1. CIGRE Grid of the Future (GOTF) Symposium
STATCOM Application to Address Grid Stability and Reliability: Part II
Monday October 12, :00-11:30 AM
Paper Session 1B
System Analysis, Grid Control and Reliability Technical Track
Authors:
Donald Shoup, P.E., MEPPI
Nick Tenza, MEPPI
Greg Reed, University of Pittsburgh

2. Summary of Part I
FACTS and Other Technologies are a Valuable Asset to the Planning Toolbox
Use where tradition tools are saturated in use and are not feasible
Use to address new concerns driven by changing landscape of system
Tools Required are Changing Based on the Continually Changing Landscape of the System and Its Operation
System changing more rapidly with multi-dimensional challenges
Adaptability increasingly important
FACTS Have Many Years of Operating Experience
FACTS are used to address rotor angle, frequency, and voltage stability
FACTS have been used for 30+ years
Studies are Required to Determine the Optimal Solution
Optimal solution is not obvious and avoid ‘general conceptions’
Optimal solution may be a combination of many technologies
FACTS devices are a tool when traditional solutions are saturated in their use, cannot respond quick enough, or an not practical for cycling (repeated operations in short amounts of time)… Plus reasons when constructability, permitting, security, and adaptability are of concern.
System strength, loading conditions, and generating conditions/dispatch are all important to consider for evaluating solution options.
About thirty years ago SCs were replaced by SVCs for variable reative power compensation and voltage control in new schemes. The main reason for this was improved dynamic voltage performance, lower investment and lower operational costs.
These reasons are even more valid today. Why his new interest in SCs?
There are two main Reasons:
Wind and solar power with converters are replacing active power from thermal power plants but not their short circuit power or inertia. Denmark.
Installation or upgrade of HVDC LCC installations are being considered where the existing short circuit power is low or has already been “utilized” by existing HVDC systems, e.g. Canada, Norway 2

3. Summary of Part I (Continued)
About thirty years ago SCs were replaced by SVCs for variable reative power compensation and voltage control in new schemes. The main reason for this was improved dynamic voltage performance, lower investment and lower operational costs.
These reasons are even more valid today. Why his new interest in SCs?
There are two main Reasons:
Wind and solar power with converters are replacing active power from thermal power plants but not their short circuit power or inertia. Denmark.
Installation or upgrade of HVDC LCC installations are being considered where the existing short circuit power is low or has already been “utilized” by existing HVDC systems, e.g. Canada, Norway
FACTS accommodate changing generation conditions, i.e., renewable and intermittent resources, and distributed energy resources that can act as both a load and source. They also are adaptable to various system conditions and are fully controllable to act as the ‘perfect valve’ with prescribed reactive power and real power flows. This capability would also support the ability to address cascading outage conditions. 3

4. Review of Terminology
Rotor angle stability refers to the ability of synchronous machines of an interconnected power system to remain in synchronism after being subjected to a disturbance. (Large and small disturbance initiated events.)
Transient instability results from a lack of sufficient synchronizing torque
Oscillatory instability (i.e., power system damping) results from a lack of sufficient damping torque
Frequency stability refers to the ability of a power system to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load
Voltage stability refers to the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance from a given initial operating condition (Large and small disturbance and long-term and short-term.)
Reliability of a power system refers to the probability of its satisfactory operation over the long run. It denotes the ability to supply adequate electric service on a nearly continuous basis, with few interruptions over an extended time period
Adequacy refers to the ability of the power system to supply the aggregate electric power and energy requirements of the customer at all times, taking into account scheduled and unscheduled outages of system components.
Security of a power system refers to the degree of risk in its ability to survive imminent disturbances (contingencies) without interruption of customer service. It relates to robustness of the system to imminent disturbances and, hence, depends on the system operating condition as well as the contingent probability of disturbances.
Stability of a power system refers to the continuance of intact operation following a disturbance. It depends on the operating condition and the nature of the physical disturbance
About thirty years ago SCs were replaced by SVCs for variable reative power compensation and voltage control in new schemes. The main reason for this was improved dynamic voltage performance, lower investment and lower operational costs.
These reasons are even more valid today. Why his new interest in SCs?
There are two main Reasons:
Wind and solar power with converters are replacing active power from thermal power plants but not their short circuit power or inertia. Denmark.
Installation or upgrade of HVDC LCC installations are being considered where the existing short circuit power is low or has already been “utilized” by existing HVDC systems, e.g. Canada, Norway 4

