By Rasheek M. Rifaat, P.Eng
Independent Power Producers' "IPP" generating plants have electrical, mechanical or other interties to the outside world. Electrical interties are those connections to the grid transmission and distribution systems or the electrical host. Mechanical interties are those incoming connections from gas supply lines, and outgoing connections of the steam lines to the thermal host in the case of electrical/thermal cogeneration facilities.
In general, intertie requirements affect the design and operation of plant systems such as electrical protection and communication systems, and may stipulate the addition of some components or systems such as static excitation, power system stabilizer, plant islanding and black start capabilities, etc.
Electrical design considerations of IPP interties to the outside world may be classified as:
Plant design shall honor the conditions and optimize the benefits of the IPP agreements with gas supply, electricity sales, and in the case of cogeneration, steam sales to the hosts and the agreements with the transmission and distribution authorities and utilities. Also, design requirements will be affected by factors such as the relative size of the IPP facility, the nature of its commitments to the hosts and the transmission system, its geographical and ÒelectricalÓ locations etc. While 50 MW plants would be considered large plants in certain systems, the same plants will be mid size or small plants if installed in other systems.
Hence, caution must be exercised when classifying plants, for design purposes, based on their size, nature, cycle etc without considering the intertie system. With that in mind, and for the purpose of this article only, IPP plants will be classified in a system similar to the Alberta system:
i. 20 MW or less may be considered a small plant
ii. 25 MW to 125 MW may be considered mid-size IPP plant
iii. larger than 125 MW may be considered large IPP Plant
To maximize the benefits of this article and avoid confusion, it should be noted:
a. Many of the definitions, limitations, conditions and other intertie considerations have been or are in the process of being identified by the present Alberta Transmission Administrator (TA also known as EAL or ESBI). This article is not intended as an explanation or a companion document to the EAL documentation. While EAL documentation lists the official acceptable guidelines for connections to the Alberta system, this article provides general explanations of engineering topics that affect the design requirements of the intertie.
b. The examples given in this presentation are intended to facilitate and demonstrate the concepts listed in the article. They are not cookbook cases in as far as plant design is concerned and they should be used in their conceptual context as intended.
PROTECTION SYSTEM DESIGN
CONSIDERATIONS
Characteristics of Intertie Protection System
Intertie protection system, as other protection systems, will normally be required to meet high standards of:
Selectivity or "discrimination"
Indicates the protection system ability to isolate only faulted zones, without disrupting services to other zones.
Economics
Reflects minimizing cost without relinquishing essential system performance.
Intertie Configurations and Protection System Design:
To depict typical effects of power grid system configuration on the protection system, two typical cases are considered.
Case 1: One dedicated line connects the IPP facilities to a utility transmission or distribution substation with a dedicated breaker and protection system at each end.
Case 2: A tap off an existing line is used to tie the IPP facilities to the transmission distribution system.
From the protection point of view, Case 1 is simpler than Case 2. However, due to possible capital savings in the transmission or distribution line hardware, Case 2 (or similar) has either been used or recommended in a number of cases which the author has encountered. Let us explore some of the protection system design considerations for both cases.
System Modeling and Fault Calculations
Fault calculations had been instituted with Fortescue's development of the symmetrical component concept in 1918. Recent use of digital computers and software allows complicated systems to be modeled under different operating conditions. Analysis can be given for different fault types such as three phases, phase to phase and single phase to ground. For protection purposes, both maximum and minimum short circuit contributions are required to ensure proper protection system operations. Most utilities and system administrators possess sophisticated programs that can calculate the project short circuits under different conditions.
Protection Coordination for Dedicated Line Case
The dedicated line case (Case 1) is more elaborate than may appear. In the illustrated example, the bus configuration at the existing grid substation has an impact on the design of the protection at the IPP substation. When using distance schemes, one concern would be discrimination in the instance of having a fault on the feeder going out of utility breaker U1. The protection from the IPP facilities main breaker C0 must discriminate between a fault on either side of Breaker U1 or, better yet, on either sides of breaker U0. Typically a distance scheme will be set to trip instantly for faults within 80 per cent of the line length, as seen from breaker C0. Zone 2 will be set to cover a distance beyond breaker U0, but will trip after some time delay to allow breaker U0 or U1 to clear the fault behind the protected zone, before activating breaker C0. To accelerate fault clearing in case of a fault in the 20 per cent section, a signal can be sent from the distance protection at breaker U0 to breaker C0 to:
In some cases, it may be more prudent to use a signal from the utility substation to directly trip the IPP breaker (direct transfer trip). The protection at the IPP is then a back up protection, to ensure coverage in case of communication failure. A back up protection can be an over current if applicable, or single or multiple zone distance relay.
