CC.6.3.15.1 Fault Ride Through requirements applicable to Onshore Synchronous Generating Units connected to the Onshore Transmission at Supergrid Voltage

(a) Short circuit faults on the Onshore Transmission System at Supergrid Voltage up to 140ms in duration.

(i) Each Onshore Synchronous Generating Unit shall remain stable and connected to the network without tripping for a close up solid three phase short circuit fault or any balanced fault on the Onshore Transmission System operating at Supergrid Voltage for a total fault clearance time of up to 140ms.

(ii) Following clearance of the fault as detailed in CC.6.3.15.1(i), each Onshore Synchronous Generating Unit is required to remain connected and stable where the post fault voltage profile as measured at the Grid Entry Point is above the heavy orange line defined in the voltage against time curve shown in Figure 1D. The voltage against time profile defines the lower limit of the phase to phase voltage at the Grid Entry Point during a symmetrical fault as a function of time before and after the fault. Appendix 4 provides an explanation and illustration of these requirements.

Figure 1D

(iii) For the avoidance of doubt, the requirements of CC.6.3.15.1(i) and CC.6.3.15.1(ii)) apply only for balanced faults on the Onshore Transmission System operating at Supergrid Voltage lasting up to 140ms in duration. The post fault voltage profile is dependent upon the short circuit fault infeed at the Grid Entry Point.

(iv) NGET shall specify the following parameters in the Bilateral Agreement

a. the pre fault minimum short circuit capacity at the Grid Entry Point in MVA

b. the pre-fault Active and Reactive Power of the Onshore Synchronous Generating Unit at the Grid Entry Point

c. the pre and post fault voltage at the Grid Entry Pointd. the post fault minimum short circuit capacity at the Grid Entry Point in

MVA. (v) For Generators considering connecting to the National Electricity

Transmission System prior to applying for a CUSC Contract, the pre and post fault short circuit data at each Grid Entry Point shall be published in the Electricity Ten Year Statement. For fault ride through assessment purposes the Generator should ensure that each Onshore Synchronous Generating Unit is operating at Rated MW output at 0.95 Power Factor lead.

(vi) In addition to the requirements of CC.6.3.15.1(a)(i),(ii),(iii),(iv)and (v) each Onshore Synchronous Generating Unit shall be designed such that upon clearance of the fault on the Onshore Transmission System as detailed in CC.6.3.15.1(a)(ii) and within 0.5 seconds of restoration of the voltage at the Grid Entry Point to the minimum levels specified in CC.6.1.4, Active Power output shall be restored to at least 90% of the level available immediately before the fault. Once the Active Power output has been restored to the required level, Active Power oscillations shall be acceptable provided that:-

-the total Active Energy delivered during the period of the oscillations is at least that which would have been delivered if the Active Power was constant-the oscillations are adequately damped

During the period of the fault as detailed in CC.6.3.15.1(a)(i) for which the voltage at the Grid Entry Point is outside the limits specified in CC.6.1.4, each Onshore Synchronous Generating Unit shall generate maximum reactive current without exceeding the transient rating limit of the Onshore Synchronous Generating Unit.

(v) In the case of unbalanced faults Generators are required to satisfy the requirements of CC.6.3.15.1(a)(i),(vi) and (v).

(b) Supergrid Voltage dips on the Onshore Transmission System greater than 140ms in duration.

In addition to the requirements in CC.6.3.15.1(a) each Generating Unit shall:

(i) Remain transiently stable and connected to the System without tripping of any Onshore Synchronous Generating Unit for balanced Supergrid Voltage dips and associated durations on the Onshore Transmission System anywhere on or above the orange line shown in Figure 2D. Appendix 4 Figures CC.A.4.A.3 (a), (b) and (c) provide an explanation and illustration of Figure 2D.

