impact of the transformer’s grounding method on its...

PTCA November 2015 CONFIDENTIAL 1 1PTI Manitoba Inc. April 2017 IEEE Transformers Committee

Impact of the transformer’s grounding method on its transient performance under lighting impulse

Dr. Waldemar ZiomekSenior Expert – Power Transformers and HV Insulation

PTI Manitoba Inc., Winnipeg, Canada


IntroductionThe industry standards for grounding methods (e.g. IEEE Std 142 ) describe the system configurations and impact of grounding method on performance of the system, but do not estimate the impact of the grounding method on the overvoltages inside the neutral-grounded transformer. This study will analyze the influence of specific grounding method, i.e. directly, through reactance or resistance, on the transients developing in a transformer under lightning impulse.The calculation of internal overvoltages inside the transformer can be performed using different methods, however the most popular is a lumped parameter method in which the physical winding is represented by a system of RLC components. An equivalent circuit of the transformer is created by subdividing the winding into a cascade of equivalent RLC circuits. The RLC circuit parameters are calculated considering the ohmic value of resistance between the terminals of each winding, the value of self-inductance of each coil, L, the values of mutual inductances between the coils, M, the capacitance values found along each coil (series capacitances - Cs) and the values of capacitances between adjacent coils and between coils and grounded structural parts of the transformer (capacitances to ground - Cg). An example of such winding model is illustrated in Figure 1, excluding mutual inductances. In this example the winding is divided into n branches or inductive elements, represented in the model by ‘local’ RiLiCi components. By increasing the number of circuits by finer subdivisions, the accuracy can be extended to higher frequency.


Introduction

Figure 1 Transient response of a transformer winding to standard lightning impulse with the neutral directly grounded; a) representation of a transformer winding for transient studies as a ladder of MLRC elements (mutual inductances, Mij, not shown), b) voltage distribution along the winding vs time for a helical winding


Introduction

Figure 2 Representation of one phase of a transformer with windings coupled through capacitances to each other and to ground


Introduction

Figure 3 Example of a three-phase transformer with three circuits; TV is buried, delta-connected with one corner directly grounded; LV and HV windings are wye-connected with directly grounded neutrals

HV SYSTEM

LV SYSTEM

Buried TV


Introduction

Short examination of the model shown in Figure 3 would lead to a conclusion that prediction of such a transformer model’s behavior under a lightning impulse excitation without performing all necessary transient calculations is not possible, especially if one would need to take into account interaction between windings of a phase subjected to the impulse (transferred surge), transients flowing to connected phases, action of non-linear arrestors, if any, impact of the grounding method, etc. Only through generation of a precise model the proper transient characteristic of a transformer can be obtained. Moreover, if a transformer is connected to a specific power system, such interaction between a transformer and system needs to be taken into account, especially the presence and transient characteristics of circuit-breakers, transmission line(s), cables, etc.In all cases described above the winding neutral was grounded directly, hence the voltage in neutral would be zero. Various types of system grounding (ungrounded, directly grounded, grounded through resistance, reactance or ground-fault neutralizer) with a system characteristics are described in IEEE standard. Three grounding types relevant to this study –direct grounding, grounding through resistance or reactance - are presented in Figure 4.


Introduction

Figure 4 Selected cases for transformer grounding and corresponding zero-sequence components of neutral circuit; XG0 – zero sequence impedance of transformer or generator, RN –resistance of a grounding resistor, XN – reactance of a grounding reactor (from IEEE Std 142)


Transient study on a windfarm collector step-up transformer

RatingPower rating: 200 MVACooling type: ONAN/ONAF/ONAFWinding temperature rise: 65oC average/

80oC hot spotLiquid top temperature rise:

65oC

HV rated voltage 345 000 GrdY/ 199 200 VLV rated voltage 34 500 Y/ 19 200 VTV buriedInsulating liquid Mineral oil, Type IILTC in HV neutral for HV variation

MR VM-III-500Y-72.5/C

Terminal Winding BIL [kV]H1, H2, H3 1050

H0 110X1, X2, X3 200

X0 200


Transient study on a windfarm collector step-up transformer

The objective of this study is to evaluate the transient performance of a transformer under LI excitation (LI applied in turn to HV and LV terminals) for three different methods of the neutral grounding: directly, by resistance, or by reactance.

The study was performed on a three-phase model of a transformer, with a TV winding grounded directly in one corner and the HV neutral directly grounded, under lighting impulse for both full wave (FW) at 200kV, 1.2/50 ms and chopped wave (CW) at 220kV and 3.5/50 ms.

As the performance under LV CW impulse at 220 kV is decisive, only these results are presented here.

PTCA November 2015 CONFIDENTIAL 1010PTI Manitoba Inc. April 2017 IEEE Transformers Committee

Directly grounded LV neutral


Resistance grounded LV neutral


Reactance grounded LV neutral


Reactance grounded LV neutral –the voltage across NGR

The voltage across the neutral grounding reactance vs time during 220 kV LI (CW) applied to the LV line terminal – maximum voltage across the reactor is 138kV


Comparison of CW impulse results with different grounding methods

From the above shown results one may see that the magnitudes of overvoltages developing inside a transformer with the neutral grounded by reactance are significantly higher than those achieved with the neutral directly grounded or resistance-grounded. While LV voltage to ground does not change significantly with grounding method, the voltage from TV to ground and to LV is some 35-45% higher when the neutral is reactance-grounded, with worst case for the voltage LV-TV increasing twice for this type of grounding. These results were re-checked by performing RSG tests, which confirmed the higher transient voltages at reactance grounding, but it was noticed that due to damping and air background (vs oil) the differences between resistance/direct vs reactance grounding were around 25-35%.

