The InterNational Electrical Testing Association Journal. BY STEVE TURNER, Beckwith Electric Company, Inc.

Transcription

1 The InterNational Electrical Testing Association Journal FEATURE PROTECTION GUIDE 64S Theory, Application, and Commissioning of Generator 100 Percent Stator Ground Fault Protection Using Low Frequency Injection BY STEVE TURNER, Beckwith Electric Company, Inc. This paper covers practical considerations for proper application and commissioning of this special protection. The total capacitance to ground of the generator stator windings, bus work, and delta-connected unit transformer windings is a very important factor as will be shown. This protection can: Detect a stator ground when the winding insulation first starts to break down and trip the unit before catastrophic damage occurs Be set to trip in the order of cycles since the 20 Hz signal is decoupled from the 50 or 60 Hz power system Detect grounds close to the machine neutral or even right at the neutral thus providing 100 percent coverage of the stator windings Detect grounds when the machine is starting up or offline Reliably operate with the generator in various operating modes such as a synchronous condenser and at all levels of real and reactive power output. Special steps should be taken in the design of this protection as there are cases when it is difficult to distinguish between normal operating conditions and an actual ground fault. The protection must reject fundamental frequency (50 or 60 Hz) voltage and current signals that are present during ground faults on the stator windings. A real life example for a pump storage facility is included. STATOR GROUND FAULTS ROOT CAUSES In-service stator winding failure to ground is a common generator failure, and there are many possible causes such as mechanical damage to the ground wall insulation (category #1) and fracturing of the current carrying conductor that result in the ground wall insulation burning away (category #2). Examples of category #1 failures: ground wall insulation wear-through from a foreign object or loose component 68 SUMMER 2014

2 FEATURE fracture of the ground wall due to a sudden short circuit deficient ground wall insulation system partial discharge combined with vibration vibration sparking, inadvertent damage during maintenance wet insulation due to strand header water leak, bar vibration in the slot Examples of category #2 failures: fracture of stator bar copper conductors due to high cycle fatigue associated with vibration fracture of bar copper due to gross overheating of the copper core iron melting, failure of a bolted connection failure of a brazed or welded joint failure of a series or phase connection Category #1 failures are typically benign, unless a second ground occurs, in which case there is massive arcing. Category #2 failures are always highly destructive to the generator. Current will temporarily continue to flow uninterrupted within the stator bar ground wall insulation (i.e., welding arc) when a conductor breaks and the heat generated is extremely high. This current will flow inside the insulation until it is mechanically destroyed. Experience has shown that the copper is vaporized for perhaps 8 to 12 inches before the internal arcing breaks through the insulation wall and arcs to ground as shown in Figures 1-3. Conventional neutral overvoltage protection (59N) cannot detect grounds in the last 5 to 10 percent of the stator winding. If a failure occurs in a lower voltage portion of the winding near the neutral, a generator trip will not typically occur until some other relay protection detects there is a problem, (e.g., arcing becomes so widespread that other portions of the winding become involved). There has been recent experience with four such failures in large generators that demonstrate the lack of proper protection can be disastrous. Each of the four failures caused massive damage NETA_InteriorPages_SUM14.indb 69 Figure 1: Typical Winding Damage Resulting from Broken Stator Winding Conductor Figure 2: Typical Core and Winding Damage Resulting from a Burned Open Bar in a Slot Figure 3: Burned Away Copper Fractured Connection Ring NETAWORLD 69 5/21/14 1:27 PM