Figure 2D

(ii) provide Active Power output at the Grid Entry Point during Supergrid Voltage dips on the Onshore Transmission System as described in Figure 2D at least in proportion to the retained balanced voltage at the Grid Entry Point and shall generate maximum reactive current (where the voltage at the Grid Entry Point is outside the limits specified in CC.6.1.4) without exceeding the transient rating limits of the Onshore Synchronous Generating Unit and

(iii) restore Active Power output following Supergrid Voltage dips on the Onshore Transmission System as described in Figure 2D, within 1 second of restoration of the voltage at the Onshore Grid Entry Point for directly connected Onshore Synchronous Generating Units to the minimum levels specified in CC.6.1.4 to at least 90% of the level available immediately before the occurrence of the dip. Once the Active Power output has been restored to the required level, Active Power oscillations shall be acceptable provided that:

- the total Active Energy delivered during the period of the oscillations is at least that which would have been delivered if the Active Power was constant

- the oscillations are adequately damped.

For the avoidance of doubt a balanced Onshore Transmission System Supergrid Voltage meets the requirements of CC.6.1.5(b) and CC.6.1.6.

(iv) NGET shall specify the following in the Bilateral Agreement a. the short circuit capacity at the Grid Entry Point in MVA b. the pre-fault active and reactive reactive power of the Onshore

Synchronous Generating Unit at the Grid Entry Pointc. For the avoidance the pre and post fault short circuit capacity at the Grid

Entry Point would be expected to be the same.

CC.6.3.15.3 Other Requirements

(i) ……..

(v) Onshore Synchronous Generating Units are required to satisfy the requirements of CC.6.3.15.1(a) and CC.6.3.15(b) unless the protection scheme for internal electrical faults requires the disconnection of the Onshore Synchronous Generating Unit from the Transmission System. The protection schemes and settings shall be agreed with NGET prior to issue of an Energisation Operational Notification (EON) and shall not jeopardise fault ride through performance.

(vi) Notwithstanding the requirements of CC.6.3.15.3 (v) the under voltage protection at the Grid Entry Point shall be set by the Generator to the widest possible range unless otherwise agreed with NGET.

Appendix 4C – Fault Ride Through Requirements for Onshore Synchronous Generating Units

CC.A.4C.1 Scope

The fault ride through requirements for Onshore Synchronous Generating Units are defined in CC.6.3.15.1(a) and CC.6.3.15.1(b). This appendix provides illustrations by way of examples only of CC.6.3.15.1(a) and CC.6.3.15.1(b) and further background and illustrations to CC.6.3.15.1 and is not intended to show all possible permutations.

CC.A.4C.2 Short circuit Faults at Supergrid Voltage on the Onshore Transmission System up to 140ms in duration

For short circuit faults at Supergrid Voltage on the Onshore Transmission System up to 140ms in duration the fault ride through requirement is defined in CC.6.3.15.1(a) and Figure 1D which is reproduced in this Appendix as Figure CC.A.4C.1D and termed the voltage against time curve.

The voltage against time curve is not a voltage – time profile that the Onshore Synchronous Generating Unit would be required to meet to an Onshore Transmission System fault, nor a voltage duration curve where each point on the profile (ie the heavy orange line) represents a voltage level and associated time duration which connected Onshore Synchronous Generating Units must withstand or ride through. Rather the voltage against time curve represents permitted boundary of the post fault voltage profile following clearance of a fault at the Grid Entry Point for a period of up to up to 140ms.

Figure CC.A.4C.2D

The Fault Ride Through requirements are based on a voltage-against-time profile at the Grid Entry Point, which reflects the worst voltage variation during a fault and after its clearance (retained voltage during a fault and post-fault voltage recovery) which is to be withstood. Onshore Synchronous Generating Units are required to stay connected and stable for at close up solid three phase balanced or unbalanced short circuit fault lasting up to 140ms in duration and the post fault voltage response as measured at the Grid Entry Point should remain on or above the heavy orange line shown in Figure CC.A.4C.3D(a).

