Mining Earth Leakage Protection with Variable Speed Drives

Transcription

1 Ampcontrol Application Note Mining Earth Leakage Protection with Variable Speed Drives.1

2 Introduction Mining electrical systems have evolved various protection systems due to the unusual environments and equipment they use and the hazards they present. In particular, they include: a) Earth Fault Current Limitation, usually consisting of a resistor connected between the supply transformer star point and earth (NER). b) Earth Continuity monitoring and protection. c) Earth leakage protection d) Earth fault lockout protection. As described in Appendix C of AS/NZS , the protection scheme is intended to ensure that when persons are exposed to touch potentials, the level of voltage and time exposed before protection systems trip is limited to an acceptable level. The acceptable levels are given in Figure C1 for 50Hz touch voltages. A properly designed protection system also limits the risk of fire, arcing and hazardous arcing faults, but this is a secondary aim. To ensure that the touch potentials are managed to acceptable levels, the complete electrical system needs to be assessed and a protection scheme implemented accordingly. These protection systems were originally devised to protect against touch potential hazards resulting from earth fault currents that are, of course, driven by the power supply (50 Hz). Consider, for example, that an earth fault occurs in a mobile machine (rubber tyres) powered by a trailing cable. The earth fault current will flow through the fault to the machine frame and return to the supply transformer star point via the trailing cable earth conductors (primarily). The voltage drop caused by this earth fault current flowing through the total earth impedance will result in a potential rise above earth for the frame of the machine, and it is this potential rise that presents the touch potential hazard to anyone who may touch the machine while their feet are on the ground. As described in Appendix C of AS/NZS , using Figure C1 the system assessment must determine the earth fault limitation current and earth continuity relay trip settings that will protect people based on the achievable earth leakage clearance times and knowledge of the cable lengths and the resulting pilot and earth impedances. Variable Speed Drives Variable speed drives (VSD s) are now finding wide use in mining applications. Most of these drives use variable frequency outputs that are produced by rectifying the supply to DC and then inverting this DC voltage back into AC using a high frequency carrier and pulse width modulation (PWM) to produce variable frequency currents in

3 the motor. They complicate the situation in several ways: 1) They introduce a new and complex voltage source into the power system. This may mean that earth faults can now be direct current (DC) in nature or may be driven by the inverter of the drive and so have a frequency that is primarily that of the drive PWM carrier frequency (say 1000 Hz for example). 2) To minimize interference with protection and control systems, many drives employ Electromagnetic Compatibility (EMC or EMI) filters that consist primarily of a capacitive circuit between the input of the drive and earth. This provides a path for the earth fault currents that represents an alternative path to the NER. In fact, it is the intention of the filter to provide this alternative path for the high frequency currents that flow (through the motor and cable stray capacitances) to earth under normal (non fault) conditions. They will also provide an alternative path under fault conditions, particularly if the fault is driven by the high switching frequency drive output. It has also been shown that when one or more drives and filters are in use, and an earth fault occurs, there can be circulating currents between the drives and filters and/or the fault location. The fault current magnitudes may then greatly exceed the nominal current limitation value (typically 5Amps) determined by the NER. These large currents may cause touch potentials that greatly exceed the expected values. 3) Most earth leakage protection relays approved for use in mining applications are designed to detect 50 Hz currents, not DC or high frequency currents so that the relays may not trip, or if they do trip they may take longer than expected. The overall result is that with standard earth leakage protection relays and electrical system assessments based only on consideration of faults driven by the supply system (50 Hz), protection performance is unlikely to be adequate when variable speed drives are used in mining applications. A Real World VSD Earth Fault Example The following diagram shows the schematic of a system used to examine these issues and actually measure the currents that can flow when an earth fault occurs in a system using VSDs and an EMC filter. These are measurements taken on actual

4 Ampcontrol Application Note mining equipment in an underground situation. It demonstrates that the EMC filter alone changes the situation. The test is applied to a situation where one machine is supplied by a VSD with an EMC filter and the other machine is a DOL machine. The NER is selected to limit the normal 50 Hz fault current to a level of 5 amps RMS (7.07 amps peak). At a time when the VSD is energized but NOT operating (so there is no high frequency source) an earth fault is applied to one phase of the DOL machine. The results are displayed above. The recorded currents are those seen by the current transformers feeding the earth leakage relays (within the IPC relays). Outlet 1 feeds machine 1 which has the VSD and filter. The current seen by the E/L for outlet 1 is shown in purple while that seen on outlet 2 is shown in blue. Note that when both outlets are connected, the peak current seen by both relays reached about 30 amps (well over four times the limitation level). These high currents are a result of circulating currents between the earth fault location and the EMC filter. The circuit rings at a frequency

