DO UTILITIES STILL NEED STAND ALONE DISTURBANCE RECORDERS?

Transcription

1 Proceedings of the 14 th International Middle East Power Systems Conference (MEPCON 10), Cairo University, Egypt, December 19-21, 2010, Paper ID 304. DO UTILITIES STILL NEED STAND ALONE DISTURBANCE RECORDERS? Mohamed A. EL-HADIDY Dalal H. HELMI (*) Maha S. ABDELHADY Egyptian Electricity Transmission Company (EETC) Egypt Abstract The analysis of faults and wide system disturbances has always been a fundamental foundation for a secure and reliable electrical power supply. Lessons learned from the analysis of blackout events during last years, prove that the source and quality of data and information used in the analysis of such events are of top importance. The resolution of the recorded data varies for different system events, ranging from hundreds of samples in one cycle to few numbers of samples in a minute. As an example, from recording of Power Quality event s point of view, the requirements for recording voltage impulsive transients, temporary, sags, swells, harmonics and trend profile have quite different requirements of sampling frequency, data filtering, accuracy, time span, etc. Also for system wider vision, the requirements to record system large disturbances and dynamics, oscillations, reclosure, voltage collapse and out of step conditions are quit different from fault recording requirements. The new competitive business environment forces utilities to formulate their strategies regarding productivity, quality and reliability of service, asset management, maintenance strategies, etc. One of the critical considerations for selection of multifunctional Intelligent Electronic Devices (IED) may be their role in supporting the above mentioned functions, which in turn means that the IED evaluation criteria should include the capability for their use in defining existing and new applications. In the world-wide trend implementing IEC within the substations automation systems, there exist big debates about the necessity to use stand alone Disturbance Recorder. Most of utility personnel think the multifunction IED can also do all recording tasks (as add-on function) that the Disturbance Recorder can perform; i.e PQ monitoring, Fault Recording, fault location, Phasor Measurement and System Dynamic etc. This paper presents a technical comparison between the above mentioned two points of view: built-in recording within distributed IEDs and stand alone disturbance recording. It studies the effect of some factors such as analog and digital filtering process, noisy analog signals and sampling frequency on the recorded data. Triggering possibilities and recording capabilities are compared. The results obtained from this research threw some light on this issue which is important to both utility engineers and device manufacturers, through providing deep comparison between the gain and loss the utility will accordingly reach as a result of their selection. KEYWORDS: Power system Disturbances, Substation Automation, IEC 61850, Disturbance Recorder, protection IED, Bay Controller (BCU). 1. DEFINITION OF THE PROBLEM The standard IEC61850 Communication Networks and Systems in Substations supports all communication for substation tasks like control, protection and monitoring. It covers communication between all Intelligent Electronic Devices (IEDs) from the process level (data acquisition, sensors and actuators) over the bay level (protection, monitoring and control tasks) up to the station level. Typical devices on process level are intelligent switchgear interfaces or unconventional (non-magnetic) instrument transformers. The goal of IEC61850 is providing interoperability between IEDs of different suppliers. In addition, the standard should have a long lifetime in line with the long lifetime of switchgear. Nevertheless, the standard should not stop technological innovations and advanced operation philosophies of the utilities. The migration to IEC compliant substations require more than just good IEDs and tools and a 897

2 general introduction into the new possibilities. The training is most important for utility engineers (people ware) that are involved in system integration. Experts from vendors gain a lot of experiences with the system integration of the many projects already underway. Many utilities require IEC but do not care about detail. Specifying the requirements for Substation Automation (SA) needs a different approach than substation engineers have used in the past to construct new substations. To certain extend, the technical requirements for substation automation systems are independent of IEC For any given substation, the basic functions are: protection, revenue metering, local control for circuit breakers and switches, interface to SCADA for remote control and data gathering, alarming, monitoring of power equipment, sequence of event recording (SER) and last but not the least disturbance recording (DR) equipment. Before IEC was published there was no need to specify a unique formal and detailed specification. With the introduction of IEC this and several other new aspects will become part of the procurement specifications [1]. There are several use cases how to build, design, engineer, configure, operate, maintain, extend, and