New alternatives aid medium voltage automatic transfer for critical power systems

Transcription

1 POWER MONITORING SYSTEMS CAN RESOLVE GROUND RELAY TRIPPING New alternatives aid medium voltage automatic transfer for critical power systems By Reza Tajali, P.E. Power Systems Engineering, Square D Company To assure critical power in many industrial and commercial facilities, medium voltage power is often switched between alternate sources. In these power systems, each individual feeder often supplies multiple transformers. The automatic transfer operation energizes all the transformers and their loads in one block, resulting in transformer inrush current on phase relays and ground relays. Where a ground relay tripping occurs, maintenance personnel may solve the problem by increasing the relay settings. The problem is that increasing the settings reduces the sensitivity of the relay, which reduces the likelihood of catching a fault online; late detection of faults allows more arcing and other damage. Recently, a large commercial facility was undergoing a major construction project to expand its processing center. Solving certain electrical problems at the site included effective responses to ground relay tripping. In the facility s power distribution system (Figure 1), two low-resistance grounded utility power transformers provide alternate power sources at KV. The grounding resistance on each transformer limit the magnitude of ground fault current to 600 amperes. When one source loses power, the electricity is switched at KV to the other source through an automatic transfer system. The transfer connects the entire affected load to the alternate source in one block. The metal-clad feeder circuit breakers are equipped with digital microprocessor based relays that include the 50/51 three-phase overcurrent and 51N ground relay functions. Three 1200/5 amperes bushingmounted current transformers are connected in a residual ground fault circuit to provide ground fault detection. Identifying the problem During construction, one of the utility sources was accidentally tripped. The automatic transfer system switched the power to the alternate source as expected. However, upon completion of the transfer, the 51N function of the feeder protection relay tripped.

2 The construction crew manually re-energized the tripped feeder, but the 51N relay tripped again. That process repeated itself, leaving half of the construction site without power. The crew checked downstream equipment for ground faults but found no evidence of insulation deterioration. They finally managed to restore power after some of the downstream load was switched off. Everyone s attention was focused on the relay and it s alleged malfunction, yet no actual ground fault was present. To establish a benchmark for the performance of the relay the team measured the exact current in the residual circuit that the relay sees. The metal clad switchgear was equipped with POWERLOGIC circuit monitors that provide full function power quality analysis. The circuit monitor wiring was modified to insert a current coil in series with the ground sensing element of the relay to measure the same exact residual current as is seen by the relay (Figure 2). The current swell detection capability of the circuit monitor captured oscillograms of the events. We asked the construction crew to perform several switching actions, which included energizing and deenergizing blocks of load. One test entailed energizing all the transformers on one feeder together. We were trying to cause the relay to trip and obtain an oscillogram of the tripping current. The current sensed as ground fault by the relay saw a transient reaching up to 1000 amperes due to the switching action (Figure 3). That current corresponded to 4.16 amperes in the CT secondary residual circuit. This occurred with no ground fault in the system. We wondered how to get 1000 amperes (4.16 amperes CT secondary current) in the residual circuit while there was no actual ground fault. The answer involved current transformer saturation and the residual method of ground detection. Inrush current interacts with residual circuit When system voltage is applied to a power transformer, a current transient occurs, which is known as magnetizing inrush current. It occurs because the transformer has some magnitude of remaining flux from the previous energization. If the voltage impressed on the transformer demands a different value of instantaneous flux, a transient condition occurs and significant current will flow for a few cycles. As individual transformers may be de-energized at different points of the voltage wave, they will have

3 different magnitudes of remnant flux. The resultant total inrush current obtained by energizing multiple transformers is random. An oscillogram of inrush current obtained through the test circuit of Figure 2 showed that the offset wave dies after several cycles (Figure 4). This characteristic inrush current is very rich in second harmonic. As no actual ground fault was present in this case, the sum of instantaneous currents in the three phases equaled zero. This is dictated by the physical principles of conservation of charge and Kirchhoff s law. The residual circuit of Figure 2 adds up the currents in the three phases. Therefore, this residual current must be zero. However, our tests indicated a random pattern of residual current of large magnitude. Current transformer saturation The signal produced by the current transformers in a transient inrush condition does not accurately depict the current flowing on their primary side because of current transformer saturation. The saturation occurs due to the direct current (DC) component of the inrush current. In one of the tests, the C phase current showed evidence of severe current transformer saturation (Figure 5). Therefore, the mechanism involved is as follows: 1. The waveshapes of the inrush current (DC offset) on the three phases cause saturation of the CTs to different levels. 2. The saturated CTs produce incorrect waveshapes in their secondary circuits. 3. Net residual currents appear in the secondary circuit due to incorrect waveshapes. Circuit monitors to the rescue The data obtained form the circuit monitors provided two key pieces of information: 1. The residual current exists during transformer energization and is of large magnitude. 2. The waveshape of the current in the CT secondary circuit indicates severe saturation. At this point, equipment shutdown was scheduled and the current transformers were evaluated. It was discovered that the tap settings on the CTs were installed incorrectly. Once the tap settings were corrected, the residual current was reduced, but it was not eliminated. Corrective actions In this example, incorrect setting of the CT taps was the primary cause of the severe transformer saturation and it was easily corrected. However, it is generally possible to improve the transient response