A similar concern will apply to breaker U0 protection, with the IPP plant power having different bus configurations. If the bus configuration, and the main plant breaker is connected directly to the transformer, the utility distance protection could be set up to the transformer high impedance without difficult accuracy considerations. Having more than one feeder on the plant side of the breaker adds requirements for discriminating between faults in the intertie zone and faults, as, on the plant feeder. A blocking or permissive signal will be required from the C0 protection to the U0 protection, which means additional communication requirements.
In a small IPP plants connected to a distribution system, coordinated over current schemes are sometimes proposed for the back up protection of the intertie zone at breaker C0. Some design concerns shall be examined.
i. The difference between maximum and minimum fault currents from the plant shall be considered. A minimum short circuit current contribution from the plant side will exist when the smaller generator is running while the large one is down.
ii. Over current schemes use time stepping for coordination with other relays on the grid side. Time stepping results in delaying the operation of the protection. With the delay, the generator impedance will increase from the sub-transient impedance (X") value to the transient impedance (X') value. Corresponding generator current decay will occur, which may affect the protection scheme operation.
iii. Delay in operation may have adverse impact on system stability under outside faults.
Consequently, other protection systems such as differential (pilot wire) distance, voltage controlled, or directional overcurrent systems are used.
Protection Coordination for a Tap-off or Tee-off Case
In this case additional concerns need to be examined. Prior to the addition of the IPP facility, the line would normally have had its own existing protection schemes. A new situation is created for the protection system, as the line becomes a multi source terminal line instead of having only two sources, one at each end. The existing protection may have limitations to be adjusted for the effects of in-feed. In-feeds make the line apparent impedances, as seen by a distance relay at either terminal, different from actual impedances. The relative "strength" of each source, as well as the fault location will affect the "apparent impedance" as seen at each fault-feeding terminal.
The definitions of strong and weak sources depend on their ability to deliver fault current, with the strong source being able to deliver high fault current and having less source impedance. Having parallel sources with different strengths and line impedances will make apparent impedance, as seen at any source terminal, different from actual line impedance. Normally, the apparent impedance is used for setting. However, considerations are given to what would happen if one source trips first, and the "apparent" impedance changes at the second source. The relay is required to properly perform under such conditions. Further complications can result from having additional taps such as UC1. In such cases, a protection scheme may require communication channels to improve its discrimination ability and further identify the fault location.
Ground Fault Protection
In the cases of bi-directional energy flow between the utility and small IPP plants located at industrial facilities, the existing power transformer may be used.
The transformer connection may be delta on the transmission or distribution system side. In this case, a single phase to ground fault will not drive fault current from the IPP plant side. Instead, once the grid side trips, the system becomes floating. The faulted phase voltage will be at ground level, while the voltage of the two healthy phases will rise above ground to 173 per cent (/3) of their normal voltage values. In many cases, the insulation of the line or the devices connected to that line will not tolerate such voltage rise.
Furthermore, other customers connected to the line will be subjected to voltage rise, which can be of concern especially if the ground fault clearing time is long, due to the lack of fault current, contribution from the IPP side. On the transmission level, the TA documentation states that the grid system shall operate grounded which implies that the IPP will have either a WYE grounded high voltage side or an artificial grounded system. A remote transfer trip can ensure that the IPP plant will trip once the other end trips. However, contingencies shall be considered under different conditions, which may require the artificial grounding. In general, if a system is to be designed with new equipment, a grounded WYE will be the recommended transformer HV side connection.
For most cases where a grid side solidly grounded WYE connected transformer is used, the coordination concerns will be similar to those of phase protection requirements. Additional discussion is given to line-ground fault due to the fact that it may be more frequent than three phase faults. Many utilities attempt to reclose after opening one or more poles of the breaker and clear the fault. Reclosing on the IPP side has its concerns as will be discussed later.
POWER SYSTEM DESIGN
CONSIDERATIONS
Power system considerations have major impacts on the design of the IPP electrical system. For example, short circuit levels and current flows under both normal and abnormal conditions are all factors that will shape the selection of the plant main components. Our discussion here will concentrate on the intertie portion of the plant design.
