z�

� (2)

where Krel is the reliability coefficient.

(1) Fault type judgment

a. Check negative sequence current exists or not to judge the fault is symmetrical or asymmetrical fault.

b. For asymmetrical fault, use zero sequence current to judge it is ground fault or not.

c. The criterion to determine phase A-to-ground fault is that ( ) ( )BC AB BC CAm I I m I I� � � � � � �� � � � (3)

Where , ,AB BC CAI I I� � �� � � are the difference of line currents, and

m is usually set as 4~8. The criterion for other two phases can be achieved similarly.

(2) Other parameters

a. The equivalent source impedance can be acquired by

2011 The International Conference on Advanced Power System Automation and Protection

building equivalent model for the fault system using the superposition principle.

b. The equivalent voltage can also be achieved by using the relay measured voltage, fault current and equivalent source impedance.

Comparing to the variables in traditional relay setting, the proposed adaptive setting algorithm has the following characte-ristics,

a. The coefficients are not constant, and they depend on the operating mode, wind farm capacity, and fault types.

b. The coefficients must be calculated online to accommodate the integration of wind farm and changing operating mod-es.

Model system in Figure 6 is configured in software Mat-lab/Simulink to prove the effectiveness of the proposed algo-rithm.

Figure 9 Simulation model system

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Fig.10 Fault current at F2 measured by R1

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Fig.11 3 node 500 kV equivalent system Fault current at F2 measured by R1

Seen from Figure 10 and 11, according to the proposed adap-tive relaying algorithm, the setting of relay R1 and R2 are calcu-lated online by considering the change of system configurations. If a fault occurs at F2, fault current measured by relay R2 is larger than its setting value so it can correctly trip for the fault, and the fault current measured by relay R1 is smaller than its setting so that it would not mis-trip for the fault outside its protection zone. The proposed algorithm promises the coordination between two relays and guarantees their selectivity.

4 Conclusion

The proposed protection algorithm has following characteris-tics:

1) The setting is determined by the operating mode of the dis-tribution system based on real-time calculation, while the tradi-tional setting is constant.

2) The setting value can adapt to the disconnecting and con-necting changes of the wind farm by calculating the impedance of the system using both measured voltage and currents. The adaptive relays can trip the faults within the protection zone correctly and not act for the faults beyond its protection zone which shows the proposed algorithm has a better sensitivity, se-lectivity and reliability comparing to the traditional setting rules, which can be proved through theoretical analysis and case studies by simulation.

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