## 3 LE Identification Method Considering the Fluctuation of Wind Power Output
### LE Identification Criterion  
LE in a power system usually occurs after the topology of the grid has been changed. When a contact line in the system is removed due to a fault, its active power will be transferred to other normally operating branches in the system, causing an overload on the normal line. If it is possible to quickly estimate the active power transferred by each branch in the CLS of the removed branch after the line is removed, then compare this estimate with the APMV, the cause of the line overload can be determined. If LE occurs, the backup protection should be correctly blocked before the line reaches its thermal stability limit.  
The basic idea of identifying LE is as follows: if branch \(k\) is overloaded after faulty branch \(l\) is removed, the active power of the branch \(k\) after the faulty branch \(l\) is removed is estimated according to the active power distribution of the power grid before the faulty branch \(l\) is removed, and the APEV of the branch \(k\) is compared with the APMV of the branch \(k\) after the branch \(l\) is removed. If the two are basically the same, it shows that the overload of branch \(k\) is caused by the LE. The LE identification criterion based on active power is shown in Equation (5).  
\[\left|\mathrm{P}_{k}^{t}-\mathrm{P}_{k}^{\prime}\right|<\varepsilon\mathrm{P}_ {k}^{\prime} \tag{5}\]  
where \(\mathrm{P}_{k}^{t}\) is the APMV of the line \(k\) uploaded by WAMS after the power grid topology changes; \(\mathrm{P}_{k}^{\prime}\) is the APEV after active power redistribution; \(\varepsilon\) is the threshold value considering the transient transition process and various errors of the power grid. The factors affecting the value of \(\varepsilon\) are as follows:  
* The maximum error caused by the change in the amplitude of the bus voltage at the protection installation.
* The maximum error caused by the transient oscillation process of the power grid.
* The influence of branch capacitance current.
* The margin required to ensure the reliability of the criterion. Based on the above four factors, \(\varepsilon\) can take 30% [24]. Equation (5) is called the steady state criterion and is applicable when the system's operating state does not change much after the faulty branch is removed.  
Define \(\left|\left(\mathrm{P}_{k}^{t}-\mathrm{P}_{k}^{\prime}\right)/\mathrm{P}_{k}^{ \prime}\right|\) as the LEE. If \(\left|\left(\mathrm{P}_{k}^{t}-\mathrm{P}_{k}^{\prime}\right)/\mathrm{P}_{k}^{ \prime}\right|<\varepsilon\), it shows that the LE identification criterion is satisfied. It is concluded that the branch overload is caused by the LE.  
According to the circuit superposition principle, the active power transmitted by each line in the CLS of the faulty line after the faulty line has been removed can be considered to be obtained by superposition of the pre-fault active power and the APCV after the line has been removed. The active power transmitted by each line in the network before the faulty line is removed can be obtained based on the measurement value of WAMS, and the key to identifying the LE is to estimate the active power of each line in the network after component removal based on the real-time measurement value of WAMS. In the following, the traditional calculation method is first introduced and the pitfalls of this method in high wind power penetration power systems are analyzed, based on which a calculation method for the APEV applicable to high wind power penetration power systems is proposed.