GENERATOR JOINT VAR CONTROL: INTEGRATION ISSUES AND IMPACT ON SYSTEM TRANSMISSION CAPABILITY

By Chris Fuchs, David Apps, Nick Chopra and Wilsun Xu

BC Hydro has identified Joint Var Control (JVC) as the chepaest means to extend transmission capability limits to meet load forecasts. JVC controls generators such that a common point voltage is maintained while coordinating machine var output in a multi-machine plant. It does not involve installation of additional var capacity.

In this article, some of BC Hydro's experiences in designing the JVC scheme for its Mica generation station are presented.

Limitations of Line Drop Compensation Line Drop Compensation is a supplemental regulating input to a generator's excitation system. It simulates a voltage beyond the generator terminals using only the generator terminal voltage and current as inputs without the express need of additional transducers. By altering the setting of the LDC components, the generator can be made to control a range of voltages beyond its terminals by adjusting the percentage of transformer impedance. In a standard multi-machine installation with dedicated generator transformers, the limit of controllability is reached when the compensated transformer impedance approaches 100 per cent. Beyond that limit machine VAR output between units becomes, with only slight changes in voltage setpoint. Consequently the closer the control voltage is to the generator terminal, the higher the degree of controllability.

Another thing to consider when setting the LDC is to allow the generating units to be as responsive to system conditions as possible. This lets each unit to contribute as much VAR as possible to the system in the case of an outage; hence the higher the LDC setting, the more output VAR becomes available to aid the system when it is most needed.

A compromise between these conflicting objectives leads to a solution that ranges between the terminal and the high side of the unit transformer. BC Hydro has determined that an LDC setting of 60 per cent of the step-up transformer impedance allows all machines to control their own output voltages.

The setting is also dependent on how accurately the voltage regulator is able to control voltage. A voltage regulator that drifts or oscillates could create large VAR changes even at relatively low LDC settings. The 60 per cent trade-off in setting allows the local operating constraint to be satisfied but limits generator participation in system-wide events such as voltage instability.

Joint Var Control An alternate method for controlling a voltage point regardless of these limits is possible by using generator joint voltage control (JVC). A JVC scheme determines what the reactive plant output schedule should be in order to maintain a prescribed common point voltage level for a

multi-machine installation and distributes this amount among all constituent on-line units which are on joint var control.

High and Low Side JVC There are two basic types of joint var control based on high side and low side voltage regulation. The latter is designed such that the operators control the output of the 'lead' unit by manually raising or lowering its voltage setpoint. Automatic VAR balancing control circuits then cause the 'follow' units to track the changes in the lead unit output. As such, low-side JVC does not require a system reference voltage. In effect, the plant voltage is controlled to the level of the lead unit's voltage regulator; hence, it is controlled at the "low side" of the step-up transformer. This form of JVC is currently being used at Kootenay Canal and at the Peace Canyon Generating Stations.

Low side JVC adjusts the voltage setpoint of all units and as such does not require any additional telemetered inputs beyond what is already provided for in each unit's voltage regulator. No actual regulation to a common point is provided. By simulating a common voltage point using LDC for the lead unit's regulator, coordinated plant control beyond the limitation of single unit LDC is theoretically possible. This form of low side JVC is not used by B.C. Hydro. High side JVC requires a voltage signal from the point where coordinated plant voltage control is needed and hence the common point voltage can be physically controlled, providing tighter voltage regulation than can be achieved with low side JVC.

Typical High Side JVC Scheme High side JVC control can be implemented by converting the high side voltage into an equivalent VAR plant schedule. JVC as implemented at GMS and Mica is based on approximating the VAR requirement necessary to maintain a remote voltage beyond the high side of the unit transformer at the desired setpoint by the voltage droop between the the controlling end and the remote end:

Assuming all the combined machine and transformer impedance of unit i at a n-unit plant is denoted by Xti , then the equivalent impedance seen by a machine looking out from it's internal voltage is given by:

where Xeqi is the common system impedance beyond the transformer high side to the point where JVC intends on controlling the voltage .

The contribution of Vti toVB is given by:

The resulting feedback voltage at VB is then the contribution from both the plant generating units and the infinite voltage which can be derived from the high side voltage:

The approximate total VAR demand from all plant units is just the sum of all the droop approximations or

Comparing the output VAR demand with the schedule needed to maintain the common point voltage gives a VAR feedback error. This error is then added to the VAR schedule and distributed amongst all units on joint var control.

The units may not be identical in design; it is entirely possible for one generator to reach its VAR limits before the others exhaust their reserves. There are several ways in which the

reactive power can be dispatched to each of the plant units: apportioning vas such that all units reach their limits simultaneously dividing the total VAR schedule equally amongst all units scheduling according to predetermined rules.

