An Analysis of a Permissive Overreaching Transfer Trip Scheme at a 120kV Substation

Transcription

1 An Analysis of a Permissive Overreaching Transfer Trip Scheme at a 120kV Substation Russell Louie and Mehdi Etezadi-Amoli Abstract This paper describes a post fault investigation into an undesired operation of a protective device in a permissive overreaching transfer tripping scheme at a 120kV substation. The paper includes a system description, an event analysis, and a proposed solution. Index Terms Permissive Overreaching Transfer Trip (POTT) scheme, Protective relaying, Transmission lines. S I. INTRODUCTION IERRA Pacific Power Company (SPPC) is the main power supplier for Nevada and northeastern California. The Company was founded over 150 years ago and was responsible for delivering power to a rapidly increasing number of silver mines in Virginia City. In 1999, SPPC merged with Nevada Power Company and expanded its coverage area to over a million customers. Until the 1960s, nearly all of the power distributed in northern Nevada was purchased from other suppliers. Presently, SPPC continues to build generation in an attempt to decrease outside dependency. The Reno-Tahoe International Airport is one of the largest power consumers in the Reno area. It is fed by a 120 kv sub transmission system, as shown in Fig. 1. In this figure, significant transmission ties into the rest of SPPCs system at Steamboat, Mira Loma, and N. Valley are shown as equivalent generators. During a major storm, the 127 line tripped at both ends and successfully reclosed automatically. At the same time, the N. Valley 174 line terminal tripped and locked out. With the 174 line out of service, loads at Rusty Spike and Airport are supplied solely from Steamboat and Mira Loma. Due to the significant load at Airport, this creates a strain on the system that would require load tripping during summer peaks. The main objective of this analysis is to understand why the N. Valley relay tripped and to determine the proper settings of the protective scheme to prevent similar undesired operations in the future. R. Louie is a graduate student in electrical engineering at the University of Nevada, Reno (UNR), Reno, NV ( louier@unr.nevada.edu) M. Etezadi-Amoli is a professor of electrical engineering at UNR ( etezadi@unr.edu) II. SYSTEM DESCRIPTION The lines that make up the system in Fig. 1 are protected for line to ground faults by distance relays and ground overcurrent relays. The 127 line is protected by electomechanical phase distance and ground overcurrent relays in a POTT scheme with automatic reclosing at both ends. The 174 line is protected by Schweitzer Engineering Laboratories (SEL) relays using phase distance, ground distance, and ground overcurrent elements in a POTT scheme. The N. Valley terminal uses SEL 321 and SEL 221F relays while the Rusty Spike terminal uses SEL 321 and SEL 311C relays. The 172 and 173 lines are both protected by SEL 321 and SEL 311C relays using phase distance, ground distance, and ground overcurrent elements in a stepped distance scheme. Automatic reclosing is not used because the majority of the 173 line and all of the 172 line are underground. A. Tools for Fault Analysis SEL relays provide two types of reports for analyzing a system whenever there is a fault: Tabulated Sequential Event Repots (SER) and Graphical Event Reports. The SER provides a display of elements that are asserted and deasserted during a fault. It is taken from the relay and relates the time and date to the elements that asserted and deasserted as the fault took place. Fig. 2 shows the SER report for the Rusty Spike substation during the storm. SEL Event Reports were studied to analyze the type of fault, the fault time, the fault currents, and the behavior of the relay elements. Figs. 3 and 4 show the Event Reports for N. Valley and Rusty Spike. A dark line associated with the element name implies asserted status and a light line implies deasserted status. The SEL definition keys are as follows: N. Valley, TRIP and 3PT (3 phase trip) are the trips issued by the relay. KEY and Xmt Perm S (transmit permissive signal) are different relay elements, but essentially the same data, transmitting the permissive signal to the other end of the line. Rcv Perm Sig is the permissive signal received. 52A is the status of the circuit breaker (asserted element is a closed breaker). Rusty Spike, 52A, TRIP, and KEY are the same as described above for the N. Valley relay. TMB1A is essentially the KEY relay element. PT, PTRX, and RMB1A are elements

