Extended bus differential protection method based on limited wide area
Dividing of limited wide area
The proposed EBDP method divides the power system into several protection areas at the core of the bus. In each protection area, protection relay devices collect data from various locations and operate to isolate the fault independently. Each protection area neither crosses each other in topology nor overlaps with others. The protection areas do not exchange data with each other either. Considering that the differential protection is superior in selectivity, sensitivity, speed, and reliability, each protection area is configured with EBDP which protects not only the core bus but also the adjacent transmission lines and transformers connected to this bus.
In this method, the transmission lines can be divided into two categories: terminal line and contact line. The terminal line is connected to the terminal load directly, and breakers and CTs are only installed at the beginning of the terminal line. Power flows from the bus to the terminal load via the terminal line. While for the contact line, it is connected between two buses and functions as a power exchanger with breakers and CTs installed on both ends of the line. Power flow direction in the contact line cannot be determined due to the connected intermittent DGs. In terms of the two-winding transformer, it can be considered as a special contact line connected between two buses with two different voltage levels.
For any bus, the definition in this method is that when the contact lines or transformers are connected between this bus and a certain power source, the power direction of this branch as for the bus is considered as “positive”. Otherwise, the power direction of the branch for the bus is considered as “negative” if the branch is connected to a load. According to this definition, if power flows from the contact lines or transformers to this bus, the direction is “positive”. Similarly, for the load lines or transformers connected to the load, the power direction is from the bus to the load, so the direction is considered as “negative”.
The protection area in the proposed EBDP method normally includes the bus and the lines or transformers connected to this bus with the “positive” direction. The power flow direction on the contact line is not fixed because of the high penetration of DGs connected to the micro-grid. However, its power flow direction can be determined during a certain period of time. Therefore, the contact line must be included in one certain protection area with either beginning terminal bus or end terminal bus.
Figure 1 shows a typical power network with micro power generations, and the dividing of the protection area in the proposed EBDP strategy will be demonstrated clearly in the following paragraphs.
In Fig. 1, S1 denotes an infinite power system, S2 is a distributed generation, T1, T2 are the transformers with the “positive” direction for Bus II and Bus I, respectively. T3 is the load transformer with the “negative” direction for bus I. L1, L2, L3 are all contact lines connecting two buses, L4 is the terminal line, D1-D12 are the circuit breakers. As shown in Fig. 1, the power flow direction on line L1 is from Bus II to Bus I, and power on line L2 and L3 flows away from Bus II. Transformers T1 and T2 inject power to Bus II and Bus I respectively, whereas power flows out of Bus I from the transformer T3 to the terminal load.
Therefore, for Bus II, T1 is considered “positive” direction, and the protection area includes Bus II and T1, namely the area between D1 and D3. For Bus I, T2 and L1 are considered as “positive” whereas L2, L3, and L4 are considered “negative” direction, and the protection area includes Bus I, L1 and T2. Their protection areas are indicated in Fig. 2. Similarly, the wide protection area for Bus III and Bus IV are also shown in Fig. 2. Thus, for the given power network in Fig. 1, the dividing of the LWA is indicated in Fig. 2. There are four protection areas in this network, and thus only four EBDPs are needed to protect all the components in this network. For the terminal line, the backup protection configured at the beginning can operate to isolate the fault if necessary.
When the power flow direction changes, the protection area will also change. For example, if the power direction on line L1 changes, namely this power network injects power to the infinite power grid S1, L1 will be included in the area of Bus II. Thus, the contact line can be included to one certain protection area depending on the power flow direction. Therefore, the EBDP should include an extra dynamic switching module to achieve the dynamic dividing of the protection area. This module will change the protection area based on the following principle: when power direction change on the contact line is detected, the direction will be considered as the same direction for the first 100 ms; after 100 ms, if the direction change is completely done, this module will change the dividing protection area quickly.