4 of current transformers by providing transformers with a larger magnetic core. Such large transformers may be easy to install during the switchgear manufacturing process, but they are difficult to install in the field. Another common approach is to replace the residual circuit with a core balanced current transformer. The core balanced CT is basically a large window current transformer, which encircles the three phase conductors. In an ideal geometric configuration, the flux produced by the three phase currents would cancel each other. These CTs are not as prone to saturation and provide more accurate ground current waveforms. However, making these physical changes may not be possible in the field. Other corrective options The 51N relay used in this application was a microprocessor based relay. As such, it provided us with the capability to manipulate the protection curves in various ways. Relay manufacturers, usually choose one of three approaches to address the transformer energization problem: Manipulating protection curves; 2 nd harmonic restraint; Cold load pickup restraint. Manipulating protection curves Digital relays offer a variety of ways to manipulate their protection curves. This capability can be used to great advantage when nuisance tripping is a problem because the entire tripping characteristics can be modified in the field. The inrush current dies off within a few cycles (Figures 3-5). So, if we prevented the relay from tripping instantaneously, the relay could ride through the transient inrush condition. This can be accomplished by the instantaneous delay setting. But the new time current curve would also have to coordinate with the upstream relays, requiring similar adjustments in the upstream relays. This kind of adjustment involves a tradeoff. Inserting the referenced time delay increases the amount of potential damage to downstream equipment in case of a true ground fault. In this application, the justification for this adjustment was that the ground current available from the source was limited to 600 amperes due to low resistance grounding. 2nd Harmonic Restraint

5 2nd harmonic is amply present in the transformer inrush current waveform. Figure 6 shows the Fourier Transform (frequency spectral analysis) of the waveform presented in Figure 4. The large second harmonic component is clearly evident in the spectral analysis. Aside from transformer inrush, this harmonic is not at all normal to power systems such as the one in this facility. The shape of the inrush current wave generates the 2nd harmonic. As a matter of comparison, power system fault currents which also have large DC offsets - do not have much 2nd harmonic content. Because 2nd harmonic is somewhat unique to transformer inrush, it is used to restrain relays during transient inrush conditions. This technology was originally developed and applied to transformer differential relaying. Essentially, the relay restrains itself from operating if a certain amount of 2nd harmonic current is present in the current that flows through the relay. A typical restraint setting is 20%. That means that if the 2nd harmonic is larger than 20% of the fundamental, the relay will be restrained from operation. In this case, the percentage of 2nd harmonic in the transformer inrush current was very large (Figure 6). Cold Load Pickup Restraint Cold load pickup is a terminology that applies to distribution line protection. When a de-energized line is suddenly energized, inrush current will flow to the line and to the transformers and loads connected on the line. Relays must be set so they do not trip under these cold load conditions. Some new microprocessor relays have expanded this concept and provide programming capability so that specific tripping functions can be restrained in a cold load condition. The cold load condition is determined by a digital input from the 52a contact of the circuit breaker. So with this technology, any time the circuit breaker is closed, we can restrain the ground fault trip for a pre-determined period. Old problem, new conclusions The cause of nuisance tripping in this case was a transient current in the residual sensing circuit due to saturation of current transformers. This was primarily caused by incorrect CT tap settings. However, nuisance tripping due to transformer inrush current is not new to electrical power industry and is

6 especially troublesome with differential relaying schemes. When combined with current transformer saturation, the problem shows up in radial ground fault relaying schemes. The traditional method for solving this problem has been to raise the instantaneous trip setting of the relay until the relay no longer trips under transformer inrush. But raising this setting would allow more ground fault current in the case of a true ground fault, and coordination with upstream relays limits the allowable settings. Current transformers with larger magnetic core provide better accuracy. Also core balanced current transformers improve the accuracy of the signal delivered to the ground relay. However, physical limitations of switchgear might prevent retrofitting of the current transformers. Microprocessor relays offer special trip curve adjustments, which can be useful in preventing nuisance tripping. Other solutions for this type of problem include harmonic restraint and cold load pickup restraint. The key to choosing the correct solution is in understanding all the available options. # # # Reza Tajali is Senior Staff Engineer for Square D Company s Power Systems Engineering group. He has over 20 years of experience with electrical power distribution and control, and holds two United States patents on switchgear products. In his present function he leads the engineering team in the Midwest region of the United States. His team of engineers support industrial and commercial customers with power system design, analysis and power quality improvement plans.

7 600A 50/ 51 51N 2500KVA 12.47KV 480Y/277 Other XFMRs Figure 1: System One Line Diagram Powerlogic Digital Relay 50/ /5 51N Figure 2: Modified circuit monitor connection diagram to capture the exact residual current.

8 bkr 163 w3s _06_48_new>B_AMPS-AMPS( )(14:06:48) Current (A) Time (ms) Figure 3: Measured Residual Current bkr 163 w3s _06_48_new>A_AMPS-AMPS( )(14:06:48) Current (A) IV. CURRENT TRANSFORMER SATURATION Time (ms) Figure 4: Phase A Current, Transformer Inrush Current

9 bkr 163 w3s _06_48_new>C_AMPS-AMPS( )(14:06:48) Current (A) Time (ms) Figure 5: Phase C Current, Note the Effect of Current Transformer Saturation 600 DERIVED>FFT2-AMPS( )(14:06:48) Current (A) Frequency (Hz) Figure 6: Fourier Transform of Phase A Inrush Current Waveform. Note the large 2 nd harmonic component.