The scope of this document is to provide guidance as to how to extract the relevant information from a grid code document and a connection agreement, which will drive the electrical section of the Pegasus configuration. This document links the extracted information to Pegasus, so that with the grid code, connection agreement information and customer filled-out check lists etc. the Pegasus configuration can be made.

A glossary is provided at the end of this document to define many of the industry specific terminology and abbreviations.  Terms located in the glossary are indicated by this style of formatting.

Once the configuration is made, this document provides a guideline for how to evaluate the gathered information. The evaluation aims at determining whether the project has a size or a complexity calling for expert review. Secondly the guideline provides a way to evaluate whether the configuration is likely to be correct.

For the purpose of this document a simplified schematic is shown below.

                                                                              Electrical schematics

The drawing above has a turbine to the far right, a LV bus bar with a switch; all bus bars have switches, a turbine transformer and a MV bus bar. This is what we normally sell as the turbine, some times with a scope split. The collector system is the cabling interconnecting one or more turbines to the sub station. Again the sub station has a MV bus bar on the right side of the sub station transformer and a HV bus bar on the left side of the sub station transformer. The sub station has the Point of Interconnection or POI either on the left side of the transformer or the right side of the transformer.

Part 1: How to read a grid code & a connection agreement.

There is normally always a need for reading these two documents, all though the customer might already have decided what he needs, indeed he might have additional requirements or requirements contradicting our findings in these documents, in which case, we will need to resolve this with the customer.

The grid code is a generic document specifying the requirements to all types of electrical energy generation in a non-project specific manner. The project specific information is written into the connection agreement, which may or may not stay within the frame of the grid code, secondly derogations might be given to the grid code, which is then written into the connection agreement. The connection agreement defines amongst other things POI and voltage level. Just as we might need to resolve contradictions between customer requirements and our own findings, we will also some times need to resolve contradictions between the grid code and the connection agreement.

1 Identifying Grid Code sections relevant for Wind

Starting with the grid code, the first step is to identify and eliminate the sections of the code document not pertaining to wind. Some European utilities have separate grid codes for wind, which makes this easy, others separate in other ways:

• Sections clearly marked as      pertaining to wind

• Generation type marked as wind      generation

• Thermal, gas, steam, hydro and      nuclear type of generation is not wind

• Asynchronous or induction type      generation with or without converters is wind, (GE’s 1.5 model family is      an example of this.

• Synchronous type generation      with or without converters is wind, (GE’s 2.x, model family is an example      of this.

• Dispatchable generation is      typically not wind. Dispatchable generation means a type of      generation, which you can start and stop at will via remote control, in      some grid codes it is still assumed that wind cannot be curtailed even      though we can do that.

The next step is then to try to identify the boundaries of the plant. Some requirements pertain to the interface between the wind park sub station and the grid; other requirements pertain to the generator terminals either LV or MV.

1.1 Sub station interface

Defining all or most of the requirements in this interface is getting to be the norm. Secondly this interface is normally designated the POI, the point of interconnection. In most cases the sub station is outside our scope, thus we can identify the sections not relevant to us:

Sections pertaining to the number of circuit breakers, the protection integrated in these circuit breakers and the general design are not our scope.

Transformer design is also not our scope, however we need to take notice of the on-load tab changer if present.

• Voltage level and range,      standard is +- 10 %, we have some under voltage capability, but almost no      over voltage capability, so take notice of that and check the latest ERDB      turbine documents.

• Frequency level and range, for      example for 50 Hz turbines, standard is 47,5 to 51,5 Hz with      some over and under frequency capability. Check the latest ERDB turbine      documents.

• Active power flow / power curtailment      means a WFMS

• Reactive power flow / Power factor      control means WFMS. Power factor = 1 will typically mean 0,95 generators,      power factor = 0,95 means 0,90 power factor generators, power factor below      0,95 in this interface means capacitor banks, go call an expert.

1.1.1 LVRT requirements

LVRT requirements are handled in different ways. The requirements are found in sections pertaining to non-standard grid incidents, non-standard grid voltages, and handling of grid faults or similar.  Some requirements are defined in the sub station interface; others are defined in other interfaces. Our equipment’s ability to comply to a set of requirements is if the retained voltage is higher than our lower operating limit and the duration is shorter than our maximum operating time duration for any given package. Secondly if we have a margin on the voltage, we handle longer time duration. The point here is that it is current over time, which is the determining factor, with a higher retained voltage, the current is lower and that we can handle longer time duration.   This situation is to be handled by experts only.