In the case of the Mica plant, all units are of the same design and rating and thus the second allocation rule the simplest was used. Varying allocations may be necessary for derated units or to prevent some units from operating too close to their thermal limits. If one or more units reach their limits before the others, the design will automatically reapportion the remaining VAR to the units with spare capacity. Figure 2 shows a 4 machine high side JVC configuration with transformers having equal impedance which allocates VARs equally amongst the units (ie n=4 and Xti =Xt).

Joint Var Control Integration Issues and Design Requirements Joint Var Control is by no means new technology. B.C. Hydro's Gordon M. Shrum Generating station (GMS) JVC installation was deployed in the late 1960s. The technology used by the JVC controller is dated and slow. It has now reached the end of its service life and is rarely, if ever, used. A new design would have to be flexible enough to be able to be incorporate new control modes and fast enough to be able to deal with system conditions. A few requirements that JVC must satisfy include:

• act fast enough in order to respond effectively to voltage collapse • capable of being integrated into an overall system voltage control scheme (yet to be

developed) • flexible enough to allow modification of the VAR allocation control method • operate harmoniously with other real or reactive power controller action • fail-safe operation in event of loss of centralized control • should preferrentially be incorported into a multi-function plant controller • must be cost-effective

JVC versus PSS Two concerns with regard to PSS/JVC interactions were identified:

The first concern was that if the JVC response was very rapid, it would have the tendency to cancel out PSS action. Time simulations and eigenanalysis later proved that this interaction did not manifest itself at the proposed JVC response time.

The second concern was related to the power setpoint ramp ups which result in temporary VAR disturbances as a result of PSS action. These VAR disturbances, it was feared, would cause undesirable JVC action. Field tests later proved that judicious choice of JVC gain and deadband eliminated this problem.

JVC and OEL interactions Overexcitation limiters on the B.C. Hydro generators step the field current (Ifd) down to the maximum field current (Ifdmax) in a time inversely proportional to Ifd-1.05*Ifdmax. The time constant associated with this action is on the order of 30 seconds. It was initially unclear how JVC would operate when units with different levels of VAR output would behave when one or more of the units was driven into it's OEL control mode with the remainder on joint var control. JVC automatically balances VAR output between plant units.

Units with unequal MW output willhave different amounts of output VAR while on JVC, as a result, some or all units will reach theirOEL limits at different VAR output levels. The moment that the limiter takes control overvoltage regulation, the contribution from that particular machine no longer reacts to JVC control changes. High voltage JVC will divide the remainder of the VAR no longer being output from the units on OEL control to the units remaining on JVC. Time simulations have confirmed that the VARs are redistributed to the remaining units on JVC.

JVC/OEL control versus AGC The new EMS system installed at the Burnaby Mountain System Control Center (SCC) has an Automatic Generator Control (AGC) function which reduces the generator power setpoint if the unit is above its capability curve. AGC ensures intertie MW flows are maintained according to the current operating schedule and that system frequency remains within a small margin of the 60 hertz system frequency base. The EMS software controlling the AGC function uses a model of each generator's capability curve to dynamically determine the machine limits based in part on each unit's terminal voltage. When a unit exceeds either its rotor or stator limits, AGC will automatically reduce the generator's power output at a rate of 0.0067 per unit per second or less (or 20% change in power setpoint in 30 seconds).

This only causes problems if the generator's operating point is in region A as shown in figure 4.

Initially it was feared that contingencies resulting in operation beyond the OEL would cause the AGC to ramp the generator power setpoint down significantly or even to zero. Simulations of this event showed that AGC action causing real power output reductions would also lower the reactive output power by a similar proportion. Thus the trajectory of the operating point following a contingency (e.g. line outage) resulting from AGC power reduction would angle downwards from point 2 to point 3 of figure 4.

It was also determined, that there was only one single 500kV line outage which would cause a plant's output to rise above the rotor limit for the heavy winter base case. This single exception was already being handled by an operating order which would trip one unit off-line effectively rectifying the situation. The rarity of this situation and the effect of AGC on unit reactive output effectively allayed all concerns that plant operation under these exceptional conditions on JVC would behave rationally.

System Implementation and Assessment

Effects of JVC on System Planning and Operation In order to be effective, JVC must be part of an overall planning and operating discipline. The system must be planned and operated such that all major generating installations are off-loaded in terms of reactive power output. This allows the generators a necessary margin of VAR reserve with which to respond to severe contingencies when they occur. Without this allowance, JVC would be much less capable of preventing or slowing voltage collapse. The operational version of this rule has been, in practice, to load generators to only about a third of their upper Mvar limit.

Ideally no VAR output from all plants would give the maximum protection from JVC under voltage collapse scenarios.

It was also apparent that plants which are in or adjacent to the main load center do not require JVC for voltage collapse prevention. Under collapse conditions these machines will output the maximum amount of reactive power almost regardless of the voltage setpoints of the units. Moreover, the time simulations demonstrated that this reactive support is highly responsive to system conditions and is quickly extracted because of the voltage droop experienced by each of the plant's units.

