Impact of IEC61850 Based Digital Substation on Busbar Protection

Xiaotong Xu

The University of Manchester

Impact of IEC61850 Based Digital Substation on Busbar Protection

Xiaotong Xu

The University of Manchester

A new international standard called IEC61850 is applied to the substation system recent years.This standard has many advantages,and it has brought a positive influence on the development of the power system.This paper concentrates on the impact of IEC61850 on the busbar protection which is one of the most important parts in the electrical power system.It begins by introducing the digital substation based on IEC61850,and then a more suitable busbar protection architecture is selected by comparing the integrated busbar protection and distributed busbar protection.

Busbar protection；integrated busbar protection；distributed busbar protection

Ⅰ.Introduction

The busbar is one of the most important elements in the electrical power system,because many transformers,generators and loads are connected to it.Because the busbar is the convergence point of supply circuits which contain large current magnitude,so a single bus fault may be equal to a lot of simultaneous faults.In order to ensure the system stability and reduce the damage to electrical devices,busbar protection with high speed is needed(Elmore 1994;Mohan and Chatterjee 2010).

For the transmission bus,the bus differential protection scheme is always applied to it.The principle of the bus differential protection is to analyze all the current data from primary devices which are connected to the same bus.However,a problem exists in this kind of protection scheme.If ratios of current transformers are different or the configuration of the substation which uses bus differential protections may be changed,the bus differential schemes will be more expensive and complex.In order to reduce costs and decrease maintenance requirements,many distribution and sub-transmission buses do not have bus differential protections,and only backup protections which have long fault clearing time are used to clear bus faults.Due to the increased voltage sag duration caused by long fault clearing time,the power quality problem can not be neglected in this case(Apostolov 2009).

The power quality problem is able to be solved by applying the new communication standard called IEC61850.Using this standard, not only can IEDs(Intelligent Electronic Devices)make communication with each other in the substation or the whole network,but also the special peripheral equipments are replace by merging units(Apostolov 2009;Brent and Self 2006).Therefore,the costs are reduced, and it is able for distribution and sub-transmission buses to install bus differential protections.

Due to the advantage of this standard,it is significant to study the theory of busbar protection deeply,and discuss the impact of IEC61850 on the busbar protection.Based on the previous work about this field,this paper will focus on choosing a suitable architecture for the busbar protection in the digital substation which is based on IEC61850.

After a general introduction about the dissertation,substation architecture which is based on IEC61850 is discussed.Next,integrated busbar protection and distributed busbar protection are explained and compared so that a suitable busbar protection architecture can be chose.Conclusion are written at the end of the paper.

Ⅱ.Digital substation based on IEC61850

The international standard called IEC61850 is published by International Electrotechnical Commission in 2004.It is applied to substation network,and brings many benefits to the power system.In a substation automation system which is based on IEC61850,the functionality of the system is improved a lot by using the peer-to-peer communications with high speed,and no device is required to be added to the substation(Apostolov 2004).Nowadays,complicated microprocessor technology is applied to the relay.As a result,the cost of substation is reduced because a number of units is replaced by one device(Brent and Self 2006).

The architecture of substation automation system based on IEC61850 is shown in figure 2.1.Process bus and station bus are two main parts in this figure.

Figure 2.1 Architecture of digital substation based on IEC61850

Process bus consists of merging units(MUs)and Ethernet switches.Data is measured by instrument transformers,and this data is digitized and transformed to sample value Ethernet packets through merging units.Then the data is sent to intelligent electronic devices(IEDs). What is more,generic object oriented substation event(GOOSE)and generic substation status event(GSSE)information can be sent via this network.Because of this property,status and logical data,tripping and blocking commands are transferred to IEDs.The function of monitoring,control and protection is completed by IEDs which get the sample value Ethernet packets.Hard wiring is reduced by using this kind of communication.Station bus is shown at the next level in figure 2.1. Automation,control and monitoring function is achieved in station bus.Many devices are connected to the station bus,they are substation human machine interface(HMI),substation IEDs,supervisory control and data acquisition(SCADA).The main function of station bus is similar to process bus.Both of them are used for sending messages(Zadeh et al 2011).

Compared to traditional protection scheme,there are three advan-tages of protection which is based on IEC61850.The first one is that a great quantity of data is able to be collected by one IED.Secondly,the condition of many elements in the substation can be received by one IED.Last but not least,an IED sends many kinds of signals to other IEDs,such as tripping and blocking commands(Zadeh et al 2011).

Ⅲ.Busbar protection architecture based on IEC61850

Integrated busbar protection and distributed busbar protection are two kinds of busbar protection schemes which will be introduced, and the difference between the architecture of these two protection schemes will also be discussed in this part.The architecture of integrated busbar protection is conventional,and distributed busbar protection is based on the integrated busbar protection.

