Insulation Coordination Studies for a 132 kV Submarine Cable Interconnection

A Case Study Description Implemented in the Middle East Region

 



 

 


Abstract—The concept of insulation coordination is well known, however, the exact and detailed method performing these studies are not practiced to the same extent as regular power system analysis and studies. A study case is presented in this paper, where power frequency temporary over voltage, switching frequency and lightning over voltage studies [fast and very fast front over voltage studies] are performed strictly in accordance with the IEC standards 60071 parts 1 and 2. The power frequency over voltage studies were performed using standard power system analysis tools such as load flow, short circuit and transient stability studies. The statistical switching and lightning over voltage studies were performed using the EMTP software. The details of the studies are presented in this paper.

Keywords-Insulation coordination, Power Frequency Overvoltage Studies, Switching Frequency Overvoltage studies, Lightning overvoltage studies, Selection of withstand levels, Surge arrester applications.

A.       Introduction

The utility operating company in a middle east country region has been operating the offshore oilfield in island U for over 40 years. Over the years, various installations were upgraded / added to the existing complex consuming significant amount of spare power generation capacity. The facilities in the island U is now facing up-grades for new process installation as the utility envisages various business opportunities in and around its facilities in the island. Consequently, the electrical local load growth demands additional power generation. Therefore, upgrades of the existing power generating system are envisaged to meet these demands.

In relation to the above, the operating utility in the island U intends to meet the additional load demand at the island, by means of providing a sub-sea cable link, of approximately 40 KMs, from D Island power system to the U island power system. In respect of this proposed tie-in various engineering studies, power system studies and insulation coordination studies were performed. This paper outlines the insulation coordination studies performed and presents the summary of the studies.

B.       Description of the system.

The islands of U and D both have gas turbine generators with the island D having the surplus generation. The 132 kV sub-sea cable link is initially planned to operate at 33 kV level and later to be upgraded to 132 kV operating voltage level. The studies were performed considering that the initial operating voltage will be at 33 kV level. Though the proposed sub-sea cable is adequately sized, the initial proposed operating conditions envisaged a maximum export of about 8 MW power from the island D to U, which is lower than the cable capacity.  The single line diagram of the system considered for the analysis is shown in the figure 1.

C.      Insulation Coordination Study

Insulation co-ordination procedure consists of the selection of the highest voltage for the equipment together with a corresponding set of standard rated withstand voltages which characterize the insulation of the equipment needed for the application. The optimization of the selected set of withstand voltages Uw may require reconsideration of some input data and repetition of part of the procedure till satisfactory results are obtained.

The first step in the insulation coordination is the determination of the representative over voltages in the system [Urp] to which the electrical circuit is subjected to , under various operating conditions and switching phenomena.  IEC standard 60071 classifies these over voltages as

a.       Low frequency continuous over voltages. Power Frequency Load flow analysis used to determine these over voltages.

b.       Low frequency temporary over voltages. These are determined by transient stability analysis and unbalanced short circuit studies involving ground faults.

c.        Transient slow front overvoltages. These are determined by statistical switching , line energization studies with or without pre-insertion resistors or other means of over voltage control.

d.      
Text Box:  Figure 1: System SLD

Transient fast front switching overvoltages. These are determined by statistical switching studies, with unbalanced faults, fault removal, and switch reclose with or without fault removal on energized lines.

e.        Transient very fast front lightning over voltages. These are determined from the lightning over voltage studies.

