In DS, the power flow solution is “the” initial system condition criteria used for initializing all DS models. Due to the nature of stability, and the model’s equations, we find that a few adjustments are needed in the power flow solution technique to get an appropriate initial condition power flow.
As noted in the next section, entry into the DS Focus includes solving the system power flow to establish our initial conditions. Thus after entry into the DS Focus, the user is presented with the solved power flow on the oneline of the system. The user may notice a solution that is not exactly equal to the solution in the Power Flow Focus. This is caused by a few subtle changes that are necessary to prepare the power flow initial conditions to line up with the needs of DS simulation methods and DS models.
2.2.1 Generators
Generators are converted to internal Thevenin sources behind equivalent impedances, to represent the internal behavior of the machine and its interaction with the network. Thus, swing sources and power flow voltage control no longer exist. All automatic control is supplied by the generator’s excitation system and governor system, if they are included.
2.2.2 Motors
Due to the need for motors to be represented as active sources in DS, motors have a modified behavior in the power flow solution within the DS Focus.
First, since the regular power flow does not contain the ability to model motors as Thevenin sources during balanced switching, motors are converted to a PQ generator internally. This means that the motor load is treated as a constant MVA load (instead of constant current or impedance as specified in the motor data dialog) during the DS Swing Bus Power Flow. This slight change could then cause a system to take a few more iterations to solve, and could present slightly lower voltages than those seen when solving the system in the Power Flow focus.
NOTE: If the user notices that DS entry is not allowed due to lack of a power flow solution, there are induction motors being modeled and the system solves just fine in the Power Flow focus, try increasing the number of iterations in the Power Flow Options dialog. This can be accessed in the PF focus, under Tools.
Then, try re-entering DS Focus.
In addition, induction motors have an issue in matching var requirements determined by the machine equations with vars specified in the database. To properly initialize the motor, we have elected to ignore vars specified in the database, and match vars determined by machine equations. This then causes the need to repeat several power flow solutions (iteratively), as the machine equations are initialized to supply updated var requirements. Upon completion, the initial power flow solution will create a match between power flow voltage and var conditions, so that induction motor machine equation var requirements match the power flow.
Finally, there are differences in how single and grouped motors are treated. Refer to “Induction Motor Modeling – Part 4”, for an in depth discussion of this behavior.
2.2.3 MCCs and Panels
In the DS Engine, MCC’s and Panels have no ability to model dynamic motor response. Their motors specified will be treated as passive loads in the DS Engine. If motor starting is desired for a motor in an MCC, we recommend adding a single motor on a bus connected off of the MCC.
Note also, that since the motors of MCCs and Panels are represented as passive, there is no short circuit contribution from them during a fault in DS.
In a future revision, detailed modeling of MCC and Panel motors will be considered.
2.2.4 Behavior of UPS’
In the Power Flow focus, a UPS is modeled as a load on its primary, and a swing generator on its output. This falls in line with a UPS holding voltage and supplying power as needed, as long as it is operating within its rating.
In the Short Circuit focus, a UPS has a fault contribution like a generator on its output according to entered data.
In the DS Focus, a UPS is modeled as a fixed Thevenin voltage source on its output, and a load on its input. During a simulation, changes in the output loading are not reflected on the primary, and no voltage control occurs. We thus recommend not simulating switching actions on the secondary of a UPS, as they will provide no automatic voltage and power response, and the changes will not reflect onto the high side of the UPS.
In a future revision, full UPS automatic control will be considered.
2.2.5 Transformer Tap Behavior
Since transformer tap changing is typically timed in an actual system with a 20 to 60 second delay, transformer tap changing is disabled during a dynamic simulation.
2.2.6 Protective Device Behavior
All Protective devices modeled in Power Protector are simulated in the DS Engine if appropriate data is supplied and the Power Protector feature is enabled (has been purchased). For users without Power Protector, no protective devices are transferred into the DS Focus.
Modeling of protective devices includes:
Fuses
LV Breakers
Relays
Under-Frequency Relay Action
Contactor Drop Out Action
ATS Auto-Transfer Action
Over-Voltage Relay Action
Under-Voltage Relay Action
Source Inverter Solid State Blocking Action for Faults
In all cases, for devices that include a minimum and maximum curve for device operation (for example, an uncertainty band or fuse min melt and max clear curve) the more severe max clear curve is used to determine when a device will be tripped. This will thus keep a fault condition on longer, and corresponds to a consistent tripping action that matches the EasyPower Arc Flash tool. More specifically, we note the following for each protective device:
Fuses Fuses are simulated using an accumulated I2T action. When current through a fuse causes the trip time to drop below 1000 seconds, I2T energy begins to accumulate.