3 to the generator and collectively had a total cost, including repair and loss of generation, close to $500,000, This demonstrates that failure of stator windings in the last five percent of the winding is not uncommon. One hundred percent stator ground fault protection is provided by injecting a 20 Hz voltage signal (64S) into the secondary of the generator neutral grounding transformer through a band-pass filter. The band-pass filter passes only the 20 Hz signal and rejects out of band signals. The main advantage of this protection is it provides 100 percent protection of the stator windings for ground faults including when the machine is off-line (provided that the 20 Hz signal is present). APPLICATION Figure 4 illustrates a typical application. A 20 Hz voltage signal is impressed across the grounding resistor ( ) by the 20 Hz signal generator. The band-pass filter only passes the 20 Hz signal and rejects out of band signals. The voltage across the grounding resistor is also connected across the voltage input (V N ) of the 64S function. The current input (I N ) of the 64S function measures the 20 Hz current flowing on the secondary side of the grounding transformer and is stepped down through a CT. It is important to note that the relay does not measure the 20 Hz current flowing through the grounding resistor; it measures the 20 Hz current flowing into the power system. The 20 Hz current magnitude increases during ground faults on the stator winding and an overcurrent element that operates on this low frequency current provides the protection. 20 Hz Voltage and Current 64S Relay Measurements The following method shows how to calculate the 20 Hz voltage and current measured by the 64S function. Figure 4: 20 Hz Injection Grounding Network 70 SUMMER 2014

4 Grounding Transformer Turns Ratio (N) Assume that the turns ratio of the grounding transformer is equal to: N 8, (1) Capacitive Reactance The total capacitance to ground of the generator stator windings, bus work, and delta connected transformer windings of the unit transformer is expressed as C 0. Generator step up transformers have delta connected windings facing the generator so capacitance on the high side is ignored. The corresponding capacitive reactance is calculated as follows: X CO 1 2π f o C o (2) Assume the capacitance to ground is 1 microfarad: X CO 1 2 π (20Hz) (10 6 F) 7,958Ω primary Reflect the capacitive reactance to the secondary of the grounding transformer: X CO 7,958Ω N 2 7, 958Ω ( ) 8, Ω secondary Grounding Resistor ( ) The ohmic value of the grounding resistor can be sized as follows so as to avoid high transient overvoltage due to ferroresonance 2 : X c0 3 (3) Ω sec Ω secondary A value of 2.5 ohms secondary is used for this example. 20 Hz Signal Generator and Band-pass Filter Characteristics The 20 Hz signal generator output is 25 volts RMS and the band-pass filter has a resistance equal to eight ohms. V source 25 0 volts (4) R Filter 8Ω secondary (5) Stator Insulation Resistance ( ) is the total insulation resistance from the stator windings to ground. A typical value for nonfault conditions is 50,000 ohms primary. Reflect the insulation resistance to the secondary of the grounding transformer. R s 50,000Ω pri N 2 45Ω secondary 50,000Ω pri ( ) 2 8, Current Transformer The current input (I N ) of the 64S function measures the 20 Hz current flowing on the grounded side of the grounding transformer and is stepped down through a CT. CTR 400/5 80/1 (6) Grounding Network These are all of the elements needed to mathematically derive the grounding network and determine the 20 Hz signals measured by the 64S function. Figure 5A shows the insulation resistance and the stator windings referred to the primary of the grounding transformer. Figure 5B shows the insulation resistance and the stator windings reflected to the secondary of the grounding transformer. NETAWORLD 71

5 Figure 5A: 20 Hz Grounding Network Referred to Grounding Transformer Primary Figure 5B: 20 Hz Grounding Network Refl ected to Grounding Transformer Secondary X C total capacitance to ground R stator stator insulation resistance to ground neutral resistor R Filter band-pass filter resistance V N voltage drop across I N current from system I BANK current flow in I T I BANK + I N 20 Hz Current (I N ) Measured by 64S Function The current input (I N ) of the 64S function measures the 20 Hz current flowing in the secondary of the grounding transformer and is stepped down through a CT. As noted previously the relay does not measure the 20 Hz current flowing through the grounding resistor. Z T 8 + ( 2.5) / /( 45) / /( j) j Ω secondary The total 20 Hz current supplied by the signal generator is determined as follows: Total 20 Hz Current Supplied by Signal Generator The 20 Hz signal generator looks into the band pass filter resistance (R Filter ) which is in series with the parallel combination of the following: Z CO Rs I T I T V CTR Z T (8) V ( j Ω) Therefore, the total loop impedance of the 20 Hz grounding network can be expressed as follows: Z T R Filter + Z CO + Z CO + Z C O + Z CO (7) ma 20 Hz Current Measured by 64S Function (I N ) during Nonfaulted Conditions The 20 Hz current measured by the 64S function is the ratio of the total current that flows into the primary side of the grounding network (Z CO //R s ): 72 SUMMER 2014