A.Integrated busbar protection

With the purpose of having a better understanding of the integrated busbar protection,three main elements are introduced firstly.The first one is named merging unit.Voltage and current data from corresponding transformers is sent to MUs,then the data is converted to integrated protection units(IPUs)after being transformed to digital signals.The intelligent execution unit(IEU)is the second component. Both circuit breakers and switches are controlled by IEUs,and the condition information of switches is delivered to IPU through IEUs. The last one is IPU which seems to be a platform,all protection information from the whole substation is centralized in IPUs.The sample values from MUs and IEUs are reserved in IPUs,too(Peng and Lijun 2010).

Figure 3.1 Architecture of integrated busbar protection

In figure 3.1,the busbar protection architecture of integrated system is presented.In this system,two IPUs are installed so that they are the back for each other.By calculating the sampled values from MUs and analyzing the condition information of switches in the IPUs,the protection operation is decided.Subsequently,commands of alarm or circuit breaker tripping are sent to IEUs to protect the devices in the substation(Peng and Lijun 2010).

B.Distributed busbar protection

Figure 3.2 shows the configuration of distributed busbar protection.MUs,transducers ECT/EPT and ICBs are in the process level, meanwhile field protection units(FPUs)are in the bay level.At the right top of the figure,Devices of substation which contains exchangers and GOOSE can be found.By applying the same communication standard,IEDs is able to make communication with the network even though they are produced by different factories(Zou et al 2010).

Assuming N bays are connected to the busbar.The working process of distributed busbar protection system will be discussed.Each FPU calculates and analyzes the condition of busbar separately after all sampled value from MUs and status information of switches in whole bay units are sent to them.Taking bay 1 as example,FPU-1 sends tripping signals to the ICB in bay 1 via GOOSE network when an internal busbar fault happens and discovered by FPU-1,at the same time,the substation receives these protection messages through station bus.Therefore,the faulted busbar can be isolated from the network by tripping all ICBs in bay units.In summary,the duty of FPUs is to detect the fault which happens on the busbar and control relevant ICBs.(Zou et al 2010)

Figure 3.2 Architecture of distributed busbar protection

C.Comparison and selection

In the integrated busbar protection system,current transformers, voltage transformers,isolator auxiliary contacts and switch tripping contacts of each device are connected to the busbar protection panel which is located in the main control room.But there are some problems existing in this system.One of the problem is that a large number of electrical cables are needed due to the far distance from the main control room to switches.Because of the complexity of secondary circuits,it is difficult for operator to regulate and control the network in practice.This is the second problem.What is more,the worst case is that the connection of busbar may be changed sometimes,since the configuration of the substation cannot remain same all the time.

These problems may be solved by applying the new distributed busbar protection.The traditional integrated busbar protection is divided to several busbar protection units in the distributed busbar protection system.These protection units are installed in different circuit protection panels,and the data in these protection units makes communication with each other via computer network.

In the distributed busbar protection system,each protection unit can collect the data from its own circuit,and the digital data which is transformed from original data is sent to all the other protection units. Based on the data from its own protection unit and the other units,the calculation of busbar differential protection is done in every protection units.If the calculation result indicates that an internal busbar fault happens,the circuit breaker will operate to separate the fault.Otherwise,all circuit breakers do not operate when there is no fault or an external fault occurs.

Compared to the traditional integrated busbar protection,the reliability of the network is improved by applying distributed busbar protection.Because if one protection unit is disturbed or operates incorrectly due to inaccurate calculation in the distributed busbar protection scheme,only the faulted circuit will be isolated from the network, and the whole busbar will not be affected.It is important to the electrical power system with extra high voltage busbar particularly.There is another advantage for using distributed busbar protection as the construction of the busbar is able to be changed easily,so it may contribute to the development and revolution of the electrical power system. In conclusion,Distributed busbar protection may be applied to the power system in the future and become more and more popular,and it will bring many benefits to the healthily operation of the network.

Ⅳ.Conclusion

With the development of digital substations based on IEC61850, it will bring more and more benefits to the healthy operation of the electrical power system.In this paper,the architecture and benefits ofIEC61850 based digital substation are introduced firstly.The most important part is to compare and select a suitable busbar protection architecture in the IEC61850 based digital substation,so the architecture of integrated and distributed busbar protection is explained.Compared to traditional integrated busbar protection in which all components are centralized in one busbar protection panel,the architecture of distributed busbar protection is more reliable and the configuration of the busbar is easier to change in the distributed busbar protection. Because elements in the network are divided to several protection units in the distributed busbar protection,so incorrect operation of one protection unit has little effect on the whole system.In summary,distributed busbar protection which is based on IEC61850 is a good choice for digital substations.However,in the real life,some technical and financial problems may delay the application of distributed busbar protection.In order to have a better electrical network,the problem existing in the conventional substation should be solved as soon as possible.

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Bibliography：Xiaotong Xu,female,born in August of 1990,native place：Jilin City Jilin Province,MSc in Electrical Power Systems Engineering of the University of Manchester.

徐晓彤（1990-），女，吉林省吉林市人，英国曼彻斯特大学2013级电力系统工程研究生。