service substations when taking IEC compliant devices and tools into account [2]. These cases extend from utility puts traditional specification of substation automation as in the past and requests IEC in general terms and buys turnkey substation (vendor or third party may be the system integrator), up to Utility builds all substations (utility or third party may be the system integrator) and also Utility extends, replaces IEDs, maintains, services the system. In the first case there is little knowledge about IEC required framework. In the last case, there is a comprehensive knowledge and experience required from utility personnel. The crucial issue for utilities is to understand and to define the role the utility people want to work in conjunction with IEC There is also another aspect we need to briefly look at the skills needed to efficiently use tools. Tools could only help to reduce costs and save time when the user of the tool knows what the tool provides and how to use it. Training and experience is crucial to tap benefits from a tool. One main point makes conflict to some of utility engineers is that the recording capabilities of protection IEDs can replace the multifunction disturbance recording IED. Several utilities engineers who have been using protection relays to do the Digital Fault Recorder (DFR) duties now see IEC61850 as the means to re-introduce in a more cost effective manner, as the sampling rate and recording done in the bay control unit may be sufficient for monitoring everything. Protection IEDs are and will still circuit orientated devices. During real world-wide system disturbances, system analysts were struggling without proper disturbance data. In addition, Relays can not monitor all power quality problems today. These issues and more will be illustrated in the next sections. 2. CATEGORIES OF POWER SYSTEM DISTURBANCES A wide range of abnormalities can affect the operation of the power system such as: weather, fault conditions, equipment failure, incorrect operation of the protection and control systems, load and demand mismatch, human errors, to name a few. These problems can occur at any time and in various combinations, and it is the task of the utility's engineering, protection and operation staff to explain what has happened. And of course, management also wants to know what had happened and the corrective actions to ensure that it will not happen again. This type of diagnostic work requires a lot of information. The latest approved NERC PRC-002 [3] and PRC-018 [4] standards ensure that Disturbance monitoring equipment is installed in a uniform manner to facilitate development of models and the analyses of events. The disturbance data is reported in accordance with regional requirements. Also, these captured events are used to play-back the incident to test the protection IED and validate system models. 2.1 Time Frames of Power System Operation In power systems, various events happen at various time frames. Some of these events can be categorized as follows [5]: Time Period Micro-seconds Milli-seconds Cycles Events Switching surges harmonics Fault clearing Example Breaker strikes/ Lightning Variable speed drives / arc furnace Relay and breaker operations 898

3 Seconds Minutes Hours / days Load flow changes System stability Load variations Governor and exciter response Reaction to dispatcher action New generation schedules The requirement for proper disturbance analysis needs different types of recording. The categories mentioned in the above table need different recording resolutions vary from high scan rate (the first four rows) up to slow scan recordings (the last two rows). Power system abnormalities can be classified in three main categories: Power System Transients, Power System Dynamics and Power Quality (PQ) Problems. 2.2 Power System Transients Short term abnormal conditions that can affect the transmission level of a power delivery system include, but not limited to, transmission line faults, switching over-voltages, transformer winding faults, station bus fault, breaker fault / abnormalities, improper operation etc. These transient conditions are very short in time and their analysis needs a lot of data and information occurring in few seconds or even milliseconds. It is vital for the sound operation and maintenance of the power system to capture data as they occur and use it in finding the exact reasons behind the abnormalities and/or malfunctioning. The number of samples required to capture these phenomena may reach hundreds /cycle. Figure 1 shows an example of current impulsive transient. between generation and load occurs in power systems, or when sudden diverts in power flow takes place, electric utilities need to: capture slow oscillatory swings that are poorly damped, capture frequency transients resulting from sudden losses of generation or load, capture power system disturbance data to support analyses of the events, and obtain experience in recognizing disturbances as a precursor to the development of emergency state and unconventional transient state control. Fig.2: frequency record with slow scan rate Figure 2 shows an example of the slow scan recording of system frequency during a disturbance for 1800 second. 