Realization of extended bus differential protection
The EBDP consists of a “wide current differential protection (WCDP)” to protect the whole protection area and several “current differential protection (CDP)” devices to protect one specific component. The wide current differential protection functions as the starting element, and the current differential protection is responsible for the protection of the specific component if the wide current differential protection detects the fault in the power system. In order to enhance the reliability of the starting element, the WCDP adopts the ratio restraint differential protection and the coefficient is generally set at 0.3. The CDP isolates the fault by tripping the exact circuit breakers, and also adopts the ratio restraint differential protection with the coefficient generally set at 0.5. Take the EBDP I as an example, the WCDP takes all the components in this protection area as a whole. If a fault occurs in any component, the sum of the current will not equal to zero. Thus, the WCDP will start protection and the specific CDP for a certain component will detect and isolate the fault. As showed in Fig. 3, the red frame indicates the whole protection area for the WCDP, whereas the three dotted lines indicate the three CDP areas: L1 line current differential protection, Bus I current differential protection, and T2 current differential protection. Therefore, the EBDP in Fig. 3 includes four parts, i.e. one WCDP and three CDPs.
The wide current differential protection is based on the Kirchhoff’s law as:
$$ {I}_{op}\ge K{I}_{res} $$
(2–1)
$$ {}_{I_{op}=\left|{\displaystyle \sum_{i=1}^n{I}_i}\right|} $$
(2–2)
$$ {I}_{res}={\displaystyle \sum_{i=1}^n\left|{I}_i\right|} $$
(2–3)
where I
op
is the operating current, I
res
is the restrain current and I
i
is the branch current of the components connected to the bus. k is ratio restraint coefficient and is generally set at 0.3.
The current differential protection adopts the conventional transmission line protection, bus protection and transformer protection.
The logical relationship between the wide current differential protection and the current differential protection is shown in Fig. 4.
In normal operation, only the WCDP criteria is calculated to monitor the power system behavior in each EBDP. The EBDP will be started when a fault occurs, and several CDPs will be activated to calculate and operate to isolate the fault by tripping the exact breakers. Meanwhile, other EBDPs will not be started by the WCDP because there is no fault occurrence in their protection areas.
Analysis of action behavior of EBDP
This subsection will clarify the action behavior of the EBDP by taking the network in Fig. 1 as an example.
Figure 5 indicates the corresponding fault current directions when a fault occurs at the point F1. L1 is considered as “negative” direction for Bus I. However, during the first 100 ms period after the fault, its direction is “positive”, and thus, L1 is included in the protection area of Bus I. After the fault occurrence in this protection area, the WCDP will start the EBDP, and then the L1 line current differential protection, T2 current differential protection and Bus I current differential protection will be inputted for calculation. Obviously, the Bus I CDP and T2 CDP will not operate and the L1 line CDP will operate to trip the breakers D3 and D4 because the fault occurs on line L1. After 100 ms, L1 is removed from this EBDP protection area. Under the worst condition, if the fault is not isolated within 100 ms, the backup protection (such as the overcurrent protection and the distance relay) will operate to isolate the fault.
Figure 6 shows that if the fault occurs at the point F2, the WCDP will start the EBDP. The line L1 current differential protection and T2 current differential protection will not operate whereas the Bus I current differential protection operates to open the breakers D4, D5, D7, D9, D11 and D12 to isolate the fault.
It can be concluded that for fault at either F1 or F2, the EBDP II, III and IV will not be started by the corresponding WCDP, effectively preventing the misoperation of other EBDPs.
Figure 7.1 shows that if the fault occurs at the point F3 at the beginning of line L1, L1 is considered “positive” for Bus I, and “negative” for Bus II during the 100 ms period after the fault. Therefore, L1 is included in the protection area EBDP I. The WCDP of the EBDP II will start the EBDP II because the fault is inside the protection EBDP II. The Bus II CDP will then operate to trip the breakers D2 and D3. Figure 7.2 indicates that 100 ms after the fault, L1 will be considered “negative” for Bus I and “positive” for Bus II. Thus, L1 will be included in the protection area EBDP II. The WCDP of the EBDP II will start the EBDP II and the Bus II CDP will operate to isolate the fault at the point F3 at the beginning of line L1. Therefore, it can be concluded from Figs. 7.1 and 7.2 that regardless whether the L1 is included in EBDP I or EBDP II, the whole wide area relaying protection works effectively.
In addition, the proposed EBDP is more efficient in normal operation as only the WCDP of each EBDP is checked to monitor the whole distribution network, leading to reduced CPU usage. In the event of a fault condition, only the WCDP in the protection area where the fault occurs will start the corresponding EBDP and the specific CDP operates to trip the circuit breakers and isolate the faults. Therefore, the proposed EBDP strategy is more applicable for complicated distribution networks. In addition, each EBDP protection area is divided dynamically according to the specific power flow direction, which is adaptive for the micro-grids with high penetration of DGs.
