High Voltage Dry-Type Air-Core Shunt Reactors

K. PAPP* M. R. SHARP D.F. PEELO Trench Austria GmbH Trench Limited Consultant

Austria Canada Canada

SUMMARY Dry-type air-core shunt reactors are now being used more frequently on high voltage power transmis-sion systems to limit overvoltages. Recently, high voltage dry-type air-core shunt reactors have been designed, manufactured and installed directly connected to the transmission systems at voltages up to and including 345 kV. Applications at 500 kV are presently being considered. These ratings require appropriate analysis in terms of switching transient overvoltages and electrical and magnetic clear-ances since dry-type air-core reactors have some saliently unique characteristics as compared to traditionally applied liquid-immersed units. Dry-type air-core reactors have a stray magnetic field that extends beyond the periphery of the equip-ment. Hence the magnetic clearances to surrounding metallic objects and for personnel must be estab-lished. High voltage dry-type air-core reactors are typically made with a modular winding design that allows use of a lower cost partial phase spare unit. This design technique also allows use of shunt connected surge arrester protection of each series connected winding module. High voltage direct connected liquid-immersed iron-core reactors have a higher inherent capacitance value to ground than dry-type air-core devices. Hence the magnitude and/or frequency of the switching transient overvoltages can be significantly higher where dry-type air-core units are employed. The transient overvoltages to be considered are the transient recovery voltage (TRV) at current interruption which stresses the circuit breaker and the reignition overvoltage which stresses the reactor. The TRV is the significant quantity for the circuit breaker or other load break device. This paper discusses these important aspects of dry-type air-core reactors and their ramifications with respect to the application of these devices for shunt connection on high voltage power transmission systems. Further, information concerning reactor protection, calculation methods for magnetic field levels around reactors, guidance regarding specification and type testing and appropriate methods to mitigate TRV frequency and magnitude, where necessary, are provided. KEYWORDS Air-Core Dry-Type Shunt Reactor - Magnetic Field - Shunt Reactor Switching Overvoltage * klaus.papp@trench-group.com

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INTRODUCTION Shunt reactors are used in power systems for voltage control. They may be connected either to the tertiary winding of a power transformer, or directly to the station busbar, or anywhere along a transmission line. In the two latter cases these reactors were usually of liquid-immersed, iron-core design. Reactive shunt compensation is one of the most common applications of dry-type, air-core reactors. Dry-type air-core shunt reactors have been employed for over 50 years at voltages up to 69 kV. Figure 1 illustrates the design concept of a dry-type, air-core reactor.

1) winding 2) winding conductor 3) spider 4) terminal 5) duct stick 6) protective roof/shed 7) base insulator 8) mounting bracket

Figure 1: Air-core dry-type reactor A dry-type air-core reactor consists of a cylindrical winding (1) made of one or several concentric layers of film/glass tape insulated aluminum conductor (2). Aluminum is preferred versus copper be-cause of its lower specific weight and the lower investment cost of the winding at a comparable level of losses. All layers are electrically connected in parallel by welding their top and bottom ends to me-tallic cross arms, commonly referred to as spiders (3). Each spider carries a terminal (4) for electrical connection of the reactor. The individual layers are configured such that radial voltage stress is virtu-ally nil and the remaining axial voltage stress results in surface stress values and turn-to-turn steady state operating voltages that are well below specific limits derived from field experience. All layers are radially spaced by several glass fiber sticks (5) which form air ducts necessary for the cooling of the winding. Cooling is provided by natural convection of ambient air, which enters at the bottom end of the winding and exits at its top end. The winding is impregnated/encapsulated by epoxy resin resulting (after curing) in a mechanically strong and compact unit. Finally, protective coating is applied to the winding to improve track- and weather resistance. In case of adverse pollution conditions at the site of installation, shunt reactors for sub-transmission and transmission systems may be equipped with a protective roof/shed (6). The reactor is mounted on several base insulators (7) and mounting brackets (8). The rating of the insulators depends on the specific system requirements at the site of installation. Dry-type shunt reactors are usually of single phase design and three wye-connected units are mounted side-by-side forming a three-phase shunt reactor bank with the neutral earthed or unearthed depending on system requirements.

