Driven by the need to maintain power quality while reducing the impact of system events, utilities are demanding more from protection and control equipment. The way forward is the use of Intelligent Electronic Devices and integrated substation systems
Dr. Alexander Apostolov, Areva T&D
hile the selection of protection and control equipment is still driven by the need for maintaining system stability, more and more importance is given to reducing the effect of short circuit faults or other abnormal system conditions on sensitive loads. Utilities today are consequently looking for new solutions based on state-of-the-art Intelligent Electronic Devices (IED) and integrated substation protection, control and monitoring systems. Such systems provide an economical way of locally or remotely delivering metering, monitoring, control, protection and recording functions to the different users in a utility or industrial facility.
Areva T&D implements local analysis functions in different types of IEDs and integrates them into substation automation systems where analysis tools are used for substation or system level analysis.
Figure 1. Single-phase-to-ground fault (primary event) waveform record and results from post-fault analysis
The successful analysis of different power system events requires a good understanding of the phenomena and of the operating principles of primary and secondary system equipment, as well as of the limitations imposed on the analysis given the available data. A proper classification of the types of events is a pre-requisite in the development of a system for automatic event analysis.
The types of events that can be typically analysed within a substation automation system may be broken down into several categories, such as:
- Shunt (short circuit) faults
- Series (open conductor) faults
- Breaker switching
- System parameter variations
- Equipment failure
- Protection operation
- Control system operation
- Operator action.
The problem with the analysis of system events, such as the ones listed above, is that they are usually complex events that fall within several of the categories. For example, breaker closing is an event that may be the result of an operator’s action or protection operation. A protection operation may be the result of incorrect setting under maximum load conditions but it may also be the result of a short circuit fault.
Because of the above, it is necessary to identify two main classifications of events:
- Primary event: this is a single event that may result in a sequence of related events of different types
- Secondary event: any event that is the result of a primary or other secondary event (caused by the same primary event).
Several simultaneous secondary events may be associated with a single primary event. Or, they may represent a sequence of secondary events. An example will help clarify these definitions:
Primary event: Single phase-to-ground fault in phase A
Secondary event 1: Voltage variation (sag) in phase A
Secondary event 2: Voltage variation (swell) in phase B
Secondary event 3: Voltage variation (swell) in phase C
Secondary event 4: Protection operation
Secondary event 5: Breaker trip
Secondary event 6: Autoreclosing relay operation
Secondary event 7: Breaker close.
If we look at the above listed secondary events, we see that events 1 through 3 are simultaneous, while events 4 through 7 are sequential. The importance of automatic event analysis is the requirement for converting data into information that must be properly tailored for the various users.
The hierarchy of the power quality event analysis system is determined by several different factors:
- The type of protection, control, monitoring and recording devices in the substation
- The substation size and configuration
- The communications architecture.
The solution: IEDs and substation automation systems.
The fundamental principle in the design of a power quality event analysis system is that it is performed as close as possible to the location of the primary event. The practical implementation of this principle will be significantly affected by the type of protection, control, monitoring and recording devices in the substation. For example, if the substation is equipped with electromechanical and solid state relays and centralized digital fault recorders, the event analysis hierarchy will be significantly simplified due to the fact that there is a single data source. In this case also, the system will operate in an off-line mode by analysing the records after the events occur.
Modern multifunctional IEDs with monitoring, control and protection functions are typically being integrated in hierarchical substation protection and control systems. Because of the high sampling rate and the availability of multiple recording modes, specialized power quality monitoring or disturbance recording devices are used as the primary recording devices.
Figure 2. Simplified PACiS communications architecture performed by the individual multifunctional IEDs
Multifunctional protection devices are used as backup recording devices. Their sampling rate is much lower – typically 16 to 48 samples per cycle for the waveform capture and without disturbance recording capabilities. However, some devices allow waveform capture of more than 10 s that will be sufficient for capturing many typical system events. Since power system events are identified in many cases based on the rms values of the voltages or the harmonic content of the waveform, further processing of such waveform files will be required. The harmonic content of the waveform can be determined using off-line analysis of the records, but the specifics of the recording (analog filtering, sampling rate) implemented in the relay has to be taken into consideration.
The IEDs are integrated in substation automation systems. They serve as devices that directly interface with the process providing data to the upper layers of the system hierarchy and executing commands received. The functional architecture of a substation automation system is multi-leveled. All IEDs are connected to a substation local area network (SLAN). They represent the lower level, directly related to the individual power equipment in the substation – transformers, distribution feeders, transmission lines, buses, etc.