6 Table 1: 20 Hz Current Measurements I N I T I N Z C o Z Co + (9) ( j) / /(45) ma ( Nonfaulted) 20 Hz Current Measured by 64S Function (I N ) during Ground Fault on Stator Windings A typical value to represent the insulation resistance of the stator windings when it is breaking down during a ground fault is 5,000 ohms primary. If the calculations for equations 7 through 9 are repeated for a fault resistance equal to 5,000 ohms primary (4.5 ohms secondary), then the 20 Hz current measured by the relay is as follows: I N ma (5,000 ohm primary ground fault) If the calculations for equations 7 through 9 are repeated for a fault resistance equal to 1,000 ohms primary (0.9 ohms secondary), then the 20 Hz current measured by the relay is as follows: (primary) I N (secondary) 50,000 Ω ma 5,000 Ω ma 1,000 Ω ma PRACTICAL CONSIDERATIONS Here are three important aspects to consider when applying 100 percent stator ground fault protection by 20 Hz injection: slight change in fault current measured by relay rejection of fundamental frequency (50 or 60 Hz) voltage and current signals under-frequency inhibition Slight Change in Fault Current A very large capacitance to ground (C 0 ) coupled with a small value for the grounding resistor ( ) can result in very little margin between the fault and nonfault current measured by the 64S function. Generator windings and isophase bus work (see Figure 6) are both sources of high capacitance to ground (ex., a long run of bus from the generator to the step-up transformer). I N ma (1,000 ohm primary ground fault) Table 1 summarizes the 20 Hz current measured by the relay for nonfaulted and faulted conditions. Often the 64S protection can detect the ground for insulation resistance is much higher than 5,000 ohms primary. The 64S function can easily distinguish between nonfault and stator ground faults for this example. Set the pickup of the 64S, 20 Hz tuned, overcurrent element above the current measured during normal operating conditions but below the current measured for a stator ground fault equal to 5,000 ohms primary. Figure 6: Iso-Phase Bus Work NETAWORLD 73

7 Figure 7: Primary Side Current Distribution Figure 8: Current Flow in Secondary Network Figure 7 illustrates why there is not a significant change in the magnitude of the total neutral current when there is high capacitance. If the capacitive reactance is low enough, then only a small portion of the total neutral current flows through the insulation resistance. To overcome this challenge, calculating the real component of the total neutral current can reliably detect a stator ground fault for such a system. Consider the following grounding network parameters: N 8, C 0 10µF Z CO 0.25 Ω secondary Table 2: 20 Hz Current Measurements for High Capacitance (primary) j Ω secondary Determine the 20 Hz current measured by the 64S function for nonfaulted and stator ground fault conditions using the equations presented in the previous application section as shown in Table 2 below. I N (secondary) 50,000Ω ma 5,000Ω ma 1,000Ω ma The 64S 20 Hz tuned overcurrent element pickup cannot be set such that it reliably discriminates between nonfaulted and stator ground fault conditions. The solution is for the relay to calculate the real component of the 20 Hz current with respect to the neutral voltage. To do so, first determine the 20 Hz voltage measured by the relay voltage input (V N ). The 20 Hz voltage is equal to the drop across the grounding resistor due to the portion of the total current flowing through this branch of the grounding network (i.e., I BANK ). Use Figure 8 above to determine the corresponding equations (10 13). 20 Hz Current Flowing through the Grounding Transformer I Bank I T I N (10) 20 Hz Voltage Drop Across the Grounding Resistor V N I Bank (11) Real Component of 20 Hz Current Measured by 64S Function Calculate the real component of the relay current based upon the angle between the relay neutral voltage (V N ) and current (I N ) as shown in Table 3. V N I N (12) I Real I N i COS( ) (13) 74 SUMMER 2014