In the near future this capability is to be expanded with the ZVRT package, which means that the retained voltage is zero and that the duration of zero voltage is 200 ms max. The capability is not fully defined yet. The drivers for this are US project being commissioned after end of 2007.

1.1.2 Line drop compensation

This function is included in the WFMS and is needed if and only if the point where we measure is not the same is the one in which we need to control. The distance between the two locations and the cable and transformers between are needed to let the control handle this situation and achieve the correct control of power factor, voltage etc in the control point.

1.2 Interfaces likely to be found on the single turbine installation

The main items that change if the interface moves to the turbine, LV, or the turbine transformer (MV) are:

• Reactive Power: The requirement corresponds      directly to the generator capability

• Active Power flow / Power curtailment:      Can be implemented in the turbine, does not necessarily require WFMS

• Protection equipment and circuit      breaker capabilities: We have specifications and description      documents covering these, they will have to be compared with the      requirements

• Transformer design and      location:  We have specifications      and description documents covering these, they will have to be compared      with the requirements

There are other sections of the grid code related to command & control and to signals & communication. Since these sections are for the WFMS / SCADA specialists to handle, they are not covered here.

2 The Grid connection agreement

The grid connection agreement may or may not stay within the frame set by the grid code, thus contradictions do happen, in most cases the connection agreement take precedence so read both documents and ask the costumer to clarify with the utility company. The agreement will handle local project specific issues should they give rise to requirements different from the requirements in the grid code, so the grid code might state that certain things are handled in the connection agreement or it might state the requirements directly. The connection agreement in North America is called LGIA (Large Generator Interconnection Agreement) Secondly quite often this agreement is supplemented by an internal GE Wind document called the grid checklist. If the customer has filled it out completely and it gets to be part of the contract, there is normally no need to check the LGIA. Checklists are not as common in Europe as in North America.  In Europe and Asia poles the Applications Engineer may consider filling out the Grid Checklist on behalf of the customer.

Noteworthy issues, which might not be handled in the grid code, are:

• Local grid disconnections: we      rarely see this, however if we do, it will decide our choice of gearbox      in Pegasus.  We will need to      specify a more robust gearbox to handle a larger than normal number of      disconnects since these events put a large amount of physical stress on      the gearbox.

• Fault level or fault current      & duration: This has to fit to our switchgear specification. Our      equipment has to be able to handle currents higher than the ones stated in      the agreement The utility companies are normally conservative people, thus      adding a safety margin of our own, will most often just be a margin on top      of another margin, so matching the connection agreement specification      numbers will normally do nicely.

• Energy metering details: The      commercial issues between our customer and the utility company are handled      here, so here the information on metering is also found. This can be in a      separate metering section or “hidden” amongst the commercial issues. There      are specific F&A codes in Pegasus to specify commercial vs. standard      level metering.  Standard level      metering is the default configuration for GE WTGs.

3 Additional sources of information

This chapter aims at describing what other sources of information, which are needed for completing the configuration in Pegasus. Often when the grid code and the connection agreement defines the interfaces handled by these document as in the sub station, the turbine transformer and the switch gear mounted next to this transformer might need be covered by these documents, hence the information needed for the correct configuration of this equipment in Pegasus will have to be located in other documents. Secondly the scope of supply of these pieces of equipment might not belong to GE, but is the responsibility of the customer, however it is still the responsibility of the application engineer to make sure that the equipment delivered by the customer is according t6o our specifications and thus will work with our turbine. So regardless whether this is in GE or customer scope, special attention to this equipment is needed.

The additional information is often found in kick-of meeting protocols, draft contracts or checklists. Looking at the electrical schematics in this document, the pieces of equipment are the LV switchgear next to the turbine, the unit transformer or turbine transformer and the MV switchgear in front of the transformer, so asking tech ops or the customer for information pertaining to these pieces of equipment might also work.

The first two pieces of information needed are whether the transformer is in GE scope and whether it is to be located inside the tower or not. For customer scope transformers, we have scope split documents and specification documents, which will handle most issues. Once the transformer is in our scope, the configuration variants from Pegasus can be followed. (See below) In the following, the limitations to the configuration variants are listed.

• With an external transformer      all types are possible, but the preferred one is standard oil.