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A dry-type shunt reactor is not enclosed in a grounded steel tank and all parts of the reactor must be considered to be live. Therefore, dry-type shunt reactors must be installed such that accidental contact by station personnel is not possible. Two methods that can be used to achieve this are fencing, and elevating the reactor at a safe distance above ground by selecting the length of the mounting brackets accordingly.

One of the major advantages of air vs. liquid-insulated shunt reactors is the simplicity of the insulation between the phase reactors and to ground which is provided by the surrounding air and by the support insulators on which the equipment is resting. Assuming adequate striking distances, faults external to the reactors are very unlikely to occur. Therefore the widely used differential relay protection for liquid-insulated shunt reactors is not very meaningful for dry-type units. As for all shunt reactors the detection of turn-to-turn failures is very difficult and early failure detection is sometimes nearly impossible. Best practice of turn-to-turn failure protection is to monitor the neutral through a voltage transformer if the neutral is ungrounded or a current transformer for shunt reactor banks with the neutral grounded. Guidance for shunt reactor relay protection is found for example in [1].

MAGNETIC FIELD CONSIDERATIONS As the name implies, air-core reactors do not have an iron core and therefore the magnetic field is not constrained and will fringe out from the winding ends and will occupy the space around the reactor winding. The strength of this stray field depends on the unit power rating of the reactor, the higher the rating, the higher the magnetic field level. Figure 2 shows the magnetic field plot of an air-core reactor. The selected example is one phase of a 69 kV, 50 Mvar (50 Hz) wye connected 3-phase reactor bank.

Figure 2: Magnetic Field Plot

Reactor data: reactance (50 Hz) ............... 95.2 Ohm inductance .......................... 303 mH winding length .................... 2.7 m mean winding diameter ...... 2.56 m turns number ...................... 450 current ................................ 418 A(rms) The winding is cylindrically shaped so that the magnetic field generated by the winding possesses rotational symmetry and is, in addition, symmetrical to the winding midplane.

The field strength quickly drops off with increasing distance from the reactor, since a solenoid has a magnetic dipole characteristic, so that the field decays with the third power of distance. The presence of the magnetic field must be taken into consideration in dry-type shunt reactor installations for two aspects:

a) To ensure that the magnetic field of the reactor does not induce undue currents in nearby metallic geometries (station earthing grids or rebar of the concrete platform on which the reactor is mounted, etc.) to avoid overheating and undue losses. There are some simple rules of thumb that can be em-ployed. Clearance to small metallic parts not forming closed loops should be at least one-half the coil diameter radially from the edges of the reactor. Larger geometries or closed loops should be located at least one coil diameter from all the surfaces of the reactor. The reactor manufacturer can provide

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guidelines about magnetic clearances as well as recommendations for the design of foundations and nearby station structures.

b) Maintaining the limits for exposure of the station personnel to magnetic fields. Guidelines on hu-man-exposure limits to magnetic fields are proposed by several national and international organiza-tions. The ICNIRP guidelines [2] for human exposure limits to electric and magnetic fields have gained widespread support in the global electric power industry. The latest version of these guidelines recommends a reference level for 50 Hz or 60 Hz magnetic fields of 1 mT, for occupational exposure. The reference levels are expressed in terms of the effective field strength, that is the root of the sum of the squares of the rms field components along three orthogonal axes. Biot-Savart's law may be applied to calculate the magnetic field of a cylindrical winding [3]. The external magnetic field of a dry-type air-core reactor winding at a greater distance from the winding may be approximated by the field of a current loop as shown in Figure 3. This approximation holds for coils having a winding length shorter than about three times the winding diameter. The field produced by a current carrying winding loop in a distance r of more than around three times the loop diameter may be approximated according to [3] by the equations (1) and (2). (1) (2)

|B| ........ magnitude of the magnetic field 0 ...... permeability in air (0 = 4S.E-7 H/m) n ........ turns number I ......... current D ....... loop diameter (= mean winding diameter) r, 4 ..... coordinates as per Figure 3 f(4) ..... directivity function as per equation (2)