Selection of Main Output Transformer Voltage Range and Tap Changer
Small and mid-size IPP facilities will have a common generator bus and A common main output transformer . The facility will have a station service transformer that will probably be fed from the common bus. With that arrangement, the main output transformer design needs to address the following concerns:
In the case of the common bus scheme, there could be a bi-directional flow of both real power and vars. Consequently, voltage drop over respectable main output transformer impedance will be noticeable. System designers are required to identify:
Designers will perform load flow runs based on these limits and select the best voltage taps on the high voltage side. It is assumed that the low voltage side of the transformer could be fixed to match the generator nominal voltage (in the shown case).
Designers will also verify that under such conditions the common and station service bus voltages are within the acceptable limits to run the plant during start up and running conditions.
Designers will also verify that the plant motors can start under start up conditions.
If satisfactory results cannot be obtained with the use of off-load tap changer transformers, on-load tap changers may be used. The capital expense and maintenance requirements associated with the use of on-load tap changers prompt designers to always try their best to avoid using on-load tap changers wherever possible. It may be noted that, if required:
Islanding
If the IPP is connected to local loads, islanding may increase the reliability of the power supply to the local loads, especially if the incoming line reliability is limited. The concerns associated with islanding include:
With regard to item 3.2.1, riding over the disturbance, a dynamic stability study shall be conducted to ensure that the provided protection is fast enough to separate the island from the faulted intertie line before the generators and machines slip out of stability. The stability checks are typically monitored verses the fault clearing time of the back up protection, which is commonly larger than the primary protection.
In regard to item 3.2.2, maintaining voltage and frequency upon islanding, there can be one of three cases:
With regard to item 3.2.3, re-synchronization is a matter of hardware availability. In some small IPP schemes the synchronization is allowed only at the generator breaker which may require the addition of synchronizing scheme at the intertie breaker. In some cases, an intertie breaker may not be there and will be required to be added, if islanding is required.
It may be noted that, in addition to increased power supply reliability within the island, some machines will benefit from reducing the number of shutdowns by allowing them to island with the load in case of line interruption. The selection of islanding will depend on the system requirements in each case. It may also be noted that in the case where a "neighbour" to the IPP can island with the facility, a proper legal contract may be required to avoid future complications.
Reclosing
In general, the following may be noted:
Static Excitation and Power System Stabilizers
The TA and system operators try to avoid cascade tripping of generating facilities that will result in total blackouts. Their concern will be in the case of a remote fault or mild system disturbance, as many generators remain connected to the system, supplying its requirements of real power and vars and try to "ride over the disturbance".
If the IPP machine is larger than a certain size, the TA (with its association with other regional authorities such as WSCC), may require a static excitation system on the generator, a power system stabilizer, or both. Such facilities allow faster and more efficient responses when a disturbance occurs. Although the criteria for selecting a static excitation and/or PSS is the jurisdiction of the TA. The TA's technical staff are willing to go to some length to explain the requirements which justify the extra expenses associated with the selection of static excitation.
It may be noted that some machine suppliers now claim that brushless systems have advanced in their response capability to satisfy the requirements of the transmission authorities. The selection of brushless or static excitation needs to be finalized at a very early stage of the project due to its association with ordering the generator.
Black Start
The TA has discussed the black start conditions in their documentation. From a technical point of view, black start entails enlarging the in-plant diesel generators to allow the supply of all loads that are required for starting the facilities when the intertie line is dead.
If the facilities are equipped with black start, the TA may request that the IPP start their facilities at any time, and back feed the system via the dead line. This means that the line breaker shall be allowed to close on the dead line. The facility's service system should be designed such that all start up loads can be fed from the upgraded emergency generator and that there are live transfers allowing the switch over to normal supply once the facility has started. Since the use of the black start is not expected to be very frequent, the IPP operator shall set a regular testing schedule to ensure the readiness of the black start diesel generator and its associated facilities so the system is ready when actually required.
COMMUNICATION SYSTEM DESIGN CONSIDERATIONS
Communication systems are required:
The design of the communication systems can be affected by:
Historically, Power Line Carriers (PLC) were used extensively to provide protection signals and connect generating facilities with other parts of the transmission system. PLC involves considerable amount of dedicated hardware such as wavetraps, terminals etc., which result in considerable capital investment, especially for IPP. PLC also have limitations on the amount of information that can be carried on a specific system.
Fiber optics and dedicated telephone lines are used in more and more applications nowadays. Microwave towers are common in remote areas and have been successfully used.
Conclusions
IPP interties affect the plant protection, communication and other systems. As early as possible, intertie considerations should be addressed to avoid subsequent modifications that may be costly.
Rasheek M. Rifaat is Principal Electrical Engineer at Delta Hudson Engineering Ltd. located in Calgary, ET