• With an inside transformer only      compact oil and dry types are possible with a preference for dry types.

• The exact voltage HV level is      needed; a number like 11.5 is just as likely as any other, however if it      is over 36 kV system voltage, it is probably wrong.

• If voltage over 24 kV system      voltage, the number of switched feeders are limited is 2 or less and fused      might be used as a safety device.

• A motor driven circuit breaker      does not work with a fuse switch

4 The link to Pegasus

The F&A codes associated with the information gathered are the codes beginning with WF in the turbine design and the codes beginning with WK3E in the Aux. Design.

Gathering all the information from tech-Ops (Grid Checklist and Scope of Supply) and reading through the documents as suggested above should provide enough information to make a Pegasus configuration. With the configuration completed, the part I is completed. If you cannot complete part I, you will need to approach tech-Ops for the missing information or look into whether we have done something similar before. In part II the Pegasus configuration and the project data will be evaluated.

Part II The evaluation

With a complete draft configuration in place, now is the time to review some details for potential required changes or fine-tuning of this configuration. There are 3 numbers we need, first the number of turbines, then the apparent power of turbine model to be installed in the project and lastly the so-called short circuit power or short circuit capacity in the POI.

Starting with the weakest system condition for which the wind plant is to operate unconstrained, considering all reasonable commitments of system generation under light-load conditions and any line or transformer outages for which the wind plant will not be curtailed, calculate the short-circuit capacity for one outage beyond this condition.

If this N-1 condition is too weak, an alternative is to ensure that the wind plant is curtailed or tripped immediately in the event of this contingency.  The critical event then becomes the most severe contingency for which tripping or curtailment is not performed.

Utilities also work with a max short circuit power, this is used for protection setting purposes and not relevant for the check we do here. The short circuit power numbers might not have been made available to you so far, so you might need to ask the customer.

Example

We need to calculate the short circuit ratio for this project:

In this example 10 1.5 turbines is installed in a POI with 500 MVA of short circuit power, so the ratio is 29 Be aware that in our official document the rated apparent power and the rated active power is used, thus the 1.5 is a 1.7 when using the apparent power.

So if there are clear indications behind every piece of your Pegasus, either due to clear documents or clear customer decisions and your ratio is >> 15, go ahead, your configuration should be OK. If a WFMS is selected, be sure to communicate this clearly to the assigned WFMS engineer for the project.

In the following we use the ratio, you just calculated, the limits are for guidance only!

If the ratio is between 10 and 15, there are certain elements in the Pegasus configuration you will need to have a look at:

Turbine Code WF8      Voltage Ride Through

If LVRT is not already included, then ask the customer to consider this and get a clear statement to his decision With a ratio between 10 and 15 the grid is weak without being critically weak, however events in the grid might lead to voltage collapses, hence the need for LVRT, so that the turbines stay on-line. “We should make sure to be part of the solution and not be part of the problem”.

Turbine Code WFAA, WFAB, WFAC        0,9 power factor generator

If the 0,90 generator is not already included, then ask the customer to consider this and get a clear statement to his decision. Likely explanation is the use of cap banks.   Having the ability to feed extra reactive power into the grid can help stabilize the grid either with cap banks or with our turbines.

Aux. Equipment Code WK3EA to WK3EH WFMS system

If no WFMS is selected, then ask the customer to consider this and get a clear statement to his decision. Secondly suggest that the WFMS shall run in a voltage control mode.

If the ratio is below 10, it is time to gather all your material with your findings and go see an expert.

Glossary

60 Hz Vs 50 Hz We have two grid frequencies in the world for public supply of electricity, 50 and 60 Hz and we have indifferent voltage levels when measured in absolute values. The issue is not to let ourselves get confused by this, in stead see it as two distinct systems, so we only need to identify which system we are working with, meaning the frequency and the voltage level found in this system.

Active Power

This is the power that is capable of producing work, meaning driving a machine that is a torque is associated with it. The energy from this power is the one you sell as a power provider.

Circuit breaker, see switchgear

Gearbox in Pegasus

This Pegasus code is associated with the impact electrical grid events can have on the mechanical components of the gearbox. In spite of the design and functionality of our control system, abrupt disconnection of the turbine from the grid can lead to a higher strain on the gearbox, so if this happens too often, the gearbox might not last as long as we would like it to. This kind of disconnection is mostly seen in grids, where demand cannot always be met and frequent power cuts is the norm

Grid checklist

In some markets customers have accepted to provide their information and requirements as a checklist, which get to be part of the contract. From a global perspective as this is not adopted in all markets, you will some times need to do without this. In Europe and Asia poles the Applications Engineer may consider filling out the Grid Checklist on behalf of the customer.