Figure 3: loop, equivalent to a reactor winding

Using (1) and (2) in lateral direction (4 = 0, f(4) = 0.5) the magnitude of the magnetic field at greater distances may be estimated by

(3) For the shunt reactor winding as per Figure 2, the required minimum lateral distance from the coil axis estimated by equation (3) is 4.6 m. This result is less than 1% off from a full analytical calculation by [3]. For shunt reactor banks consisting of three phase reactors the magnetic clearance gained by single phase calculation is even higher than that found by the superposition of the magnetic fields of all three reactors assuming that all three coils do have the same sense of magnetization. HV APPLICATION In the past dry-type air-core shunt reactors have been applied at distribution class voltage levels. Typi-cally they have been connected to the tertiary winding of a power transformer. Common ratings have been in the range of 10 to 50 Mvar per phase with reactor currents greater than 1000 A and reactances in the 3 to 50 ohm range. Recently, however, due to the advances made in materials, and application of novel design techniques, high voltage dry-type air-core shunt reactors have been designed, manufactured and installed, directly connected to the transmission systems at voltages up to and including 345 kV. One factor driving this demand increase is the integration of renewable generation such as wind parks to the grid. The con-

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necting overhead lines or cables require shunt compensation with ratings for reactive compensation in the range of some tens of Mvars. Usually such wind parks are located in environmentally sensitive areas thereby favoring dry-type air-core technology. Because these reactors have a low investment cost, require minimal maintenance, there is no oil spill or fire hazard, installation is fast and protection is simple, they are well suited for remote installations at any point along the power transmission lines. Because dry-type air-core reactors are usually custom designed for each specification, the size, shape and layout of reactor modules can be modified to suit the particular requirements of each particular installation For such transmission class shunt reactors, current ratings are often less than 100 A and reactance values can be in excess of 5000 ohms. These ratings require appropriate analysis in terms of steady state voltages and switching transient overvoltages. The main design parameter for dry-type shunt reactors employed at sub-transmission or transmission systems is the steady state voltage drop along the surface of the reactor. The length of the winding is chosen to provide abundant creepage distance along the winding surface to cope with the continuous voltage stress, considering the pollution condi-tions at the site of installation. Shunt reactors connected to 110 kV systems are designed for a winding length of about 3.5 m which translates into a reactor height (over spiders) of about 4 m. This dimension corresponds with a typical maximum height for road transportation on a truck. Dry-type shunt reactor windings of this length will have a high safety margin to withstand the transient and dynamic overvoltages associated with this voltage class. This feature is of particular importance for shunt reactors subjected to frequent switching. For system voltages of around 230 kV two single units, stacked one above the other and connected in series are required to keep the winding voltage stresses within acceptable limits. For even higher volt-ages more than two series connected winding modules are required which is achieved by a separately side-by-side mounted reactor stack connected electrically via cables or buswork. Standard phase to phase and phase to ground electrical clearances used by the utility are applicable for theses reactors. Figure 4 shows a 345 kV, 20 Mvar, 60 Hz, shunt reactor bank. Each phase consists of totally 4 series connected winding modules each having a winding height of 3.55 m. The space requirement for this 345 kV dry-type air core shunt reactor bank is around 17 m x 12 m. However, because the reactors are typically custom designed, the coil dimensions and coil orientation can be adjusted to suit site conditions. Figure 4: 345 kV, 20 Mvar (60 Hz) 3-phase shunt reactor.

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The transient voltage distribution along a chain of series connected reactors is non-linear and it is gov-erned by the capacitances of the individual winding modules between turns and to ground. Figure 5 shows a simplified electrical schematic of one phase of the 345 kV example reactor.