5. Study Application Example
20,000 MW of load pocket
40% of the load is served by remote generation 200 miles away from the load center and is not available for reactive power support
Load pocket is expected to continue to have generation retirements because of aging generation plants and clean air legislation
Load is expected to continue to increase in the load pocket
Of the total load pocket, 40% of the load is estimated to be single-phase residential air- conditioner (a/c) motor load.
Mechanically Switched Capacitors (MSC) provide compensation on the order of magnitude of one-third of the total load in the region (7000 Mvar).
Under-Voltage Load Shedding (UVLS) and Under-Frequency Load Shedding (UFLS) are not used to mitigate short-term voltage stability here.
Key Contingency:
To Generation
Primary Clearing
Example 345 kV Bus
Hung Breaker
Backup Clearing
(x3 Lines)
To 138 kV
x2 Lines
The test system has 20,000 MW of loads that are connected to a 138 kV system with a strong 345 kV backbone
40% of the load is served by remote generation 200 miles away from the load center and is not available for reactive power support. The remote generation consists of a large nuclear plant, coal plants, gas turbines, and wind generation.
Load pocket is expected to continue to have generation retirements because of aging generation plants and clean air legislation. The local generation will be replaced with remote generation, which will further exasperate voltage problems.
Load is expected to continue to increase in the load pocket.
Load is summer peaking. Of the total load pocket, 40% of the load is estimated to be single-phase residential air-conditioner (a/c) motor load.
Mechanically Switched Capacitors (MSC) provide compensation on the order of magnitude of one-third of the total load in the region (7000 Mvar).
Under-Voltage Load Shedding (UVLS) and Under-Frequency Load Shedding (UFLS) are to be avoided as means for mitigating short-term voltage stability issues here. 5

6. Results
This figure shows the voltage at a key bus in the area of interest as a function of delayed clearing time. Here we can see that as the clearing time increases from 2 cycels to 10 cycles (or from top to bottom), the voltage profile at this bus detaeriorates. In fact with a clearing time of more than 6 cycles, the bus voltage collapses. Also, the voltage recovery requirement does not meet for the clearing time of more than 2 cycles.
Furthermore, 6

7. Various Re-Enforcement Options for
Results (Continued)
Various Re-Enforcement Options for
10 Cycle Clearing Time
Surrounding generators also loose synchronism and trip as shown in the top plot. Hit Enter
This plot sows the voltage at the same bus, as before, but with SVC, STATCOM and SC to provide reactive power to support the voltage. To bring the voltage back to 0.9 p.u with 1.0 second without stalling any motors,
If using SVC : We need 2*400 Mmvar at 4 locations (total of 3200 Mvar ) required.
If using STATCOM: 2*300 Mvar at 3 locatios with 125% overload for 3 s rewquired (total of 1800 Mvar.) without any overload, 4 locations with a total of 2400 is required.
If using SC : 2*300 Mvar at 3 locations (total of 1800) required.
Even though all three solution options provide the desired result, a detailed study must be done to determine the best solution for a given application 7

8. Sensitivity Analysis
This table shows the result of the sensitivity analysis. There is no significant affect on the output of these devices when the load increases upto 2%.
But when the stuck breaker time is reduced from 10 cycles to 6 cycles, there is a significant reduction in the VAR output of the devices. This reduction further increases when the breaker-and half bus scheme is adapted to reduce the the number of outaged elements.
And the Last row shows that loosing around 2000MW of generation, referring to FERC ORDER 754 which says that system should be stable when upto 2000MW of generation is lost, the VAR output increases significantly. 8

9. SVC, STATCOM, and Synchronous Condenser
Example Dimensions (+250 Mvar): 25m x 40m = 1000 m2 (10,764 ft2)
Example Dimensions (+300/-100 Mvar): 78m x 78m = 6,084 m2 (65,488 ft2)
This table shows the result of the sensitivity analysis. There is no significant affect on the output of these devices when the load increases upto 2%.
But when the stuck breaker time is reduced from 10 cycles to 6 cycles, there is a significant reduction in the VAR output of the devices. This reduction further increases when the breaker-and half bus scheme is adapted to reduce the the number of outaged elements.
And the Last row shows that loosing around 2000MW of generation, referring to FERC ORDER 754 which says that system should be stable when upto 2000MW of generation is lost, the VAR output increases significantly.
Above is 100 MVA GCT-Based STATCOM
Note the footprint of a STATCOM is reduced by 50% or
more than that of an SVC (38m x 76m for comparison). 9

10. Summary
Refer to Paper for Table on “Summary of Characteristics Associated with the Solution Options” (note actual characteristics will vary from that given in table)
Studies are Required to Determine the Optimal Solution
The STATCOM is an effective solution. Of further consideration for the STATCOM solution is the use of new switching technologies such as the Modular Multilevel Converter (MMC), providing lower losses than conventional designs. Continued advancements in semiconductor technology require less semiconductor devices with increased current and voltage ratings than conventional designs.
The synchronous condenser provides an alternative solution option with its inherent, initial sub-transient reactive power response to a fault, subsequent ability to provide a transient reactive power response far in excess of its steady state capability, and small footprint. Application of a STATCOM or a synchronous condenser may be a choice between maintaining transmission level interconnected power electronics-based technologies versus rotating machinery.
The test system has 20,000 MW of loads that are connected to a 138 kV system with a strong 345 kV backbone
40% of the load is served by remote generation 200 miles away from the load center and is not available for reactive power support. The remote generation consists of a large nuclear plant, coal plants, gas turbines, and wind generation.
Load pocket is expected to continue to have generation retirements because of aging generation plants and clean air legislation. The local generation will be replaced with remote generation, which will further exasperate voltage problems.
Load is expected to continue to increase in the load pocket.
Load is summer peaking. Of the total load pocket, 40% of the load is estimated to be single-phase residential air-conditioner (a/c) motor load.
Mechanically Switched Capacitors (MSC) provide compensation on the order of magnitude of one-third of the total load in the region (7000 Mvar).
Under-Voltage Load Shedding (UVLS) and Under-Frequency Load Shedding (UFLS) are to be avoided as means for mitigating short-term voltage stability issues here. 10