Transformer Open Phase Detection - Russell Patterson

1 1 Abstract detecting an open primary Phase on a grounded-wye Transformer is difficult when the Transformer is unloaded. This paper describes the problem and presents an unconventional method that allows Detection of open primary phases using conventional protection class current transformers and readily available digital relays. Index Terms open Phase , current Transformer , offsite power. I. INTRODUCTION HIS paper describes a problem where an open primary Phase on a power Transformer at a generating station may go undetected. An innovative solution to this problem using conventional protection class current transformers (CTs) and digital protective relays is presented. This problem has been amplified due to its occurrence on auxiliary transformers used for offsite power at nuclear power stations.

2 II. THE OPEN Phase PROBLEM A. Regenerated Voltage When one primary Phase is opened to an unloaded or lightly loaded grounded-wye Transformer the voltage on the opened Phase on the Transformer can be regenerated and present as normal to voltage balance protection. This regeneration is affected by the secondary winding connections and core construction of the Transformer [1, 2]. When the Transformer is sufficiently loaded it is possible to detect this degraded condition using voltage balance or current balance relaying. The case of concern is when the Transformer is unloaded or lightly loaded and the primary terminal voltages appear healthy even though one Phase is open as shown in Fig. 1. Fig. 1. Simplified generating station one-line When this condition exists the Transformer is not ready for service and is unable to supply healthy power to the auxiliary R.

3 W. Patterson is owner of Patterson Power Engineers, Chattanooga, TN 37402 USA (e-mail: loads. Furthermore, there is no indication the Transformer is in this condition. B. Offsite Power Supply Generating stations are typically designed as shown in Fig. 2 with an alternate source arrangement to serve auxiliary loads when the normal source of auxiliary power is not available. For increased security this alternate source may be served via a path that is independent of the primary transmission path and will generally be referred to as offsite power. In the normal configuration the auxiliary loads are being supplied by the unit auxiliary Transformer (UAT). When the normal supply is degraded ( due to a fault) the auxiliary loads are Fig. 2. Simplified generating station one-line transferred to the offsite supply so that adequate, healthy power is supplied to the various auxiliary loads required to support the generation process (fans, pumps etc.))

4 In many cases the station auxiliary Transformer (SAT) is unloaded or only lightly loaded until the auxiliary loads are transferred to it. In these cases the open Phase condition may exist without having been detected resulting in an unhealthy alternate source of power. When the transfer occurs the open Phase condition manifests itself as negative sequence current to the connected auxiliary motor loads and as undervoltage on the auxiliary bus. Loads then trip by protective relay action. This is a very serious defect for a nuclear plant as reactor coolant pumps and other critical loads are necessary for safe control and shutdown of the nuclear process. Many examples have been given of this problem resulting in starting of the diesel generators as a last ditch supply of shutdown power (diesels start after the auxiliary supplies fail).

5 III. Detection A. Scheme Description The solution described in this paper takes advantage of the fact that open circuiting a CT that has current flowing in Transformer Open- Phase Detection R. W. Patterson , Senior Member, IEEE T 2 its primary winding will produce a voltage spike on the CT s secondary. As previously mentioned, when the power Transformer is sufficiently loaded there are other preferable methods for detecting the open- Phase condition. The method described here is beneficial for use when the Transformer is unloaded or lightly loaded and overcurrent Detection isn t feasible. A typical excitation current for an unloaded power Transformer is in the range of 1-3% of rated current with banks constructed of grain oriented steel being as low as of rated. The scheme is shown in Fig. 3 and describes as follows.

6 A set of wye-grounded CTs from the terminals of the protected power Transformer are wired to a digital relay through the relays current inputs and then through normally open contacts that are in parallel with the relays voltage inputs. When the power Transformer is adequately loaded such that the secondary current flowing through the relays current elements can be dependably detected then no special action is required to detect the open Phase as simple undercurrent or negative sequence overcurrent will suffice. However, when the current is too low to measure with the relays current elements the CT open circuit scheme can be used. The scheme works by opening the normally closed contacts and detecting the spike in voltage that should occur if the CT in that Phase has primary current flowing. If the primary Phase current is absent then no spike in voltage will Fig.

7 3. Open- Phase Detection scheme be detected. The normally closed contact need only be opened for a brief pulse and then closed back. The voltage relay being used should be capable of operating quickly on instantaneous voltage. There are modern high-impedance bus differential relays available with this characteristic as well as the needed current elements. The scheme can either be manually initiated or automatically occur on some pre-determined schedule. B. Scheme deficiencies Care should be taken when applying the scheme so as to ensure dependable operation. For example, the configuration in Fig. 4 may fool the scheme due to the parasitic current drawn by the connected capacitive load ( CCVTs). Fig. 4. Capacitive loading may defeat scheme This effect is more significant at higher voltages such as 500kV and may be mitigated by judicious location of the CTs.

8 Susceptibility to this deficiency can easily be evaluated based on the specifics of the connected equipment. A further concern might be that the opening of the CT secondary may occur near a current zero and will not produce a satisfactory kick. This can be mitigated by operating the contact several times, appropriately spaced and at staggered intervals. C. Testing To date, only a small C10, 50:5 window CT has been bench tested with approximately 170mA of primary current. The resultant secondary voltage waveform upon open circuiting the secondary is shown in Fig. 5. This is not representative of the CTs that will be available in actual installations but is useful for proof of concept. Note that the magnitude and duration of the spike will depend on the actual circuit parameters and a fast operating relay is required.

9 The filtering used in the relay will have a direct impact on operating speed and bench testing candidate relays will be necessary. Fig. 5. Secondary voltage for open-circuited C10, 50:5 CT with 170mA primary current A set of C800, 600:5 bushing CTs has been ordered for testing and will be more representative of what will be found in field installations. The results of this testing will then be used to fine tune the scheme. 3 IV. REFERENCES [1] J. Horak, G. F. Johnson, A Practical Guide for Detecting Single-Phasing on a Three- Phase Power system , Western Protective Relay Conference, Oct, 2002. [2] Analysis of Station Auxiliary Transformer Response to Open Phase Conditions , EPRI, Palo Alto, CA: 2012. 1025772 V. BIOGRAPHIES Russell W. Patterson (SM 2002) received his Bachelors of Science in Electrical Engineering degree in 1991 from Mississippi State University in Starkville, MS and his Masters of Science in 2013 from the University of Tennessee in Chattanooga, TN.

10 He began his career as a field test engineer for TVA and has over twenty years experience in utility generator and transmission protection. Russ managed the system protection department for TVA until his retirement in 2008 to enter full time consulting. Russell owns Patterson Power Engineers, a consulting firm headquartered in Chattanooga, TN with eight full time protection engineers. He is a member of the IEEE Power system Relaying Committee where he is chairman of the Line Protection Subcommittee and a member of the Rotating Machinery Subcommittee. He is a senior member of IEEE, a member of CIGR , and a registered professional engineer in multiple states and with NCEES. He is also an adjunct lecturer at the University of Tennessee and at Georgia Tech where he teaches graduate classes and short courses on power system protection.