The I2T trip value is updated on each time step corresponding to the present current flowing through the fuse. When the accumulated I2T meets or exceeds the I2T trip value, the fuse is tripped (actually the EasyPower switch on the oneline is opened to simulate this).
Note that the energy accumulated in the fuse is not reset during a given simulation. This memory action is performed since a typical dynamic simulation runs for 10 to 50 seconds, and we believe this is not long enough to allow any significant dissipation of heat from the fuse. From this memory action, multiple faults through a fuse can contribute to a faster fuse blowing action, which in reality would exist in the field.
LV Breakers Low Voltage Breakers are simulated using a time accumulation method.
When current through a LV Breaker causes the trip time to drop below 1000 seconds, then a timer is used (accumulating time) to trip the device as long as the current remains above the devices pickup setting. When the accumulated time exceeds the trip time at the current point on the devices TCC (specified and updated by the present current flowing through the device), the device will trip.
The device instantaneously resets if the current drops below the pickup setting.
Relays Relays are simulated using time accumulation as a simulated induction disc turns.
This assumes that digital relays are performing a similar action. Thus the device is simulating travel time and tripping in accordance to the time dial setting.
When current through a Relay causes the trip time to drop below 1000 seconds, the disc simulator starts timing. When the time passing by meets or exceeds the trip time from the relays TCC based on present current through the device, the device will trip.
If the current drops below the pickup setting before tripping, the device will simulate travel-back of the induction disc (again assuming digital devices will do the same). This travel-back assumes that the full travel-back of any relay is 60 seconds when on the maximum time dial, with this effect ratio’ed accordingly to other time dial settings. From this travel-back action, we are simulating memory, and thus are including the capability of a relay to trip faster on a second fault application.
UF Relays Under-Frequency Relays use a single under-frequency setting and timer. When the bus frequency drops below the setting, and has stayed below for the time specified, the device will trip its specified breaker. The relay performs an instantaneous reset if the frequency goes above its setting.
Under-Frequency Relays are presently only able to be connected onto a Current Transformer (CT). In a future revision, we anticipate adding Potential Transformers (PT) to the equipment pallet.
Contactors Contactors operate like the Under-Frequency Relay, and trip after voltage has dropped below its setting for the time specified. The device resets instantaneously.
ATS ATS’ only perform automatic operation from left (Normal) to right (Emergency) as seen on the oneline. Their behavior is as follows:
Assuming we are originating on the Normal side, if the source is lost (voltage drops below the Trip Voltage setting for a time longer than the Delay on Start setting), the ATS is prepared for transfer. If the voltage on the Emergency side is above the ATS Required Voltage setting, then the transfer will continue. If not, the ATS remains on the Normal side.
The ATS model does not presently simulate the Neutral position in its transfer.
Therefore, if transferring, it stays on the Normal Source side until the Neutral Delay time and Mechanical Delay time are satisfied. To simulate the Neutral position, an additional bus would need to be simulated in the network, and that has not been implemented in the present version.
The ATS will not auto-transfer-back if the Emergency Source is lost. The action presently modeled is a one-way transfer.
OV Relays Over-Voltage Relays use a single over-voltage setting and timer. When the bus voltage goes above the setting, and has stayed above for the time specified, the device will trip its specified breaker. The relay performs an instantaneous reset if the voltage goes below its setting.
Over-Voltage Relays are presently only able to be connected onto a Current Transformer (CT). In a future revision, we anticipate adding Potential Transformers (PT) to the equipment pallet.
UV Relays Under-Voltage Relays use a single under-voltage setting and timer. When the bus voltage goes below the setting, and has stayed below for the time specified, the device will trip its specified breaker. The relay performs an instantaneous reset if the voltage goes above its setting.
Under-Voltage Relays are presently only able to be connected onto a Current Transformer (CT). In a future revision, we anticipate adding Potential Transformers (PT) to the equipment pallet.
Inv Block Inverters (when no DS data is specified) include a blocking action when the fault current (specified in the inverter data dialog), has stayed above 102% FLA for the time specified. Upon blocking, the inverter current injection is removed from the network model and the inverter does not interact with the grid.