Bay computers perform functions at the second level. A substation computer is also connected to the SLAN and performs multiple functions based on the data and information available from the IEDs at the power equipment level and the bay computer’s level. It represents the substation level in the hierarchy. Typical functions include the Human Machine Interface (HMI), alarm and event logging at the substation level, settings, control, etc. It also includes centralized power quality analysis functions.
The event logs from multiple devices during short circuit faults, voltage sags, swells and other power system events can be analysed at the substation level in order to determine the cause and the effect on different customers. A simplified diagram of the communications architecture for such a substation automation system is shown in Figure 2. This is an architecture that uses switches instead of hubs to eliminate the effects of collisions on the performance of protection functions based on high speed peer-to-peer communications between the different IEDs.
Figure 3. Analysis hierarchy
IEC 61850 – the new international standard for substation communications – is enabling the development of advanced and economical solutions that improve the performance of the system while at the same time reduce the equipment, engineering, commissioning and maintenance costs. Areva T&D’s IEC 61850-based substation automation system, PACiS, is designed to take full advantage of this new technology. The Generic Object Oriented Substation Event (GOOSE) messages defined in the standard are used in a distributed disturbance recording system to allow inter-triggering of records between different IEDs. For example, when a protection IED detects an event that requires some specific type of recording, it will immediately send a GOOSE message to a group of recording IEDs to trigger recording. Each recording IED will have to be configured to receive GOOSE messages from different protection IEDs and to select a recording mode based on different bit pairs in the GOOSE message.
When the different protection, monitoring and control functions are performed with multifunctional IEDs, substation automation system architecture becomes much more complex.
Power quality monitoring devices continuously calculate hundreds of power quality related parameters, such as harmonics, inter-harmonics, and THD and analyse them to determine if there is a trigger to record a power quality or other system event.
At the same time, the protection IEDs continually analyse sampled data and store it in a data buffer. This data is processed in order to detect the inception of a fault that will trigger the protection elements of the relay and record the fault samples. IEC 61850-based SAS have an advantage from the analysis perspective because they define reporting models and high-speed peer-to-peer communications used for distributed recording functions. They are based on GOOSE – asynchronous reporting of an IED’s status to other peer devices enrolled to receive it. It is used to replace the hard-wired control signal exchange between IEDs for different protection purposes.
Figure 4. Power quality events (voltage sags) on a CBEMA plot
The post fault analysis of the data is performed based on the stored data in the relay memory and is available immediately after the relay operates. There are several different types of analyses performed by the relay summarized in the fault record: faulted phase selection, fault currents and voltage calculation, sequence components and fault location calculation. The calculated fault voltages can be used to determine the parameters of a voltage sag or swell, while the measured fault duration will provide the duration of this power quality event.
Disturbance records are an extremely important part of the analysis of power system events. They can be used to determine the effects of wide area disturbances on sensitive loads connected to the distribution system. The user should be able to look at the complete record, but also to zoom into a specific section of the record in order to see the details of the waveform. Such tools are usually part of the analysis software that is located at the substation computer or at remote engineering stations.
The analysis hierarchy is similar to the generic functional hierarchy of the system. The upper level of the analysis hierarchy may also include automatic extraction of waveforms, disturbance and event records, as well as programs that analyse the records and classify the performance of the involved IEDs. All these functions are incorporated in substation or system level analysis tools such as Areva T&D’s e-terraanalyst software. They use both operational and non-operational data to determine the cause and the characteristics of different power system events.
Figure 5. e-terraanalyst software incorporates numerous analysis functions
All levels of the hierarchical power quality event analysis system will produce not only reports related to a specific event, but will also provide an analysis of historical data to show the frequency and characteristics of multiple power quality events over a period of time. This information can then be correlated to interruptions in manufacturing processes and be used to determine the requirements necessary for improving the protection systems.
The analysis of power quality events is only possible if waveform and disturbance records from different IEDs are available. One of the advantages of Substation Automation Systems-based solutions is that they provide primary and backup recording devices with different characteristics.
Three different types of records with appropriate sampling rate ranges and record lengths are identified:
- Low-speed abnormal system conditions
- High-speed abnormal system conditions
- Waveform capture.
The combination of waveform capture and high- or low-speed disturbance recording triggered by the same system condition allows the recording of long events, while at the same time the details of the transitions from one state to another are recorded in the waveform capture.
In order to allow the user to ‘zoom-in’, all recording types should run in parallel, as required by the application, power system condition and triggering criteria specified by the user. This is made possible because the same triggers can be used for the different types of recordings and because accurate time stamps are based on the time-synchronization feature in the IEDs.