A new coordinated backup protection scheme for distribution network containing distributed generation

2.1 Coordinated substation (CS) protection

The CS is an application concept that uses the unified communication protocol and strong information network of smart grid, and at process level, accomplishes synchronous sample value acquisition, action command execution and breaker states uploading. Based on the needed local information, bay level includes a number of protection IEDs to implement protection. At the same time, modularization structure of IED helps in harmonious interaction and coordination of functions to support reliable fault identification which is independent to backup protection. The coordination of IEDs includes information sharing, redundant CPU space utilization and coordinated backup tripping. Station level is the top one which integrates useful data and events for substation management and communication with dispatching center or MS.

Part of Fig. 1 shows the structure of CS which pays more attentions on the coordination and cooperation among different protection IEDs. In this paper, the IED is an extended concept which combines local protection function with coordination concept. Every IED is an integration of functional modules and has three main parts: function module (FM), local coordination module (LCM) and remote coordination module (RCM). FM mainly is the protection algorithm part, which processes the samples received from the process bus and sends action commands to the breaker. At the same time, FM can communicate with other IEDs to share information or provide coordinated trip signal in order to improve protection reliability and security. LCM monitors the requirements of local IED, establishes appropriate tasks and sends task commands. RCM receives remote command from other IEDs and analyzes their requirements, or receives remote information and uses it in local processing. The protection of CS uses ownerless structure [17] and can work reliably even if one IED fails. In this paper, installation of the IEDs is in the standard bay as in traditional protection devices and the IEDs are based on block-based design principle which makes it easy to recombine or extend functions through standard module interface.

CS protection is a distributed substation-domain protection, and its protection range is the elements within the substation and outgoing-lines. Some coordinated IEDs implement local rapid protection judgment and other IEDs exchange information with adjacent substations for protection judgment of the outgoing-lines. If primary equipment (CT/VT/CB) is in abnormal working states, IEDs work in coordination mode and their data sharing and coordinated tripping help to improve the reliability of local substation protection.

2.2 Master substation (MS) protection

MS protection is a regional integrated backup protection (RIBP) and its protection range is the network composed of several CSs. The RIBP acquires every CS’s available data through wide area communication network to analyze regional comprehensive information. Likewise, if primary equipment (CT/VT/CB) is in abnormal working states, RIBP can acquire valid data through coordinated information analysis in corresponding CS. For power systems up to 110 kV, the protection scheme usually consists of single main protection and has no breaker failure protection [18]. The RIBP can help to realize dual-configuration protection and improve reliability. In addition, backup protection based on wide area information optimizes general backup protection in action time while still ensuring reliability.

RIBP of MS parses the data packages and integrates all the regional information. By analyzing the integrated data, the protection algorithm unit calculates and outputs fault judgment results. Output tripping signals are sent back to communication network to reach the corresponding objective CS. Station level of CS obtains the tripping signals and relative IED in process level sends the tripping command to intelligent units (IU) and circuit breakers (CB) to isolate fault.

2.3 Communication network

In order to implement the CBP scheme, communication network is one of the key technologies. The CBP communication system includes three parts: process level communication network within CS, network communicated with adjacent substations, MS integrated data communication network.

Process level communication network within CS is shown in Fig. 1. Based on the IEC61850-9-2LE standard, the communication between bay level and process level is via high performance local area network (LAN) [7] which is the process bus (SV + GOOSE). Considering large quantities of information and network load capacity, dual process network is set in CS (shown in Fig. 1). The function module which is the protection calculation part communicates with primary equipment through LAN “SV + GOOSE A”. The coordination part, including LCM and RCM, communicates with the same part from other IEDs through LAN “SV + GOOSE B”. In order to synchronize different data sources, LAN communication needs have accurate GPS synchronous time-marker.

One CS can exchange information of outgoing-lines with adjacent substations to implement cooperation work among different substations. Network communicated with adjacent substations uses synchronous digital hierarchy (SDH) [19] technology and works in self-healing ring transmission mode. The CSs and MS connect with each other in a ring and flexible data acquisition is available. As shown in Fig. 2, CS_2 and CS_3 upload data to SDH network by SDH device, and CS_1 can download the necessary data from SDH network for CS substation-domain protection judgment.

Because the quantity of multi-CS transmission data is large and communication has high real-time requirement, MS integrated data communication network also uses SDH technology as special communication network of wide area protection (shown in Fig. 3). As communication network terminals, CS accomplishes local protection judgment and at the same time, its IEDs preprocess the sample values and package necessary data. For those data that needs to be synchronized, unified GPS synchronous time-marker should be tagged to packaged message. All the shared data is uploaded to SDH network through SDH method, and MS could download the needed data from the network. Computer simulation shows that SDH fiber communication network can satisfy transmission delay requirements of WARP [19].

The latency is another concern, which includes the sensor delay (5 ms), communication delay, processor computational time (dependent on the protection principle), and circuit breaker operation time (90 ms). The communication delay of the process level is about 5 μs which is negligible, whereas the possible longest communication delay of wide-area optic fiber network is 24 ms. For regional distribution networks, the communication delay will not be longer than 24 ms. Therefore, the protection principle should ensure the calculation to be carried out efficiently in order to isolate fault as soon as possible.

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.

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.

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.