The Active/Reactive and Voltage Constraints within WPPs and SPPs.
The Wind Power Plant(WPP) or Solar Power Plant(SPP) need to get the power out while meeting the interconnection requirements, and in general the operation or design of the plant may prevent that from happening.
What do you do?
The constraints preventing reactive power support from the wind power plant or solar power plant to the interconnecting transmission system manifest themselves in many ways. RF was designed to relieve the plant of the several constraints. Where such constraints prevent the WPP or SPP from meeting all the terms of the interconnection agreement. RF causes the reactive power to flow at the right time. Reactive following flows the capacitive reactive power to the wind turbines. In general, RF consists of a controller with a very detailed PSCAD model, a breaker and switch, and a bulk capacitor bank, connected to the high side of the station transformer. It is not that sexy of a solution and it is old school. However, what is not old school is how the constrains are managed by a team of very experienced engineers.
RF does not need to connect to the existing Reactive Power Management System such as SCADA, however there are provisions for doing so if needed.
Far WTG(S) & Collector Impedance
In the case of wind generation plants with long collector lines. Such lines going from the substation to the farthest wind turbine generators (WTG) create an impedance where the voltage may swing.
Looking from the WTG to the POI there will be voltage swings due to the many elements contributing to the resistance, reactance, or susceptance. Elements that contribute to voltage swings are 1) WTGs generator step up transformers stepping up to 34.5 kV, 2) collector lines and 3)main plant transformer.
An On Load Tap Changer (OLTC) is an automatic tap changer located on the main station transformer, which when not operating properly may cause an active-power reactive-power or voltage constraint that manifests at the Point of Interconnection of the WPP or SPP.
An OLTC is found on the Main Station Transformer and they are mechanical devices the need maintenance and do fail from time to time. An OLTC changes the winding ratio of the station transformer in small increments in an “attempt” to control the voltage on the 34.5 kV side of the transformer to regulate the voltage of the WPP or SPP. Such a device is known to fail and needs maintenance and is slow.
Depending on the design of the WPP or SPP, and the volatility of the wind or solar input, an OLTC may operate a number of times per day, and excessive operation may cause the OLTC to fail or cause costly maintenance earlier than expected. An OLTC may in part consist of vacuum interrupter switches, and according to those in industry, an OLTC diverter switch when placed in extremely demanding applications where there are an exceptional number switching operations per day, may have to be replaced at an accelerated schedule, which includes cleaning the mechanism, checking for contact wear, filtering the oil and checking its dielectric strength.( See: Vacuum interrupters as an alternative to traditional arc quenching in on-load tap-changers)
Vacuum-interrupter failure alarms and lockouts can occur for a number of reasons, some are valid and the tap changer should not continue to be operated automatically. There are a number of reasons vacuum interrupters fail, such as lost vacuum (insulation breakdown), switching transients in station triggering sensing circuit interrupter mechanism fails to open / close or a mechanical failure of the interrupter mechanism (See: Troubleshooting Vacuum Bottle Failure Alarms. )
With any failure in the OLTC or when it is too slow and cannot keep up a constraint in active-power reactive-power or voltage may manifest, when the tap is out of the desired position, causing the wind turbines to start reactive limiting. In some cases instability may occur within the plant depending on the capability of the wind turbines. At this point the plant may not supply the reactive power at the point of interconnect, or worse wind turbines go off line and there is a loss of production.
Station Transformer Impedance
Just the impedance of the Main Station Transformer (MPT) can create an active-power, reactive-power or voltage constraint within the WPP or SPP that manifests itself at the POI. The usual suspect that causes this problem is the series impedance of the MPT, which contributes to a consumption of capacitive reactive power, and a voltage rise on the collection system causing other devices to limit either their active power or reactive power output. The purpose of the MPT is to step up from the collector system voltage of the SPP or WPP to the sub-transmission or transmission level voltages.
The impedance of such transformers in power system studies is well understood, where values of inductive reactance(XL) and the X/R ratio is usually supplied from the transformer manufacturer, typical values are 7% to 10% and an X/R ratio of 30 to 1.
For example, a 230 kV/34.5kV, 100 MVA transformer with a specified and tested 10% impedance, and an X/R ratio of 30 to 1, has a high side impedance of
projected capacitive reactive power consumption at 75 percent power is 5.5 MVAR. However, more important is the voltage regulation which in turn affects the components within the collection system of the WPP or SPP. The type of constraint and magnitude value realistically depends on the voltage phase imbalance and several other factors at the station transformer.
Station Transformer Voltage Regulation
If the point of interconnection (POI) or local transmission system is taken as the infinite buss, then we can project the transformer voltage regulation from full inductive to full capacitive to be around 10%, such a swing may destabilize the WPP or SPP.
Wind Turbine Transformer Impedance
The wind turbine generator transformer impedance may also cause an active-power, voltage, or reactive-power constraint at the POI. The way it causes the constraint is like the constraint caused by the MPT. However, the long lengths of the collector cables come into the equation as well as wind or solar input volatility. With RF we can optimize such condition to minimize adverse effects constraints within the wind power plant or solar power plant.
Active Power Level
The active-power, reactive-power, or voltage-constraint at the POI due to Active Power Level is associated with several other elements within the SPP or WPP, such as transformer impedance, OLTC tap position, collector impedance, reactive production or consumption from power system equipment. As the power changes the reactive production and consumption changes, this will cause the voltage to change on the collector system and as a result the voltage at each individual WTG or solar inverter could cause a dynamic power limitation. The voltage will not be the same throughout the collector system, and equipment will reduce its contribution of reactive power and may not regulate voltage at the POI..
Constraint Due to Interconnect Voltage
The reactive constraint caused by the interconnect voltage, does not usually manifest itself, until there is a change in the voltage on the transmission providers system caused by a contingency such as an N-1 or N-2 event. Usually, the plant is configured to run within a narrow band of voltages at the point of interconnection. However, during a disturbance, when the POI voltage goes too high or too low, wind turbines and other devices are affected and may decrease their individual reactive power contribution, as a result this may cause a constraint in plant performance.
Copyright Thomas Wilkins et al. 2017 All Rights Reserved