LV neutral’s grounding method:

LV voltage to gnd[kV]

TV voltage to gnd [kV]

TV to LV voltage [kV]

TV voltage to shield[kV]

Directly grounded 220/-220 220/-200 230/-170 180/-220Resistance grounded 220/-220 230/-200 230/-160 180/-220Reactance grounded 230/-230 255/-270 320/-320 260/-280


Conclusions from a study

The industry standards for grounding methods (e.g. IEEE Std 142) describe the system configurations and impact of grounding method on performance of the system, but do not estimate the impact of the grounding method on the overvoltages inside the neutral-grounded transformer.

Grounding a transformer directly or with a resistor results in lower internal overvoltages, than those appearing when a neutral grounding reactor is used: e.g. based of this study one may state that while the LV voltage to ground does not change significantly with grounding method, the voltage from a TV winding to ground and to the LV winding is some 35-45% higher when the neutral is reactance-grounded, with worst case for the LV-TV voltage increasing twice for this type of grounding.


Conclusions from a study

As a prediction of the transformer and system performance under transients is strictly dependent of their characteristics, specific to given design, location, system configuration, and type of the neutral grounding device, such a study should be performed in the design stage for a specific transformer.

This case demonstrated that the transients developing inside the transformer under LI with the neutral grounding reactor are excessive, would lead to a failure of the transformer. On the other hand, the analysis of the voltage appearing across the neutral grounding reactor showed that this voltage is high, but it could be controlled by higher BIL of the transformer neutral or application of an arrester. Therefore a warning needs to be formed: when using the neutral grounding reactor, the transient voltages need to be thoroughly analyzed not only across the grounding device, but more importantly – inside the transformer to whose neutral this device is connected. This is typically analyzed when the NGR is defined in the specification, but may be a problem when the user of a transformer decides to change the grounding method later, e.g. after a few years in operation.


Proposal for IEEE Std C57.142

The transformer neutral grounding method

Typically, the analysis of the transformer response to a transient excitation is performed with the excited winding’s neutral grounded directly; hence the voltage in the neutral would be equal to zero, if one would ignore the ground impedance. This is a very typical situation for the large power, high voltage transformers, but in many situations the grounding method may be different and would have impact on the development of the oscillatory voltages inside the transformer under transient conditions. Various types of system grounding (i.e. ungrounded, directly grounded, grounded through resistance, reactance or ground-fault neutralizer) with a system characteristics are described in IEEE Standard 142.



The impact of the neutral grounding method on the transient response of a transformer cannot be predicted without performing the transient calculations, nevertheless one may observe the following characteristics of some of these methods:

The direct grounding will ensure that the voltage in the neutral is zero, or near zero – for both steady state and transients (the voltage rise of neutral during the system faults is not discussed here),

The resistive grounding will have a damping effect on the transients – both inside and outside a transformer, as resistance will dissipate the energy of transient current; it is stated in [1] that “a system properly grounded by resistance is not subject to destructive transient overvoltages”;

The grounding by reactance will sustain the transient state and lead to overvoltages as reactance will store and release

the electric energy, We: 𝑊𝑒=1

2𝐿𝑖2

In other words, one may expect that the presence of the reactance in the neutral will promote and uphold the transient overvoltages inside and outside the transformer; as the energy stored in the reactor flows through the system including a transformer – one may expect its possible adverse impact of the transient voltages inside the transformer. The standard [1] addresses the presence of reactance in the neutral only for the system faults and states as follows: “in reactance-grounded system, the available ground-fault current should be at least 25% and preferably 60% of the three-phase fault current to prevent serious overvoltages (X0 ≤ 10X1)”.



As the transformer and system performance under transients is strictly dependent of their characteristics, specific to given design, location, system configuration, etc., there is no universal description covering all possible cases. Some specific theoretical and experimental work related to the methods of transformer grounding can be found in the references [2-4], but a detailed analysis of a specific case is always required to fully understand the response of a given transformer to transients.

Under typical circumstance the neutral grounding method is communicated to the transformer manufacturer and its impact on the transient voltages can be taken into account during design and can be mitigated by developing the enhanced insulation system or application of the internal, or external non-linear resistors.

The situation becomes more complicated if the transformer grounding method is changed later, e.g. by a new owner of the substation, and the transformer manufacturer is not consulted for possible impact of a new grounding method on the oscillatory overvoltages developing inside the transformer under transient excitation. This may result in the excessive internal voltages breaking down the transformer insulation system and leading to a transformer failure.


References

[1] IEEE Std 142 – Recommended Practice for Grounding of Industrial and Commercial Power Systems, 1991 (revised in 2007)[2] C. Tian et al, “Lightning Transient Characteristics of a 500-kV Substation Grounding Grid”, 7th Asia-Pacific International Conference on Lightning, November 1-4, 2011, Chengdu, China[3] M.Saied “Effect of Transformer Sizes and Neutral Treatments on the Electromagnetic Transients in Transformer Substations”, IEEE Transactions On Industry Applications, VOL. 31, NO. 2, MAR-APR 1995[4] J.C.Balda et al, “ Measurements of Neutral Currents and Voltages on a Distribution Feeder”, IEEE Transactions on Power Delivery, Vol. 12, No. 4, October 1997

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