The undesired outages of generating units during the July 1996 Outages in the Western Interconnection and the August 2003 blackout in the Eastern Interconnection have resulted in updates to reliability standards which secure, improve, and optimize generator response during power system disturbances. The North American Electric Reliability Corporation (NERC) has recently issued Standard PRC-019-2 which specifies reporting and review standards for generator protection coordination. Because the skill requirements to conduct the review are not normally included in plant operations, outside experts are brought in that have a knowledge of what may be available in terms of information and data at the plant, the technical knowledge to conduct the coordination assessment and the experience to identify needs and deficiencies that are critical to presenting a credible review report.
In recent work, Pterra, acting as an external resource, developed approaches to conducting the review for compliance with PRC-019-2 for several legacy power plants. Such power plants have been in operation for many years, but may have changed ownership at least once, and where test results and data may not be readily available. This article discusses the general review approach, and applies this to a sample a 230-MVA Steam Turbine Generator Unit in a combined-cycle power station.
First, some basic concepts on generator capability and excitation systems.
A synchronous generator must observe operating limits for its real and reactive power output in order not to exceed thermal capability for its various components. The plot of real versus reactive power limits under various conditions for a specific machine is known as its generator capability curve (GCC). Figure 1 shows the GCC for a sample cylindrical rotor steam-turbine generating unit. Many coordination requirements for protection and limiters can be directly evaluated using the GCC, and is a basic tool for conducting PRC-019-2 compliance reviews.
The GCC consists of three regions. These are the rotor winding limit in the overexcited region (curve A-B), stator winding limit (curve B-C), and stator end iron limit in the underexcited region (curve C-D). The rotor or field winding limit, and the stator winding limit are associated with the i2t capability of the rotor winding, and of the armature of the generator, respectively. When the generator is supplying reactive power below its rated power factor, an increase in exciting current is observed to flow in the rotor windings, and thus an increase in conductor temperature in the field winding. In the underexcited region, a reduction in exciting current in the rotor circuit causes a significant increase in excitation being absorbed by the generator. The rotor retaining rings (end core) approach saturation due to leakage flux crossing the air gap which originates from the stator and produces localized heating. This is usually referred as the core end heating limit and is due to localized heating in the end-turn region of a generator when operating underexcited.
Another important concept is that of generator excitation.
Synchronous generators depend on the presence of a magnetic field to produce electric power. The excitation system is the component of the power plant which creates and controls the magnetic field, which in turns controls the electrical field. There are many varieties of excitation systems, and this leads to most of the complexities involved in review coordination under PRC-019-2. A block diagram of the typical components of an excitation system is shown in Figure 2.
The exciter produces dc current to power up the field winding of the generator. The amount of current and voltage produced by the exciter is controlled appropriately by the automatic voltage regulator (AVR). The AVR uses input from the system side of the generator to compare with its reference signal and keep the generator operating at its voltage schedule within its GCC. During disturbances, the output of the generator changes significantly based on conditions in the external system, and may lead to operation in the under or over excited regions. Limiters and protective circuits control the amount of reactive power produced or absorbed by the generator to ensure that the generator operates within its GCC. The means and individual components in the exciter can vary significantly from machine to machine. Furthermore, the limiters and protection must coordinate among themselves to avoid undesired operation.
Areas of Coordination
Using the GCC as a basis, we have three classifications for the general topics for coordination under PRC-019-2: (1) over excited operation, (2) under excited operation and (3) transient overloading. Each is discussed in the following sections.
Under Excited Operation
A generator is operating under excited when it is absorbing reactive power from the system. In the GCC, this is associated with the lower portion of the curve. The typical components to be coordinated are underexcitation limiters (UEL) and protection (UEP), and loss-of-field (LOF) relays, also known as ANSI Device No. 40. The coordination is reviewed both internally, within the generator and excitation system, and externally, in the grid or power system. Internally, the UEL, UEP and LOF relays should allow operation within the over excited region of the GCC. In some cases, an exception is made where older generating units stator end units are particularly degraded or over voltage issues restrict the number of VARs the generator can absorb, the protection and limiters may restrict under excited operation to a small percentage of the limits allowed by the GCC. Externally, the coordination is confirmed against the allowable VAR absorption for stability, often using the steady-state stability limit (SSSL) or alternatively using computer simulation to the test the protection and limiter settings.
The limiters and protection should allow the generator to absorb reactive power within its capability and prevent undesired tripping of generating units during stable power swings and disturbances in the power system. To visualize the coordination, the settings of the UEL and Device 40 are plotted in the P-Q plane (sample provided later in this article) together with the GCC and SSSL curves. Note that the SSSL curve is an approximation that may be sufficient for compliance purposes. However, Pterra recommends transient stability simulations to confirm the coordination of the limiters and protection under various credible disturbances. When conducted, it is useful to plot the response of the system with the equipment settings in the R-X or G-B planes (samples provided later in this article).
Over Excited Operation
Conversely, a generator is over excited when delivering reactive power to the system. Operating points under this condition are plotted on the upper portion of the GCC. Typical coordination components are overexcitation limiters (OEL), field winding over current and stator winding over fluxing (also referred to as volts/hertz) protection, or ANSI Device No. 24.