LV

Within a given electrical system this is defined as Low Voltage, for 50 Hz systems it is in our case 690 V and for 60 Hz it is 575 V

LVRT (Low Voltage Ride Through)

This is the ability to handle collapses in voltage for a short period of time without having to disconnect the turbine from the grid and stop the turbine. The collapse in voltage is due to faults in the grid, which are then cleared by protection equipment disconnecting the faulted equipment. Once the fault is cleared the voltage will return. This entire sequence of events takes a fraction of a second, who is handled by the turbine; it rides through this feeding only reactive power into the grid. Once the voltage returns, the turbine returns to normal pre-fault operation.

MV

Within a given electrical system this is defined as Medium voltage, which is from 6 kV to 34.5 kV nominal voltage.

On-load tab changer

Transformers have the ability to connect two voltage levels by having two different numbers of windings on the same iron core. The ratio between the two coils of windings can be changed with a tab changer. The on-load tab changer has the ability through a motor drive and a controller to do this. Adding a control scheme and a voltage measurement device, means that this can be used for voltage control controlling the voltage on one side of the transformer relative to the voltage on the other side of the transformer. Sub stations are typically equipped with on load tab changers this is by far the simplest and cheapest way of doing voltage control and the preferred method even though it is slow.

Power Curtailment

The ability to, in a controlled fashion, to decrease or limit the out put power either on turbine or park level.

Power Factor

This is the ratio between reactive power and active power. Both powers are expressed as vectors and the power factor is expressed as the cosine to the angle between the two power vectors.

Per Unit Value (PU)

Within an electrical system, where the voltage and the frequency is defined, you can denote all nominal values 1 pu, this means that the nominal voltage is denoted 1 pu voltage, the nominal power for a particular machine is denoted 1 pu power and the frequency is denoted 1 pu frequency. All other quantities, such as current or resistance are derived from voltage, power and frequency. For our 1.5 50 Hz product, we denote 1.5 MW 1 pu power, 690 V 1 pu voltage and 50 Hz 1 pu frequency.

POI

Defines the point or the interconnection between utility company equipment and our or customer equipment, so a way to administratively define what is “ours” and what is “theirs”. This is also known as pcc, Point of Common Coupling.

Reactive Power

The reactive power in a grid is responsible for maintaining the electrical and magnetic fields found surrounding all pieces of electrical equipment, some components have a behaviour leading them to produce reactive power as a side effect to their operation and other components have a behaviour leading them to consume reactive power as a side effect to their operation. It is part of the responsibilities of the grid operator to maintain a reactive power balance between production and consumption in the grid, therefore certain requirements are posed to us as suppliers of equipment meant for grid connection.

Retained voltage

Grid failures lead to a voltage collapse from the nominal voltage value down to a retained voltage value. In grid codes this is related to a required fault ride through behaviour, where a fault scenario is defined. In this scenario, the retained voltage is the voltage that the grid will drop down to from the nominal voltage during the fault, once the fault is removed, the voltage will return to its nominal voltage after a period of time.

Short circuit Current

At a given point in the grid the current that would flow to a point in case of a short circuit is calculated.

Short Circuit Power

Short circuit power is calculated in a point in the grid and is used as a way to see the strength of the grid in that point. The method is to calculate the current, which would flow to that point in the grid if a short-circuit where to happen and with the voltage level, calculate the short circuit power. Related are thus short circuit current and short circuit impedance

Short circuit impedance

With the short circuit current and the voltage level, the impedance is calculated.

Switchgear

A piece of equipment able to separate one part of an electrical circuit from another part of the electrical circuit. As an example switchgear is able to separate or disconnect the turbine from the grid.

WindFREE Reactive Power description

This function utilizes the converter as a reactive power source, so when the turbine is not running, reactive power is still available. The level of reactive power available is depending on the size of the converter.

WFMS (Wind Farm Management System) (commercial name: WindCONTROL)

This is a control system working on a park level, not on turbine basis. It works by sending set points of various nature to the turbines, who adjust their mode of operation accordingly and thus achieves what the control sy