Figure CC.A.4C.3D(a)

NGET does not require the actual voltage recovery curve to be of the shape of the voltage-against-time-profile. The actual voltage recovery curve will have a free controlled response during the post-disturbance recovery period that will be primarily dependent on the short circuit strength of the Transmission System at the Grid Entry Point and secondly on the behavior of the Generating Unit under post fault conditions. An example of trip condition would be were the post fault voltage profile transits through the heavy orange line is shown in Figure CC.A.4C.3D(b).

Figure CC.A.4C.3D(b)

Figure CC.A.4C.4D shows an equivalent single machine network. Under these conditions, NGET shall provide the pre fault and post fault short circuit level at the Grid

Entry Point and specify the pre fault operating conditions of the Onshore Synchronous Generating Unit.

Insert Fig CC.A.4C.4D – Equivalent network – Example to be included depending upon option agreed by Working Group.

Workgroup discussion – Are further examples and studies required here.

Details relating to compliance and simulation with this requirement are defined in CP.A.3.5.

CC.A.4C.2 Supergrid Voltage dips on the Onshore Transmission System greater than 140ms in Duration

For balanced Supergrid Voltage dips on the Onshore Transmission System having durations greater than 140ms and up to 3 minutes, the fault ride through requirements are defined in CC.6.3.15 (b)(i) and Figure CC.A.4C.2D below which is reproduced in this Appendix as Figure CC.4C.5D and termed the voltage-duration profile.

This profile is not a voltage-time response curve that would be obtained by plotting the transient response at a point on the Onshore Transmission System to a disturbance or the free and uncontrolled post fault voltage profile following a 140ms fault as described in CC6.3.15.1(a). Rather, each point on the profile (ie the heavy orange line) represents a voltage level and an associated time duration which Onshore Synchronous Generating Units must withstand or ride through.

CC.A.4C.5D (a),(b) and (c) illustrate the meaning of the voltage-duration profile for voltage dips having durations greater than 140ms.

Figure CC.A.4C.5D

Figure CC.A.4C.5D (a)

Figure CC.A.4C.5D (b)

Figure CC.A.4C.5D (c)

CP.A.3.5 Fault Ride Through

CP.A.3.5.1 The Generator shall supply a time series simulation study to demonstrate the capability of an Onshore Synchronous Generating Unit to meet the requirements of CC.6.3.15 by submission of a report containing:

(i) a time series simulation study of a 140ms solid three phase short circuit fault applied on the nearest point of the National Electricity Transmission System operating at Supergrid Voltage to the Onshore Synchronous Generating Unit.

(ii) a time series simulation study of 140ms unbalanced short circuit faults applied on the nearest point of the National Electricity Transmission System operating at Supergrid Voltage to the Onshore Synchronous Generating Unit. The unbalanced faults to be simulated are:

1. a phase to phase fault2. a two phase to earth fault3. a single phase to earth fault

For an Onshore Synchronous Generating Unit the simulation study should be completed with the Onshore Synchronous Generating Unit operating at full Active Power and maximum leading Reactive Power. NGET shall specify the minimum pre and post fault short circuit level in the Bilateral Agreement.

(iii) a set of time series simulation studies of balanced Supergrid Voltage dips applied on the nearest point of the National Electricity Transmission System operating at Supergrid Voltage to the Onshore Synchronous Generating Unit. The simulation studies shall include:

1. 33% retained voltage lasting 0.14 seconds2. 50% retained voltage lasting 0.45 seconds3. 85% retained voltage lasting 180 seconds

For an Onshore Synchronous Generating Unit the simulation study should be completed with the Onshore Synchronous Generating Unit operating at full Active Power and maximum leading Reactive Power. NGET will specify the minimum fault level at the Grid Entry Point in the Bilateral Agreement. For the avoidance of doubt, the pre and post fault minimum fault levels at the Grid Entry Point shall be the same.