2.4 Power Quality Problems The most important PQ phenomena are: Short duration variations of supply voltage: dips (sags) & swells. Long duration variations: over-voltages, undervoltages & interruptions. Voltage distortion: Harmonics & Interharmonics. Voltage fluctuations and Flickers. Figure 3 shows an example of transformer inrush (high decaying harmonic content) Fig.1: Impulsive Transient Records with high scan rate 2.3 Power System Dynamics Power systems can be subjected to dynamic problems due to certain physical phenomena, which have to be counteracted by appropriate protection and control schemes. Such phenomena are: power oscillations, angle instability, frequency instability, voltage instability and cascade outages. When sudden unbalance Fig.3: Transformer Inrush current with high scan rate Utilities have always striven to achieve high reliability. Now some major utility customers contract with electric utilities to maintain high reliability and power quality or pay a substantial penalty. Disturbance Recording is necessary to monitor compliance with the reliability and PQ contract clauses. The communications flexibility and 899

4 automatic polling features of most Power Quality or Disturbance Recording IEDs make them ideal for a system-wide network. These devices should be part of billing metering equipment for customers with these types of contracts. 3. FUNCTIONS OF DISTURBANCE RECORDERS The impact of a single point failure is far, may be, too costly to be acceptable. The DRs provide efficiently the data necessary for the following types of analysis [5]: Relay system operation: This is the traditional task of DRs, especially for complex protection schemes using communication signals and during relay failure. System disturbance monitoring: Long time power system features could be captured for additional system studies. Such features are power swings and phase measurements. Modern DR s record P, f, VAR's,.etc, continuously for days. Substation monitoring: Trend curves for usage, loading of equipment, calculated values of power factor, frequency, harmonic,.etc could be recorded, monitored and analyzed. PQ compliance: DR s also can be used for monitoring compliance with reliability and PQ contracts saving substational penalties to power utilities. Information system reliability: Redundant data obtained from DRs can support the SCADA systems. Fault location: This saves the maintenance time and effort, and increases TL availability and system reliability. Maintenance management: Monitoring accumulated interrupted fault currents can predict the need for CB maintenance. DR can also provide data for planning and operation. System stability/state estimation: Phase shifts between different busses could be measured synchronously using GPS system. Maintaining the stability limits on power lines is possible with increased power sales, thus, some investments can be saved. The applications of Phasor Measurement (with 1μsec accuracy or higher) aid in most of offline, on-line and closed loop control studies that support the enhancement of power system performance. After defining different power system disturbances and recording needs, we will consider if the recording capabilities of the Protection and Disturbance Recording IEDs can meet them all. 4. PROTECTION IEDs VERSUS DISTURBANCE RECORDERS 4.1 The main functions For Protection IED, the primary function is to detect Electrical Power System conditions which can cause damage to equipment, danger to personnel and to the operation of the system. The relay then isolates the problem and/or eliminates the conditions as quickly as possible. Secondarily, the digital relay records the conditions which are monitored for a period of time, beginning with a prefault period and continuing until after the fault is cleared. A relay may also obtain fault data and equipment performance on devices such as circuit breakers. Relays are usually installed for a single line or a piece of equipment such as transformer. The capacity or total number of faults stored is limited by the available memory. Most relays do not record and store data if no fault is detected. Usually, the settings for starting the relay are much higher than the level of DR s triggering. The primary function of Disturbance Recorder is to gather as much data as possible to allow detection and analysis of normal and/or abnormal conditions, to verify system modelling, and confirmation of system planning information. Secondarily, the recorder will obtain information on performance of all monitored equipment such as circuit breakers, relays, generator systems, etc. Recorders allow time and system correlated date. They receive data from all available sources including external inputs from relays, other recorders and transducers. A recorder will obtain the desired data by proper selection of quantities monitored for different equipment. 4.2 Sampling Rate Although the emerging processor technology was suited to the development of numerical relays, there still existed a problem of digital multiplication. Any numeric relay, other than the simplest application, will execute a large number 900