2 for the permissive signal received from the other end of the line. Steamboat Mira Loma 127 line 173 line Airport 172 line Rusty Spike 174 line N. Valley Fig. 1. System one-line diagram. DATE TIME ELEMENT STATE 12/14 12:19: P1 Asserted 12/14 12:19: PTRX Asserted 12/14 12:19: KEY Asserted 12/14 12:19: P1 Deasserted 12/14 12:19: PTRX Deasserted 12/14 12:19: KEY Deasserted 12/14 12:19: LOP Asserted 12/14 12:19: LOP Deasserted 12/14 15:29: PTRX Asserted 12/14 15:29: KEY Asserted 12/14 15:29: PTRX Deasserted 12/14 15:29: KEY Deasserted 12/14 15:37: KEY Asserted 12/14 15:37: KEY Deasserted 12/14 15:37: PTRX Asserted 12/14 15:37: PTRX Deasserted 12/14 15:37: KEY Asserted 12/14 15:37: OUT102 Asserted 12/14 15:37: OUT103 Asserted 12/14 15:37: PTRX Asserted 12/14 15:37: P1 Asserted 12/14 15:37: P1 Deasserted 12/14 15:37: IN101 Deasserted 12/14 15:37: KEY Deasserted 12/14 15:37: PTRX Deasserted 12/14 15:37: OUT102 Deasserted 12/14 15:37: OUT103 Deasserted 12/14 15:49: IN101 Asserted 12/14 16:06: P1 Asserted 12/14 16:06: PTRX Asserted 12/14 16:06: KEY Asserted 12/14 16:06: P1 Deasserted 12/14 16:06: PTRX Deasserted 12/14 16:06: KEY Deasserted 12/14 16:25: P1 Asserted 12/14 16:25: PTRX Asserted 12/14 16:25: KEY Asserted 12/14 16:25: P1 Deasserted 12/14 16:25: PTRX Deasserted 12/14 16:25: KEY Deasserted 12/14 17:03: KEY Asserted 12/14 17:03: OUT102 Asserted 12/14 17:03: OUT103 Asserted 12/14 17:03: PTRX Asserted 12/14 17:03: KEY Deasserted 12/14 17:03: IN101 Deasserted 12/14 17:03: PTRX Deasserted RSK P Date: 12/17 Time: 07:37: RUSTY SPIKE 174 LINE FIDSEL-311=C-R105-V0-Z00303-D CID=98 Fig. 2. Sequence of Events Report. 12/14 17:03: OUT102 Deasserted 12/14 17:03: OUT103 Deasserted 12/14 17:04: IN101 Asserted 12/14 21:22: KEY Asserted 12/14 21:22: KEY Deasserted 12/14 21:22: KEY Asserted 12/14 21:22: P1 Asserted 12/14 21:22: P1 Deasserted 12/14 21:22: OUT102 Asserted 12/14 21:22: OUT103 Asserted 12/14 21:22: PTRX Asserted 12/14 21:22: KEY Deasserted 12/14 21:22: IN101 Deasserted 12/14 21:22: PTRX Deasserted 12/14 21:22: OUT102 Deasserted 12/14 21:22: OUT103 Deasserted 12/14 21:24: IN101 Asserted 12/15 06:27: KEY Asserted 12/15 06:27: KEY Deasserted 12/15 06:27: PTRX Asserted 12/15 06:27: PTRX Deasserted 12/15 06:29: KEY Asserted 12/15 06:29: KEY Deasserted 12/15 06:29: PTRX Asserted 12/15 06:29: PTRX Deasserted 12/15 06:29: KEY Asserted 12/15 06:29: KEY Deasserted 12/15 06:29: KEY Asserted 12/15 06:29: OUT102 Asserted 12/15 06:29: OUT103 Asserted 12/15 06:29: PTRX Asserted 12/15 06:29: KEY Deasserted 12/15 06:29: IN101 Deasserted 12/15 06:29: PTRX Deasserted 12/15 06:29: OUT102 Deasserted 12/15 06:29: OUT103 Deasserted 12/15 06:38: IN101 Asserted 12/16 00:00: KEY Asserted 12/16 00:00: OUT102 Asserted 12/16 00:00: OUT103 Asserted 12/16 00:00: PTRX Asserted 12/16 00:00: IN101 Deasserted 12/16 00:00: KEY Deasserted 12/16 00:00: PTRX Deasserted 12/16 00:00: OUT102 Deasserted 12/16 00:00: OUT103 Deasserted 12/16 00:17: IN101 Asserted