Figure 5: simplified electrical schematic. Both, the capacitance between turns -and to ground are represented by lumped capacitors, the ratio of which is crucial for the transient voltage along the reactors. The voltage nonlinearity factor is ex-pressed by the square root of the capacitance to ground vs. the capacitance between terminals. The maximum peak transient overvoltage across reactor 1 in this example was found to be around 75 % of the total peak voltage of 1550 kV applied at the HV terminal. This is about three times the voltage at linear distribution, i.e. the nonlinearity factor is around three which is typical for a HV shunt reactor made up by four single winding modules. A high voltage dry-type air-core reactor made with a modular winding design allows use of shunt con-nected surge arrester protection of each series connected winding module. The arresters may be mounted inside the winding. Care must be taken to select arresters having metallic fittings of limited size made of non-ferrous steel or aluminum to avoid undue heating by the magnetic field of the wind-ing. The arresters will clamp the transient voltage of the winding modules effectively protecting the shunt reactor and will also limit the magnitude of reignition overvoltages. Further, the capacitance of the surge arresters will add to the capacitance between the terminals of each winding module and thus will lower the nonlinearity of the transient voltage distribution of the HV shunt reactor. TESTING OF HIGH VOLTAGE SHUNT REACTORS Recommendations for acceptance testing of dry-type air core shunt reactors is provided in the inter-national standards, IEC 60076-6, 2007 Power transformers- Part6: Reactors, and IEEE C57.21, 2008 IEEE Standard Requirements, Terminology, and Test Code for Shunt Reactors Rated Over 500 kVA as well as many national standards. Both IEC and IEEE provide specific testing recommendations for both oil immersed and dry-type air-core shunt reactors. For impulse testing of high voltage dry-type shunt reactors consisting of more than one reactor module it is recommended to fully assemble the reactor with all modules connected in series but with the surge arresters removed. The full test voltage shall be applied to the HV terminal while the neutral terminal is earthed. SHUNT REACTOR SWITCHING: GENERAL CONSIDERATIONS Shunt reactor switching represents a unique interactive and severe duty for circuit breakers. The duty is not standardized because shunt ratings are application dependent and users should select circuit breakers according to the application. Guidance in this regard is discussed later.

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The general circuit for the first-pole-to-clear is shown in Figure 6. The transient recovery voltage (TRV) imposed on the circuit breaker is the difference between the source voltage and the load side oscillation. The load side circuit and oscillation is characterized as a series RLC circuit with a precharged capacitor [4]. The capacitance CL is initially charged to the voltage at the shunt reactor at current interruption, which then rings down to zero at a frequency given by L and CL. The peak of the TRV occurs when the load side oscillation peak is of opposite polarity to the source voltage.

sV : Source voltage peak value ppk : First-pole-to-clear factor

L : Shunt reactor phase inductance K : Neutral shift equal to 1ppk

LC : Load side capacitance Figure 6: Shunt reactor switching general case:

circuit for first-pole-to-clear.

The TRV trvv has a cosine1 waveshape and is given by:

(4) where ka is the suppression peak overvoltage, ds is the degree of damping in the circuit and tg is ge-neric time based on periods of the undamped oscillation [4]. The application dependency and interac-tive nature of switching can be understood by considering the variables other than the source voltage: x kpp = 1+K is dependent on the shunt reactor neutral grounding arrangement, being 1 pu, 1.5 pu or

1.2 to 1.35 pu when the neutral is directly grounded, ungrounded (isolated) or grounded through a neutral reactor, respectively.

x ka is dependent on the circuit breaker arc voltage and its ability to chop current. For SF6 circuit breakers, ka increases with arcing time and the TRV becomes more and more onerous at succes-sive current zero crossings.

x ds is the degree of damping in the load and is given by ds = R/Rc where R is the actual resistance in the load circuit and Rc is the resistance value that would give critical damping. For shunt reac-tor circuits, R is generally very low and ds

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To illustrate the use of the above equation, consider a dry-type shunt reactor rated at 30 kV, 100 Mvar, 50 Hz in the three configurations below:

Configuration ppk sd 1 pu gt Ps

Oscillation frequency kHz

Neutral grounded 1 0.033 2.67 59.6 Neutral ungrounded 1.5 0.033 3.27 48.7 Neutral grounded and RC damper : 20R and

F 15.0 C

1 0.023 65.55 2.43

Assuming an ideal circuit breaker (ka = 1), the calculated TRVs are shown in Figure 7. With the re-actor neutral grounded, the TRV has a peak value of 45 kV at a frequency of 59.6 kHz as shown in Figure 7(a). The effect of having the neutral ungrounded is shown in the same figure, i.e. the peak value increases by 1 pu voltage with a decrease in the frequency. The reason for the large increase in the peak value is due to axis of oscillation of the load transient now being the shifted neutral [4]. The addition of the RC damper line to ground with the neutral grounded has a major effect on the fre-quency with the peak value virtually unchanged (Figure 7(b)). The large decrease in the rate-of-rise of the recovery voltage (RRRV) is beneficial for the circuit breaker regardless of type (low probability of reignition) and for the reactor (lower turn-to-turn stresses).

Figure 7: TRVs for switching of 30 kV, 100 Mvar, 50 Hz dry-type reactor.

An actual measurement for the neutral grounded case with the RC damper is shown in Figure 8. The trace shows that the load side oscillation is very underdamped with an amplitude factor of 1.9 or greater. The peak value of the TRV for the circuit breaker will therefore approach 2 pu voltage as indicated in Figure 7(a).

Figure 8: Load side oscillation on switching out 30 kV, 100 Mvar shunt reactor with RC damper.

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The selection of circuit breakers for shunt reactor switching requires special attention. For applications at 100 kV and above, where the load current is usually less than 500 A, type testing is carried out at a minimum current of 100 A [5]. The intent of the test is not to demonstrate interrupting capability but rather to derive the chopping characteristic of the circuit breaker which can then be used to calculate performance in the field as described in [6]. If the actual load current is less than 100 A, then a type test should be performed at that current. For applications at load currents greater than 1000 A, current interrupting capability for the expected TRVs is the issue [5, 6]. For this reason circuit breakers should not necessarily be selected on the basis of the shunt reactor rated voltage, i.e. a circuit breaker with a higher rated voltage may be required to meet the TRV requirement. At current interruption the circuit breaker is stressed by the TRV and the shunt reactor by the load side transient both to ground by ka pu and across the winding by (ka + K) pu. The frequency of both transi-ents depend on the load circuit inductance and capacitance (Figure 6). For dry-type reactors, the value of CL is five or more times lower than that for oil-filled reactors and the above frequency is two or more times higher. The remedy for this, if necessary for the selection of the circuit breaker, is to add capacitance as was done for the 30 kV, 100 Mvar reactor discussed earlier. In the event of a reignition in the circuit breaker, a perfectly normal event, the load side voltage recov-ers to the source voltage but overshoots producing a reignition overvoltage. This overvoltage is a char-acteristic of the circuit involving the source and load side capacitances and the inductance of the asso-ciated circuit loop [6]. The exposure of the shunt reactor to this overvoltage is limited in magnitude by the reactor surge arrester and its frequency, usually hundreds of kHz, is not considered a design issue. BIBLIOGRAPHY [1] IEEE Std C37.109-2006: IEEE Guide for the Protection of Shunt Reactors [2] International Commission on non-ionizing Radiation Protection ICNIRP: Guidelines for

Limiting Exposure to Time-Varying Electric and Magnetic Fields (1 Hz - 100 kHz). Health Physics 99(6):818-836; 2010.

[3] W.R. Smythe, Static and Dynamic Electricity, McGraw-Hill, 1968 [4] D.F. Peelo, Current Interruption Transients Calculation. John Wiley & Sons Ltd., 2014. [5] IEC 62271-110 High-voltage switchgear and controlgear Part 110: Inductive load switching. [6] IEC 62271-306 High-voltage switchgear and controlgear Part 306: Guide to IEC 62271-100, IEC 62271-1 and other IEC standards related to alternating current circuit-breakers.