D.      Power Frequency Overvoltage Studies

The objective of load flow studies is to examine the power frequency over voltages in the system under all possible operating conditions. These operating conditions involved no load cases or complete loss of load conditions as well, to assess the extent of over voltages under no load conditions. A list of sample study cases considered for load flow, short circuit and Transient stability studies is as follows

1.       Two generators of island U in operation along with all D island generators at Peak Load

2.       All generators of both islands in operation at peak load, sharing loads

3.       Two U generators in operation at peak load with loss connection between D island and U island.

4.       Two U island generators in operation along with all D island generators at No Load

5.       All D island generators along with two U island generators in operation with reduced output at Peak Load such that there is maximum flow in Sub-Sea cable from island D to U

6.       All U island generators in operation along with all D island generators except one gas turbine at peak Load such that there is maximum flow in Sub-Sea cable from U to D

For each of the network configurations, the short circuit studies were also performed to compute the over voltages on healthy phases during ground fault conditions.  Further transient stability studies were performed for the following disturbance scenarios and events

-          3 Phase faults and fault removal by isolating the faulted circuit , to determine the over voltages , their duration , upon fault removal.

-          Loss of load conditions resulting in over voltages.

E.       Switching Frequency, Fast Front, Very Fast Front Overvoltages

Statistical switching studies were performed using the ATP-EMTP software for the following switching scenarios with and without proposed surge arresters. These studies involved

a.       Line Energization studies with receiving end open ended

b.       Single phase fault, with single phase reclosing after fault clearance

c.        Single phase fault, with single phase reclosing after unsuccessful fault clearance

d.       Single phase fault, with 3 phase reclosing after fault clearance

e.        Single phase fault, with 3 phase reclosing after unsuccessful fault clearance

f.        3 phase fault, with 3 phase reclosing after fault clearance

g.        3 phase fault, with 3 phase reclosing after unsuccessful fault clearance

Apart from the above lightning over voltage studies of a typical 33 kV overhead line was also considered for the insulation coordination calculations to arrive at the conservative values , as the lightning was not applicable for the sub-sea cable system.

F.       Modeling of Sub-sea cable for EMTP studies

In electromagnetic transient simulations there are basically two ways to represent transmission lines/cables:

i.         Lumped parameter models: Nominal and exact PI-models

ii.        Distributed parameter models/traveling wave models: Bergeron and frequency-dependent models

 

Nominal PI-model: The nominal PI-model is one of the simplest representations that can be done of a cable line. It includes the cable's total inductance, capacitance, resistance and conductance (usually not considered) modeled as lumped parameters.

 

Exact PI-model: The exact PI-model, sometimes also called the equivalent pi-model, is a more advanced version of the nominal pi-model that considers the distributed nature of the impedance and admittance. This model is accurate when used in the frequency domain for a single frequency and is normally used to validate other models.

 

Bergeron model: The Bergeron model is a constant-frequency model based on traveling wave theory. The cable is considered to be lossless and its distributed resistance is added as a series lumped resistance. Typically, the model is divided into two sections, it can be divided in more sections, but the differences in the results are minor. This model is a constant-frequency model and its use is only recommended for the cases when only one frequency is considered.

 

Frequency Dependent (FD) models: As the name indicates, FD-models are models that have frequency-dependent cable parameters. When compared with the previous models, the use of the frequency domain increases the results accuracy. In FD-models all calculations are performed in the frequency domain and the solutions converted to time domain by the using transformations such as Fourier-transform or Z-transform.

 

For this study Exact PI model was considered.

G.      Sample Simulation Results and Plots

Text Box:  Figure 2: Voltage profile at one end of the Sub-Sea cable [without Surge Arrester]

Text Box:  Figure 3: Voltage profile at one end of the Sub-Sea cable [with Surge Arrester]

H.      Conclusions/Recommendations

The lightning, switching overvoltage and insulation co-ordination studies were carried out for the 33kV sub-sea cable system. The model representing the 33kV system is carried out as recommended in IEC 60071-2, and in accordance with the next extend as required by EMTP. All studies were based on the relevant international standard, i.e. IEC 60071-2.

References

[1]     IEC 60071-2, "Insulation Co-ordination - Part 2: Application guide", 1996

[2]     IEC TR 60071-4, "Insulation Co-ordination - Part 4: Computational guide of insulation co-ordination and modeling of electrical networks, 2004

[3]     IEC 60099-4, “Surge arresters – Part 4: Metal-oxide surge arresters without gaps for A.C. systems”