3 Fig. 3. SEL report of N. Valley. Fig. 4. SEL report of Rusty Spike.

4 A. Event Description Relay targets indicated that a temporary line to ground fault occurred on the 127 line. The relays at Mira Loma and Steamboat successfully tripped and reclosed after the fault cleared. At the same time, the N. Valley 174 terminal tripped while the Rusty Spike terminal remained closed. Because of its trip time, it was initially suspected that the N. Valley relays misoperated due to an instantaneous overreaching element. However, as shown in this paper, this was not the reason for the undesired operation. Understanding the behavior of the relays in this system is a vital part in solving this problem. The electromechanical KD and CEY relays on the 127 line detect ground faults in a zone scheme. In a 3-zone scheme, the first zone covers up to 80% of the line, the second zone covers 120% of the line, and the third zone covers beyond the second zone and into the third line. The timing of these zones is such that the shorter zones trip at shorter times. Also, a communication procedure was implemented into the adjacent relays to coordinate the proper trip sequence. This communication allows the breakers closest to the fault to trip first, minimizing the outage as much as possible. SEL relays work much like the eletromechanical relays with the exception that they are able to help locate faults and read fault currents and provide this information along a 60 Hz time axis. This makes it possible to accurately analyze a particular relay operation. Clearly, the overcurrent relays operate according to fault current characteristics of the line, and are coordinated with the other overcurrent protective elements. III. EVENT ANALYSIS In order to determine why the N. Valley 174 terminal tripped, the following three possibilities were analyzed. a. The fault occurred in the instantaneous protection zone of the N. Valley relay, causing it to trip without delay. b. An underreaching reverse distance element at Rusty Spike prevented the blocking capability of an echo keying scheme. c. An inactive or improperly coordinated reverse reaching ground overcurrent element at Rusty Spike prevented the blocking capability of an echo keying scheme. IV. SOLUTION The first possibility assumes that the phase to ground fault occurred at the closest distance to the Mira Loma substation. With this assumption, N. Valley would see the minimum possible impedance with the maximum amount of fault current. The impedance between Mira Loma and N. Valley was found as follows: Z = Z + Z + Z (1) (Primary)NVR 174Line(pu) 172Line(pu) 173Line(pu) = j j j (2) = ( j0.0392) (3) Therefore: Z p.u. (4) (Primary)N VR = Using the PT (1000:1) and CT (1200:5) ratios and converting all values to ohm we have: 2 (1200 / 5) 120 Z = = Ω (5) (Secondary)NVR The setting of the instantaneous ground distance element (zone 1) at N. Valley was 0.18 Ω. Because the value calculated in equation 5 is well beyond the reach of N. Valley s zone 1 setting, there is no way this element could have caused the undesired trip. After simulating a fault on the Mira Loma bus, it was determined that the N. Valley ground relay would see about 1600 A primary or about 6.7 A secondary. The instantaneous ground overcurrent element at N. Valley was set at 50 A secondary (12000 A primary). With this setting, a fault anywhere on the 127 line would not have been picked up by this element. Because neither of the N. Valley instantaneous elements could have seen the fault on December 14 th, it was concluded that the first possibility could not have happened. The second possibility was that the misoperation was caused by an underreaching reverse distance element at Rusty Spike designed to block the echoing capability of the relay. The N. Valley 174 line has an active POTT scheme in addition to the usual direct tripping elements. As a result, it will trip when its communications scheme trip mask (MTCS) setting is asserted and it also receives a permissive trip from Rusty Spike. MTCS at N. Valley has the following elements: MTCS=M2P+Z2G+51NP Therefore, a fault in the zone 2 phase distance (M2P), zone 2 ground distance (Z2G), or ground overcurrent (51NP) elements will cause the relay to trip if a permissive signal is received. POTT schemes require permission from both terminals of a line to achieve faster trip times for an internal fault. When one of the line terminals is open, its protective elements are unable to detect a fault and cannot send the permission to trip to the other terminal [3]. In order to overcome this, the SEL relays have the ability to echo a received permissive signal. The signal is echoed if the following two conditions are met [3]: 1) Permissive signal must be received for a set amount of time. 2) No reverse fault detected by reverse reaching elements. The trip logic in Fig. 5 shows that in order for a trip to occur, the PTRX (permissive) element and a forward element of zone 2 must be asserted, and the Z3RB (reverse reaching zone 3 elements of other terminal) must be deasserted.

