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Variable-speed double-fed induction machines now have a rotor-dedicated protection relay. Called P348, it is the first of its kind. Built from scratch, it overcomes issues of very low frequencies and offers fast protection thanks to a patented protection system incorporating a new algorithm.
Although double-fed induction machines (DFIMs) have been around for some time, it was the rise of wind power for electricity generation that brought them into widespread use. Double-fed induction is also the design of the variable-speed motor-generators used in pumped storage, which is the most efficient way to store renewable power on a large scale rather than use it or lose it.
So far, so good.
However, Alstom Renewable Power voiced concern over rotor protection in its new DFIM. The machines had standard protection for the stator winding, not for the rotor. And they relied on the power electronics’ system to detect overcurrent and overvoltage faults. What Alstom Renewable Power wanted was a rotor-specific, independent protection relay.
The project, which brought together several countries and Alstom sites, would eventually yield the now commercially available – and patented – P348 protection relay.
Lloyd and his team had to address 2 major issues: how to measure low frequencies and deliver fast protection.
Single line diagram of variable speed machine protection
However, round about the time the project began, new kinds of sensors were becoming available – optical and Rogowski sensors that could measure DC and work at low frequencies.
The project team first explored the potential of optical instrument sensors which, rather than providing a 1-Amp or 5-Amp analogue current output, say, would output a data stream of sampled values of the primary current from these non-conventional types of CTs. They effortlessly measure low frequencies, but were too expensive and so were not selected for this application. That left the Rogowski CTs. “We were unsure of the Rogowski CTs”, says Lloyd, “because of the phase shifts they produce and attenuation at low frequencies.” However, testing at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland would dispel those misgivings. “To measure voltage at low frequencies, we went for resistive voltage dividers.”
The second major issue was to ensure fast protection. Protection relays that used standard Fourier-based algorithms were not designed to protect at low frequencies because the protection algorithms typically need at least one cycle of data before they trip. To extract the fundamental component, Fourier algorithms are used where the sampling rate is tuned to tracking the fundamental frequency. “Tripping speed slows right down when the tracked rotor frequency is low. At a frequency of just 1 Hz, that’s one second per cycle – enough for the machine to be in danger of being damaged,” says Lloyd. “You need to be operating within the 10 ms to 20 ms range.”
The development team had to develop new algorithms from scratch. To achieve fast protection they designed peak protection algorithms that used a fixed sampling rate and used directly sampled overcurrent and overvoltage values. Plus rms (root mean square) measurements for overcurrent and over-voltage protection. Although rms is not as fast as peak, it does offer the advantage of a steadier measurement.
In early 2014, the new protection relay was first tried out at the EFPL in Switzerland in the Electrical Machines Group directed by
Dr Basile Kawkabani. Rogowski CTs and voltage resistive divider sensors were integrated with a merging unit and primary converters, and faults were applied on the rotor circuit and the stator. “The Alstom Research Centre in Villeurbanne had designed converters that created a data stream from the low voltage output from the sensors,” adds Lloyd. “They had also adapted the filters to measure low frequencies. The connected merging unit produced a digital output of sampled values in a format compliant with the recent standard
IEC 61850-9-2 LE, which defines the protocol for connection to the P348 relay.”
Results were encouraging, while simulator testing back in the UK demonstrated that the innovative peak and rms protection delivered fast, accurate protection. Factory acceptance tests are under way and the Systems team at Alstom Grid in the UK is currently completing the cubicles. The first commissioning is slated for the summer of next year.
Testing at EPFL
The Electrical Machines Group, headed by Dr Basile Kawkabani at the École Polytechnique Fédérale in Lausanne, Switzerland, was the venue for testing the new protection relay on a small 3.3 kVA DFIM.
Three-phase Rogowski current sensors were connected to the rotor and stator, while voltage dividers were connected to the rotor terminals. All the sensors were connected to an
IEC 61850-9-2 LE protection relay input through primary converter and merging units. The faults applied to test protection were: three phase, phase phase, and phase to ground. The currents supplied to the relay for the tests were increased by winding 320 primary turns on the Rogowski CTs to represent the larger currents that would be present on a large machine.
The peak and rms rotor and stator overcurrent protection was set to 120% of rated current.
The rotor and stator peak and rms overcurrent stages began operating soon after the fault kicked in. The peak protection picked up faster than the rms as expected and the protection stages tripped in the expected time in order to ensure correct peak time delayed protection operation.
The peak three-phase fault current on the real machine at EPFL was 49 kA on the rotor and 6.8 kA on the stator as shown. The three-phase fault current at 5 Hz is many times the overcurrent protection setting so that the overcurrent protection can easily detect these faults and trip quickly.
Machine parameters
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Representative test bench DFIM
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Large hydropower DFIM
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Rated apparent power
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3.3 kVA
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300 MVA
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Stator line voltage
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400 V
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18 kV
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Rotor current
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20 A
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6 kA
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Rotor line voltage
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0 to 60 V
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0 to 5 kV
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Pole pairs
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2
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6
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Rated speed
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1500 rpm
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500 rpm
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Slip range
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±10%
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±10%
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