8 Table 3: 20 Hz Current Measurements for High Capacitance Including Real Component (primary) I N (secondary) R e (I N ) 50,000Ω ma ma 5,000Ω ma ma 1,000Ω ma ma A 20 Hz tuned overcurrent element that operates on the real component of 20 Hz current measured by the 64S function can reliably distinguish between nonfaulted and stator ground fault conditions when there is high-capacitive coupling to ground on the stator winding. A good rule of thumb is to use the real component of 20-Hz current for sensitive protection if C 0 is greater than 1.5 microfarads and the grounding resistor is less than 0.3 ohms secondary. The user can follow the commissioning instructions that appear at the end of this paper to determine the total capacitance to ground (C 0 ). If the values for and C 0 do not clearly fall under the category defined by this rule of thumb, then use the equations provided earlier in this paper to determine if use of the real component of neutral current is necessary. Figure 9 illustrates the fundamental voltage drop (50 or 60 Hz) across the grounding resistor as a function of the ground fault location along the stator windings. Table 4 shows the voltage drop as the fault location moves from the neutral end of the stator windings to the output terminals. Thegrounding transformer is rated 110 volts secondary; the grounding resistor is sized 0.32 ohms secondary; and the CT ratio is 400:5 (80:1). The corresponding fundamental component circulating current is shown as well. If the fundamental current is not well rejected, then high magnitude circulating current can saturate the neutral current input and, as a result, the protection will measure a value of 20 Hz current less than the actual value. Saturation causes the following problems: delayed operation or, even worse, no operation at all less than 100% coverage of the stator windings as the ground fault location moves towards the generator output terminals. NOTE Saturation is most likely to occur when the grounding resistor is sized less than one ohm secondary. NOTE If the 64S function uses the real component, it should also use the total neutral current as a backup for the case of a solid short circuit located right at the machine neutral (i.e., the real component equals zero for this case since there is no reference voltage). Rejection of Fundamental Frequency (50 or 60 Hz) Voltage and Current Signals Fundamental component voltage and current present at the relay measuring inputs during stator ground faults can prevent the 64S function from operating properly unless they are well rejected. Note that these signals are not eliminated by the band-pass filter since they are due to the fundamental voltage drop across the secondary of the grounding transformer. Figure 9: Fundamental Voltage across as Function of Ground Fault Location NETAWORLD 75

9 Table 4: Fundamental Voltage Drop Across Grounding Resistor and Circulating Current ( 0.32 Ω) Fault Location V S I S I S /CTR 100% (phase side) 110 V (110 V)/0.32 Ω amps amps 90% 99 V (99 V)/0.32 Ω amps amps 80% 88 V (88 V)/0.32 Ω 275 amps amps 70% 77 V (77 V)/0.32 Ω amps amps 60% 66 V (66 V)/0.32 Ω amps amps 50% 55 V (55 V)/0.32 Ω amps amps 40% 44 V (44 V)/0.32 Ω amps amps 30% 33 V (33 V)/0.32 Ω amps amps 20% 22 V (22 V)/0.32 Ω amps amps 10% 11 V (11 V/0.32 Ω amps amps 0% (neutral side) Table 4 provides magnitudes of the variables at various fault locations along the winding. Underfrequency Inhibition Sometimes it is necessary to block the 64S function during conditions such as startup when the system voltage measured by the relay is 40 Hz or lower. The third harmonic of 7 Hz is very close to 20 Hz during startup when the generator is transitionally through the lower frequencies and can cause unwanted operation. COMMISSIONING INSTRUCTIONS Figure 10 illustrates how to configure the 64S protection for commissioning. NOTE Use the 20 Hz signals captured during normal operating conditions to determine appropriate overcurrent pickup settings. Determine the total capacitance to ground and calculate the overcurrent pickup setting based upon these field measurements as a check. Stator Ground Fault at Machine Neutral Configure the power system as follows: High side breaker is open. The generator is offl ine. Generator terminals are connected to delta windings of the generator step up transformer. Switch F 1 is closed. Switch F 2 is open. 20 Hz signal generator is online. Place a single line-to-ground fault at location F 1 (switch F 1 closed) and measure the 20 Hz I N. This measurement corresponds to a short circuit applied at the neutral of the machine (see Figure 11 on the following page). The total 20 Hz neutral current (I N ) should be very close in magnitude to 39 ma. 76 SUMMER 2014

