CS protection implements local rapid fault judgment and coordination working mode and can help to improve protection reliability. MS integrated backup protection, which is known as RIBP in previous discussion, analyzes regional comprehensive information to recognize fault effectively in case CS protection fails to clear the fault.
The following discusses the detailed CS protection and MS protection configuration principles.
3.1 CS protection based on differential principle
The principle of current differential protection is not affected by changeable power flow. Therefore, this type of protection can identify faults correctly and rapidly without the need for VTs at every node to provide directionality. In CS protection, differential principle is applied and multiple IEDs can cooperate by information sharing and function coordination to implement extended differential protection. The basic principle is to use the nearest non-fault data when local data has failure and to broaden trip boundary when protection or breaker does not operate correctly.
Based on the CS concept in Framework of coordination backup protection scheme section, the IEEE 14-node system is divided into two substations (CS_1 and CS_2) in Fig. 4, and the two CSs dispose substation domain protection separately based on differential principle. As an example, fault 1 (F1) in the IEEE 14-node system as shown in Fig. 4 is considered and the coordination working mode is analyzed. In this paper, positive current direction is defined as from bus to the line. \( {\overset{.}{I}}_{471} \), \( {\overset{.}{I}}_{781} \) and \( {\overset{.}{I}}_{791} \) are the first-terminal currents of line4-7, line7-8 and line 7–9, respectively. \( {\overset{.}{I}}_{472} \), \( {\overset{.}{I}}_{782} \) and \( {\overset{.}{I}}_{792} \) are the second-terminal currents of the same three lines, respectively. The basic differential current dif
1 of line 7–8 is \( \left|{\overset{.}{I}}_{781}+{\overset{.}{I}}_{782}\right| \). If the local data \( {\overset{.}{I}}_{781} \) has failure, it can be replaced by \( {\overset{.}{I}}_{472} \) and \( {\overset{.}{I}}_{791} \) through information coordination with other IEDs. The information sharing is achieved through the process bus LAN “SV + GOOSE B”, and the differential current dif
2 should change to \( \left|{\overset{.}{I}}_{782}-\left({\overset{.}{I}}_{472}+{\overset{.}{I}}_{791}\right)\right| \). Furthermore, the differential current dif
3 can also be expressed by replacing \( {\overset{.}{I}}_{472} \) with \( {\overset{.}{I}}_{471} \).as \( \left|{\overset{.}{I}}_{782}-{\overset{.}{I}}_{791}+{\overset{.}{I}}_{471}\right| \).
3.2 MS protection based on centralized searching method
For a large-scale network, only some key nodes have PMU measurements due to the cost. In general, those key buses and buses having several feeders should be regarded as the key nodes. As DG node voltages are necessary for their converter control and condition monitoring, the DG nodes should also be the key nodes.
When there is a fault in the network, node voltages have no directionality according to fault sequence analysis. This is because that voltage drops occur at the nodes at or around the fault and low voltage protections will all act. The fault point has the lowest voltage and voltage increases with the increase of the electrical distance. Thus voltage information can reflect fault location to some extends, though when voltage data is only available at the key nodes, it is difficult to diagnose the fault. Obviously, when there is a fault, most power from the power sources will go to the fault point through a minimum impedance path, referred to as the fault power path in this paper. The fault power flow of this path is the biggest and direction is to the fault point. Based on the selection of the fault power path and search tree model, together with current information comparison, fault location can be identified effectively. Thus this paper proposes a MS protection based on centralized searching method, and based on that backup protection can be realized.
The key nodes sequence from initial results are searched and the node with the maximum objective function value (Z* shown in formula 1) is found to be the fault power path. The termination condition is c ≥ (k + p + 1) where k and p are the number of general key nodes and DG nodes respectively.
$$ \begin{array}{l}\left\{\begin{array}{l}{Z}^{*}= Z\left({x}^{*}\right)={\displaystyle \sum_j re\left[{\overset{.}{U}}_i*{\overset{.}{I}}_{i. j}\right]}-{\displaystyle \sum_j re\left[{\overset{.}{U}}_{i. load}*{\overset{.}{I}}_{i. j. load}\right]}\\ {} c= c+1\end{array}\right.\\ {} s. t.\kern0.4em re\left[{\overset{.}{U}}_i*{\overset{.}{I}}_{i. j}\right]<0\& re\left[{\overset{.}{U}}_{i. load}*{\overset{.}{I}}_{i. j. load}\right]<0\\ {}{c}_0=0\end{array} $$
(1)
Where,
x
∗: feasible solution
\( {\overset{.}{U}}_i \): voltage vector of node i
\( {\overset{.}{I}}_{i. j} \): current vector of the line between node i and j
\( {\overset{.}{U}}_{i. load} \): voltage vector of node i (normal operation)
\( {\overset{.}{I}}_{i. j. load} \): current vector of the line between node i and j (normal operation)
re[ ]: calculation of real component
c: iteration counter
The second-step search appoints fault power path as the search target and works to find the fault using current information.
➀ When it is a node fault, voltage at the fault point is approximately zero and the vector sum of the node currents is not zero based on Kirchhoff’s current law. The features discussed above can be used to judge node fault. ➁When it is a branch fault, the fault current flows through non-fault branches which is the same as load current. For the fault branch, the contralateral line current is zero if it is a single-terminal power network. On the contrary, if it is a double-terminal power network, the contralateral line current is positive but the direction information is hard to calculate due to the lack of voltage data. Thus, a fault criterion based on current data for double-terminal power network is necessary.
Neglect the line charging capacitance and system impedance, line impedance and generator transient reactance are inductive. In this paper, phase angle change is defined as the absolute value between the positive sequence component of the fault current and the non-fault current. For the non-fault branch, the phase angle changes of the two terminals are small whereas for the fault branch, the phase angle change of one terminal is small but the other is large. Thus the fault criterions based on current information are given as
$$ \left|\left| \arg \left(\frac{{\overset{.}{I}}_{K_{(1)}. i}}{{\overset{.}{I}}_{normal. i}}\right)\right|-\left| \arg \left(\frac{{\overset{.}{I}}_{K_{(1)}. j}}{{\overset{.}{I}}_{normal. j}}\right)\right|\right|>{A}_{rel} $$
(2)
where, A
rel
is around 5∘ ~ 10∘. The branch, whose two-terminal current data satisfies either criterion, can be judged as the fault line. In this paper, the criterion is named as two-terminal current phase comparison (TCPC) criterion for line protection. The criterion needs no voltage data and fault judgment is independent from directional element.
If the line is connected with single-terminal power system, the second-terminal current of the feeder line is compared with unbalanced current to judge whether it is a line fault. If the line is connected with two-side power system, line fault is judged by the TCPC criterion. The second-terminal node is judged by bus differential criterion to decide whether it is the fault node.
3.3 Time coordination analysis
CS protection judges fault independently and coordination of information and functions helps to trip reliably. The two-step search of MS protection needs small calculation time, and thus the tripping output works with minimum delay. When the CS protection has judgment failure, backup protection will operate. However, if CS protection needs coordination among many IEDs, time required for communication and coordinated judgment will be long and tripping output time may be longer than that of the backup protection. In that case, the backup protection should output tripping signals immediately in order to clear the fault.