5 For the N. Valley relay, as long as the MTCS is asserted, the permissive signal is sent. Therefore, if the fault is seen by its zone 2 ground distance element (Z2G), the permissive signal is sent to Rusty Spike. If Rusty Spike s reverse reaching zone 3 distance element can not see the fault, it will echo the permissive signal back to N. Valley, allowing it to trip. A simulation of the N. Valley and Rusty Spike ground distance elements is shown in Fig. 6. It can be seen that the reverse reaching element of Rusty Spike easily overreaches the zone 2 element of N. Valley. Therefore, the second possibility could not have happened. The final possibility was that undesired trip was caused by an inactive or improperly coordinated zone 3 reverse reaching ground overcurrent element at Rusty Spike. As described for the second possibility, this zone 3 element is designed to block the echoing of the permissive signal. All overreaching overcurrent and impedance (distance) elements that initiate permission to trip must be coordinated with the remote-end reverse reaching elements to ensure that they do not overreach the reverse elements. It is important to enable and use the same types of relay elements in the reverse directional relay for blocking as are used in the forward directional relay for keying permission [5]. Therefore, if the ground overcurrent element (51NP) of the N. Valley MTCS sees a fault, the reverse reaching ground overcurrent element at Rusty Spike must also see the fault in order to block echoing of the permissive signal. The ground overcurrent element at N. Valley is directional and taken from the 51NP setting. The 51NP element has a setting of 480 A, which covers most of the 127 line. Therefore, MTCS will be asserted for nearly any fault on the 127 line. With MTCS asserted, the permissive signal is sent to Rusty Spike. Rusty Spike requires that the permissive signal be received for at least two cycles before echoing. As shown in Fig. 3, the December 14 th fault lasted for approximately four cycles. With the first condition met, the only thing capable of preventing Rusty Spike from echoing the signal is its zone 3 reverse reaching ground overcurrent element. After reviewing the setting sheets for the Rusty Spike 174 relay, it was determined that the reverse reaching ground overcurrent element (50G3P) was not activated. Because of this, there was no echo blocking for anything picked up by the N. Valley ground overcurrent element (51NP). Since the 50G3P element is used strictly for blocking permissive signal echoing, a very liberal setting can be used to ensure it overreaches the 51NP element of N. Valley. V. CONCLUSION An investigation and analysis into an undesired trip of a protection scheme at a 120 kv substation has been presented. Based on the timing of the relay operation at N. Valley, it was initially assumed that the relay misoperated by overreaching. However, after an extensive analysis, it was determined that this relay operated correctly and that the trip was caused by the remote terminal s inability to correctly block echoing of a permissive trip signal. The use of advanced communication schemes can lead to overlooking critical elements and undesired trips. In this particular case, a commonly unused zone 3 reverse reaching ground overcurrent element was ignored and an unnecessary outage occurred. To correct the undesired operation of any given relay scheme, it is critical to understand the function of the protection scheme, the relays being used, their settings, and the output from fault recording devices. This information will allow correct diagnosis for a given undesired operation and lead to proper settings. It is essential to carefully choose a coordinated protection scheme impervious to system variances. ACKNOWLEDGMENT We like to thank Mohammed R. Hashemi, Jason Gunawardena, and Holly Hoff for their contribution to this project. We also like to thank Gene Henneberg, a consulting engineer at Sierra Pacific Power, for answering our questions and his valuable knowledge and advice. Fig. 5. Trip Logic for SEL 321 [3].

6 NVR174-21G Ty pe=sel 321 MHO4 PTR=1000:1 CTR=240:1 Min I= 5.00A Zone 1: Z=0. 18 s ec Oh 78.1 deg. T= 0.0s Zone 2: Z=0. 44 s ec Oh 78.1 deg. T= 0.60 s Zone 3: Z=0. 20 s ec Oh deg. T= 0.1 0s Zone 4: Z=0. 77 s ec Oh 78.1 deg. T= 0.90 s Line Z= 0. 28@ sec O hm ( O hm) RSK174-21G1 Type=SEL 321 MHO4 PTR=1000:1 CTR=200:1 Min I= 2.50A Zone 1: Z=0. 14 s ec Oh 64.2 deg. T= 0.0s Zone 2: Z=0. 35 s ec Oh 64.2 deg. T= 0.67 s Zone 3: Z=0. 20 s ec Oh deg. T= 0.1 7s Line Z= 0. 23@ sec O hm ( O hm) Fig. 6. Distance settings at N. Valley and Rusty Spike [1]. REFERENCES [1] ASPEN Software, Academic Version, 1999, ASPEN, San Mateo, CA [2] M. Etezadi-Amoli, R.J. Salgo, Protective System Performance Analysis, Proceedings of the 1998 World Automation Congress, pp , May [3] A. Guzman, J. Roberts, K. Zimmerman, Applying the SEL-311 Relay to Permissive Overreaching Transfer Trip (POTT) schemes, SEL Application Guide, Pullman, WA, [4] SEL-311C Instruction Manual, Schweitzer Engineering Labs, Pullman, WA, [5] Application Guide for Echo Keying Logic on Permissive Overreaching Transfer Trip Schemes, WECC Relay Work Group, May 5, Russell Louie received a BSEE in May 2008 from the University of Nevada, Reno. He currently works for Sierra Pacific Power Company and is continuing his graduate studies at the University of Nevada, Reno. Russell was born in Oregon and has lived in Fallon, Nevada for 18 years. Mehdi Etezadi-Amoli received a BSEE in 1970, MSEE in 1972, and Ph.D. degree in 1974 from New Mexico State University. From he worked as an assistant professor of Electrical Engineering at New Mexico State and the University of New Mexico. From he worked as a Senior Protection Engineer at Arizona Public Service Company in Phoenix, AZ. In 1983 he joined the faculty of the Electrical Engineering Department at the University of Nevada, Reno where he is responsible for the power system program. His present interest is in power system protection, large-scale systems, fuzzy control, neural network applications, and renewable energy. Dr. Etezadi is a Registered Professional Engineer in the states of Nevada and New Mexico.