The GCC coordination can be checked by plotting the GCC with the OEL and other protection components in the P-Q plane. Note that during system disturbances, the generator is generally required to provide transient reactive power that will exceed the limits shown in the GCC. The generator can tolerate high levels of reactive power for short durations because of its inherent thermal capacity. The system needs the transient reactive power to ride through power and voltage fluctuations during system events. Hence, the OEL and protection settings should allow for some time delay before applying any forcing reduction or tripping. Timing plots are useful for checking coordination sufficient for PRC-019-2 compliance. For improved rigor, dynamic simulations may also be conducted to verify performance with the OEL and protection settings.
Backup Overload Operation
Generators should be provided with back-up protection in the form of distance relays (ANSI Device No. 21) for phase faults in the power system. This is to ensure that the generator will not be damaged ensuing from supplying high current for a prolonged period. A distance relay with mho characteristics is frequently sufficient in this application. To check the coordination, the settings of the relay are plotted in the R-X plane together with the loci of impedances and GCC transformed from P-Q to the R-X diagram.
The approach described above is applied to an example where we review protection coordination for an existing generator, a 230 MVA, 18 kV, 60 Hz, 0.85 p.f. Steam Turbine Generator (STG). The protection scheme applied to the generating unit is illustrated in Figure 3.
The GCC for the sample STG is shown in Figure 4.
The generator is equipped with UEL and LOF relay. These are shown plotted on the P-Q plane together with the GCC and SSSL in Figure 5.
The UEL is set to limit the STG from absorbing reactive power to less than the GCC rating. This is a conservative setting since it does utilize the full capability of the STG, however, there may be operational reasons for setting the UEL in this manner. The LOF relay is set at higher levels of reactive power absorption that the UEL permits. Since LOF can occur with failure of the excitation system, this setting ensures there is backup protection to under excitation for the study generator. The LOF has two Zones. Zone 1 is set to be at about the same level of VAR absorption as the SSSL; i.e., the generator will trip at the level when the system may approximately become unstable. In doing so, the system may be able to recover. Zone 2 provide backup protection in case Zone 1 of the LOF fails. The settings of the UEL and LOF relays are adequately coordinated with the generator’s GCC and the system SSSL, although simulations may be needed to confirm the stable system response.
Figure 6 shows the coordination for under excited operation as viewed on the R-X plane.
The R-X diagram plots the apparent impedance as seen from the generator terminals. Hence, whereas normal operation of the generator is within the GCC on the P-Q plane, normal operation on R-X plane is outside the GCC. Each protective device must then be coordinated in restricting the operation within this limit. Using this general concept, the same conclusions regarding setting coordination using the P-Q diagram can be made here. However, now, if a stability simulation is performed, the locus of impedance points can be plotted in R-X diagram, where the impedance trajectory is expected to encroach UEL curve much sooner. The results of stability simulation will also confirm the accuracy of the time delay settings for the LOF relay.
The study generator is likewise equipped with OEL with settings for instantaneous, fast delay and continuous limitations for over excited conditions. These settings are plotted on the P-Q plane as shown in Figure 7. The OEL is further equipped with time delay settings that determine the allowable duration for each of the limits. The diagram shows the coordination of the OEL settings with respect to the GCC. However, the time delays should be confirmed using transient stability simulations.
The study generator is also equipped with over fluxing protection via Device 24. A time plot of the V/Hz magnitude as allowed by industry standards, the Device 24 settings and the generator and step-up transformer damage curves is shown in Figure 8.
IEEE Standard C37-102 specifies that the continuous limit for V/Hz of a generator is to be 1.05 times nominal, or 105%. Device 24 for this example is equipped with two time definite and inverse time settings. These are shown in Figure 8. All the settings are coordinated with each other as indicated in this time plot of V/Hz. However, the plot tells that the generator is the limiting element in the event of core saturation because of overexcitation. Because the damage brought by overexcitation is very severe, it is important that the generator and transformer manufacturers should be consulted first before applying settings of V/Hz protection.
The study generator is equipped with a Device 21 relay with two-zone mho characteristics for backup protection on close-in faults. Plotting the Device 21 zone elements, system impedance, and GCC, the resulting coordination curve in R-X diagram is illustrated in Figure 9. The maximum load impedance at rated power factor angle (RPFA) should be calculated to check that it will not encroach the settings of Zone 2 protection. Normally, the reach of Zone 2 protection should not exceed 50-67% of generator load impedance to prevent the generator from tripping during stable power swings.
Figure 9 shows that the phase back-up protection is coordinated to the capability of the generator as the present settings does not encroach the GCC. The generator can fully maximize its overexcitation capability. When a stability simulation is carried out for a fault in the power system, the trajectory of generator load impedance is expected to cross the GCC and will lie within the Zone 2-GCC region. The large margin observed between the Zone 2 and GCC region indicates that the present settings of Device 21 for Zone 1 and 2 are sufficient to prevent the undesired tripping of generator during grid disturbances and voltage collapse scenarios, while still providing backup protection for close-in faults.
This article presented a generator protection coordination method based on use of P-Q, R-X and inverse time curves. The method is illustrated through an actual review of coordination for a steam turbine generating unit based on the requirements set forth in NERC PRC-019-2 standard, technical reports, and international standards dealing with ac generator protection.
Since the observations being drawn for each protection coordination requirement were based on the plots and diagrams, it is suggested to carry out a positive sequence dynamic simulation to confirm stable response of the settings.