Voltage Collapse Time Frame and Transmission Capability Margin Increases The primary question of whether JVC is effective in preventing voltage instability is wholly dependent on the amount of time in which voltage collapse occurs. A dynamic simulation was performed for a system loading pattern that evolves into a full blown voltage collapse scenario for a single 500kV line outage. The simulation required developing a JVC and over-excitation limiter (OEL) model specific to BC Hydro. On-line tap changers (OLTC) are important components strongly implicated in the voltage collapse so the existing OLTC model incorporated in the EPRI Electrical Transients Modeling and Simulation Package (ETMSP) was used to model all existing base case OLTCs.

The time frame of a voltage collapse scenario establishes the primary design criteria that the JVC must meet. The final design target established from simulations for the BC Hydro JVC was that the voltage setpoint should be re-established within 15 to 25 seconds for a 3% change in system voltage. The main constraint to this requirement is the mechanical potentiometer based voltage setters used by the generator regulators. This mechanical setpoint control is common to the regulator of most B.C. Hydro machines and it's ramp rate varies between plants but is generally capable of rates of 30 per cent per minute.

Studies demonstrated the equivalent amount of VAR support that JVC provided over the base case. Figure 5 shows the interpolation of a series of time simulations giving the equivalent amount of Mvar capability that JVC represents for two severe contingencies.

According to the figure JVC provides about an equivalent of 75 Mvars and 50 Mvars of voltage support to the system at the load center (Ingledow Substation) for a 5L42 and 5L82 outage respectively. This is the difference in the amount of extra shunt capacitor support needed between similar cases with and without JVC to halt voltage collapse. The power flow base case used for these simulations did not have minimal VAR output from all plants - when most plant output was reduced as much as possible, the JVC was equivalent to about 180 Mvars at the load center.

JVC Over-Reaching Control beyond Plant Perimeter A natural extension of JVC is to control the voltage into the power system beyond the transformer high side. In theory JVC can control into any amount of system impedance to achieve control objectives.

Controlling voltage beyond the plant's substation somewhere into the power system (over-reaching control) makes the system even more responsive to voltage disturbances. If the over-reaching control is set to a point beyond a network branch forking into two or more separable and distinct electrical paths then no distinct physical control point will be recognizable; consequently this type of control is a coordinated form of LDC making a plant look like a single unified VAR source; unless a remote telemetered voltage from a load center is used as the JVC controlling bus voltage input.

Over-reaching control of this type is antithethical to the design of the power system. Voltage profiles for a given base voltage level are meant to be flat or have a minimum amount of voltage droop. It is the change in voltage angles that make for real power transfers across the system. Transporting VAR across the system works on the basis of a discernible transmission voltage gradient: the higher the gradient the higher the plant VAR demand. The over-riding physical constraint on generator support is of course the amount of VAR that a plant can supply.

There are three limits to over-reaching JVC:

• Plant Mvar limits as defined by stator thermal protection settings • Machine rotor limits as defined by OEL settings • Machine terminal voltage limits defined by Volts/Hertz and over-voltage protection

settings (generally set at about 1.1 p.u. voltage).

Transmission equipment BIL limits will generally not be a concern during heavy loading conditions that characterize voltage collapse. Another constraint on over-reaching voltage control is for the case of two plants within short electrical distance of each other; the B.C. Hydro GMS and Peace Canyon plants qualify in this regard. Two such plants both equipped with JVC can miscoordinate VAR allocation in much the same way as two units under LDC control with high compensation settings This case can be dealt with by placing both plants on the same JVC system and this would require telemetry between both plants.

Results of a study attempting to quantify the increase in VAR support using over-reaching control is shown in table 1. Each of the plants were independently placed on over-reaching control. There are two lines in the table for each plant one each for two stressed system planning base cases. The amount of impedance that is being over-reached is given as a percentage of the equivalent total line length. The amount of increase in the system MW transmission capability for over-reaching control is compared to 100 per cent LDC (or high side JVC). There is a significant increase in the system's power transfer capability under this type of control from 144 to 254 percent of what high side JVC can provide. Note that the plants are generally constrained by the terminal voltage limit of 1.1 per unit.

Over-reaching control if implemented may mean the difference between system voltage stability and its collapse; if not it may provide a short time period for the EMS or system operators to assess the situation and act accordingly.

Conclusions The support studies for preceding the Mica JVC design and implementation demonstrated the feasibility of using high side JVC without any adverse generator controller or AGC interactions. A measure of the amount of equivalent VAR support that JVC provides has also been calculated; moreover, time simulations have also demonstrated that JVC can delay voltage collapse or even prevent it. Operating experience to date on the Mica JVC system has shown it to be reliable without creating any adverse affects on installed equipment. ET

C. Fuchs, D.Apps, and N. Chopra are with B.C.Hydro. W. Xu is with the University of Edmonton. This paper was originally presented at CEA's Electricity'97 Conference held in Vancouver last April.