5 determined by the stray inductances and capacitances and the EMC capacitance but there are many frequencies present because the step transient produced by application of the fault contains many frequencies. In the example above, there is only one non operating drive and an EMC filter and the only source of excitation is the transient caused by application of an earth fault. The situation can be far worse if multiple drives and filters are supplied in parallel and the drives are operating (producing a high frequency output voltage). The simulation results shown above suggest that, when a fault occurs on one of two machines that are supplied from operating VSDs with EMC filters, then very large high frequency earth leakage currents [I(XC11A) & I(XC21A)] may result and potentially dangerous touch potentials [V(G1) & V(G2)] can be expected. Of the AS/NZS 2081 compliant earth leakage relays currently available for use in mines, most are unlikely to respond properly to these high frequency earth fault currents and even if they do, the tripping times will be unpredictable. Any protection system design based on the calculations described in Appendix C of AS/NZS is unlikely to be suitable for such cases. Improving Protection The design of earth leakage relays used in mining applications is required to comply with AS/NZS The new version (AS/NZS 2081:2011) of this standard came into operation on Nov It has various changes and better recognizes that system protection needs to be assessed in accordance with AS/NZ and with the changes in the mining electrical environment. For example, the new standard allows higher protection sensitivity settings where it can be demonstrated to be safe. The Following extracts from AS/NZS 2081:2011 provide some key statements of interest to our subject: 1.1 Scope: Appendix B: What this means is that when Variable Speed Drives (or other non 50Hz sources) are used in a mining electrical system then the standard approach needs to be modified and interpreted to ensure that protection is adequate. The key factors to consider are as follows: 1). The sensitivity of the human body to electric shock varies with frequency. In general, for frequencies above 50 Hz, for a given exposure time, the allowable touch voltage magnitude increases with frequency. For example, at 10 khz, the allowable touch voltage (actually the let go voltage) is about 5 times that at 50 Hz. 2). When EMC filters are used (with an internal shunt capacitive path to earth) then this forms a path for earth fault currents that is an alternative to the NER. When considering touch potentials at a mobile machine (for example), strictly speaking we must examine the impedance of the filter at the frequency of interest (say the drive carrier frequency) in order to determine the earth fault current that will flow when a fault occurs in the

6 machine. The earth leakage tripping time, (or other fault clearance / current limiting time), must then be used to ensure that the touch voltage and exposure time form a safe system. Some EMC filters may have fuses or current limiting circuits that act quickly to limit the earth fault current that flows during any earth fault. Care must be taken when multiple filters are connected to a single supply. In this case there are many modes of possible earth fault that need to be considered and actual earth fault currents may exceed the current seen by any single filter. 3). We must have an earth leakage relay that will accurately sense earth fault currents of any frequency from DC to the maximum frequency of interest. It can be seen that this is not a trivial matter and it is likely that the industry will need to adapt to this new and complex environment. The Ampcontrol VSD Guard Earth Leakage Relay The VSD Guard relay is a panel mounted earth leakage relay and event analyser. It is significantly different from any earth leakage relay currently used in mining applications and should provide a key tool in understanding, monitoring, protecting and analysing systems using Variable speed Drives. Features: Complies with AS/NZS 2081:2011 International patent, application number PCT/AU2011/ Detects earth leakage currents at frequencies from DC to 10 khz and provides adjustable trip set points and trip delay times. A high resolution colour graphics display, supported by Ethernet communication to external devices, provides the ability to display the harmonic spectrum of earth leakage currents being monitored, in real time, or allow the user to see the signal waveform (oscilloscope mode). A data recording facility allows events (such as an earth fault trip) to be captured, stored indefinitely, transmitted to external devices and be analysed and displayed on the screen (or surface computer) to extract the component frequencies and magnitudes of the earth fault currents. The VSD Guard can be set up in several ways: 1). Standard Earth leakage Mode: The relay will see all currents between DC and 10 KHz and trip if the true RMS level of leakage current is above the trip level (adjustable from 0.1 to 5 Amps). This mode is fully compliant

7 with AS/NZS 2081:2011 and would be used in most cases. 2). Advanced Modes: These modes allow a modified form of operation for demanding applications based on the guidance given by AS2081:2011 in the extracts given above. The settings allow increased trip levels at higher frequencies to take into account the reduced sensitivity of the human body to touch potentials at these frequencies. While the standard requires the system designer to analyse all applications to ensure that touch and step potentials are safe, for some situations a higher level of analysis can be required and such analysis may allow higher trip levels to be set on the protection. In this case the Standard Mode protection can be disabled using password protected access. The Advanced Modes available are: Weighting of currents according to AS Figure 10, to reflect the fact that higher currents should be allowable at higher frequencies because the allowable touch voltages will be higher. One trip level will be set but this will be modified to allow higher currents at higher frequencies. Fully adjustable trip levels at different frequencies. The spectrum of the leakage currents is divided into bins of adjacent frequencies and a trip level may be selected for each bin. Learn Mode. Over an adjustable period of time, the relay records earth leakage currents that may flow through motor and cable stray capacitances during normal and worst case operation. This information may then be used to set trip levels on frequency bins that are conservative and still reliable, or simply to learn about the electrical environment (in any mode). System Design in Applications Requiring Advanced Operating Modes In some applications, operation with the VSD Guard in the standard mode may not be acceptable and the advanced operating modes may be attractive. With complex environments that contain multiple drives and filters a careful analysis of the system is recommended. The characteristics of the filters, drives and cables and the system circuit (single line diagram) must be known. Information about the possible frequencies that the various drives can produce (carrier frequencies) will also be required. In some cases simulation of the entire circuit will be required in order to estimate the maximum currents and so touch potentials that may occur under fault conditions. Once the currents and frequencies are determined the protection settings of the VSD Guard relay may be specified. It is acknowledged that this may be a challenging process and it could take some time for users to develop the required engineering knowledge and familiarity with simulation tools. Ampcontrol is available to assist users in this area. For further information, contact Ampcontrol Electronics, Cameron Park. PH: