OPEN-PHASE DETECTION TECHNIQUES FOR CRITICAL STANDBY SUPPLIES

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Mervyn Jennings
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1 OPEN-PHASE DETECTION TECHNIQUES FOR CRITICAL STANDBY SUPPLIES U AJMAL, GE Grid Solutions UK Ltd, usman.ajmal@ge.com S SUBRAMANIAN, GE Grid Solutions UK Ltd, sankara.subramanian@ge.com H Ha GE Grid Solutions UK Ltd, hengxu.ha@ge.com G LLOYD, GE Grid Solutions UK Ltd, graeme.lloyd@ge.com S HOSSEINI, GE Grid Solutions UK Ltd, sharaf.hosseini@ge.com Dr.J BLAKE, GE Grid Solutions US, jim.blake@ge.com C.F.CHOW, GE Grid Solutions Singapore, chin-fei.chow@ge.com 1 ABSTRACT Open-phase Detection (OPD) is of prime concern in the nuclear industry [1]. Events at several nuclear power plants have shown that the open-phases on the HV side of a transformer may be difficult to detect by existing plant instrumentation and electrical protection schemes. If an open-phase condition is not quickly detected and the transformer not isolated from the system, tripping of critical motor driven loads and loss of plant safety systems can occur. Breaker poles failing to close, a broken conductor, failure of a transformer bushing or line insulator, a loose connection or a blown fuse can all lead to an open-phase condition. Two methods of implementing OPD are described in the paper. The first method uses simple undercurrent detectors and relay logic which is being site trialled in the USA. The second method is a new method proposed for OPD using sequence components and phase identification using total energy flow per phase which has been tested in Matlab (Simulink). Both methods provide a fast, simple and cost effective solution to detect an openphase on the HV side of a transformer. An open-phase condition on heavily loaded to moderately loaded transformers can however be reliably detected and protected against with conventional protection relay schemes. Conversely, reliable open-phase detection for all transformer connections which are lightly loaded or unloaded is challenging. An unloaded power transformer with its primary winding energized, draws a magnetization current in each phase in the range 50 to 750 ma with a significant harmonic content. In addition, an open-phase condition on the HV side of a transformer can be very difficult to detect from the LV side only as the current magnitude and phase angle measured on the LV side depend on the transformer s core and winding configuration as well as the loading level. This paper describes the issues from loss of a phase for nuclear power station auxiliary transformers and proposes a new universal protection scheme applicable to most power transformer configurations. The new scheme uses optical CTs to reliably and accurately measure the very small currents during an open-phase condition using a protection relay with an IEC LE process bus input. KEYWORDS: Nuclear, Open-phase, Auxiliary, Generation 1

2 2 INTRODUCTION An open-phase condition on the HV side of a transformer can be very difficult to be detected from the LV side as the current and voltage response depends on the transformer s core and winding configuration. Commonly used core and winding configurations of power transformers include: Core design: Three limbed core (core type), five limbed core (shell type) and five limbed core with buried delta. Winding configuration: YNd, Dyn, YNyn, Yyn and YNynd Transformer primary-side currents can be an effective indication of an open-phase condition. Extensive simulation studies, have established that with an open-phase on the HV side of a transformer, in some cases the three phase voltages and currents on the LV side could be fairly balanced, particularly under low transformer loading conditions. The severity of the current and voltage imbalance on the LV side depends on the winding configuration as well as the core construction. An unbalance on the LV side of a standby auxiliary transformer for a nuclear power plant can lead to the following: Unbalanced voltage conditions at redundant safety buses. Tripping of equipment such as essential service water pumps, centrifugal charging pumps and component cooling water pumps. Preventing the onsite electric power system from supporting plant safety systems. A lightly loaded or unloaded transformer may continue normal operation for days with acceptable balanced voltages and currents on the LV side even when there is an open-phase condition on the HV. The condition may be detected either during visual inspection, or when the loading of the transformer has increased substantially. The protection relay, monitoring the LV side, can only detect the condition when the load has increased substantially and this will mainly be via the overcurrent element (negative phase sequence or sensitive earth fault) protecting the LV side. A universal protection scheme for HV side open-phase condition of Standby Auxiliary Transformer (SAT) is described in this paper. This scheme is based on a very sensitive, highly accurate and reliable current measurement system that uses optical CTs, where the open-phase condition is automatically detected and alarmed. There is consensus among all parties involved that an alarm rather than a trip command may be more beneficial as the condition can be investigated and remediated in a controlled manner without loss of safety functions. This scheme is universal in the sense that it applies to all transformer winding and core configurations and to a wide range of transformer loadings from no-load to full-load. The OPD scheme discussed in this paper has been used in the field trial of a particular nuclear power plant. All OPD schemes used for automatic and reliable detection of open-phase conditions must be calibrated and tuned to the plant distribution system during the commissioning stage because it depends on the design and configuration of the onsite and offsite power system. 3 TRANSFORMER RESPONSE An open-phase fault, for the purpose of this paper, is defined as a series single phase open fault between the source and the transformer HV winding. The measuring point is at the HV bushings of the transformer. Transformer responses to open-phase faults depend on several factors with the following four being dominant: 1. Source grounding type 2. HV/Primary Winding Configuration 3. LV/Secondary Winding Configuration 4. Core type (3-5 legged) The above criteria affect the presence and level of sequence currents in the transformer. The ratio of these currents to the positive sequence current can be used to detect the presence of an HV open-phase fault. During development of the OPD scheme, various transformer types were modelled to study their open-phase responses. For open-phase faults on the HV side, grounding of the primary winding will determine the ratio of zero sequence to positive sequence currents (I 0 /I 1 ) and the ratio of negative sequence to positive sequence currents (I 2 /I 1 ). This is discussed below. 2

3.1 Grounded Primary (Yn) Transformers with a grounded primary winding exhibit a significant increase in I o /I 1 (close to 0.

Simulation results are shown in Figure 1.

faulted winding being regenerated. However, no power will be transferred via the faulted primary phase winding to the secondary phase winding.

1 Open-phase response for grounded primary transformer; Positive (Red), Negative (Green) and Zero (Blue) sequence components shown. 3.

3 3.1 Grounded Primary (Yn) Transformers with a grounded primary winding exhibit a significant increase in I o /I 1 (close to 0.9pu) current ratio during an open-phase fault. This is mainly attributed to current re-routing through the transformer earth connection. Simulation results are shown in Figure 1. In addition to the current response, for Y n - transformers, flux interaction between the primary and secondary windings will result in the voltage in the primary faulted winding being regenerated. However, no power will be transferred via the faulted primary phase winding to the secondary phase winding. Hence, current response is used for OPD in this case. Figure.1 Open-phase response for grounded primary transformer; Positive (Red), Negative (Green) and Zero (Blue) sequence components shown. 3.2 Ungrounded Primary (Y or ) Transformers with an ungrounded primary side will carry a high I 2 /I 1 ratio (close to 0.8pu) as shown in Figure 2. This is due to change in current direction in one of the un-faulted phases. Voltages in healthy primary windings will drop to approximately 0.5pu of the pre-fault voltage. In this case, both the current and voltage can be used for an OPD scheme. Time (s) Time (s) Figure.2 Open-phase response for ungrounded-primary transformer; Positive (Red), Negative (Green) and Zero (Blue) sequence components shown. 3

4 3.3 Loaded versus Unloaded Transformers The current and voltage behaviour described above can be detected by traditional protection schemes when a transformer is carrying significant (>5% rated) load. However, detecting an open-phase fault when the transformer is unloaded or lightly loaded (<5% rated) is not possible with traditional protection schemes. In such cases, optical CTs with digital filtering (to eliminate noise at such low current levels) can be used to provide accurate primary current measurements on the transformer HV side. Typical primary current measurements would be between 50mA to 750mA for unloaded transformers. This current will mainly consist of the transformer magnetising current and any mutual inductance current caused by the HV line. 3.4 System Balance Even though it may initially seem that an open-phase condition on the HV side of a power transformer will result in imbalances on the LV side with enough significance that can be detected by monitoring the LV side, there have been extensive simulation studies that all have confirmed that the impact not only depends on the transformer core design but also on the winding configuration as well as the loading of the transformer. System Balance HV side Phase A Open (Unloaded Transformer ) Configuration and core Primary Voltage (pu) Secondary Voltage (pu) design Phase A Phase B Phase C Phase A Phase B Phase C YNyn (Shell type) Dyn (Core type) YNd (Core type) YNynd (Shell type with Tertiary Delta) Simulation results above indicate that under no-load condition and when there is an open-phase condition on Phase A of the HV side, if the secondary or the tertiary is delta, regardless of the core design the system voltage remains balanced. For example in the case of YNd (core type) the system is balanced because the flux in the three limbs are balanced on the LV side, resulting in the voltage being re-created to 1 pu in the HV phase that has become opened. From the table above it can be concluded that: YNd - lost phase is always re-created irrespective of the core design. Primary Delta or un-earthed Y - the lost voltage is not re-created. YNyn - System balance depends on the core construction. 3.5 Magnetizing current Transformer primary side current magnitude can be an effective indication of an open-phase condition. Transformers that are normally unloaded such as SAT, detection of an open-phase condition via conventional current transformers (CTs)is difficult due to the low magnitude of magnetising currents. CTs are adapted for significantly higher currents than those under unloaded or lightly loaded conditions. A possible solution is by monitoring of the magnetization currents on the HV side of the transformers using Optical CTs together with an adequate numerical relay. Generally, an unloaded SAT with the HV side energized draws magnetization current which is rich in harmonics and is in the range of ma. Figure below depicts a typical SAT magnetizing current. Figure.3 Transformer magnetising and excitation current 4

1 Use of Undercurrent Elements with Relay Logic (Technique-1) Figure 5: Depicts the relay logic used covering both unloaded as well as loaded transformers.

5 The figure below depicts the magnetization current of unloaded SAT before and after HV side phase A opened. Figure 4: Before phase A (L1) being opened (left), After phase A (L1) being opened (right) 4 OPD SCHEMES 4.1 Use of Undercurrent Elements with Relay Logic (Technique-1) Figure 5: Depicts the relay logic used covering both unloaded as well as loaded transformers. The relay logic employs the measurement of the individual phase currents together with a zero sequence check for the case when the transformer is unloaded. In the case of long incoming lines and an open-phase condition at a long distance away from the measurement point, line charging should also be taken into account which could mask the effect of an open-phase condition if the detection is merely based on low current measurement. In this case a negative sequence test should also be incorporated into the relay logic for the unloaded case. 5

4.2 Use of Sequence Components and Summation of Energy (Technique-2) This technique is based on three design modules.

An overview of the scheme is shown in Figure 1. The OPD design model takes primary (HV) current and primary (HV) voltage as inputs.

In design module 2 - open-phase fault detection, input current and voltage components are compared against set criteria. This is the criteria for OPD and for elimination of non-open-phase fault types.

3 Design Module 1 - Discrete Fourier Transform (DFT) & Sequence Calculation This module, shown in Figure 6, converts measured voltages and currents into sequence components via a Fourier transform.

6 4.2 Use of Sequence Components and Summation of Energy (Technique-2) This technique is based on three design modules. When combined with the versatility of configurable logic of a numerical relay, the scheme can be used to detect open-phase faults in several different transformer construction types. An overview of the scheme is shown in Figure 1. The OPD design model takes primary (HV) current and primary (HV) voltage as inputs. Symmetrical components of these currents and voltages are derived in design module 1 - discrete Fourier transform (DFT) and sequence calculation. In design module 2 - open-phase fault detection, input current and voltage components are compared against set criteria. This is the criteria for OPD and for elimination of non-open-phase fault types. Finally, design module 3 - fault detection, calculates the sum of real energy flow per phase for phase selection. Figure.6 OPD Technique-2 Scheme block diagram 4.3 Design Module 1 - Discrete Fourier Transform (DFT) & Sequence Calculation This module, shown in Figure 6, converts measured voltages and currents into sequence components via a Fourier transform. In Matlab, the DFT algorithm is implemented by the following equation: N 1 2 k 2 j N X ( n) x( n N k 1) e N k 0 (1) Where N is number of samples per cycle (N=T0/Ts), T0 is the cycle period of the fundamental frequency, Ts is the sampling interval, x is the input signal, k is the integral variable, and n is the time of the present cycle. The DFT algorithm is the preferred method of sampling measured analogue values as it is feasible to implement in protection relays. Figure.7 DFT and sequence components derivation block diagram representation 6

4.4 Design Module 2 - Open-phase Fault Detection The voltage and current sequence components output by module 1 are analysed in module 2 to detect an openphase condition, as shown in Figure 5.

current, I 1 is the positive sequence current, and I 0 is the zero sequence current. The same notation applies to voltage components.

7 4.4 Design Module 2 - Open-phase Fault Detection The voltage and current sequence components output by module 1 are analysed in module 2 to detect an openphase condition, as shown in Figure 5. The criteria used for detection can be divided into the following: Fault Detection Block: Ratios used for comparison are I 2 /I 1, I 0 /I 1, V 0 /V 1, V 2 /V 1, where I 2 is the negative sequence current, I 1 is the positive sequence current, and I 0 is the zero sequence current. The same notation applies to voltage components. The criteria are based on the tested behaviour of sequence currents and voltages during open-phase conditions on the HV side of various transformer construction types. In an actual application of the scheme, apart from the fault type and transformer construction type, operating values of these ratios will be affected by the system standing imbalance, capacitive charging currents and mutual inductance with adjacent conductors. Figure 3 shows the criteria used for a Y- or Y-Y n type transformer. Non-Open-Phase Fault Detection: To provide selectivity, the algorithm includes functions for detecting several types of non-open-phase faults both on the LV and HV side. These faults will normally be picked up by either the transformer protection scheme or the HV line protection scheme. Figure.8 OPD criteria Technique-2 5 CASE STUDY (TECHNIQUE-1) GE Grid Solutions has carried out three field trials in collaboration with Florida Power and Light (FPL). In the first field trial the GE Grid Solutions OPD system was installed at FPL s Alexander substation in May, 2013 on a 230 kv, 55 MVA wye-wye power transformer. The second field trial of the GE Grid Solutions OPD system was done at the EFACEC Transformer factory in Savannah, GA in April, Three phase testing was done on a 138 kv Delta-Wye transformer and included both open-phase and grounded phase tests, with excitation voltages ranging from 0.9 pu to 1.1 pu. A third OPD trial system was permanently installed at the FPL Martin plant (fossil plant) in July 2014 on a 238 /6.9/6.9 kv three winding Y-Y-Y power transformer. The GE/FPL OPD systems together with Real Time Digital Simulation (RTDS) testing have all demonstrated that the GE OPD system can reliably detect the presence of open, grounded, and high impedance grounded phase faults on the HV side of both loaded and unloaded power transformers. The third field trial is discussed here in detail. 5.1 GE Grid Solutions/FPL Martin plant field trial GE Grid s solution is to install COSI (compact optical sensor intelligence) non-conventional current transformers on the HV bushings of the off-site 3 winding 238/6.9/6.9kV Y-Y-Y power transformer. The photo below shows the COSI optical current transformer installed at the site. 7

8 Figure.9 Photos of field trial GE Grid Solutions s COSI optical CT measures small currents with high resolution and excellent stability. The ring, as shown on the photo above, is not required to be centred on the current carrying conductor because the measurement and the data transmission are carried out optically making the system immune to electromagnetic noise/pickup. A GE Grid Solutions transformer protection relay (P645) with IEC LE Process Bus capability is used to measure the current. The relay is programmed to detect open-phase conditions, including grounded phase and high impedance grounded phase conditions. This relay can be set to be picked up as low as 2 ma with a 2 ma resolution and as fast as 55 ms. 5.2 Data processing algorithm The OPD system is designed to detect the open-phase condition when the transformer is either loaded or unloaded. In the case when the transformer is unloaded, current measured by the relay is typically in the range of 50 ma to 750 ma in which case the response time required can be several seconds. However in the case when the transformer is loaded, current measured by the relay is higher than 1A and the response time needs to be much faster. In order to implement the OPD system to operate in both loaded and unloaded conditions, a dual stream digital signal processing approach has been designed within the same relay. To do so the raw digital data from the COSI current transformer is split into two separate digital streams. One stream is heavily filtered using a frequency tracking comb filter with narrow pass bands located at the fundamental power frequency and several higher harmonics to provide a low noise signal. This allows for the measurement of currents down to 2 ma to be used for when the transformer is unloaded. An input signal from a local conventional voltage transformer is used to provide a frequency reference for the comb filter while redundancy is taken care of in case of a lost or corrupted signal. The second stream is lightly filtered using a weak band pass filter at around 60 Hz and the data from this stream is used for determining the open-phase condition when the transformer is loaded. The objectives of the OPD system were: Test 1 To detect HV side grounded phase condition (phase A, B and C) when the transformer is unloaded. Test 2 To detect healthy condition on the HV side when the transformer is unloaded. Test 3 To detect healthy condition on the HV side when the transformer loaded. Test 4 To detect single phase OPC on HV side (phase C at approximately 120 meters away from the measurement point) when the transformer is unloaded. Test 5 To detect double phase OPC on HV side (phase B and phase C at approximately 120 meters away from the measurement point) when the transformer is unloaded. 8

9 The results of the tests are shown in the table below. Test Transformer Phase A State/Current A Phase B State/Current A Phase C State/Current A Neutral Current A 1 Unloaded Grounded / Grounded / Grounded / Unloaded Healthy / Healthy / Healthy / Loaded Healthy / Healthy / Healthy / Unloaded Healthy / 1.41 Healthy / Opened / Unloaded Healthy / 1.07 Opened / Opened / It is concluded from the above test results that under all conditions the current magnitude is above 2 ma (the accuracy limit of the relay). Also note that the neutral current increases under open-phase condition compared to healthy condition. This is due to the fact that capacitive currents are not cancelled out while under healthy condition the capacitive currents cancel each other out resulting in a small neutral current. 6 CONCLUSION The proposed OPD techniques uses a combination of phase current, sequence components and energy summation to detect and identify an open phase. The requirement for a fast trip is not compromised, as in lightly loaded conditions; the HV side can safely trip after several seconds. The scheme however, is dependent on an accurate measurement of primary phase current down to 5% of transformer rating. This is provided by GE s COSI Optical CTs. Site trials have established that GE Grid Solutions optical fibre measurement technology has excellent potential to improve present power system protection systems and to enable novel protection schemes that will meet the demands of future networks. 7 BIBLIOGRAPHY [1] United States Nuclear Regulatory Commission Advisory Committee on Reactor Safeguards Washington, DC Branch Technical Porition 8-9 on Open Phase Conditions in Electrical Power Systems (ML14057A433, May 2014,1-4 ) 9

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