5 of multiplications whilst performing its protection function. Since multiplication on a microprocessor was, at the time, achieved by a series of shifts and additions which took a relatively large number of clock cycles to execute, this led to relay algorithms being very conservative in the number of multiplications used since multiplications used up processing time. Note that all the calculations performed by the relay must be completed in the time between the A/D converter samples. Digital relaying algorithms are usually based on 50/60Hz signals. Disturbance Recorders do not need such time, hence they can have high sampling rate which enables capturing fast transient phenomena which relays can t capture 4.3 Data Conditioning [6] Filtering of the applied signals may be both analog and digital, and together with the sampling rate determine the frequency response of the recorded information. The use of measurement windows and associated measurement algorithms can have an impact on the data contained within the record. Triggering of the recording device may be dependent upon a calculated value for a specific parameter, or a magnitude of an input, or the position of an input contact, or a combination of these Analog filtering for ac signals Any digital device, including relays and DRs, must include an anti-aliasing feature, usually achieved using an analog filter. DRs traditionally sample at 64 to 512 samples/cycle and have their cut-off filters set well above 1kHz, yielding comparatively good spectral coverage. Relays traditionally sample from 4-20 samples/cycle, but relays are now available that sample at 64 to 128 samples/cycle. However, protective relays, which use only filtered quantities for main protection and control functions, have their analog filters are set comparatively low. As a result, the signal spectrum effectively recorded by these devises is limited to few hundred hertz. Another aspect of analog filtering is the design of the filter itself. When high order filters are used, their response may not be ideally flat over the pass-band frequencies. This should be considered when a detailed harmonic analysis is performed using records produced by protective relays. The analog filter of digital relays is typically a low-pass filter, allowing the sub-harmonics and dc components to pass through. However, the frequency response of the input magnetic modules at low frequencies may alter the low frequency components (at the level of a few Hz). This should be considered when analyzing sub-harmonics and decaying dc components. Not all recorders use magnetic input modules for isolation. Many have dc response capability Digital filtering for ac inputs The primary purpose of the digital fault recorder is to record the applied signals within the spectral limitations of the equipment. The recorded data must have a high level of accuracy in terms of representation of the input signal. The accuracy of this data is impacted by the performance of any digital filtering applied to the ac inputs. A DFR rarely applies digital filtering, as the DFR only captures data for later display and analysis. Protective relays do apply digital filtering that have a shorter group delay which leads to shorter relay operating times. The recorded data from a digital relay can provide very good information, but the user will need to understand how the data is captured and any limitation in presenting the data. Limitations include slow sample rate, limited response to high frequency, dc filtering, and software filters, depending on the method used by the relay. Figure 4 illustrates the same event captured two ways from the relay and displayed by the vendor s display program. The first capture uses 4 samples/cycle of digitally filtered data and the second uses 16 samples/cycle of sampled data with no special filter. Note that the filtered data lags behind the sampled data (this time delay is the result of the filter) and does not show the dc offset. Another important observation is that the filtered value does not show the true waveform (e. g. peak current and the moment of full current interruption). This may be acceptable for a relay under certain conditions, but not acceptable for a recording and event analysis. Other methods used, on a limited basis, include converting the sampled waveforms to RMS or other application specific values. In all cases, specific information is lost and it is important that the user understand what filtering or algorithms are used in order to properly analyze the values. 901

6 5. IINTEGRATION OF RECORDING CAPABILITIES Fig. 4: Unfiltered vs. Filtered Data The electric utility SA system uses a variety of devices integrated into a functional package by a communications technology for the purpose of monitoring and controlling the substation. SA systems incorporate microprocessor-based intelligent electronic devices (IEDs), which provide inputs and outputs to the system (as shown in Fig. 6). 