10 Figure 10: 20 Hz Grounding Network for Commissioning I sc 25 8ohmsX mA NOTE This is the first step and is a quick check to see if all the wiring is correct. Normal Operating Conditions Configure the power system as follows: High side breaker is open. Generator terminals are connected to delta windings of the generator step up transformer. Switch F1 is open. Switch F2 is open. 20 Hz signal generator is online. Preferably the generator is online. Measure the following 20 Hz signals: V N (neutral voltage) I N (neutral current) Re(I N )(real component of neutral current) These signals correspond to normal operating conditions. V N and I N are the 20 Hz signals applied to the relay inputs. Record V N, I N, and Re(I N ). Modern numerical generator relays can meter these values. The signals recorded in Figure 11 were captured while commissioning a large pumped storage hydroelectric unit while operating in the motoring (pumping) mode. Figure 11: Short Circuit at Machine Neutral Figure 12: Numerical Generator Relay 20 Hz Metering NETAWORLD 77

11 CONCLUSIONS This special protection provides 100 percent coverage of the stator windings for ground faults under all operating conditions including variable real and reactive power output and when off-line. Grounds on the last five percent of the stator windings do occur and can be disastrous if not quickly detected. Traditional protections (59N, 27TH, 27D) may not reliably operate for faults in the last five percent of the stator winding. The total capacitance-to-ground of the generator stator windings, bus work, and delta-connected unit transformer windings is a very important factor and must be known to ensure the protection settings are correctly determined. This value can be determined during commissioning. There are cases when it is difficult to distinguish between normal operating conditions and an actual stator ground fault unless special steps are taken in the design of this protection. A good rule of thumb to decide if the real component of 20 Hz current is necessary is when C 0 is greater than 1.5 microfarads and the grounding resistor is less than 0.3 ohms secondary. Use the real component of the 20 Hz current measured by the relay for these cases. The protection must reject fundamental frequency (50 or 60 Hz) voltage and current signals that are present at the relay measuring inputs during ground stator ground faults. Use an under-frequency element that operates on the system voltage to block the 64S function if nuisance tripping occurs during either startup or shutdown of the generator. REFERENCES 1. Maughan, Clyde V., Stator Winding Ground Protection Failures, ASME POWER Macon, Russell C., The Art and Science of Protective Relaying, Wiley , 1956 Steve Turner is a Senior Applications Engineer at Beckwith Electric Company, Inc. His previous experience includes working as an application engineer with GEC Alstom for five years, an application engineer in the international market for SEL, focusing on transmission line protection applications. Steve worked for Progress Energy, where he developed a patent for double-ended fault location on transmission lines and was in charge of all maintenance standards in the transmission department for protective relaying. Steve has both a BSEE and MSEE from Virginia Tech University. He has presented at numerous conferences including Georgia Tech Protective Relay Conference, Western Protective Relay Conference, ECNE, and Doble User Groups, as well as various international conferences. Steve Turner is also a senior member of the IEEE. 78 SUMMER 2014