4.4 Triggering Capabilities. Triggers are necessary to initiate recording for both Protection IED and Disturbance Recorder. Protection IEDs have limited triggering capabilities compared with recording IEDs since DRs have in addition to H/W binary contacts, another software triggering channels in which the user can formulate a specific function to trigger the recorder. For continuous recording, DRs triggers provide markers into the key pieces of data during an event. The ability to share triggers between multiple sites is also available in order to capture a wide-area view of an event. The analog channels in a relay are fixed as current or voltage channels. This limits the ability of the relay to record and trigger on the measurements actually available in a substation. Relay triggers are based on specific protection function events, and are generally high or low magnitude triggers [7]. There is little flexibility to apply different types of triggers for a specific measurement. Relays may supply some pre-configured calculated, particularly zerosequence current, but have no capability to provide user-defined calculated triggering signals, or triggers on user-defined channels. Figure 5 shows example how the sensitivity of the triggering value may capture valuable information. This record is for intermittent faults occur as a result of arcing at certain points in the electrical system where insulation is weakened due to pollution. Fig. 5: High scan record for intermittent fault Fig. 6: SA system functional diagram In addition to the recording capabilities in both protection and disturbance recorder IEDs, there is also the recording capability in the Bay Control Unit (BCU) which, from the recording point of view, has, to certain extend, some recording capabilities as found in protective IED, taking into consideration the lower sampling rate (20 sample/cycle) and the time tagging resolution for events and sampled measurement (1ms). Full integration from the capabilities of all IEDs should be achieved. Modern multifunctional IEDs with monitoring, control and protection functions are typically being integrated in hierarchical substation protection and control systems [8]. The installation of advanced multifunctional protection, control, power quality monitoring and recording devices result in a very efficient solution that meets all requirements for primary and backup monitoring and recording in substation automation system. Because of the high sampling rate and the availability of multiple recording modes, it is obvious that power quality monitoring and specialized disturbance recording IEDs can be used as the primary recording devices. Multifunctional protection devices and the BCU can be used as the backup recording devices. 902

7 Within the environment of IEC61850, DRs are becoming more important. Being Cross triggered by GOOSE messages from protection IED, will ensure records are captured regardless of system configuration. This is because the settings for starting the relay are much higher than the level of DRs triggering. CONCLUSION This paper tries to demystify the recording capabilities of Protection and Disturbance Recorder IEDs. From recording point of view, stand alone disturbance recording system is advantageous compared to relays and BCU s. DRs have more processing power, higher sampling rate, advanced filtering, data logging capacity and triggering capabilities. Recording capabilities in Protection IEDs and BCU s can t replace stand alone DRs IED s. The state-of-the-art of the DRs IEDs enables the analysis of the performance of wide electrical network during different system disturbance events (Transients, Dynamics, PQ.). They render a useful tool for predictive maintenance of system equipment such as CB, CT, VT,., and also of the protective and control devices. They play a vital role in improving the performance and increasing the capabilities of power systems. They can help the operators and planners to take correct decisions. The selection of proper IED to do specific function is highly relied on the voltage level and desired application. At substation level, within the environment of IEC61850, full integration from the capabilities of all IEDs should be achieved. Disturbance Recorders can be used for primary recording functions, and Protection IEDs and BCUs can be used as backup (up to certain extend). During the analysis of wide system disturbances, it is critical to understand as quickly as possible what happened and why it happened, so steps can be taken to ensure that the disturbance does not reoccur. This can t be done without the detailed synchronised data provided by Disturbance Recorders IEDs. BIBLIOGRAPHY [1] International Standard IEC , Communication networks and systems in substations -Part 4: System and project management, First edition [2] Karlheinz Schwarz Impact of IEC on System Engineering, Tools, People-Ware, and the Role of the System Integrator ( DistribuTECH , San Diego, USA). [3] NERC PRC-002 standard [4] NERC PRC-018 standard [5] Mohamed A. EL-Hadidy, Dalal H. Mostafa, Samir A. Ezz El-Arab "Role of The Monitoring Systems in Improving the Performance and Capabilities of Interconnected Electrical Networks, (Arab League Seminar, April 2005, Cairo, Egypt). [6] IEEE final report on Considerations for Use of Disturbance Recorders. [7] Dennis Denison, Rich Hunt Using a Multiple Analog Input Distance Relay as a DFR (2005 Fault and Disturbance Analysis Conference Georgia Tech Atlanta, Georgia April 25th 26th, 2005) [8] Alexander APOSTOLOV, Damien THOLOMIER Analysis of Recording Elements in Wide Area Monitoring Systems ( /solutions). 903